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.