Free heat has to be one of the most comforting phrases in the English language, especially for large open spaces like garages. Luckily, you can construct an easy and inexpensive solar heater capable of warming your garage even in freezing temperatures.
Solar heaters come in a wide variety of sizes, shapes, and configurations, all of which can help cut down on home energy costs. Their prices vary widely, and they're always worth looking into, but just by investing in some raw materials, you can even assemble one yourself.
If you're clever about sourcing, you might be able to salvage all of the materials needed for this project. Once you finish, you might find this activity as addicting as potato chips when your power bills start dropping. Using only scrap lumber, caulking, a handful of lag screws, black aluminum screen material, and some clear plastic sheeting, you can harness the power of the sun at will.
Building The Framework
The framing for your space heater should be built against the south-facing wall of the structure it'll serve, so it catches more sunlight in Winter than it does in Summer. Southeast or southwest will work if your wall doesn't face due south. On average, your panel should produce warm air at between 84 and 104 degrees F. and will need to be 1/4 the size of the garage. In colder regions, you can add 1/3 more square footage to the total to account for the difference in temperatures.
Paint The Framework
If you choose to paint the framework, it's a good idea to leave the surfaces facing the wall unpainted to allow moisture in the heart of the wood to evaporate over time. Otherwise, it'll reduce your solar heater's service life by promoting decay from the inside out. By the time you can see the problem, it's too late.
Connect the boards to which you'll attach the aluminum screen, running a 2x2 horizontally across the frame. Notch out the middle board to fit a 2x2 to support the finished unit's glazing.
Attach The Frame To The Wall
One way to accomplish the weighty task of attaching the unit to the wall is to find the correct height and mark it on the wall studs at either end, then snap a chalk line across the wall, or use a straight-edge to draw a line and check it for level then nail a ledger board along the bottom of the line.
The heater's frame will rest on the edge while you and your helper drill out the holes from the inside—then you can attach it with 6-8, 4-inch lag screws. You can also use a floor jack to raise and level the frame, but a ledger board gives the heavy wooden unit extra support and structural integrity.
Seal The Frame
Using canned spray foam insulation, seal the 2x4 to which the screen attaches, and seal off all gaps between the frame and the garage wall.
Lay out a 4x16 hole between each stud on the bottom and top of the unit, and cut them out with a reciprocating saw. Beware of cutting into any electrical wiring, plumbing, live steam, or natural gas lines. It's probably no coincidence that assumes rhymes with kabooms, but it's a fact that a little bit of knowledge is just enough to level half of a city block. Never assume anything that you should know.
Trim the inside edges of the vent holes with foil tape and then cut out some of the aluminum screening to fit over the vent holes. Cut out a plastic flapper valve from the plastic sheeting for each one of the upper vents and staple them to the top line of foil tape to hold them in place while you affix the screen to the trim with another layer of foil tape.
Only the top edge of the flapper shall be taped down, and it should be large enough to completely cover the vent at night when the heater isn't in use, as it'll work in reverse and pull the warm air out of the garage and cold air in. You can staple or tack cardboard cutouts over the flapper valves during the hot Summer months to close them off.
Building The Heat Exchanger
Cut the screening to fit and staple it securely to the wooden inserts inside of the framework. Use a width that will allow both sides to bear on a wooden edge and trim off the excess with a razor knife wherever necessary. Attach the glazing that you've chosen securely to the frame, and call it a day.
Clear plastic sheeting works just as well as glass for this application, but it's far cheaper to replace plastic sheeting in the long- run. You can get really fancy and install ducts and a thermostat-regulated fan, or you can go with the easier convection unit and avoid having any moving parts or electric bills.