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
*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
(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
Step 4: Insulate the Back
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
Step 6: Making the Coils
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
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
*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
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.........