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Swimming Pool and Hot Tub Spa Heater Operation

Pool and Spa Heaters Terminology

By heating the pool or a spa just a few hours each day, can extend the swimming season several weeks into the spring and fall. In warmer climates, a heater can allow you to swim all year round. Here is a glossary of basic terms that are commonly used for gas fueled heaters.

Btu British thermal unit. A unit of measurement used to define the capabilities of heaters. One Btu is capable of raising the temperature of one pound of water by one degree Fahrenheit.
Burner The component of the heater where the combustion of fossil fuel takes place. In a pool and spa heater, the burner is typically located in a heater's combustion chamber.
Bypass valve A valve that maintains a constant flow of water through the heat exchanger, thus preventing damage to heater components.
Combustion The act or process of burning - and technically described as "rapid oxidation usually accompanied by heat and light."
Direct-fired heater A type of heater in which the heat produced by the burning of fossil fuels is exchanged directly to water flowing through a heat exchanger. These heaters use ignition systems of two basic types

Millivolt ignition, in which heat from a continuous pilot is used to generate electrical energy that opens and closes the main gas valve and operates the system's safety controls; and

Automatic spark ignition (also known as an intermittent ignition device, or IID), in which the pilot is lit only during heater operation and requires an outside electrical hook-up to spark the pilot and run the system's control and safety devices.

Fireman switch A device ensuring that the heater shuts off for a period of time - say, five to 15 minutes before the recirculation pump shuts off. This switch allows the water to cool off the system before it stops running, thus prolonging the life of the heat exchanger. (Due to varying designs of combustion chambers, not all units require use of a fireman switch.)
Flow switch A safety device that prevents the heater from firing if there isn't adequate water flow through the system.
Header A manifold in a heater that directs the flow of water into and out of the heat exchanger.
Heater efficiency The ratio of heater output and input. Heater output is energy, expressed in BTUs, transferred to the water. Heater input is the energy used to generate that heat. Heater efficiency is calculated by dividing heater output by heater input.
Heat exchanger A device with coils, tubes or plates that absorbs heat from any fluid, liquid or air and transfers that heat to another fluid without intermixing. Most common to the pool and spa industry is a copper-finned heat exchanger.
Heater input adjustment A regulator built into the gas valve in a gas fired heater that controls the gas input to the burners and combustion chamber.
Heat loss The natural drop in water temperature as heat is transferred to the surrounding air. The majority of heat loss in a pool or spa system flows from the uncovered surface of a pool or spa.
High limit switch A component of the temperature control system that limits the temperature of water in the heater - no higher that 140 degrees in most.
Intermittent heating A method of heating in which water temperatures are raised only when the pool or spa is going to be used.
Liquid propane gas The liquid form of propane gas, a heavy hydrocarbon occurring naturally in petroleum.
Natural gas A mixture of gaseous hydrocarbons, chiefly methane, occurring naturally underground, often in association with petroleum products.
Pilot A small, fossil-fuel burner that is kept lit in order to light the principal burner. In spark ignition heaters, the pilot is lit only when the heat is being used. In a millivolt system, the pilot is on continuously, and heat from the pilot's small flame is used to generate low-voltage electricity to operate the system's valves and safety controls.
Pilot generator The component in a millivolt system that transforms heat from the pilot into electrical energy. Also referred to as a thermal coupling or thermocouple.
Pressure switch A device that will not allow the heater to fire unless there is adequate water pressure in the system. A pressure switch must be adjusted according to differences in the level of the water's surface vs. the location of the heater.
Soot A black, powdery, carbonaceous substance created by an improper air-fuel mixture in combustion of fossil fuels. Soot is the by-product of incomplete combustion.
Temperature maintenance A method of heating whereby the water is maintained at a constant temperature.
Temperature rise The difference between the desired water temperature and the temperature of the surrounding air.
Therm A unit of thermal measurement equal to 100,000 BTUs. This unit is commonly used in gas billing by utility companies.
Thermostat A temperature-control device that shuts off the heater when the water reaches the desired temperature. In a temperature maintenance system, this device will activate the heating system when the water drops below a set level.
Venting The system responsible for the introduction of air for combustion in the combustion chamber and for dispersal of the spent fossil fuel, or flue products, into the surrounding air.

Gas Heaters

A typical gas-fueled heater uses natural or propane gas as the heating fuel. The water enters through one port of the front water header, then through the nine heat exchanger, and then out of the other port. Most of the heaters, water goes through at least four of the tubes and picks 6 to 9 degree F on each pass before leaving the heater.

The exchanger tubes are made of copper due to its excellent heat conductivity, there by transferring the heat effectively to the water. The tubes have fins to absorb heat even more efficiently and are topped with sheet metal baffles to retain the heat. However, improper water chemistry can easily attack this soft metal and dissolve it into the water.

There is a flow control assembly on the front header. This spring-loaded valve is pressure sensitive, designed to mix cool incoming water with hot outgoing water to maintain the temperature. Temperature control is achieved by flow regulation rather than direct temperature regulation. This design keeps the outgoing water no more than 10 to 25 degree F above the temperature of the incoming water. This prevents condensation and other problems that greater temperature differentials would create. When water temperatures are over 115 degree F, minerals suspended in water deposit in the heat exchanger. The design of the unit is such that it allows 100 gpm with 1 1/2 inch plumbing or 125 gpm with 2 inch plumbing.

The other major component of the gas-fueled heater is the burner tray. This assembly can be disconnected from the cabinet for maintenance or inspection. Depending on the size of the heater, there will be 6 to 16 burners, the last one having a pilot mounted on it. Individual burners can be removed for replacement. The combination gas valve regulates the flow of gas to the burner tray and pilot and is itself regulated by the control circuit.

Gas-fueled heaters are divided into two categories based on the method of ignition. They are millivolt or standing pilot heaters and the other being electronic pilot heater.

The Millivolt or Standing Pilot Heater

This type of heater has a pilot that is constantly burning. The heat of the pilot is converted into a small amount of electricity (750 millivolts) by a thermocouple, powering a number of switches. Together, the control switches constitute a control circuit. When electricity passes through the entire control circuit, the main gas valve opens and the burner tray gets flooded with gas. The gas is then ignited by the pilot. The temperature of a pilot flame is more than 1,100 degrees F.

The Control Circuit

The control circuit is a series of safety switch devices that test for various conditions in the heater to be correct before allowing the electrical current to pass on to the main gas valve and fire up the unit. Following the flow of electricity, a control circuit includes the following items.

Fusible link

A simple heat-sensitive device that is located on a ceramic holder near the front of the gas burner tray, is the fuse link. If the heat becomes too intense, the link melts and the circuit is broken, there by cutting the power in the circuit. The most common cause of high temperatures in the tray are: the heat built-up due to improper ventilation, debris in the tray that is burning, or the low gas pressure which make he flame to roll over.

On-off switch

A small toggle-type switch, on the face of the heater next to the thermostat control, is the on-off switch. Often it is a remote switch. Manufacturers recommend that a remote on/off switch for a millivolt heater should be located in 20 to 25 feet proximity from the heater. This is because with less than 0.75 volt passing through the control circuit, any loss of power caused by heat loss, means that there might not be enough electricity to power the gas valve when the circuit is completed. This is particularly true on cold days when heat loss is greater. Another reason is that as the thermocouple wears out, the electricity generated decreases, there by reducing the power.

