Part III: Solar Space HeatingChapter 4. Solar Air Heaters
A Fan-Powered Wall Collector for a Garage or Shop
A larger wall-mounted collector with a thermostat and a fan, sized for a workshop rather than one window.
- Difficulty
- Intermediate
- Cost
- $180–420
A wall collector is the window heater's larger cousin. It hangs on the outside of a south wall, pulls room air through a low opening, runs that air across a black absorber, and blows it back through a high opening. A fan makes the loop reliable when the collector is large and the ducts are short and horizontal. A backdraft damper keeps the same holes from cooling the shop at night.
The reference size here is 4 by 8 feet, about 32 square feet of glazing, built to heat a garage or a shop on sunny winter days. In full sun that area intercepts on the order of 3 kilowatts. A careful air collector might deliver a third of that as useful heat, sometimes more, sometimes less if the glazing is dirty or the flow is wrong. Call it roughly 800 to 1500 watts in good sun. That takes the edge off a shop. It does not replace a furnace in a house, and it does nothing useful under heavy overcast.
Build the window-sized heater first if you have never handled glazing, paint, and a damper. The physics are the same. The wall unit adds weight, a fan, a control, and two holes in the wall.
Difficulty
- Level
- Intermediate
- Time
- Two or three weekends
- Skills
- Framing a square box, flashing a wall, and either plugging in a listed fan or hiring an electrician
Cost estimate
$180–420
Glazing is the swing. A 4 by 8 foot twinwall sheet is often the largest single cost. A used blower and scrap aluminum land near the low end. New polycarbonate, a listed snap-disc control, and metal duct land near the high end.
| Piece | Typical cost |
|---|---|
| 2×4 frame and plywood back | $40–70 |
| 1 in foil-faced polyiso | $30–50 |
| Twinwall polycarbonate, 4 by 8 ft | $70–160 |
| Aluminum absorber and high-heat paint | $25–45 |
| Duct, damper, screws, flashing, sealant | $30–55 |
| Fan and a listed temperature control | $25–60 |
Materials
| Item | Qty | Notes |
|---|---|---|
| 2×4 framing and exterior plywood | A 4 by 8 ft back | The box is about 4 inches deep inside. Actual 2×4 depth is 3.5 inches, plus the absorber standoff. |
| Foil-faced polyiso, 1 in | One 4 by 8 sheet | Line the back. Do not use expanded beadboard. It softens when the collector stagnates. |
| Aluminum sheet or corrugated aluminum | About 4 by 8 ft | The absorber. Two layers of black aluminum screen are an alternate if sheet is hard to find. |
| Flat black high-heat paint | One quart or two spray cans | Rated for at least 400 degrees F. Gloss latex reflects more and may blister. |
| Twinwall polycarbonate, 6 or 8 mm | 4 by 8 ft | UV side out, as the sheet is labeled. Tempered glass is heavier and wants a stronger frame. |
| Inline fan or blower, about 80 to 150 cfm | 1 | Roughly 3 cfm per square foot of collector is a workable target. A 32 square foot box wants on the order of 100 cfm. |
| Backdraft damper | 1 | In the upper duct, so night air cannot fall back into the shop. |
| Listed temperature control | 1 | A snap disc on the absorber or a line-voltage thermostat rated for the job. See the electrical note before you wire anything. |
| Metal duct or two short collars, 6 in | Two penetrations | Low inlet, high outlet. Seal them to the wall air barrier. |
How the loop is arranged
Mount the box on the outside of a south wall that the site survey says is actually sunny in winter. The absorber stands about an inch off the insulated back, and the air passage is in front of the absorber, between the black metal and the glazing. That is the same order as the window heater: insulation, absorber, air, glazing.
Bring shop air in through a low hole and return it through a high hole. Warm air rises on its own, so a fan blowing from the low hole to the high one moves air the way it already tends to go. Keep both runs short. A long duct in a cold garage wall throws away a lot of what the absorber just collected.
A common flow target for a screen or sheet absorber is a few cubic feet per minute for each square foot of glazing. Too little flow and the absorber runs very hot, loses more heat out the glazing, and can soften cheap plastics. Too much flow and the air comes back only a few degrees warmer, which feels disappointing even when the energy is real. Around 3 cfm per square foot is a reasonable place to start for this box. Measure the outlet temperature on a sunny day before you decide the fan is wrong.
