Part III: Solar Space HeatingChapter 4. Solar Air Heaters
Build a Simple Window-Sized Solar Air Heater
A shallow south-facing box that moves room air across a black absorber, with flaps so the window does not become a night-time hole.
- Difficulty
- Beginner
- Cost
- $50–110
A window-sized solar air heater is a shallow box hung on a sunny window. Sunlight passes through a clear cover, a black metal sheet turns that light into heat, and room air moving across the sheet carries the heat back inside. At night, two light flaps fall shut so the same window does not pour cold air into the room.
This guide builds one reference unit with about 6 square feet of opening (24 by 36 inches, or 610 by 914 mm). That is enough to take the chill off a small bedroom or office on a clear winter day. It is not a furnace, and it will not carry a whole house. If you want a shop or a garage, use the fan-powered wall collector.
The box in this plan sits on the outdoor face of a plywood panel that fills the window opening. The window itself can stay open behind that panel, or the lower sash can be raised, because the panel is the weather seal. Sun hits one layer of glazing, not the existing window plus a second cover.
Difficulty
- Level
- Beginner
- Time
- A weekend to build, plus one sunny day to test
- Skills
- Measuring, sawing, drilling, painting, and weatherstripping a window
Cost estimate
$50–110
The swing is almost entirely the glazing. A cut piece of twinwall polycarbonate, or a tempered storm pane you already own, lands near the low end. Buying a full 4 by 8 foot sheet and using only part of it lands near the high end, with leftover cover for a second project.
| Piece | Typical cost |
|---|---|
| Two 8 ft 1×4 boards | $8–14 |
| Plywood panel for the window | $12–20 |
| 1 in foil-faced polyiso, a small piece | $10–18 |
| Glazing, 26 by 38 in | $0–45 |
| Aluminum sheet, about 24 by 32 in | $10–16 |
| Paint, tapes, weatherstrip, screws, flap film | $15–25 |
Materials
| Item | Qty | Notes |
|---|---|---|
| 1×4 pine or fir | Two 8 ft boards | Actual size is about 0.75 by 3.5 in. This is the box depth. |
| Plywood, 1/4 in | One panel the size of the window opening | This is the room-side closure, not the absorber. |
| Foil-faced polyisocyanurate, 1 in | About 8 sq ft | Back and side strips. Do not use beadboard that melts at a low temperature. |
| Aluminum sheet or coil flashing | 23 by 32 in | The absorber. Black aluminum insect screen, two or three layers, is an alternate if sheet is hard to find. |
| Flat black high-heat paint | One 12 oz spray can | Grill or stovepipe paint rated to at least 400 degrees F. Not glossy latex. |
| Twinwall polycarbonate, 6 mm, or tempered glass | 26 by 38 in | Ordinary annealed glass is the wrong cover. It breaks into large sharp pieces. |
| 1×2 stop, or aluminum angle | About 12 ft | Holds the glazing on the front of the frame. |
| Closed-cell foam tape, 1/2 in | One roll | Gasket under the glazing and under the stop. |
| Neutral-cure silicone | One tube | Joints in the wood box. Keep it off the polycarbonate edge if the maker says it is incompatible; foam tape can do that job alone. |
| Foil HVAC tape | One roll | Seams in the insulation. |
| Exterior screws | A handful of 1-1/4 in and 2 in | The box is screwed, not glued, so the panel can come out. |
| Weatherstrip | Enough for the panel perimeter | Silicone bulb or high-quality foam. |
| Mylar sheet or 2–4 mil polyethylene | Two pieces larger than the vents | The backdraft flaps. Heavier material will not open on its own. |
| Optional 120 mm 12 V fan, snap disc, and 12 V supply | One set | Only if thermosiphon flow is too weak. Do not connect this project to household mains. |
Tools
A tape measure, a square, a saw, a drill, tin snips, a caulk gun, and a place to spray paint with real ventilation. An inexpensive infrared thermometer makes the first-sun test clearer, but a hand hovering near the outlet (not on the metal) will do.
How the pieces make heat
The absorber
The absorber is the black surface that turns sunlight into heat. Flat black paint absorbs most of the visible light that reaches it. It also radiates heat well. That is one reason a single cover is enough on a first build: the metal only has to stay warm enough to hand its heat to a stream of air.