Many manufacturers now sell dual thermostat heaters, so that when different temperatures for pool and different temperatures for the spa are desired, then you just have to flick the remote to the desired thermostat.


Thermostats are the ones that control temperature. They fall into two categories: mechanical and electronic. The mechanical thermostat is a rheostat dial connected to a metal tube that is filled with oil. At its end is a slender metal bulb, which can sense the temperature of the water coming out of the heater. These thermostats are precisely calibrated but the settings is by trial and error to achieve desired temperature due to instillation variation. Pool heater thermostats generally are color-coded around the face of the dial, showing blue at one end for cool and red at the other end for hot.

Thermostats usually do not allow water in the pool or spa to exceed 103 to 105 degree F, although they can be set higher. Also, they do not generally register water cooler than 60 degree F, so if the water is cooler than that the heater will continue to burn even if the thermostat is turned down. Therefore, to be sure that a heater is off, turn it off from the switch.

The electronic thermostat uses an electronic temperature sensor. These are more precise than mechanical types; however, they are also not given specific temperatures, but rather the cool to hot, blue to red graduated dials for settings.

High-limit switches

High-limit switches are small, bimetal switches designed to maintain a connection in the circuit as long as their temperature does not exceed a predesigned limit, usually 120 to 150 degree F. The protection value is similar to the fusible link and often two are installed in the circuit, one after the other, for safety and to keep the heater performing as designed.

The first high-limit switch is usually a 135 degree F switch, and the other is a 150 degree F switch. Where the fusible link detects excessive air temperatures, the high-limit switch detects excessive water temperatures. They are mounted in dry wells in the heat exchanger header. Sometimes a third switch, called the redundant high-limit, is mounted on the opposite side of the heat exchanger for added safety.

Pressure switch

The pressure switch is a simple device at the end of a hollow metal tube, which in turn is connected to the header so that water flows to the switch. If there is inadequate water flow in the header there will not be enough resulting pressure to close the switch. Thus, the circuit will be broken and the heater will shut down. Although preset by the factory for 2 psi, most pressure switches can be adjusted to compensate for abnormal pressures caused by the heater being located unusually high above or below the water level of the pool or spa.

Automatic gas valve

The automatic gas valve is often called the combination gas valve because it combines a separately activated pilot gas valve with a main burner tray gas valve and sometimes a separate pilot-lighting gas line combined with the pilot gas valve. After the circuit is complete, the electricity activates the main gas valve which opens, flooding the burner tray. The gas is ignited by the pilot and the heater burns until the control circuit is broken at any point, such as when the desired temperature is reached and the thermostat switch opens, if the on/off switch is turned to off, if the pressure drops, the pressure switch opens and breaks the circuit.

There are several different designs of automatic gas valve. For millivolt-powered units, the terminal board will have three terminals. Terminals 1 and 2 are the neutral and positive lines from the pilot generator to start the control circuit. When the circuit is complete, power arrives at terminal 3 which opens the main gas valve.

On 25-volt units, there is a pair of terminals to power open the pilot valve and to return the current to the common or neutral line of the intermittent ignition device. Another pair of terminals power open the main gas valve and return the current to the neutral line of the intermittent ignition device.

The gas plumbing of the automatic gas valve is self-explanatory. The large opening (1/2or1/4 inch) on one end, with an arrow pointing inward, is the gas supply from the meter. Note that it has a small screen to filter out impurities in the gas, like rust flakes from the pipe. The hole on the opposite end feeds gas to the main burner. The small threaded opening is for the pilot tube and a similar hole is for testing gas pressure. These are clearly marked. Automatic gas valves are clearly marked with their electrical specifications, model numbers, and most important, Natural Gas or Propane. Black components or markings usually indicate Propane.

All combination gas valves have on/off knobs. On 25-volt units, the knob is only on or off. With standing pilot units, there is an added position for pilot when lighting the pilot. As a positive safety measure in most, you are required to push the knob down while turning.

Electronic Ignition Heaters

An electronic spark ignites the pilot when a heater with electronic ignition is turned on. This in turn ignites the gas burner tray in the same manner as described previously. In all other respects, these heaters operate the same way as those already discussed. Where the control circuit on the standing pilot heater is powered by millivolts, the electronic ignition heater is controlled by the same kind of circuit but is powered by 25 volts ac.

Regular line current at 120 or 240 volts is brought into the heater and connected to a transformer that reduces the current to 25 volts. This voltage is first routed into an electronic switching device called the IID (intermittent ignition device), which acts as a pathway to and from the control circuit. From here the current follows the same path through the same control circuit switches as described previously.

When the circuit is completed the current returns to the IID, which sends a charge along a special wire to the pilot ignition electrode creating a spark that ignites the pilot flame. The IID simultaneously sends current to the gas valve to open the pilot gas line. When the pilot is lit, the heat generates a current that is sensed by the IID through the pilot ignition wire. This information allows the IID to open the gas line to the burner tray, which is flooded with gas ignited by the pilot.

Natural Versus Propane Gas

There is not much differences in heaters using natural gas and those using propane gas. The gas valve is clearly labeled Propane. Because of different operating pressures, the gas valve is slightly different although it looks the same as a natural gas model, as are the pilot light and the burner tray orifices. The heater case, control circuit, and heat exchanger are all the same as for a natural gas model. Most manufacturers make propane heaters in standing pilot/millivolt models only.

Natural gas is lighter than air and will escape if the burner tray is flooded with gas but not ignited for some reason. The odor added to natural gas can be detected if you are near by.

With propane, however, the gas is heavier than air and if it floods the burner tray without being ignited it tends to sit on the bottom of the heater. Because it remains undissipated and cannot be detected by smell, if it ignites suddenly, it will do so with violent, explosive force. Rarely is the heater itself damaged-the explosion takes the line of least resistance, which is out through the open front panel. Never position your face in front of the opening and always use the safety checklist before trying to detect any thing wrong in the system.

Electric Heaters

Electric heaters are used where gas heaters are impractical and also when you are heating a spa. Because of the cost of operation, the slower recovery and heating time, and the high amps required with the corresponding heavy wiring and electrical supply, electric heaters are useful in small portable spas.

The components are similar to gas heaters except the heat is derived from an electric coil that is immersed in the water flowing through the unit. This is also true of the small in-line electric heaters used in small spas. Often these in-line units have no control circuits or they might have only a thermostat control because the other controls are built into the spa control panel itself.
Several sizes of electric heater are manufactured, rated by the kilowatts consumed and, therefore, the BTUs produced. Here is a comparison of the energy use and output of the most common models:

  • 1.5-kilowatt (1500-watt) heater = 5119 Btu
  • 5.5-kilowatt (5500-watt) heater = 18,750 Btu
  • 11.5-kilowatt (11,500-watt) heater = 37,500 Btu

Each of these generally consume about one-third more power to start up than to run at the designated wattage.

