Solar Heating My Pool's Hot Tub?

I have seen it done on a smaller scale but not for a large hot tub (hundreds of gallons). I imagine to moderate the heat you would need to build a heating loop with a fairly large insulated storage tank, then use a dump loop that is thermostatically controlled, opening up to release that water into the hot tub, along with refilling the tank from the pool skimmers.

@clmason: I have a set up pretty much as you describe. My solar pool water heater pump switches over automatically when the temperature there is x degrees higher than the pool temp. I can switch over the water to route from the solar heaters (black rubber sheets you describe) into the hot tub only.

It works but (once you get bodies in) some of the hot water flows out of the hot tub into the pool via the spill over wall and there's no storage or insulating system to keep the water hot over time. On a cool/warm but sunny afternoon the hot tub can be really pleasant but it's never really 'hot' without turning on the gas heater, at night the water has cooled quite a bit and obviously solar doesn't help.

Here in Florida we tend to use the solar heaters to keep the pool pleasant through the winter; the hot tub really needs a gas/electric heater if you want to use it at night though.

I live in the (SF area) east bay and have several Fafco pool heating panels on my pool. I have a navy blue electric cover, my house, Fafco panels face west and south. In the summer I can get my 20K + gallon pool 105 + degrees with this set up very easily. During the day the outside temp can be 100 but will drop to the 50's at night. We use the pool at night and it feels pretty nice jumping into a big bath tub. I am sure you could get a regular size hot tub rocking and rolling with the correct set up. We have a pool style gas heater on our regular hot tub, its cheap to heat it in the winter and we don't use it in the summer, I have not bought about doing anything with it.

"Swimming Pool" solar heat systems generally use a lot of square feet of un-glazed panel to raise a very large volume of water (the whole pool, 20 to 50 thousand gallons) a small increment in temperature (a few deg up to maybe 25 F, as a normal max). "Domestic hot water" systems generally a much smaller sq ft of glazed panels to heat a small volume of water (a few hundred gallon pre-heat or buffer tank) a much larger increment in temperature (60 or 70 Deg F in winter).

For a hot tub, you are somewhere in the middle. Medium amount of water, medium temperature delta. You could make either kind of system work... but, having "DIY" calculated and installed a couple of solar pools, I'd lean more toward glazed panels and a buried buffer tank. Obviously, once the sun sets, no more heat. So the only way to keep a circulating tub hot, particularly with air blowers on, is to have a massive thermal sink of some sort that is hotter than the tub. So... some glazed panels, and an insulated, buried, buffer tank of 3 to 5x capacity the hot tub, that can target to 120F or 130F before sunset... and a SUPER RELIABLE failsafe mixer to prevent scalding... could be done.

Of course, I'm just thinking off the cuff.


But... if there was ever a gas heater... it will be MUCH cheaper just to replace that, unless you plan to run it a lot.



And, when I had solar pools and not gas, I used to do what erosinc mentioned... heat the whole pool to hot-tubbish temps. Very easy to do, and very nice swimming experience at night night in the summer. Not so nice the next day.