Introduction: 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

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.

Step 2: Frame

The panel itself is light weight, so the frame doesn't have to be super strong, just solid enough to keep its shape. The frame also acts as the walls of this panel, trapping the hot air inside to improve convective heating from the surfaces not directly heating from solar radiation or conduction from the metal backing through the piping, and prevents convective losses from air colder than the water in the pipes (we hope) blowing through. The frame is a simple rectangle. Measure and cut the 2x3 or 2x4 boards at 6' each, this yields 2x6' and 2x2' boards. Screw them together to form a simple rectangle the size and shape of the roofing tin. The tin should fit nicely with minimal overlap. Make sure you do this on a flat level surface or the frame might come out twisted or irregular. To make it more air-tight and rigid, you can use construction adhesive in the joints along with the screws. Drill pilot holes or countersink them as you see fit.

(its easier to do these next steps now before the tin is in place,
Drill 2 1" holes near one corner of the frame, side by side close to the bottom edge (where the tin will be attached), to be for the inlet and outlet lines. 1" is large enough to pass the lines and a hose adapter or line splice, so if you dont plan to use one of those, or have them outside the frame completely, use a 3/4" or other bit to mode snugly fit the lines.

Place several nails, or more preferably screws about 1/2" from the bottom (tin side) along the inside of each board, leaving 1/4-1/2" sticking out. Space them about every 1-2'. These will be used later to tie down the coils. Its much easier to place these now than after the tin is in place.

Step 3: Attach the Tin

Lay the tin over the frame, lining it up for best centered fit. Optionally put a bead of silicone caulk along the top of wood where it will interface with the tin to better seal the panel from drafts. Fit the corrugated foam thingy between the tin and the wood on both corrugated ends, you will probably need to stretch it to fit properly. Using the Roofing nails or some short screws, secure one corner, double check alignment and the foam (it likes to move if you don't hold the panel down), then secure the diagonal opposite corner and end of its foam strip. Nail the other corners to secure the tin and foam, and then nail down every valley in the corrugated tin starting in the middle and securing the middle most valley each time. Going along the flat/long sides nail the tin every 6" or so also starting in the middle and dividing the distances until you get to about 6". This is to keep any slop from being propagated along to one corner where it becomes a large gap. Again, make sure the frame is on a flat surface for this, nailing while its not flat will give you a twisted frame (mine came out with a minor twist).

Step 4: Insulate the Back

Now we will make a frame to hold the foam and insulation board on the back of the panel. Cut your frame material into the same dimensions as the main frame, using the 2' sections made from the 1x2's (gives an extra 1" for the corrugated tin to fit) and longer sections with the 1x3's. Build the frame the same as you did for the main one too. Secure the frame in place with wood strips cut from the leftovers of the 1x2's and 1x3's, nailed/screwed to the side of the main wooden frame, and then nailed/screwed into the sides of the new frame. To seal the ends, use two or the foam insulation strips (the ones that fit the tin's corrugation).

Now the fun: use your spray foam (the stuff for "Big Gaps" is probably best here) and go back and forth along the troughs in the corrugation on this side. One can will not come close to filling the box, so use it mainly to fill the troughs and act as glue to hold a slab of foam board insulation.

Step 5: Paint the Thing Black

Using the high-heat paint, give the interior of the panel a nice flat-black coating. Use several light coats of paint to get a nice even finish. Since the bare tin is bright and shiny  its fairly easy to find the missed spots. Spray the wood as well, though pay more attention (and use more of the paint) on the tin itself.  Once done, let it dry before mounting anything in it. The next step can be done while waiting.

Step 6: Making the Coils

The next step is to setup the pipes that go into the box. This is the part I did the most experimenting with. I tried a few variations of pipe setups, including an array of the tiny 1/4" lines running from a manifold, but found that the more complex the setup, the only things gained were leaks. Power from the sun is a blanket rate over an area, so the more surface area of panel, the more power you get into your system. The size of the elements doesnt matter as much as the surface coverage. The 1/4" test also confirmed that. Since its such smaller diameter, it took a lot more to cover a lot less of the area of the panel, and choked off the flow rate, and ended up not as efficient as my final design.