Solar Heaters

A solar heater transfers the heat of the sun to the water. A typical solar installation consists of solar panels from which the water should go before it passes through the heater. In this way, some amount of heat can be gained from the sun first, and then the gas heater adds additional heat if desired. Sensors detect the heat on the panel and open motorized valves to divert the water to the panels before it gets to the heater. If the panels are cold, water flow will bypass the solar panels and go directly to the heater.

Solar heating systems are controlled by time clocks and/or thermostats because, in summer, the panels might add too much warmth to the water and some means of regulation is needed. They also have simple on/off toggle switches to completely disable the system.

Most of the solar heaters work by cycling water through solar collectors and back to the pool. These are called open loop systems. They employ unglazed collectors, made from plastic or metal, to absorb heat from the sun and transfer it to the pool water. Other solar heaters employ glazed collectors, that contain antifreeze fluid. As the fluid is heated, it is sent to coils inside a heat exchanger, which then transfers the heat to the pool water. These are called the closed loop type of systems. A newer type of collector is made of Lexan with polycarbonate refractor.

The major manufacturers install and service the units that they sell, so it is best to your benefit to understand the design and have the warranty from the manufacturer.

Usually the solar heaters are mounted on the roof, facing south. But with the flexibility of the designs available today, the panels may not face directly south and can be mounted on fences, shade structures garage roofs and slopes.

Heat Pumps

It is the heat pump in which heat is transferred to that water by taking the warmth out of the air that is created by compressing a gas. Pool and spa water circulates through the unit the same way as the other heaters, but does not pump any more heat than any other design of pool and spa heater.

A compressor in the unit exerts pressure on a gas, usually Freon, and generates heat. The water is circulated through a heat exchanger that is warmed by contact with the hot gas. The gas cools from contact with the water and is recompressed and heated to start the cycle all over again. The Freon used in heat pumps is a nonflammable, noncorrosive gas, which makes it suited to this application. Freon does not contain the chlorine component of the Flourocarbon that makes it environmentally hazardous.

Though expensive, heat pumps are energy efficient and last a long time. They are not effective spa heaters because they take a long time to heat the water in the spa. Because they rely in part on taking warmth from the air, the hotter the surrounding temperature, the better and quicker they work.

Unlike gas-fueled heaters, heat pumps are rated like air conditioners, expressed in tons. In this rating, a ton is the amount of energy required to keep one ton of ice at 32°F for 24 hours. As a rough rule of thumb, one ton equals 15,000 Btu.

Oil-Fueled Heaters

These are not very common, and are designed identically to gas-fueled units but they burn #2 diesel fuel instead of natural or propane gas.

Makes and Models

Three firms dominate the pool and spa heater market-Raypak, Teledyne Laars, and Hydrotech (formerly Purex)-with products that are remarkably similar. Several smaller companies market spa heaters. For now you need to know that heater models are based on their size as expressed in output of heat, measured in BTUs. Each manufacturer produces models of similar size; for example, 50,000, 125,000, 175,000, 250,000, 325,000, and 400,000 Btu.

Teledyne Laars makes a good reliable unit with almost everything in a convenient place inside the heater for easy service work. They design the pilot assembly to be removed without pulling out the entire burner tray.
The wiring for the transformer on the Teledyne Laars is located in tight quarters behind the intermittent ignition device which is inconvenient only when installing the original power hookup.

Raypak makes good heaters also. On their electronic ignition models, the IID is hidden in a small compartment that is accessed by opening the main panel first, then unscrewing two or three upside-down-mounted screws to get into the second compartment. This makes quick diagnosis a longer process when troubleshooting repairs. Recently they have made this more accessible, but there are a lot of the older models still in service.

Raypak burner tray design requires removing the gas burner tray completely to work on the pilot assembly, and I have had to pull it out and turn it upside down to get at tough screws. Replacing the pilot is even tougher because the screws holding it in place are very short.

Hydrotech/Purex heaters have not been as popular as Teledyne Laars and Raypak, but their assembly and quality is comparable. Numerous other companies make heaters in addition to other equipment, but I find something comforting in working on heaters made by companies that focus their attention only on heaters.

In my opinion, A.0. Smith made the best pool and spa heaters ever built. Many are still in service and parts seem to be readily available. They were over built everything was twice as beefy and high-quality as it needed to be. As a result, they are no longer made. Legend in the pool business has it that they couldn't compete with the prices of the less expensive units on the market today and they refused to compromise their quality, making it no longer financially viable to build their units. Too bad, they were great heaters.

I have one very wealthy customer who routinely spends hundreds of thousands of dollars adding new cars to his million dollar collection, but wouldn't part with his 20-year-old A-0. Smith heater when the frequency of repairs suggested it was indeed time to do so. instead, he wanted us to keep soldering the old heat exchanger wherever it leaked and keep that baby running his pool at 85 degrees F year-round. He still has it.

A few years ago it was true to say that each manufacturer employed different designs but basically the same components and concepts. In the past five years or so, they have gone off in substantially different engineering directions so that they can no longer be viewed generically. The basic concepts are, however, the same, and Raypak, for example, has recently redesigned its cabinets so that plumbing and gas connections are in the same place as on Teledyne Laars (obviously to be more competitive when replacing an old heater).

Other manufacturers, particularly the many making small spa heaters or in-line portable electric spa heaters are not mentioned here specifically because I have no particular likes or dislikes among them, and many of them will be in or out of business before the ink is dry on this page. As noted elsewhere however, if you understand the concepts of one, you should have no trouble with the others.

Because of frequent changes in engineering and design, what I say here about a particular manufacturer might be true about some models and not others. I have tried to make comments based on the heaters I find most commonly in use today. While it is important to understand the latest technology, it is more important to understand the technology found in the most installations at the present time.


While selecting a heater sizing and cost of operation are the two basic parameters, besides the manufacturer preference.

Heater Sizing

To find the unit with a cost and energy-efficient source of heated pool or spa water, would be one with the right size of heater, should heat the water to the desired temperature, in the desired time frame. An undersized heater will heat too slowly, and an oversized unit will do the job but will increase the cost of the installation. Settling on just the right heater therefore means satisfaction and leaves you with a trouble-free heating system.

Depending on the need if the heater is going to heat a pool or a spa, the thought process behind selection differs for pools and spas.

In a pool, one of the primary factors used in calculating heater size is heat loss from the surface. In a spa, however, surface area is far less a factor, for spas are covered, which greatly reduces surface heat loss. Instead, heat-up time relative to the spa's gallonage is the critical factor.

Now you need to find out if the plans of using the heater for maintenance heating or if they are only going to turn on the burners occasionally for intermittent or spot heating. Although calculating heater size for the two types of heating strategies is basically the same, maintenance heating is typically calculated using surface area, while heaters used for intermittent heating often are sized by factoring total volume.

If you are not sure on this point, it basically breaks down to a question of usage. If the pool is used nearly every day during the swimming season or year-round, for that matter, maintenance heating is the best strategy. If the vessel is host to bathers only occasionally, however, it is far more cost-effective to heat the water only when needed.

Another critical determination is what type of heater to be put to use- natural gas, liquid propane or heating oil. In some geographic areas, natural gas is either unavailable or excessively expensive, making one of the other fuels more desirable. You also need to know if there are codes in your area governing the type of pilot-ignition system used in heaters. In some states, continuous pilots (or millivolt systems) are banned for all new installations. Intermittent ignition systems requiring an electrical hookup are required in these jurisdictions.

