UPDATE: The latest configuration works!! Yesterday when I finished filling it with water, at 1:30, it was 68 degrees. It was 72 at 2:30, 78 at 4:30, today at noon 81, 2 pm 88, now at 3 pm 91 degrees! Plus, I can see the water pushing out of the hot return pipe from the solar collector.
Here's a video of that. The debris is good for illustration of the water flow, but next I'll have to get a filter to get it out of there.http://youtu.be/VQHR0XhijPo
This is my version of a solar hot tub heater. The other Instructables I've read circulate the water through the heater using a mechanical pump. However, my system is based on a thermosiphon, in which passive heat exchange occurs through natural convection: Cold water falls down from the bottom of the hot tub into the solar heater. Warm water rises out of the heater & back up to the hot tub. This design works great for my situation since I wanted to position my heater below the level of the hot tub. Usually these heaters are mounted on the roof above the tub. But a tree shades my roof since I don't want it to be hot in the summer. It's June 12 in Omaha, & I still don't have AC in the cottage I'm building. So that's the tricky thing: If you want a thermosiphon effect, your tub has to be higher
than your solar heat collector.
This here is my second try on the design of the collector. Coiling a garden hose round & round didn't create a thermosiphon, and THANKS to your comments and info gained from the failure, I'm using 1-1/4-inch rigid PVC in place of the garden hose. Also, I don't have the box tilted at 30 degrees anymore. Now it is lying flat on a gentle sloped bit of lawn with a just brick under the highest edge, so it's close to flat, but not completely.
Please read the excellent comments below for explanation on how the thermosiphon pump works.
PVC tubes, elbows & Ts & glue
Stuff to build the box. I used 2 pallets & some 2x lumber
Sheet of rigid insulation
Black garbage bag
Black spray paint
Already thinking ahead to mastering this concept, I plan to build a wood stove that will also thermosiphon for supplemental heating.