The fan should run only when the collector is warmer than the shop. A snap disc clamped to the absorber, set to close near 90 to 110 degrees F, is a simple way to do that if the disc and the fan are listed for the way you connect them. A differential controller is more precise and more money. Either way, the fan must stop at night.
Controls and electricity
The collector is a hot box. Do not put a plug-in power supply, a plastic thermostat, or a coil of extension cord inside it.
Two approaches stay understandable:
- A listed 12 volt fan outside the hot airstream, powered by a wall adapter that stays indoors, with the on-off decision made by a switch or snap disc on the low-voltage side. This avoids opening a mains circuit.
- A listed line-voltage blower and a listed line-voltage control, wired in a box with the conductor type and the overcurrent protection the devices call for. In many places that is permit work, and it is reasonable to hire an electrician for the one circuit even if you build the wooden box yourself.
Do not invent a speed control by rewiring a motor. Do not share the shop's lighting circuit if you do not know what else is on it. A damp garage wants ground-fault protection on receptacles that the electrical code already requires. Follow that rather than adding a gadget.
Mounting, rain, and stagnation
Lag the box into studs or into solid backing, not into sheathing alone. A 4 by 8 foot collector with glazing and a plywood back is heavy enough to hurt someone if the top fasteners pull out. Paint or flash the top so water sheds over the glazing and does not sit on the upper rail. Seal the wall penetrations to the house wrap or the sheathing so you have not added a rain leak while you were adding a heater.
When the fan is off on a sunny day, the empty collector stagnates. Interior surfaces can get hot enough to scorch tape, warp vinyl, and char thin wood if you used the wrong materials. Polyiso faced with foil, metal duct, and a glazing that can take heat are there for that reason. Leave a way to open or shade the collector if you will be away in summer and the shop does not need heat. The simplest summer plan is an opaque cover, or a control that you trust, plus a glazing that survives a hot afternoon anyway.
At night the damper must close. Test it by turning the fan off and feeling for a cold draft at the upper grille. A damper that rattles open in the wind is not done.
What you should measure
On a clear winter day, record the shop temperature, the outlet temperature, and whether the fan is cycling. An outlet that is 20 to 40 degrees Fahrenheit above the shop, with the fan moving a steady stream, means the box is doing the job. An outlet that is barely warm with the fan screaming means too much flow or a damper stuck open to the outdoors. An outlet that is very hot with almost no flow means the fan is dead, the damper is shut, or the passage is blocked.
The shop may still be cold at 7 a.m. This heater has no storage. It works while the sun is on it.
Confirm the wall
Use the site survey. You want winter sun, studs you can find, and a place where two 6 inch holes do not hit a wire, a pipe, or a header you cannot move.
Build the box square
Insulate the back, paint the absorber flat black, and set it so air can pass between absorber and glazing. Seal the glazing at the edges with a cap strip and a gasket, not with a bead of caulk you will have to cut out later.
Cut the two openings
Low return from the shop, high supply to the shop. Fit collars, seal the air barrier, and install the backdraft damper in the supply.
Add the fan and the control
Keep electronics out of the collector. Use a listed fan and a listed control, plug-in and low voltage if you are not doing mains wiring. Hire an electrician for a hard-wired circuit.
Flash the top and lag into structure
The box has to stay on the wall in a wind and has to shed rain. Fasteners go into studs.
Test on a sunny day and again after dark
Outlet warmer than the room while the sun is out. No cold stream after the fan stops. Fix the damper before you declare the project finished.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
- https://www.builditsolar.com/Projects/SpaceHeating/SolAirCol/SolAirCol.htm
- https://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm
- https://www.builditsolar.com/Experimental/AirColTesting/Index.htm
- https://www.builditsolar.com/Experimental/AirColTesting/FlowTesting.htm
- https://www.builditsolar.com/Experimental/ScreenAbsorber/ScreenAbsorber.htm
- https://www.builditsolar.com/Experimental/WindowCollector/OverWindowCollector.htm
- https://www.builditsolar.com/Projects/SpaceHeating/KenSoffitCollector/Main.htm
- https://www.builditsolar.com/Projects/SpaceHeating/360SFAir/360SFAir.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.