It is also quite important to ascertain whether the available utility hookup provides adequate pressure to run the heater. Gas piping, meters and other delivery equipment must be sized correctly to ensure an adequate gas supply.

Defining the variables

As is the case with just about every other component in a pool or spa circulation system, there are variables in heater sizing that you must address. Whether you use a manufacturer's sizing chart or a generic heater sizing table, such as those provided by the American Gas Association, you need to gather and play with numbers to make the right choice. Here's the data you need:

Surface area: The main job of a vessel's heater is to offset the heat that is lost from the water's surface, particularly for maintenance style heating. Here's a rundown of the basic surface area calculations:

  • Rectangular pool: length x width
  • Oval pool: 1/2 length x 1/2 width x 3.14
  • Rectangular pool with rounded ends: length x width x .8
  • Kidney-shaped pool: length x width x .75

Things get a bit trickier with free form pools. Here, you must carefully draw an image of the pool's perimeter on standard 1/4-inch grid paper. Using a scale of 1/8-inch per foot, for example, means that each of the 1/4-inch squares on the grid will equal two feet on each side, giving each grid square an area of four square feet.

Next, you count the squares that fall entirely within the drawn perimeter of the pool. Then count all of the squares that fall approximately 3/4 within the surface area of the pool as three square feet, those with half in the pool surface area two square feet and so on. Add up the value of all of the squares on the drawing.

Generic pool heater sizing chart temperature maintenance (Outdoors 3.5 m.p.h. wind, surface area method)

Note: These heat losses are based on an assumed wind velocity of 3-1/2 m.p.h. for a velocity of 5 m.p.h. Multiply these losses by 1.25, and for 10 m.p.h. multiply by 2. 0.

Temperature Rise 10 15 20 25 30
Surface Area ( Sq Ft) Required Heater Output in BTUs/hr.
200 21,000 31,500 42,000 52,500 63,000
300 31,500 47,300 73,000 78,800 94,500
400 42,000 63,000 84,000 105,000 126,000
500 52,500 78,800 105,000 131,000 157,000
600 63,000 94,500 126,000 157,000 189,000
700 73,500 110,000 147,000 184,000 220,000
800 84,000 126,000 168,000 210,000 252,000
900 94,500 142,000 189,000 236,000 284,000
1,000 105,000 157,000 210,000 263,000 315,000

Volume: For spas and for pools in which a spot-heating technique is to be used, total water volume is used rather than surface area in calculating heater size.

To calculate volume, use the surface-area data derived above and multiply it by the average depth, thus developing a cubic-foot measurement of the vessel. To determine total gallonage, multiply the pool or spa's cubic water footage by 7.5 the number of gallons in a cubic foot of water.

Temperature rise: In most sizing charts for pools, temperature rise is the primary factor along with surface area or volume. Before you can determine temperature rise, however, you must first peg the desired water temperature. For pools, the American Red Cross recommends a range of 78-82 degrees Fahrenheit - a range that seems to satisfy most bathers. In a spa, the temperature should not exceed 104 degrees, as recommended by the National Spa & Pool Institute.

Once you know the desired temperature, you need to determine the average ambient air temperature. Most experts recommend taking the average daily temperature during the coldest month when the pool or spa will be used. When you subtract the Ambient air temperature from the desired temperature, you've found necessary temperature rise.

Heater efficiency: Expressed as British thermal units (BTUs), heater output is the energy that a heater transfers to the water. The heater input is the energy (again in BTUs) used to generate that heat. Heater efficiency (HE) is the ratio of the output to the input, expressed as a percentage. The American Gas Association requires pool heaters have an efficiency rating of at least 75 percent.

Heater-sizing charts often express the required heater output necessary to achieve the desired temperature rise for the pool's surface area or volume (see Figure 1 for a generic example). Because heaters are rated by their input, however, you must know the heater efficiency to determine what size heater is required to do the job. In other words, if you multiply the required output by .75, you will have the proper heater rating.

Manufacturers do part of the work for you in their heating charts by replacing the required output with the appropriate heater model number for the desired temperature rise and surface area or pool volume.

Heat-up time: For spas especially, the time required to heat the water to the desired temperature is important when sizing the heater. Indeed, many spa heater sizing charts use required heat-up time as a primary factor and assume a given temperature rise.

For intermittent heating in pools, heat-up time can be also very important, although many sizing charts simply assume a 24-hour heat-up time.

Typical spa heater sizing intermittent heating, gas, volume method, temperature rise of 30 degree F

Heater Input: (Btu/hr) 125,000 175, 000 250,000 325,000 400,000
Spa Volume (Gal) Minutes Required for Each 30-Degree Temperature Rise
200 30 21 15 12 9
300 45 32 23 17 14
400 60 43 30 23 19
500 75 54 38 29 23
600 90 64 45 35 28
700 105 75 53 40 33
800 120 86 60 46 37
900 135 96 68 52 42
1000 150 107 75 58 47

Plugging in the numbers Once you have determined these key factors, selecting a heater is a simple matter of plugging the numbers into sizing charts. Although they are typically easy to use, the charts are formatted in varying ways. Some plot the temperature rise on one axis with the pool volume on the other. Here, you cross-reference these two key factors to determine the proper heater output, which is listed in columns across the chart.

Other charts, most of them provided by manufacturers, list model numbers on one axis with the temperature rise on the other. Cross-referencing the heater model with temperature rise then leads you to pool sizes listed in columns on the chart. Finally, some manufacturers offer easy-to-use sizing slide rules. Here, you select the pool volume and temperature rise to determine the model heater.

For spas, heat-up time often is the critical factor. In these applications, sizing charts typically assume an increase in water temperature - say, 30 degrees - with models (or input ratings) listed on one axis and spa gallonage listed on the other. Simply pick the spa volume and the desired heat-up time to find the appropriate heater model or rating.

To determine the heater model on a chart that lists required heater output, multiply the output by .75 to come up with the heater input. (All heaters list their heater input ratings on their faceplate and in specification manuals)

Helpful Tips For those who prefer to size the heater based on their own calculations, the following formula is the basis for most heater sizing charts used in the industry and can be easily applied to either pools or spas:

Multiply the number of gallons by 8.33 (pounds per gallon) by the temperature rise. The answer is the number of BTUs required to heat the pool or spa.Here's an example using a 40-degree temperature rise in a 400-gallon spa - that is, 400 x 8.33 x 40 133,280 BTUs.

This number can either be divided by the desired heat-up time to give you the required heater output, or it can be divided by the heater capacity to give you the heat-up time a given model will provide.

Let's assume, continuing the example above, that you have a heater with an output of 266,000 BTUs. Here, 133,280 divided by 266,000 yields a heat-up time of .5 hours, or 30 minutes. Conversely, if the customer has a specific heating time in mind say, 30 minutes - the formula works like this: 133,280 divided by .5 equals 266,000 BTUs. In other words, in a 400-gallon spa, you would need a heater with an output of 266,000 BTUs to heat the water in 30 minutes.

Variables involved When it comes right down to sizing a heater, Sometimes it's not all numbers and formulas, but also the location of the pool.