This design is simple, easy, and very immune to leaks. Since I experimented, I ended up making 3 purchases of 50' spools of the irrigation line, rather than a single 150' (or 200' as they tend to sell it) spool, so I had to splice them back together. The splice you want to use if you have to do this is the simple screw-down compression type, NOT the black tube with a colored end you stick the hose into (these will leak like crazy, its only holding the pipe in by a sharp lip whereas the other one clamps the pipe). If you use a single piece of line, you don't need to worry with that at all, though putting in a splice at the input/exit of the panel will allow an easy way to disconnect the panel from the feed lines so the panel or its feed can be taken out for maintenance.

All that said, the design itself is simple: 3 flat coils of the line packed into the panel. To get the line to actually stay flat and coiled, you will need to tie them into it using string/wire. I used old/salvaged solid core Ethernet wire strands to tie them up. You will want to measure all this before hand to make sure your coils will fit properly with the ends just coming out the holes of the frame (or with enough of a tail to reach the source/destination).

To start: lay out the length of tail needed to reach the panel exit (or source/destination feed). Start coiling the first coil, from the outside-in. The tubing comes pre-coiled, so its fairly easy to do this, you just need to guide it and keep it flat. Its easier if you let the tubing warm up by leaving it out in the sun a while before you start, as it will soften a little and be more flexible and less prone to kink. Get the center of the coil as tight as it will go without kinking, with the total width of the coil just under the inside width of the panel (about 12 loops). Put a board or something else flat and heavy across the coil to hold it in place when you have it set right.

With the long end of the tubing (the end/rest of the tubing going on the the next coil) , set its direction out of the center of the first coil so that it will be easy to start the next coil (ie: tangent to it). Weight it down in that position to hold this all in place while you secure it with the wire. Starting at the outer edge of the coil where this tail crosses over the top, wrap the wire around the tail and twist it to itself to secure it. Then run the wire under and around the outermost coil of tubing and back over tail, making the wire cross between the tubes (sorta like a figure 8). Continue this for each coil trying to keep them snugly next to each other as well. Once at the center, secure the end of the wire similar to how it was started.

Move to the section where the short tail goes off the outside towards the exit and start another piece of wire there. To hitch this section together we will use half-hitches around each coil. To do this, bring the wire around the tubing, then send it under and around the wire itself. Each hitch ends with the wire pointing to the next coil. To keep things tight (keeps the coil, coiled, which is what we are going for here), pull the wire snug after wrapping under the tubing, then start the wrap around the wire about half way between coils, using your fingers to take up any slack it tries to put into the system. Pull gently to snug the hitch (wire will snap if you pull it too tight here, especially if its soft/copper like I used). Continue for each coil. Do this at least once more to get three sections tied off. Repeat the coiling and tying for coils 2 and 3, each time going from center of one coil to outside of the next. The final coil should end with the tail going up the sides of the other two coils and out next to the other tail so that both inlet and outlet lines are next to each other. Secure the tail to the outermost loop of the other coils, and to the other tail with a couple wraps of wire at crossings.

Step 7: Laying Pipe

Once the paint is dry, you can mount the tied up coils in the panel. Three coils of the proper width should fit snugly in the box. Position the coils in the panel with the crossings and wire hitches on top, and the inlet/outlet lines fed through the exit holes in the frame.

If you want to put in a splice or fittings for a quick disconnect, do this before securing the coils. Make sure they protrude through the frame long enough to enable you to hold them steady while connecting the other half (remember, once done you will not be able to touch anything inside the panel without taking things apart.

With the coils placed how you like, use more wire with the nails in the frame to run strands over the coils to hold them down and in place. You can also run segments around the outer most loop of each coil to tie it in place.