Leave the absorber shorter than the box. On the reference unit the interior is 24 inches wide and 36 inches tall, and the metal is about 23 by 32 inches. That leaves roughly 2 inches of open channel above the sheet and 2 inches below it, so air can turn into the ports. Mount the sheet on short standoffs, about 1/4 inch, so the hot metal does not press on the foam.
If you use black aluminum screen instead of a sheet, stack two or three layers with a small gap between them and let the air pass through the mesh. That is a different absorber, not a worse one. The comparison belongs in the screen, fin, and can guide, which is still a draft. Do not invent a winner from one afternoon.
The glazing
The cover has two jobs: let light in, and keep the wind off the absorber. A single layer is the right trade for this small heater.
Twinwall polycarbonate, about 6 mm, is light, safer than glass when it fails, and a bit more insulating than a single pane. Its transmittance is lower than clear glass. Tempered glass lets through more light and lasts a long time, and it is heavier. Acrylic yellows and softens; skip it here.
Cut the cover about an inch larger than the interior opening on each side so it bears on the frame: 26 by 38 inches for the reference box. Set it on foam tape. Leave a little room for the plastic to expand. Screw the stop down so you can remove it; do not bury the edge in a hard bead of sealant on all four sides.
Drill one 1/8 inch weep hole at the bottom outdoor corner of the frame, into the outdoors, not into the room. Condensation then has a way out. A speck of screen or tape over the hole keeps wasps from moving in.
The insulation
Heat should leave through the air you send into the room, not through the back of the box. One inch of foil-faced polyiso on the back is about R-6, which is enough for this thin collector. The foil faces the absorber. The foam never sees the sun, and it never touches the hot metal.
The side walls are only 3/4 inch of wood, which is a weak insulator. Line the inside of the stiles and rails with strips of the same foam before you fit the absorber, or the edges become a cold frame around a warm middle.
The airflow
Room air enters a low slot, rises in the channel between the glazing and the absorber, and leaves a high slot. For a flat sheet, keep that channel between 3/4 inch and 1-1/4 inch. Tighter than that and the flow chokes. Much deeper and a lazy layer of air sits on the metal while the rest of the channel does little.
Each slot on the reference unit is 1.5 by 10 inches, about 15 square inches. That is a practical middle for 6 square feet of collector. A tiny slot makes the box run hot and deliver less heat. A huge slot is hard to seal with a light flap.
Because the path is short and vertical, buoyancy can drive the flow without a fan. Hot air rises, the outlet is at the top, and there is no downward duct to fight. A tissue taped at the outlet should stream into the room within a few minutes of full sun. If the tissue pulls back into the box, the collector is still cooler than the room, or a flap is on backwards.
A 120 mm fan moving on the order of 30 to 60 cubic feet per minute, switched by a snap disc on the absorber that closes near 110 degrees F and opens near 90 degrees F, will push more heat on a marginal day. It is optional. Keep it at 12 volts. Mains wiring is a different project, and usually an electrician's.
The backdraft damper
After sunset the absorber becomes colder than the room. Without a damper, the collector runs in reverse and the window is a hole. A light flap fixes most of that.
Hang a mylar or thin polyethylene flap on the room side of the upper slot, hinged along the top edge, larger than the slot by about half an inch all around. Outflow lifts it. When the flow stops, it falls shut. Hang a second flap on the collector side of the lower slot, also hinged at the top, so room air can push it open on the way in and reverse flow pulls it closed.
The flap has to be lighter than cardboard. A heavy flap will not open on thermosiphon alone. Do not add a latch. If both ports are sealed in full sun, the empty collector can climb past 200 degrees F, hot enough to scorch wood and soften foam. The flaps must be free to blow open.
Sizing, in one sitting
On a clear day, sunlight at the glazing is on the order of 1,000 watts per square meter. The reference opening is 0.56 square meters, so about 560 watts arrives. A single-cover air heater that is warmer than the room often delivers something like 30 to 50 percent of that as heat, once it is up to temperature. Call it roughly 170 to 280 watts, or about 600 to 950 BTU per hour, in good sun.