  • Wind can dramatically increase the surface heat loss from a pool or spa. By making waves across the water, the wind effectively increases the surface area of the pool. The rule of thumb: In a pool with an 11 mph wind you need to increase the heater size by 25 percent.
  • Altitude is another factor that calls for a bigger heater: For each 1,000 feet above sea level, the heater needs to be four percent larger.
  • Shade also may lead you to a bigger heater. Although there is no precise rule here, if the pool is located in a shaded area, you should contact heater manufacturers, their local representatives or your local supplier for expert guidance.

Cost of Operation

The heater model shows as to how many BTUs per hour your heater uses. Dividing that by 100,000 gives how many therms per hour it uses. A therm, the unit of measurement on the gas bill. Thus it becomes easy to calculate the cost by multiplying with the number of hour the heater works, which is usually 8 hours per day.

Making it clear by using an example, and considering that the heater uses 135,000 Btu and the heater itself being a 250,000 input Btu model. When you run it for eight hours a day, then using the facts you have your operating cost calculated as:

250,000 Btu / 100,000 Btu/hour = 2.5 therms/hour

2.5 therms/hour x 8 hours/day = 20 therms/day used to run the heater

20 therms/day x cost of cents/therm = total cost /day

In the spa if you use same 250,000-Btu heater to heat the spa which would take an hour to do so. Using the same calculations, you can calculate the cost to heat the spa. Of course, the heater will run while the spa is in use and the heat loss will be considerable as jets and blowers stir up the water, you have to take this into consideration while calculating. To keep a standing pilot burning, it uses between 1200 and 1800 Btu per hour and thus the cost can be calculated in the same manner.

However, to calculate the true cost of operating your pool or spa don't forget to add the cost of electricity for pumps and motors, blowers, and other appliances.

For an oil-fueled heater the calculations can be made on the umber of gallons that are used and one gallon equals 140,000 Btu. To calculate the cost for the gas utility units, you have to know the amount of gas utilized by the heater itself and then use the conversion factor, for electricity is sold by the kilowatt-hour 1000 kilowatts consumed in one hour which equals 3412 Btu.

Installation, Repairs, and Maintenance

There are so many possible combinations of cause and breakdown for as many different types of heaters and manufacturing designs that some basic guidelines for any heater repair are listed here. Always remember to turn off the heater and allow it to cool before starting to work on it.

It is generally better to replace well-worn components rather than repair them. Also best is to get it repaired by the manufacturers' serviceman.


To ensures years of trouble-free satisfaction the heater should be properly installed. And to get the best job done follow each manufacturer's own installation guidelines at the jobsite. It consists of four basic steps like plumbing, gas, electrical and most important location which includes ventilation.

Instillation site

The American National Standards Institute offers guidelines - ANSI standard 2223.1- designed to keep the heaters. This standard sets clearance requirements based on the external temperatures of heaters, the following clearances may vary from manufacturer to manufacturer.

The rear and non-plumbed sides of the heater should have a minimum 6 inches of clearance. The water-connection side should have a minimum of 18 inches of clearance. The front of the heater should have at least 24 inches of clearance. Moreover, the following clearances are based on the National Fuel Gas Code and are universal for all gas-heater models. When installing a heater under an overhang, there must be at least 3 feet of vertical clearance from the top of the heater to the overhang, and the heater must be open on three sides. The top of the heater must be at least 5 feet below or offset 4 feet from the nearest opening to a building, such as a window or door; in addition, the top of the heater must be at least 3 feet above any forced-air inlets located within 10 feet of the unit.

All heaters must be installed at least 5 feet from the inside wall of a spa unless it is separated from the spa by a fence, wall or other permanent barrier.

Heating pad site

The heater must be installed on a level, non-combustible base such as brick or concrete. If concrete cinder block is used as a base, it must be aligned so the cells are all pointing the same way; the end should be left open. When such hollow masonry is used, the pad must be at least 4 inches high - and covered with at least 24-gauge piece of sheet metal.

If the heater is placed in an area exposed to high winds, the unit either must be installed at least 3 feet from the nearest wall, or a windblock must be constructed to help minimize the effect of wind.

The reasoning behind clearances and heating pad site is to allow proper air flow around the heater, efficient combustion and proper ventilation of the gas fuel.

Indoor Heaters

Heaters installed indoors should follow certain guidelines too:

If the heater is installed in a place where vented air comes from another interior room, the space where the heater is located must be connected to the additional airspace by two vents, and the combined area of the space where the heater is located and that additional room must represent at least 50 cubic feet per 1,000 BTUs of heater input. The Btu input of any other gas-burning appliances in that space, such as a home water heater, must also be taken into consideration.

The space must have two openings, one beginning 12 inches above the floor, the other t2 inches from the ceiling. Each model of heater has specific venting requirements to ensure proper combustion and prevent sooting. Manufacturers express venting requirements as square inches of net free air. Consult manufacturer charts for vent sizing requirements for the specific model being used.

For spaces vented to the outside, the space must also have top and bottom vents for the confined space. The openings must connect directly or be connected by ducts to the outdoors. Alternatively, the vents must connect to an area such as an attic or crawl space connected directly to the outdoors.

All indoor heater installations require a draft hood that sends combustion by-products - particularly carbon monoxide - to the outside of the building. Failure to meet this requirement can result in fire or carbon monoxide poisoning.

The diameter of the draft hood is based on the National Fuel Gas Code and may vary from model to model. Connected to the draft hood is the vent pipe, which vents products of combustion to the outside air; it must have a diameter equal to or greater than the draft hood.

The discharge opening in the vent pipe must be at least 2 feet above the roof surface and must be at least 2 feet above the highest point on the roof within a 10-foot radius of the pipe location. The vent pipe must be topped with an approved vent cap, which keeps wind and air from forcing products of combustion back down into the ventilation system.

It is best to avoid horizontal piping runs as much as possible. If horizontal runs are inevitable, the vent pipe must have a minimum 1-inch rise for every foot of horizontal run and must be supported at least every 5 feet. Avoid the 90 degree turns too.

Fuel Supply

Once venting needs are accommodated, you need to consider whether the unit is getting an adequate supply of fuel. For this, you need to look at the line running from the gas meter to the heater and determine whether it is properly sized for the job.

Gas pressure is measured as inches of water-column pressure, or WCP. Basically, WCP is a special measure of pressure per square inch; it takes 28 inches of WCP to equal 1 psi. Generally, heaters running on natural gas require between 6 and 10 inches of WCP to ensure smooth performance. Heaters on liquid petroleum require greater delivery pressures.

Heater manufacturers offer convenient pipe-sizing charts for their customers, concentrating on sizes between 3/4- and 1-1/2 inch diameters and working with runs of up to 300 feet.

Most heaters are fitted for operation at altitudes of less than 2,000 feet above sea level. For guidance on installations at higher elevations, you need to contact the manufacturer for special, high-altitude models.

The basic installation requirements for the gas connection are, a main gas-shutoff valve and union must be installed within 6 feet of the heater and outside the heater jacket; gas piping should have a sediment trap upstream of the heater's gas controls; Rigid gas-line piping must be used. (Never use flex line under any circumstances.)

