I've started a project to heat my small above ground pool with the heat from my attic. I am building the whole system from scratch using off-the-shelf components. Total system cost will be well under 500 bucks..
I am going to have a air to water heat exchanger mounted horrizontally in the attic. It will have a 12v fan on it. A small 12v pump is going to pump the water from the lowest point up into the heat exchanger.
It looks like I'll get at least 30,000+ BTU/hr of heating capability when the pool is 89 degrees and the attic is 110 degrees. I've tracked the temperature in my attic and on sunny days it stays above 90 in the attic from noon till 8pm. It peaks at 120 degrees around 3pm..
I purchased a small 12v pump that will move 3.3 Gallons per minute.
Last year I tried the plastic solar pool heater panels and while they helped, they were very ugly and the wind blew them right off my roof. I also tried a bunch of hose on the roof and the pump used 500 watts just to move the water, I'd have been better off running an electric heater.. This new system will only use 140 watts and should also cool my house a tad by cooling air in the attic.
I Can't wait to finish the plumbing, set up my pool and see how this puppy performs..