I'm having an idea which I need to run by someone with knowledge before I try it; there are small heat pumps made for above ground pools. This is one of them:
They're not super cheap, but professionally installed units are like ten times the price and probably no even available in the US. I kinda wanna give it a try. As far as I figure, I can just set the spa heater to a low temperature and run the heat pump at its set temp. External spa heaters are not entirely unheard of, right?
The spa in question is a small (125 gallon) CalSpas unit we got about a decade ago. After getting heat pumps for home climate and water heater, I'm eager to dump as many resistive loads off the electrical as possible.
I'd off start by just running inlet and outlet hoses over the top of the tub to try it out, then think about how to plumb it in more elegantly (tips on that would be cool too, if it's not a terrible idea).
Has anyone done this or thought about it? What sort of things should I look out for?