The piping must be pressure tested once installed. To conduct the test, the gas piping is disconnected from the heater to avoid damaging the heater's gas-control equipment. Here, the pipe is capped at the connection point and the gas turned on, with a soap solution applied at all joints. (Never test for a gas leak using a match or any other kind of flame.) Also, check for additional testing requirements by some local codes.

If any bubbles form when the soap solution is applied, there's a leak that must be repaired. Testing continues until there are no leaks.

Some manufacturers also suggest using the above procedure to test the burner and the pilot's tubing. In this case, care must be taken to keep the test pressure below 10 inches of WCP so as not to damage the gas control valve.

Controlling water temperatures

Properly locating the heater in the plumbing is an obvious and important aspect of heater installation.

The heater should be installed downstream of the pump and filter and ahead of any automatic chlorinating, brominating or ozonating equipment. That is, the water should be free of particulates or dirt when it enters the heater, and contact with corrosive chemicals should be kept to a minimum by their downstream placement. The heater should also be installed as close to the pool or spa in the plumbing run as possible to prevent unnecessary heat loss.

It is preferable to locate the heater level, as close to level with the surface of the pool as possible. This is because manufacturers preset their pressure switches for heater installations that are typically 3 feet above or below the surface of the pool. Consult the manufacturer's literature for specific recommendations for elevated or sub-surface installations.

Manufacturer's literature provides either pipe sizing charts based on desired flow rates and distance of the run or specific pipe-sizing recommendations this makes sizing critical.

When using PVC piping, it is important to position a heat sink between the heater and the piping - typically a metal pipe approximately 2 to 4 feet long. For best performance of the resulting PVC/metal connections, use a metal male fitting and a PVC female fitting. Where allowed by codes and manufacturer's instructions, high-temperature version of PVC (CPVC) can be connected directly to the heater. To compensate for pipe expansion, a flexible sealant should be used on all piping connections.

To avoid damage to plastic filter elements that might be caused by back siphoning of hot water into the filter, a check valve should be placed in the line between the filter and the heater. Likewise, to prevent water with high concentrations of chlorine or other sanitizing agents from backing up into the heater and possibly corroding the heat exchanger, there should be a check valve between any in-line chemical feeder and the heater.

The installation may or may not require use of an external bypass valve and/or a pressure-relief valve. Be sure to check the installation manual for a specific model's requirements and other installation conditions that may require such control devices.

Electrical Connections

Millivolt or continuous-pilot systems do not require any electrical service to the heater. In order to enhance energy conservation, however, some areas have required heaters to use intermittent-ignition systems, which do require electrical hook-ups or line voltage. For most applications, a qualified, licensed electrician must perform or evaluate this part of the job - from the circuit breaker panel to the heater.

Most heater models accommodate either 120-volt or 240-volt power. The National Fuel Gas Code requires 14-gauge copper wire for electrical service to gas heaters. Electrical wiring should be run in waterproof conduit and hard-wired into the unit; moreover, it should be run in its own conduit rather than in one shared by timer or pump wiring.

If the circulation system is run with a timer, the heater should be equipped with a separate low-voltage switch that deactivates the heater before the pump is turned off. This useful circuit is known as the heater's fireman switch. On a millivolt heater, the length of wire between the heater and the timer should not exceed 30 feet. Resistance on longer runs will reduce the millivoltage to a level that will not support reliable operation of the gas valve.

Finally, all such circuits require grounding and bonding in accordance with the National Electric Code.

Troubleshooting Tips

Gas heaters for pools and spas are more complicated than filters, pumps or motors. Heater troubleshooting resides in determining the problem's location within the system. However, it is always necessary to consult each manufacturer's service and maintenance manuals before troubleshooting a given heater model.
Heater can be a serious safety hazard, so if you are unsure about a particular situation or procedure, ask an expert at your local dealer or get help from the manufacturer.

Check the equipment room.

Check if the new unit has been installed correctly according to the manufacturer's manual. Also check that the inlet and outlet connections have not been reversed, during renovation. Alternatively, if the heater is of the intermittent-ignition device (IID) variety, the electrical hookup may not be connected. Check the circuit breaker at the main power panel and take the help of a licensed electrician. If it is an indoor installation, always make sure the unit has an adequate supply of outside air and all of the necessary venting.

Check the circulation system

The unit's safety devices will not allow it to fire, if a heater is not getting proper water flow. A common problem here is a dirty filter. So clean the filters, check the skimmer basket, the main drain and the pump basket for blockages.

Check the fuel supply

Make sure the gas line and meter are properly sized to provide adequate fuel to the heater. Then make sure the gas shut-off valve on the heater is open. If the unit is running on propane, be sure the tank has enough fuel.

Check the control settings

The top priority here is the thermostat, which should be set to a temperature higher than the current temperature of the pool or spa water. Note that the contacts in the thermostat may be frozen, in which case they can usually be freed by moving the control back and forth several times. Check the heater's toggle switch. If the heater is on a time clock, it must be in the ON position to operate.

Check the heater

If all of the above items check out, you should begin to examine the heater itself. Note before we move on that these are general guidelines; Techniques for troubleshooting pilot, ignition and safety control circuits are presented only to illustrate the logic behind heater troubleshooting; manufacturer literature must always be consulted before approaching any specific heater model.

First, you must ascertain what type of pilot the heater uses. There are two basic types; millivolt systems and IID systems.

With the millivolt system, which we'll cover first, the pilot is on continuously as a standing pilot; the heater uses heat from that standing pilot to generate electricity to run its safety, control and ignition systems. These millivolt systems have no outside electrical hookups.

Millivolt pilot heater

Use a pocket mirror to see if the pilot is burning; never put your face or eyes where you could possibly get burned. It should be clean and free of obstructions.

Check the switches.

If the pilot lights and all of the external functions and factors have checked out, the millivolt system's inability to fire is likely to be the fault of a malfunctioning component along the electrical path within the heater. At this point, the suspect parts are the pressure switch, the high-limit switch, the thermostat, the gas valve and the pilot generator.

The first step along this diagnostic line involves sidestepping the gas valve by clipping a jump wire across the valve's terminals. This enables you to bypass the chain of control devices wired along the same circuit as the valve and to check the performance of the gas valve and pilot generator. An important reminder: After troubleshooting any component, always remember to remove the jump wire.

