Cause of rusted heater?

[cmcq]

I don't store any chemicals near the heater. They're 25 feet away inside a shed, so I'd vote for the leaking supplying the moisture.

I don't understand what you mean when you say the absence of metal stains indicates a chemistry problem.

Thx
Charlie

[Pool Clown]

The improper balance may have been "eating" at the heater. Then when removed, there was nothing to eat, and deposit into the water to eventually stain.