Cheap solar hot tub/spa/pool 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

Step 1 Parts and Tools

The bulk of the cost was the pump, which could be obviated if you plumb this in directly to your pool/spa pump, though that makes it harder to power independent of the spa. Make sure your pump is rated with enough head to pump the water up to the panel with enough left over to overcome the head loss of all the piping. At first I looked at solar powered fountain pumps, but couldn't find any in my price range with enough head to work. I ended up with a 320gph pond pump* with ~10' head. I am mounting mine on the roof over the hot tub, about a 7' rise, so the 10' rating is needed (and gives it a good flow rate). Properly sealed, once the water is up to the panel the siphon effect should reduce the head back to just the loss from the flow through the pipes themselves, but the water still has to get up there first. You want the pump to be able to do this on its own without priming or other assistance because the panel will eventually drain if there are leaks, a hose comes out of the water, or you let the water sit and it boils in the pipes (has happened to me!). Manually priming it to get it going again sucks, and letting it sit in full sun without water to cool it off can soften or even melt some parts (PVC specifically).

*WARNING! (see my more detailed warning at the end of the i'ble. POND PUMPS ARE NOT RATED FOR THIS USE! YOU COULD ELECTROCUTE YOURSELF USING ONE! PLEASE USE COMMON SENSE AND GET A PUMP RATED FOR POOL/SPA USE TO BE SAFE!!!!! Continue strictly at your own risk (think: submerging the end of a LIVE 110V (220v?) power cord in a tub of water and jumping in).

Parts: 6' galvanized roofing tin panel, 2@ 8' 2x3 or 2x4, 12 wood screws (long enough to hold the boards together, so ~2.5" or be prepared to drill countersinks for shorter ones), 1" Galvanized roofing nails or corrosion resistant screws and washers (I used hardy-backer screws left over from a tiling project, with galvanized washers) , High-Heat Flat Black Rustoleum (aka Grill or engineblock paint), 150' 1/2" black irrigation line, Pump,2 @ 8' 1x2's and some "Great Stuff" type spray foam insulation (or similar 2part expanding foam) and 2-3" thick 2'x4' (or larger and trim to fit) Foam insulation board, 6' Clear Corrugated panel (poly carb or pvc, poly carb is preferred but costs a bit more. Glass is best if you can adapt this to it, be careful!), 4x corrugated to flat insulator foam strips (2 to fit the tin, two for the clear panel), wire/string, silicone caulk.

Tools: Drill, 1" bit, smaller pilot bits, hammer, screwdriver (or bit), caulk gun, saw, snips, measuring tape.

Comments

[kolraw]

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]

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]

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.

[jad51]

Is there anyway of doing this but convection rather than using a pump?

[cmac91000]

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]

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).

[nanosec12]

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

[Jaycub]

This a good idea. I saw something similar on a greenpowerscience video on youtube.