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Hot Tub Spa - Heater Troubleshooting Guide


The term "Mechanical Failure" covers everything from shipping damage to dog bite, but the most common mechanical failures in electric heaters are caused by improper handling - destroying the Epoxy End Seal, and breaking or twisting the Cold Pin until it breaks are the two leaders in this failure mode. Because they are so closely related we’ll look at them together .


Any form of rough handling can put pressure on the part of the heater that sticks out the most - the Electrical Terminal, which is welded to the Cold Pin, which in turn passes through the Epoxy End Seal to get to the Heating Coil inside of the element. If the Electrical Terminal is broken away from the Cold Pin there is obviously no way to connect electrical wires to the heater, and it becomes a throw away.

More often the rough handling results in the destruction of the Epoxy End Seal. This will allow moisture to get into the inside of the element where it will be absorbed by the MGO Filler element and will eventually cause an electrical short circuit between either the electrically "hot" Cold Pin or Heating Coil, and and the electrically grounded Outer Sheath of the heater. With luck, there will be a GFCI on the system that will open the circuit.

If you don’t know what caused the GFCI to turn the power off, you might start investigating "nuisance tripping".

Without the GFCI there will eventually be arcs, sparks, flames and anger - you can bet on that.

anatomy-heater.gif (43405 bytes)


Moisture may get inside of the element when it is off. Part of the cooling off process is to suck in any surrounding air through any crack. Then, the next time the element is energized, the moisture in the element may provide a path for current to flow, and the GFCI will trip. But, a BIG BUT, - the heater may have been on just long enough to produce enough heat to drive that moisture out of there. You come along and reset the GFCI - and the heater will come back on again. Drive ya nutty, as they say. Check that Epoxy End Seal carefully.

Never bend, push, or twist the electrical terminal. Always use two wrenches to tighten or loosen the Terminal Nut. One wrench holding the Terminal Hex to keep it from turning, and one on the Terminal Nut to do the tightening or loosening.

The trick to analyzing this failure is to carefully examine the Epoxy End Seal. Signs of severe fracture or chipping are an almost sure bet that the End Seal is no longer able to do its job - to seal. If the heater has been acting strangely - erratic- sometimes working fine and other times tripping the GFCI: look at that Epoxy End Seal.


Rarely, maybe never, is the element itself at fault when noise is reported in a heater, even though it is the element that is usually making the noise. The huge amount of water moving rapidly past the element can set up some wild vibration patterns as it twists and turns through the heater. Usually it is simply a matter of re-aligning the element by a bit of gentle bending to move it away from the heater housing; or tying the noisy part down with a clip or wire made for the purpose. Check with us.



In the normal operation the heaters that we deal with daily in pools and spas operate at temperatures only a few degrees above the temperature of the water that is flowing past them. If the thermostat in a spa is set up to maintain the water at 102º, for example, the temperature of the Incoloy Outer Sheath will be about 110º. The water flowing past the element is carrying the heat away just about as fast as the element can produce it. Everyone is happy.


One of three things can happen - A) the water flow slows down too much, B) the water flow stops, or C) the heater is somehow turned on with either no water in the housing, or only partially filled. The results will be the same: the Incoloy Outer Sheath temperature will rapidly rise - 200º - 500º - 1,000º - 1,500º in a matter of just a minute or two. This is approaching the melting point of the Sheath and it will get there quickly if it isn’t turned off by some safety device like a high limit switch. The failure can show up in several different ways:

  1. The Outer Sheath splits open. The Heating Coil wire is hanging out in all directions and the MGO Filler is cracked, smashed and blown away. The element is totally destroyed, and frequently the Stainless Steel Heater Housing may be damaged as well.
  2. Just one or two small holes are blown through the Sheath - not as dramatic as a complete, explosive meltdown, but just as costly. The element is destroyed.
  3. No holes are visible, but the walls of the Sheath are bulged outward in spots, the normally smooth surface of the Sheath are now bumpy and has become discolored. Inside of the element the Heating Coil may be broken; or it might be electrically shorted against the Sheath. Or both.


Those are three good questions. Let’s take a look at a few of the things we have learned from a lot of field experience plus a ton of hours in the engineering lab:

HIGH LIMIT PROTECTION - the primary job of the High Limit Switch in the finished product is to prevent scalding water from ever reaching the people using the product. Sure, the thermostat should shut the heater off long before the point at which the High Limit Switch is needed, but thermostats, like everything else, fail.