Step 8: Put a Lid on It

Once all the coils are reasonably in place (tilt the panel, making sure they stay as coils and relatively in place, they will probably slide a little), its time to put the lid on. If you have a sheet of glass to use, good luck, I wont go into detail on that as its not what I did and can get complicated (simple version: silicon caulk, nail down a frame to hold the glass in place), and is much more dangerous. For the corrugated pvc/poly carb, its much the same as installing the tin on the bottom. Line it up so the edges are flat on the sides and centered best you can, apply a bead of caulk between the sheet and the wood, make sure the foam strips are in place, secure the corners, then nail* or screw the panel in every corrugation valley and every few inches along the longer (flat) sides (again working from the middle out). If you use screws, also use a washer to spread the load on a wider area to avoid cracking the plastic, and only tighten it enough to hold it firm to the wood for the same reason (I used 2 washers, a spring/split washer above a flat washer, and only tightened until the split washer started to flatten). If using nails, be VERY careful to only hit the nails. Hitting the panel can shatter it (if you do this, clear packaging tape is an ok temporary repair, but will eventually dry up and flake off, requiring you to put another piece over the break or replace the whole lid). Allow the caulk to cure and your panel is ready to mount and plug in.

*Note that using screws can make it much much easier to open up again should you need to. For mine, I used screws on one-half (split length wise) of the panel so I can remove the screws on that side and bend the lid back with the corrugations.

Step 9: Mount It and Turn It On

Almost done! Find your mounting spot (preferably above the surface of the water to prevent it from siphon-draining your tub/pool if things go wrong) and set the panel in place. Angle towards the path of the sun if needed, keeping the inlet/outlet at the bottom and note that the head needed by the pump is measured to the highest point of the panel, and connect the source/output lines. One line goes to the pump (doesnt really matter which), the other should come out under the surface (prevents draining the panel). To attach to a pond pump, the 1/2" adapter will probably need to be increased in size for a tight fit. I use a wrap or two of foil tape to do this (electrical tape is too soft especially at hot-tub temps, the adhesive will turn to goo and it will fall off), making sure none of the tape protrudes over the outlet nozzle.

Thats it! Plug it in and watch any joints for leaks. It will take a minute or three for the water to travel the 150' of line before coming out the bottom. For best results, put a thermostat with a sensor in the panel to turn on only above 100F, or a timer to only turn on during the daytime (or both, to make sure your panel doesnt act as a radiator and cool the tub off in the evening after running the tub at temp. A future project will be a pic/audrino type micro-controller with temp sensors in both the tub and panel, to turn on the pump via relay when the panel is warmer than the tub. Mine is currently on an X-10 outlet with a timer programmed to turn it on/off with the sunrise/sunset (and delays to account for shadows from neighboring trees/houses), which also allows me to remote control it if the weather turns cold/cloudy.

One note on this design: with the PVC top sheet, you will want to run water through it any time its sunny outside to prevent it from going soft and possibly melting.... I left mine off one day and came back to a sagging lid with impression marks in the valleys from touching the tubing. Previous experiments using PVC pipe as line splices (1/2" pvc fits irrigation line inside it quite well, actually) showed similar results: the splices held up until run at temperature, then they started to bend and crack from the heat. Lesson learned: PVC has a low tolerance for heat.

Other note, and Warning:  The pump I used in this is a fish pond pump. While it is a grounded pump and works fine without electrocuting your fish, the pump itself is NOT rated for pool or spa use (specifically states so in the manual). The temp of a spa at usable temp is also out of bounds of operation stated in the manual. While I am fine with this for my own personal use, you should evaluate this risk on your own. I take further precautions, like running this on a switched outlet that is ALWAYS OFF with the pump unpulgged when I get in the tub, I also remove the pump from the water (leave the hose ends under the surface to keep from draining the panel). Also, ALWAYS USE GFCI! As the mythbusters tested, GFCI can save your life. Any precautions you take could be moot if the power line insulation gets nicked or some internal conductor becomes exposed to the water. If the GFCI is working, it will trip... if you find it tripped, CHECK YOUR EQUIPMENT and figure out why before using it again! You are basically dropping a 110V power cord into a big tub of water you will jump into, if that doesnt scare you.........

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