Solar Thermosiphoning Hot Tub Heater





Introduction: Solar Thermosiphoning Hot Tub Heater

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.

  • 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
Already thinking ahead to mastering this concept, I plan to build a wood stove that will also thermosiphon for supplemental heating.

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.



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Another suggestion, put some insulation around the pipe/garden hose that leads to the tub. You are dissipating a lot of heat into the ground or air as the water travels from the heater to the destination.

If you can sweat a pipe and change a fitting, my suggestion for you is to find an old steam/hotwater Radiator- the old cast-iron kind, clean it up, paint it black and build your solar box around that. The cast-iron and the larger chambers of water will act as thermal mass and even out your temperature gain. don't be afraid to make the box bigger; doubling your glazing would greatly increase your temperature, even if you leave your coil or collector the same size. also, some sliding doors have coatings to block certain wavelengths of radiation. the coatings are to prevent temperature gain and or fading. scrape at it a bit with something and see if anything comes off.

I am not sure if your design will work for just a thermo siphon. The coiled shape does not give a perfect thermocline for the water to flow through.

You would need to have a constant upward grading to obtain a natural flow, and that is not really possible with a looping system like that. But the idea is good!
I actually designed almost the exact same system. If you want to do it for a bit cheaper, try using irrigation tubing rather than a hose. The cost is a huge difference, although it is a bit less flexible and requires a bit more time and care to make.

That being said. You can buy some cheap little pumps that would work, and it is not hard to modify a submersion pump to act as an in-line pump.

I don't think it will start in the current configuration, the head to the hot tub looks like it may be lower than the total head around the coils.

So you've got maybe ten turns, each with a foot or two of head., that's ten-twenty feet of head, total.

So the head on the loops is higher than the head of the pipe to the hot tub. That will stop it drawing.

If you lay the collector flat it should start though.

Do you have at least one one-way valve somewhere in the system? I think that would help keep the water cycle flowing in one direction. Otherwise, the pressure from the heated coils will apply equally to both sections of the hose headed back to the hot tub. Give it only one-way to go and I think it will flow nicely, depending upon temp and height differential.

Living in sunny Queensland I have come across several homemade water heaters and coolers from the one system! Black is not only a great absorber of heat. It is also the best "color" to radiate heat.
In the colder months the water is either thermo siphoned or pumped to the roof during the day, heating the water. This system is turned off mid afternoon. A solar blanket helps prevent heat loss over the cool nights. During summer, when swimming in the pool is like swimming in a warm bath, the warm water is pumped to the roof at night and the black pipe dissipates the heat in the cooler night temperatures.

GREAT explanation! Thanks! I was following until that last part, where to position the hot & cold tubes on the heater.

And am I understanding correctly that I want to raise the hot tub as far over the heater as possible?

What about having a closed system, with some type of exchanger in the tub?

Some years ago a friend of mine in Rockford, Ill. built a solar water heater that did not work.
The problem was that the heat collector tube was in a "N" shape when it should have been in an "Z" shape. Hot water pooled at the top of the "N" and stopped the flow. He turned the collector 90 degrees and it worked OKAY.

The glass front box is not needed. Just fake down the hose in a single layer flat coil between two "X"s of 1"x4" laying flat on the ground in a sunny spot and attach the hose ends to the bottom of the tub.
You will have plenty of thermosiphon action in minutes.
Cover the tub with a piece of foam core or bubble wrap plastic.

Foam insulation and Rustoleum spray paint don't mix but, foam insulation and Krylon spray paint does.