Hi folks, I am new here but have really appreciated the info I have found so far, particularly regarding pool chemistry. Apologize for starting a new thread but couldn't find much via search. I want to air out this idea before I start doing something really silly.
I bought a home about 21 months ago with a pool/spa combo built around 1995. So far I have converted to salt water, replaced the pump, replaced the DE filter screens, and made some inroads into removing the copious quantities of scale. During that time the pool heater has been on life support and has actually worked to some extent. When I switched to a new propane supplier, they declared the old pool heater to be a "death trap" and would not connect to it. Sooo... time to redo the heater.
I live in Orlando, FL. The plumbing is set up for one pump to drive the pool and the spa. The spa normally overflows to the pool. The valves have to be manually realigned to just drive the spa jets. The existing heater could heat pool and/or spa. I have no desire to heat the pool.
I am looking for a cost effective way to heat the spa only and I am considering some alternate ideas. Specifically, I would like to use a tankless water heater that typically is used for household water heating and/or hydronic heating. Why, you might ask? I have installed two of them before (in normal applications) and have been very impressed. Currently on eBay there is a re-branded Takagi with remote controller available for about $850 (typically about $1400). Input is 199k BTU/hr. They are condensing units so efficiency is >90%. They are small, about the size of a carry on suitcase. As part of the implementation, I would install a second pump that would only recirculate the spa through the heater and a bypass valve and back to the spa jets. This would eliminate the need for manually realigning valves and would allow 1 button operation for the spa. The bypass is needed as the heater only passes 8 gpm and I think the jets need ~30 gpm.
Two arguments I have seen so far is whether or not a tankless heater can operate for hours at a time and can they handle the chemical environment of pool water. As far as run time goes, they can operate indefinitely and are readily used for hydronic heating. The chemical environment , I'm not so sure.
The unit consists of a standard copper heat exchanger and a stainless steel heat exchanger. The stainless can handle the acidic condensate on the outside so I think they will be fine with pool water on the inside. What I have read about copper says it is pretty resistant to corrosion in salt water environments and with the pool at ~ 3-4k ppm, it is not nearly as salty as the ocean. Of course the ocean isn't running a SWG. I have no idea how aggressive 3-5 ppm free chlorine will be on a copper heat exchanger. But aren't many of the older pool heaters built with copper? Also, there normally won't be any flow through the unit so I think that free chlorine would quickly deplete. For the record, tankless heater manufacturers specifically state they are not so be used with pool water.
There are at least 2 manufacturers making all stainless tankless heaters but their price point is >$2k.
So my questions are:
Has anyone out there done something like this? How did it go?
Does anyone have specific thoughts or data regarding chlorine and copper heat exchangers?
Would it be better for the heater to be wet or dry when not in use? i.e. should I make it drain when there is no flow.
Am I missing something else in this whole exercise?
Thanks in advance for any advice you can offer.