Will the High Limit do its job? It hasn’t had to operate for months or years - is it ready? Is it in the right place to do the job? If the High Limit is not sensing the water temperature close to the element, and the pump suddenly quits - it’s dry fire time in just a minute or two. The element will boil the water in the heater housing - and create it’s own "dry condition.

Following that, the service technician arrives and finds the heater assembly full of water again, and claims that it couldn’t have been a dry fire. Check that high limit.

FLOW / PRESSURE SWITCHES - various types of devices are used to detect whether there is any water flowing through the heater assembly. Unfortunately, when these things fail, they generally fail in the closed position and there is no indication that they are not doing their job. In most cases a spa system will work just fine with a stuck pressure or low switch - until there is a need for it.

 If a full scale dry-fire destroys the flow or pressure switch along with the element, it’s impossible to determine which went first; but one thing is certain - the heater was doing its job. It isn’t very smart - it just makes heat.



It’s a mean, angry sounding word, bringing visions of all sorts of nasty things. Even the dictionary makes it sound pretty evil "…akin to rodent" no less. Come to think of it, we’ve seen a few heaters that looked like rats had been gnawing on ‘em.

We think it’s a safe bet to say that one of the major causes of heater failure has always been corrosion. Given any chance at all, this demon will destroy a heater element or, in many cases, the entire heater.

The corrosion that we come across in our industry, particularly in spa equipment, comes in many varieties, each with its own name and characteristics - we are faced with galvanic corrosion, chemical pitting, intergranular corrosion, stress corrosion cracking, corrosion fatigue, we even have a strain of bacteria: Ferrobacillus in the Siderocapsaceae family, also called "iron eating bacteria," to contend with. Gee Whiz.


Well, a full discussion of the chemistry and electrochemistry involved with this pretty complex subject is beyond the intent of this Handy - Dandy Guide. It’s much easier to describe what doesn’t cause it than to attempt to explain all of the chemistry that does. Consider this: if all of those spas out there were filled with clean, pure water, and no one ever added any chemicals to them, the word corrosion would soon leave our vocabulary.

Unfortunately, that’s not going to happen, and all of those spas are going to continue being filled with water of every type, from the Cascade Mountain’s rain water to big-city sludge.

…and then…


Then, to add to the problem, we have learned that many, many spa owners are not following all of the instructions they’ve been given about pH, Total Alkalinity, Calcium Hardness and Total Dissolved Solids; or about how to handle those unique water problems that may exist in their spa when it is filled with water from their tap.


If we had to pick the worst offender in the corrosion list, it would definitely be LOW pH, because as wed all learned long ago: when a test sample of the water shows a low pH (below 7.0), it indicates that the water is acidic. Remember now, this is not just saying that there is acid in the water - it says that the water has become an acid. It doesn’t matter at all what kind of acid caused this to happen - hypochlorous, hydrochloric, hypobromous, muriatic or whatever - when the water becomes an acid it becomes a starving, hungry, corrosive beast looking for lunch. Acids will corrode - eat away - almost any metal in their path in order to satisfy that hunger.

 An element or heater corroded because the water has a low pH is easy to spot - it has pieces eaten away from its surface. Gone, disappeared, departed. It looks like the craters of the moon. And people are putting their bodies in there!


At the other end of the ----- we have SCALE. In some ways scale can be considered as the opposite of acid: with scale the water has too much of something and wants to get rid of it, and the nice warm heating element is a good place to deposit it. The problem is that it’s like asking a Hawaiian to run a marathon in Honolulu dressed in a fur coat, wool cap and mukluks. He’s going to have a rough time getting rid of the heat he’s generating, and probably won’t finish the race.

…and then again…


Bad-chemistry corrosion can show up where the element sheath is brazed to the bulkhead fitting, or welded to a mounting plate. Even though all of the parts are "stainless" steel, and selected just for this purpose, the ingredients (mostly iron, chromium, nickel and molybdenum) in the steel must vary - the mix has to be different for parts like the Bulkhead Fitting that needs to be machined, versus the thin, high temp Incoloy Outer Sheath that must be bent and formed.

Very special welding techniques are used at these joints to make sure that no impurities are left behind that might corrode. But, if the water gets bad enough, you may see corrosion at these weld points. It can range anywhere from a rust-like deposit to cracks and fissures in the metal.

At it’s worst it can result in "corrosion fatigue," causing the bulkhead fitting to break away from the outer sheath. This corrosion is bad stuff.


A slippery, brownish coating on the inside wall of a spa may not be any form of algae, but can be the effects of an "iron eating bacteria" that is gnawing away at the heater. This one is easy to correct - the water simply needs some sanitizer: chlorine, bromine, or ozone. A "shock" treatment would probably be in order here, a drain and a refill if it won’t go away.