Over a sunny five-hour winter day that is on the order of 1 to 1.4 kilowatt-hours (about 3,000 to 5,000 BTU). You will feel it in a small room. You will not feel it at the far end of a leaky house. These are rules of thumb, not a promise. Clouds, shade, a dirty cover, and a stuck flap all cut the number. If you want a measured answer, the testing chapter is the place, once it is written.
Vertical glazing is the right tilt for a window, and it favors the low winter sun. It also sheds some of the high summer sun, which is what you want. Face the cover within about 30 degrees of true south in the northern hemisphere, or true north in the southern hemisphere. A compass is not true south. The guide on finding true south is the one to read before you pick the window.
Build the reference heater
Pick the window
Choose a window within about 30 degrees of the true solar direction, with a clear view of the winter sun for at least four hours. Read the shade and true-south guides if you are guessing.
Do not block the only exit from a bedroom. The panel has to come out without tools in a few seconds, or use a window that is not a required egress. If the sash paint might date from before 1978, assume it may contain lead. Do not sand it.
Cut the frame
The interior opening is 24 inches wide by 36 inches tall.
Cut two stiles at 43 inches (36 inches plus two rail widths of 3.5 inches). Cut two rails at 24 inches. The rails fit between the stiles, so the outside of the box is about 25.5 inches wide and 43 inches tall. Check the corner with a square, then screw the joints. Two screws per corner are enough. Tight joints matter more than pretty ones, because leaks at the corners steal the airflow.
Insulate the back and the edges
Cut 1 inch polyiso to the interior opening and foil-tape the seams. Fit foam strips on the inside of the four boards so the wood edges are not bare. Screw the plywood panel onto the back of the frame only after you know where the window wants the panel. If the window opening is larger than the box, the plywood is the size of the opening and the box occupies the sunny part of it. The foam goes on the outdoor side of that plywood, inside the frame.
Paint and hang the absorber
Clean the aluminum, then spray it flat black outdoors, away from flames, and let it cure as the can says. Mount it on 1/4 inch standoffs so it sits in front of the foam and stops about 2 inches short of the top and bottom of the interior. The air has to get past the ends of the sheet and into the channel. If the sheet rattles, add a screw. If it bows into the glazing, add a standoff.
Glaze the front
Run foam tape around the front face of the frame. Set the cover on the tape. Run another strip of tape on the cover's edge, then screw the 1×2 stop over it. The stop should pinch the cover without cracking it. Add the weep hole at the bottom outdoor corner. Sight across the cover: it should be even, not twisted. A twisted frame will crack glass and open a leak at one corner of a plastic cover.
Cut the ports and hang the flaps
Cut a 1.5 by 10 inch slot through the plywood and the foam near the bottom, lined up with the open channel under the absorber. Cut a matching slot near the top. Ease the edges so the flaps are not sliced.
Hinge the lower flap on the collector side of the bottom slot, and the upper flap on the room side of the top slot. A light wooden batten and two screws are enough of a hinge. Blow on each flap. The bottom one should open toward the collector. The top one should open toward the room. Both should fall shut when you stop.
Set the panel and support the box
Weatherstrip the perimeter of the plywood. Fit it in the sash opening and screw it to the solid window frame, not just to a stop that can split. The outdoor box projects only about 4 inches, but it still needs those screws. A friction fit is how a box falls out of a second-story window.
Confirm you can pull the panel free quickly. Then close the rest of the sash against the weatherstrip so room air is not leaking around the panel while you are trying to judge the collector.
Test it on the first clear day
On a sunny late morning, tape a tissue at the upper slot. Within a few minutes it should stream into the room, and the upper flap should be lifted. The air at that slot should feel warm. The absorber, seen through the cover, will be too hot to touch. Do not touch it.
After dark, put a hand near both slots. You want stillness, not a cold fall of air. If you feel a draft, the flaps are too small, too heavy, or hinged on the wrong edge. Fix that before you count any savings. A collector that runs backwards at night can lose the heat it gained during the day.
What this will not do
One window box will not replace a boiler. It will not help a window that is shaded at noon in December. It will not behave if you close both flaps and leave it in the sun, or if you build the absorber out of foam, cardboard, or painted wood. Take the panel out, or cover the glazing with an opaque board, in the months when the room is already too warm.
A larger, fan-powered wall collector is a different build. So is a collector that ducts through the wall above a window. Those guides are still outlines.
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.