  1. The control components you're bypassing - the pressure switch, the thermostat and the high-limit switch - are designed to prevent the gas valve from delivering gas to the main burner when an unsafe or improper condition exists. To jump the gas valve, connect one end of the jump wire to the terminal connecting the thermostat and the gas valve, the other to the connection between the gas valve and the pilot generator; these may be labeled TH, TH-TP, thermostat or identified by color, depending on the manufacturer. If the burner lights, you know that the gas valve and pilot generator are good and that the problem lies somewhere along the string of control devices. If, however, the main burner doesn't light, you have either a problem with the pilot generator, a wiring problem in connections for the pilot generator or a bad gas valve. Manufacturers offer specific diagnostic recommendations for testing electrical current between these devices using a voltmeter. Usually, the pilot generator's output should be above 500 millivolts with the circuit open - that is, when the heater is not running. The connection between pilot generator and gas valve, should read above 200 millivolts with the heater firing. If it is below these levels, you probably have a bad pilot generator; above that, the failure to light is usually attributable to a faulty gas valve. Note, however, that a low reading also may be the result of faulty wiring. Again, a preliminary wiring check will help you avoid unnecessary effort in troubleshooting these components.
  2. The next step in most units is to jump across the pressure-switch terminals, thus electrically bypassing the pressure switch. If the burners fire, then you know that the pressure switch is not sensing proper water flow. If you know there's proper flow in the system, then the switch is either out of adjustment or defective. Consult manufacturers' literature for adjustment procedures.
  3. If the burners do not fire when the pressure switch is jumped, you move on to the thermostat. The first thing to check is that the toggle switch is in the ON position. Depending on the unit, the thermostat may need to be removed from a mounting in order access its terminals. In any case, the procedure is the same: Jump from one terminal on the thermostat to the other. If the burners fire, then you have a bad thermostat and it needs to be replaced. If they don't, you need to move on.
  4. The last diagnostic step with millivolt systems is a check of the high-limit switch or switches. If all of the previous checks have been performed properly, then jumping this switch (or set of switches) should result in the heater firing. If you replace the high-limit switch and still have a problem, check for proper water flow through the heat exchanger.

IID pilot heater

Intermittent ignition devices differ from millivolt devices chiefly in the fact that IlDs require an external electrical source to power their functions. Here, the pilot is sparked electrically only when it is needed to fire the burners.

As with the millivolt system, once external checks have been made to your satisfaction, you can be fairly certain the heater's failure to fire stems from a faulty component in the control circuit.

Troubleshooting an IID control system calls for an AC voltmeter with a 200-volt range or greater. As with the millivolt system, you will be testing a series of components in order to locate the faulty part. Set your voltmeter for the proper voltage that is, above 24 volts.

Check the transformer. Attach one voltmeter lead to one of the terminals for the system's 24-volt transformer, then probe the other terminal with the second lead. When you make contact with both terminals, you should get a reading of 20-28 volts. If the reading is low, you may have a voltage problem or a faulty transformer.

Check the circuit. With the lead from the voltmeter still in contact with the ground or common terminal on the transformer, check for voltage along the circuitry starting with the safety fuse, using the other lead of the meter as a probe. Then move to the fusible link, the highlimit switch and the pressure switch. If you have voltage at these points, the parts are good. If not, you need to replace them.

Check the temperature control. Perform the same test on the temperature control, making sure that the control is in the ON position and that it is set to a temperature high enough to call for heat. If there's voltage, the unit is fine. If the heater is equipped with an electronic temperature control, the control's sensor or probe should be checked following the procedures recommended by the manufacturer.

Check the ignition control. When an IID heater won't fire, the ignition control is another suspect. Electrically, the ignition control is tested in the same fashion as the safety devices. First, verify that it's receiving 20-28 volts. Then make a visual inspection of the unit and test for a spark at the pilot-burner electrode. Use caution around the electrode: It has low amperage but a high voltage.

Moving on, check all electrical connections to make sure they're tight. Make sure the igniter electrode has a proper spark gap; this not unlike checking for the same in an automobile spark plug. Consult manufacturer literature for a proper spark-gap spacing.

Check for a spark. With the thermostat set high enough to call for heat, you should have a spark at the pilot-burner electrode if all of the controls are good.

If there is no spark, some manufacturers recommend that you pull the wire off the ignition control and hold it approximately an eighth of an inch away from the ground terminal. If there is no arc across the space, the ignition control should be replaced. If you do have a spark but the pilot doesn't light, a good bet is that either you have a plugged pilot assembly or a bad gas valve.

If a heater does not shut off

A heater that just won't shut off can be particularly alarming. It may result in extremely high water temperatures and constitutes an extreme danger. To get to the root of such problems, you should check the gas valve. To do so, remove the wire running between the gas valve and the thermostat. If the heater doesn't shut off, you have a faulty valve that must be replaced. If the heater does shut off, the gas valve is ok, but you have a control problem. At this point, heater manufacturers recommend that you bring in a specialist to troubleshoot the system because of the seriousness of the problem.

Troubleshooting Checklist

Taking a quick look at some of the other basic problems that you can come across with heater for pool and spa:

Problem: Solution:
Symptom: The heater will not heat the pool or spa water to the desired temperature.
The system is not running long enough. Reset the time clock.
A dirty filter is keeping the heater from firing. Backwash the filter.
The thermostat is faulty or out of adjustment. Test and replace the thermostat as needed.
The pressure switch is inoperative. Given a clean filter, test the switch and replace it if necessary.
The heater is too small. Consult a heater-sizing chart and upsize the unit.
The gas system is undersized. Check gas-pipe sizing charts and upsize the plumbing.
  Check the meter and the supply shut-off valve for proper sizing as well.
Symptom: Soot has formed in the combustion chamber
Excessive water is flowing through the heater. Correct water flow and clean the heat exchanger
The air supply is inadequate. Check the installation for proper clearances and/or venting. (On indoor applications, verify the adequacy of the air supply and venting.)
The air inlet or venturi for burner is plugged. Check for debris, dirt, in the sects or small animals in the burner inlet's throat or venturi and clean them
The time clock prevents the heater from running long enough to heat the water. Adjust the time clock clean the exchanger
The gas valve regulator is out of adjustment Test for proper gas pressure and adjust the regulator as needed or replace the gas valve.
Symptom: The Heater Goes On and Off Repeatedly
The filter is dirty. Backwash the filter.
The pool's water level is low. Raise the water level.
The manual bypass is out of adjustment. Adjust the bypass.
The pressure switch is out Adjust the pressure switch of adjustment and verify that the heater shuts off when the system's pump shuts off.
Symptom: Scale is forming in the heat exchanger
The pool or spa water is excessively hard. Bring total alkalinity, pH and calcium hardness within acceptable levels.
The heater is staying on when the water flow has diminished because of debris in the filter Replace either the pressure switch or the high-limit switch..
The manual bypass valve is out of adjustment. Adjust or repair the bypass valve.
Symptom:  Heat exchanger is corroding/ eroding.
The water chemistry is acidic.. Balance the water
Excessive flow. Check the bypass valve as well as the pump sizing. You may need to install a manual bypass.
Symptom: A lazy burner flame.
Low gas pressure. Check gas pipe and meter sizes and/or adjust gas pressure as needed.
Debris, dirt or insects are plugging the burners. Clean the burners.
Symptom: The heater makes knocking or whining noises.
The heater is operating after the pump shuts off Adjust or replace the pressure switch.
Debris or other restrictions are blocking the system. Remove the blockage and flush the system.
Scale has built in the heat exchanger's tubes. Descale or replace the heat exchanger
The pressure switch is out of adjustment. Adjust the pressure switch.



When a gas heater needs help, here is a step-by-step guide to servicing its critical components.

The heat exchanger is where, as water circulates through the exchanger's copper-finned piping, it is warmed by hot air rising through the heater's structure from the burners below. With good water balance as well as proper water flow and venting a properly cared unit may last for years.

Occasionally, however, poor chemical maintenance or improper water flow will damage a heat exchanger and necessitate its replacement. The following covers the procedures involved in switching out an exchanger as well as the steps involved in another common service task: flipping the heat exchanger to reorient its plumbing to the left rather than the right side.

