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.

Hey! I felt kinda bad about leaving this instructable up after I discovered it wasn't going to work, but then you all were leaving such helpful comments, I guess this can be a work in progress. I am learning a lot.

Now I'm draining the tub & have an idea for a rebuild that probably will involve having the solar collector right next to & below the hot tub instead of 20 feet away, making a cover for the tub, possibly adding a recirculation pump, possibly solar, and of course changing the coil configuration.

What I'm picturing is maybe 2" pvc serpentining back & forth & all sloped at such an angle as to let the hot water rise. At one point I had a ladder arrangement in mind. I'm not sure how it would work. digitalia mentioned internal resistance. I'd love to have someone elaborate on what that means.

And do you all think a ladder, with 2 rigid tubes running vertically and "rungs" going across between them using T fittings would improve the thermosiphon or if the variations in volume would work against me.

My intuition says an opening on each side of the rung into a vertical tube for the hot water to rise up would decrease internal resistance, but I'm not sure.

Thanks for all your comments. I'll let you know how it's going.

Nice build!

I made a solar heater for my hot tub too, and I do not mean to take away from your hard work but you might find my experiences helpful to you.

I will skip the evolution of the design, but my final one had a 12 volt "live well pump" hooked up to a solar panel that would circulate the water through 120 feet of 1/2" PEX tubing (it was stuff that I had, your garden hose will probably work better if you continue to use it).

It provided an 8 degree temp rise at about 2 GPM. I used a 30 watt solar panel which worked fine and it was another thing laying around here so I didn't go out and buy it.

Anyway, it worked pretty good especially if you put the cover on the hot tub you could get it uncomfortably warm.

So if you have an extra solar panel laying around go buy a cheap pump, my first ones were just a wal mart 12V bilge pump for $12 ish that I dropped in the hot tub. Connect that to one end of the hose and the hot water will come out the other end regardless of your orientation.

Again, nice work and write up!


This approach may not be as cheap as you think.

I was really glad to find your instructions. I searched the web for exactly this thing.
So, I thank you very much.
However then, a few minutes later, I found details on what I believe is called "Thermal conductivity" in English.
Copper has 399 W/(m · K) while Polyethylene (is that the material of your hose?) has 35 W/(m · K)
This means, assuming your hose material is as thick as a regular copper pipe (1 mm), less than a tenth of the lenght in copper could absorb the same amount of energy in the same time.
Over here, in Germany, the price ratio of copper pipe against a PE hose (food quality) is about 10:1
The same construction built with copper pipes might be cheaper or at least more efficient.
In addition: I do not know whether PE ages. Copper prices increase year by year. in 20 years, when you do not need this construction any moore, the scrap copper may be still valueable.
What do you think about this?

1 reply

I agree with you, especially copper will last longer than plastic. In addition as you noted copper can be recycled and some of the money if not all will be recouped. Plastic on the other hand will not degrade and another cause for pollution.

Rigid tubing, like black ABS (Acrylonitrile Butadiene Styrene) plastic tubing, makes a better heat exchanger, plus its rigidity would likely help in establishing and maintaining the siphon. Instead of one coil with a dozen or more windings, 6 coils with two windings apiece, and joined at inlet and outlet, would mitigate internal resistance and improve convection. I built a lean-to water heater preheater many years ago (35 or so) positioned much the same as your setup, and it worked very well. Of course, convection wasn't what drove that system.

It will not thermosyphon in that configuration, as you have created a "thermal trap" in that parts of the coil go up and then down, heated water will not want to go down. A serpentine (back and forth) configuration will work a lot better.

Great to see this onFather's Day....! As a kid my dad did something simliar to heat our pool. He coiled a lot of hose on our roof, by running a pump to exchange water from the pool. We had plenty of sun beating down too, which obviouslybis needed here too!

keep experimenting with it!!! might need to try different coiling patterns to see what works best but at least you are trying!! let us know how it works out... I wanted to try the same thing but havent had time to make a prototype so I really want to see what works out for you!!!

I have one of those drill pumps. They don't actually have enough power to draw air out of a tube. It's better used to speed up an existing siphon. Once it has water in it though it actually pumps pretty fast.

Good idea, I'll try this. But don't you think that it's better to cover the water tank?