Part III: Solar Space HeatingChapter 4. Solar Air Heaters

How Air Collectors Work: Absorbers, Airflow, and Glazing

The heat path from sunlight to room air, and the few dimensions that decide whether a collector works.

Difficulty: Beginner
Cost: Free

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An air collector is a shallow box that turns sunlight into a stream of warm air. Sun passes through a clear cover. A dark absorber turns that light into heat. Air moving across or through the absorber picks up the heat and carries it into the room. Insulation behind the absorber keeps the heat from soaking into the wall you mounted the box on.

That is the whole machine. The window heater and the wall collector are two sizes of it. Knowing why those dimensions exist keeps the next box you build from being a guess.

Difficulty

Level: Beginner
Time: An hour of reading before you buy glazing
Skills: Following a heat path from the sun to the room

Cost estimate

Free

Understanding the path costs nothing. Glazing is what the built projects spend money on.

Materials

Item	Qty	Notes
A sunny south face	From the site survey	A collector in the shade is a wall decoration.
The temperature of the outlet and the room	Two readings	Heat delivered depends on flow as well as the temperature rise.

Where the heat goes instead of into the room

Not all of the sunlight becomes useful heat. Some reflects off the glazing. Some heats the absorber and then leaves as infrared radiation back out through the cover. Some is lost by air moving on the outside of the glazing, and some leaks out the edges and the back if the insulation is thin or missing. A hotter absorber loses more. That is why a collector with almost no airflow can feel impressively hot at a crack and still deliver little energy: the absorber is running so hot that the glazing is throwing the heat away.

Useful heat is the airflow times how much warmer that air is than the room. For a rough sense of scale, about 1.1 times the cubic feet per minute times the temperature rise in degrees Fahrenheit is BTU per hour. A trickle of air at a 60 degree rise can lose to a steady stream at a 25 degree rise. Measure both, or you will praise the wrong box.

The four parts that matter

Glazing. One clear layer is the usual choice for an air heater. A second layer cuts losses and also cuts the light that gets in, and it is heavier. Twinwall polycarbonate is one layer with a built-in air gap, which is why the project guides specify it. Ordinary window glass in front of a second cover means the sun fights two panes. Tempered glass or polycarbonate, not annealed glass that fails in long shards.

Absorber. Flat black metal gets hot. More surface in contact with the air transfers that heat sooner, at a lower metal temperature, which is what you want. A sheet, a fin, and a screen are different ways to buy that surface. The next guide compares them. High-heat paint, not glossy latex.

Air path. Air should pass the absorber, not sneak around it through a leak at the edge. The path in the window heater is between the absorber and the glazing, after the back is insulated. Flow wants a low inlet and a high outlet so heat and the fan agree. A few cubic feet per minute per square foot of collector is a workable target. The duct guide turns that into a fan and a pipe size.

Back and edges. Insulation behind the absorber, sealed edges, and a box that sheds rain. When the fan is off in the sun, the empty box stagnates and gets hotter than the materials may allow. That is a materials choice, made before the first sunny afternoon.

Insulation, absorber, moving air, glazing, then the sun. The arrow is the only part that heats the room.

What a good afternoon looks like

Outlet air plainly warmer than the room, moving as a stream you can feel, and a collector you can still touch at the frame without a burn. At night, no cold stream back into the room. If the outlet is scorching and the flow is a whisper, open the path or add flow. If the outlet is room temperature and the fan is roaring, you are moving too much air or the damper is open to the outdoors.

The collector does nothing useful under heavy cloud and nothing at 6 a.m. It is a daytime heater. Storage is a different chapter.

Name the four parts on a sketch
Glazing, absorber, air path, insulation. If one is missing, it is not this machine.
Give the air a low inlet and a high outlet
Short ducts. Leaks around the absorber are lost area.
Plan the night
A damper or a cover. The box will be colder than the room after sunset.
Judge it by flow and temperature together
A single hot reading is not a success.

Difficulty

Cost estimate

Materials

Where the heat goes instead of into the room

The four parts that matter

What a good afternoon looks like

Name the four parts on a sketch

Give the air a low inlet and a high outlet

Plan the night

Judge it by flow and temperature together

Further reading