mickey4paws said:
Does anyone know if low CH can be bad for a heat exchanger in a heater? Someone told me that, but I don't know if it's true or not. I have a vinyl pool and CH of 100.
It's not true. Saturating the water with calcium carbonate protects plaster and grout surfaces that contain calcium carbonate from dissolving. The idea of saturating the water to provide a thin layer that protects metal from corrosion is at best controversial (see
this link for some expert discussion). Basically, low pH is the worst for metal corrosion, assuming oxidizers are present (dissolved oxygen, chlorine, etc.). The next worst is having higher conductivity as from a higher TDS or salt level. For stainless steel in particular, a high chloride level is more corrosive.
If you have a non-SWG pool so have generally lower salt levels, then a copper heat exchanger should do fine. Remember that tap water used to be chlorinated at 1 ppm FC or so without any CYA so was 10 times higher in active chlorine level than our typical outdoor residential pools, yet copper pipes did not corrode. This was mostly because the pH was usually kept above 7.5 (typically 7.8 to 8.0) and sometimes corrosion inhibitors were added, such as phosphates. My tap water only has 55 ppm CH and a saturation index of -0.7. For SWG pools, copper sometimes is OK, but using cupro-nickel or titanium heat exchangers would be more resistant to the higher conductivity from the higher salt levels.