I have had this problem all too many times before when the water tank is not able to provide that fresh water needed and it is rather frustrating when you smell the water and it smells like rust. Though it can be worked around when you decide to treat it with chemicals.
That is why you should make the tank run all the time so that there would be no standing water left in the way.
Check valves at the intake of each boiler system. Sometimes valves will fail or rust away, and the stagnant water from inside the boiler can leak into your water supply. This rust-tinged water could be causing your rusty water and is considered hazardous to consume.