Water chemistry

Perhaps the biggest threat to the service life of heat exchangers is improper water chemistry - particularly with respect to pH.

Simply put, if your chemistry is too base - that is, a pH above 7.8 with alkalinity above 120 parts per million and calcium hardness above 400 ppm scale likely will form on the inner surfaces of the heat-exchange tubing. This may reduce heater efficiency and will certainly impede water flow.

In such cases, fixing the problem usually requires reaming the exchanger's tubing with a carbide-tipped auger followed by a soak in an acid/water solution and a good scrubbing.

If, on the other hand, the water is too acidic - with a pH below 7.2, alkalinity below 80 ppm and calcium hardness below 200 ppm - the water will corrode the exchanger. This will lead to high copper content in the pool and eventually, if the water is aggressive enough, will cause the exchanger to spring a leak.

Under such conditions - and depending upon the degree of damage done to the exchanger - replacement is usually called for.


Most heaters come with their inlet and outlet ports on the right side, which means you'll occasionally need to flip an exchanger to accommodate left-handed installations.

The following steps tell you the basics of both these operations, including the extra steps involved in flipping an exchanger. Note that this information is presented for illustration purposes only; always consult manufacturer literature for precise instructions and follow them carefully.

Down to business, step by step

Before digging into the heater itself, you must make certain the circulation system is off. (Note: If you are working on an intermittent-ignition device heater, cut the power at the circuit breaker or time clock!) In addition, if the heater is located below the surface level of the pool, check to make certain the appropriate valves are in a closed position.

Now you're set to remove or flip the exchanger: Disconnect the piping from the inlet and outlet fittings.

  1. Remove the faceplate and turn off the gas. Most heater models include a tool-actuated latching mechanism that requires just half a turn with a flat-head screwdriver. After opening, turn off the gas at the gas valve. Once the gas is off, the standing pilot in a millivolt system will extinguish.
  2. Remove the control panel to access the control wiring. (flipping only) These panels are attached with screws - as are many other parts of the heater. Keep track of these screws: Having a cup or other container on hand can be a big help.
  3. Clip the nylon wire ties. (flipping only) In flipping an exchanger, you will need to re-route some of the control wiring from one side of the unit to the other. Clipping the ties will give you the slack you need, but take care not to cut into the wires' insulation.
  4. Remove the inner and outer flue collectors. (A & B) These must be removed to permit access to the heat exchanger through the top of the unit. After you unscrew them, take care not to distort the shape of the collectors when removing them.
  5. Remove the faceplates. These plates are positioned on both sides of the heater in front of the headers and their plumbing connections.
  6. Remove the leads from the highlimit switches. These connections are exposed once the faceplates are removed.
  7. Remove the thermistor or thermostat sensing bulb. A note of caution in removing thermistors. The graphite paste used to enhance their temperature-sensing capability - a gray, pastey substance - is very difficult to clean from your hands and other heater parts. Experts recommend wiping the thermistor clean with a rag when removing it.
  8. Disconnect the pressure switch and siphon loop.
  9. Reposition the siphon loop and pressure switch assembly on the other side of the heater. (flipping only) In flipping an exchanger, you must reroute the connections for these devices. Manufacturers stress that with proper care in rerouting the wiring, you should not need to splice control wires. Most heaters are designed with necessary brackets and openings to accommodate flipping the heat exchanger.
  10. Lift the exchanger. With the headers still connected, carefully lift the heat exchanger, making sure not to bend or otherwise distort the mounting or "mud" clips.
  11. Inspect the fire blocks for cracks. If you find fissures or cracks, replace the insulating panels.
  12. Turn the heat exchanger around. (flipping only) Carefully reseat the unit in the opposite direction, making sure it is firmly in place.
  13. Reconnect the pressure switch siphon loop and the high-limit leads. (A & B) You do not need to switch the high-limit leads from one side to the other. Reconnecting the leads to the reversed headers does not affect the high-limit switches' function.
  14. Reinsert the thermistor or sensor into the header. A light coat of grease will adequately relace the thermistor's graphite paste. Replace the thermistor's graphite paste.
  15. Redress the wiring with nylon ties. (flipping only) A tip: take up slack by coiling excess wire around a Phillips-head screwdriver.
  16. Replace the faceplates. Making certain all of the screws are in place before tightening any of them down, remount the faceplates on both sides of the heater.
  17. Replace both the inner , and outer flue collectors. (A & B) If exchanger replacement was your objective, you're all done; if you are wrapping up the flipping procedure, all you need to do to finish the job is to remount the control-panel cover and replace the front door. Now all that remains is initiating the circulation system and turning on the gas. Always be sure to test heater functions and safety controls per manufacturer instructions.

Preventive Maintenance

Using the heater regularly is the best preventive maintenance. As noted previously, corrosion, insects, nesting rodents, and wind-blown dirt create many heater problems that can be eliminated by regular use. The heat helps to dry any airborne moisture that might otherwise rust the components. It discourages insects and rodents. It keeps electricity flowing through the circuits, preventing corrosion that creates resistance that might ultimately break the circuit completely. It burns off the odd leaf or debris that lands inside the top vents there by preventing fire. Running it for short period of times helps in maintenance of the heater.

If not then, heaters need only be visually inspected from time to time. A look around will detect sooting, gas or water leaks, or other problems before they begin. You may open the drain plug on the heat exchanger and look for scale buildup. Keep leaves and debris off the top of the heater. Look at the pilot and burner flames if they are strong, blue, and burning straight up at least 2 to 4 inches.

Spares Kit

The easiest way to work on heaters is to maintain a simple kit of spare parts that save you a trip to the supply house. It helps in troubleshooting as you can replace a part you have doubts about in test. Having spares saves you time and money and takes up little space in your toolbox.

Here's what you should have in your spare kit:

  • A Scripto lighter
  • Honeywell IID
  • Millivolt pilot assemblies (including generator); Clip-in type (Teledyne Laars) ; Screw-in type (Raypak); Electronic pilot assemblies; Teledyne Laars style with bolt-on electrode; Raypak style with clip-on electrode.
  • Orange high-tension cords. Teledyne Laars style with bare extension wire and ceramic - Raypak style (longer) with clip on the end.
  • Pressure switch with adapter for different models
  • Teledyne Laars set of high-limit switches
  • Raypak set of high-limit switches
  • Electronic thermistor
  • Transformer
  • Wire of various lengths and colors (available with connectors in kits called harnesses)
  • Teledyne Laars flange gaskets (1 1/2- and 2-inch pairs)
  • Raypak flange gaskets (1 1/2- and 2-inch pairs)
  • Small dental-type mirror for examining pilot flames

Generally, combination gas valves, thermostats, and switches are too expensive, numerous, and varied to carry as spares.

Spare parts on heaters are unbelievably expensive. Building a heater from spare parts would cost three times as much as buying a factory-assembled unit. As a result, many companies now offer replacement parts that fit the popular brands of heaters. Generally these generic brands work just as well as factory replacements, although the factory won't necessarily agree.

To ensure an uninterrupted supply of parts, manufacturers use parts from various suppliers or different models of parts from the same supplier. A part might look different, but that doesn't necessarily mean it is different. Examine it and ask your supply house.


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