Corrosion of the metal in a pool or spa heater happens when the water becomes corrosive because somebody makes mistakes with its chemistry.

There are more than 2,000,000 spas in the USA alone that have never shown any signs of corrosion - some more than twenty years old. Their owners follow the instructions of the professionals at their local pool/spa store, and maintain a good sanitizer level along with a non-corrosive pH level, and monitor their Total Alkalinity, Calcium Hardness and Total Dissolved Solids for a good water balance. They do not "throw in a little of this or a little of that." That’s like signing a Pledge of Corrosion.



It’s correct to say that the only "natural" - as in "died of natural causes" - electrical failure that a resistance-wire heater ever suffers is when the heating coil breaks, causing an OPEN in the circuit. This is also the only electrical failure ever covered by a warranty.

It’s exactly the same as a light bulb burning out - it just gets tired of the tremendous trauma it goes through each time the power is applied. Picture it - it goes from cold to very, very hot in less than 1/10th of a second - it opens a door to see who’s knocking and a thousand billion billion electrons (that’s 21 zeros!) rush in during the first second.

Because of the careful selection of components in the Heating Coil, this sort of failure is actually quite rare. (Have you ever seen a light bulb burn out while it was on? Probably never - they always burn out just as you turn them on. That’s "turn on trauma".) However, it can be brought on more quickly by switching the heater on and off rapidly - this can happen with a defective control, such as a chattering contactor or thermostat, for example. Be very suspicious if a replacement heater burns out quickly. Watch for this.

Another suspect in an OPEN situation is a reduced water flow that causes the element to operate at a much higher temperature than normal, but not high enough to cause a dry-fire condition. There may be no signs of the problem on the outside of the element - it just got tired of working too hard.



Of any piece of equipment you might find on a spa or tub today, the electric heater is probably the most simple, we’re not talking about the controls, now - just the heater itself. What makes it simple is that it is a straight resistive device. It doesn’t have any coils like motors and transformers, giving them inductive characteristics; and it isn’t anything at all like a solid state, printed circuit board mounted, microprocessor based control system. With heaters, the electrical is plain-Jane


You can learn just about all there is to know about an electric heater by applying a little knowledge and a smidgen of easy math, along with a decent test meter. We’ll start with the basic numbers:

These first two numbers are stamped or marked on the heater.

Rated Voltage: 115, 120, 240, or 230 volts

Rated Wattage: 1500,5500 watts or 5.5 kw

These next two are calculated using the above numbers:

Calculated Amperage: 12.5, 22.9, 25, or 48 amps

Calculated Ohms:5.0, 9.6, or 10.5 ohms

The math part is kept simple by using our Handy Dandy Decoder Ring, as shown. Let’s say you want to figure the amps rating of a heater marked 240 volts & 5500 watts. Use w/v: that would be 5500 divided by 240, and the answer is 22.9 amps. Simple.

The next three are measured with a meter at an operating heater:

Measured Voltage: 112, 227, or 238 volts

Measured Amps: 11.5, 21, 24.6, or 44 amps

Measured Ohms: 10.4, 9.6, or 4.9 ohms


All that follows applies to all electric heaters. The task is simple - to determine if the heater is producing heat (watts); and then make sure that it is safe - these are the facts, ma’am.

First, measure and compare the voltage at the heater terminals with the voltage at the panel board, receptacle or other power source. Don’t blame the heater for poor performance if this measured voltage is more than 10% below the heater’s rated voltage. (Do the math (v)^2 / o, to see how quickly the watts drop as volts drop.)

If the measured voltage was much different than the rated voltage, you’ll have to re-figure the calculated amps using v / o. (You can’t use w / v, because you’re not sure what the wattage is anymore.) Then, with power still on, put your clamp-on ammeter around one of the heater wires - either one, but only one. You’re now reading measured amps, which should match the calculated amps within 10%.

Turn OFF all power, then DISCONNECT BOTH WIRES from the heater terminals. Set the meter to the Ohm's scale and take the following readings at the element terminals:

  1. Terminal - to - terminal: the measured Ohm’s should read within 10% of calculated Ohms.
  2. From each terminal, one at a time, to the element sheath or element mounting plate: should read OPEN or INFINITY. ANY OTHER READING says the element is internally shorted. This is a bad element. Look for signs of dry-fire damage.

That’s it. You now know all there is to know about the electric stuff in the heater. You can now re-calculate the wattage using the voltage, amps and ohms numbers you have measured, confident in your knowledge.


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