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.
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.
Step 1: Insulate & Caulk
I used materials I had around to build the enclosure of the solar collector. The size was dictated by the glass I had. I'm hoping the heater will work in cooler weather when it's sealed up real well.
Step 2: Black Itar
I tried spraypainting foam board for a project in grade school & it melted. I could see this was going to happen here, so I used some Elmer's glue to stick down a black plastic garbage bag. And a few staples for good measure. Then I painted the wooden sides black.
Step 3: Charge It with Water
Caulk the glass after ensuring there are no leaks in the PVC connections.
Step 4: Wait for it to Warm Up
I'm still rebuilding. This green garden hose will be replaced by rigid 1-1/4-inch PVC tube plumbed into the plastic stock tank.
Now it's easy for me to see why the old design didn't create a thermosiphon pump. I'll update with results of the experiment.
Thanks for reading. Feel free to post suggestions.