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.
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
- Silicone caulk
- Concrete blocks
- Black spray paint