Picture of Cheap solar hot tub/spa/pool water heater

This instructable covers the solar heater I made from parts available at the local hardware store (or salvage) for cheap. I have yet to do true empirical measurements on its output/efficiency, but it will raise the temp of my hot tub (~460gal) from 70 to 80 in two sunny days, and keeps it in the 90s during the summer without using the tub's heating element. This allows me to keep it warm and only use the electrical element to boost the temp when I want to jump in (saves quite a bit of $$ on electrical bills), after which this will keep the temp up in the 100s for a day or two on its own. This is the result of a few experimental panels, and the finished product turned out to be about the easiest of them all to do. A more refined version with fewer connections could be made if properly planned out.

It can be built in an afternoon, possibly just a couple hours if you have the parts ready to go. The longest wait time is for paint and sealant to dry/cure.

*Trying to find more of the pics I took while building this, taken over the course of about a year (hence the new look of the wood at the start, and OLD/weathered look at the end)

** Evidently the photo tagging thing likes to move my tags up from where I put them.... working on getting it fixed, for now just imagine them a good bit lower than where they are

ZephS4 months ago

After 4 years, do you have any measurements or estimates of how much heating this saves at various times of year? (Best in KWh rather than dollars, as rates vary)


alcurb1 year ago

Regarding your concern about overheating the pump, I suggest a bubbler (a.k.a., aquarium air pump) inserted into the base of the rising tubing. The bubbles rise and move the column of water upward. The water at other end of the tubing flows freely downward towards the tub creating the necessary gravity assist to complete the water flow.

Regarding the sagging clear corrugated plastic panel, I suggest to run a tight insulated wires, as if they were guitar strings, length wise across the top of the box, then you lay the clear panel over that. The wire, if placed correctly so that they fit into the valleys of the panel would help keep the shape of the panel when or if it overheats and tries to sag.

tmack0 (author)  alcurb1 year ago
The problem with a bubbler is that it would pump air into the coils, which will reduce the surface area of water touching the tops of the coils. It will also break the siphon (doesn't take much air to do this) and end up just draining the entire setup. So far the pump is still working, going on almost 4 years now. There really isn't much to one of these, its basically a brushless motor with an impeller on the rotor shaft. No real electronics inside, just coils of wire to drive the permanent magnets of the rotor all sealed in waterproof potting material.

As to the sagging: good idea. It would need a screw or nail at each peak to get them high enough, but would prevent the sagging. Also using real glass or polycarbonate would probably work and be better suited to this application. My clear PVC is getting hazy now and has several cracks and chunks that have broken away. My next upgrade to this will be either of those. I almost got a glass lid replacement from an old glass storm door someone threw out, but it shattered on me while trying to get it on the roof (search youtube for tempered glass explosion, thats what happened). I bumped the corner on the concrete just a little too hard and poof, glass everywhere.
cobinrox2 years ago
Do you think this would work with a garden hose rather than the irrigation pipes? I have about 100 ft of black garden hose and thought this would be a good use for it. Thanks.
tmack0 (author)  cobinrox2 years ago
It probably would, though it might kink easier (especially when it heats up) and not transfer the heat quite as well. If its just laying around, couldn't hurt to give it a try, and it will likely help some. Think how water in the hose comes out steaming hot at first if its been laying around in the sun, same principle is at play here. Just make sure it doesnt get too hot and melt holes in it!
jad513 years ago
Is there anyway of doing this but convection rather than using a pump?
cdb1977 jad513 years ago
If you had your water storage (hot tub in this case) above the solar collector, you could work through convection. You would also want to simplify the flow path of the collection piping. Instead of spirals, have vertical piping that are connected at the top and bottom. As the author stated, though, this will give far more opportunity for leaks.
OK just wondering. Thought maybe too much friction loss anyway.
kolraw3 years ago
Would using copper pipe help with the heat transfer thereby heating the water more? I know it would increase the cost but it sounds like it might allow the panel to be in the sun without running water in it also.
tmack0 (author)  kolraw3 years ago
Oh, and about the part of not letting it sit in the sunlight without running water: Its mostly a problem with the transparent PVC lid. The poly pipe itself gets softer, but isnt under pressure enough to cause problems (it might flatten out a bit from vacuum, but will return to normal when the water turns on again). The lid, on the other hand, sags from the heat built up in the box, which without any water running in the system to remove, gets quite hot, and would regardless of materials the piping is made of. This sagging is the problem I try to avoid, as it actually deforms the lid and starts collecting rain water in the depression. The Polycarbonate option I mentioned might avoid this, as glass certainly would.
tmack0 (author)  kolraw3 years ago
Copper would dramatically increase the cost (and weight) of the project.
The design is similar to commercial ones (wikipedia: http://en.wikipedia.org/wiki/Solar_thermal_collector ) but with the collection tubes in front of the "absorber plate" since I used a corrugated panel instead of flat, and plastic tubing instead of metal, all to reduce costs. Once you get into copper, the price approaches/exceeds that of commercially available panels (see http://www.amazon.com/SW-37-Solar-Water-Heater-Panels/dp/B0041VM58E ). If you have it laying around though, sure, use it instead, paint it black (just be sure to use a pipe bender to avoid kinking it.

There would be gains in conductivity (coefficients of 400 vs 0.5), and since the copper is thinner, there would be gain in that as well (Q=kAΔt/x , heatflux = conductivity * area * temperature difference / thickness). http://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html has a good description of how this works out. It is a bit more complicated because instead of a (relatively) simple heat exchanger, its a heat exchanger with a radiative input power on the air side boundary layer. This adds in another Q on the right hand side of the equations.

Disclaimer: I was an ME in school, but fluids and thermodynamics were not my favorite courses. I could work all this out, but it would give me a headache and the numerical answers are already mostly done if you search the internets.

cmac910004 years ago
Sounds like a great idea but in the UK, irrigation piping has holes in it. Is this correct or do you mean what we would call hose piping which doesn't?
tmack0 (author)  cmac910004 years ago
Definitely do not want leaks in the panel, as any water that gets out of the piping will not make it back to the tub, and will also create condensation on the transparent lid (which is an easy way to tell if you have a leak), and will eventually fill the box with water and will start leaking everywhere. This is the non perforated tubing, sometimes its also called "Poly Pipe" as its made of polyethylene, or "Funny pipe" since its like pipe but flexible. Its sold in rolls and can be bent easily (more so if its warm).
nanosec124 years ago
I love this....I have been contemplating something similar, but your method is much easier to accomplish that what I had been planning. I think I will follow your instructible this winter and live up to the old motto of 'keep it simple stupid'

Thanks, 5 of 5
Jaycub4 years ago
This a good idea. I saw something similar on a greenpowerscience video on youtube.