Part III: Solar Space HeatingChapter 4. Solar Air Heaters

Backdraft Dampers and Night-Loss Prevention

Light one-way flaps and covers that stop a collector from dumping the day's heat back outdoors after dark.

Difficulty: Beginner
Cost: $10–30

published

After sunset a collector becomes a radiator. The absorber is colder than the room, so air sinks through the box and comes back cold. That reverse flow can give back a large share of the day's heat, and it can make the room next to the collector the coldest place in the house by morning. A backdraft damper is a flap that opens when air moves the right way and shuts when air tries to move the wrong way. A cover you actually put on is the other solution.

The window heater uses light film flaps. The thermosiphon heater depends on a damper even more, because nothing else stops the loop. A fan that is simply switched off is not a damper. Air will still move.

Difficulty

Level: Beginner
Time: An hour to fit, a night to test
Skills: Making a flap that moves freely and seats

Cost estimate

$10–30

Film, a light plastic or metal flap, and a stop to close against. A bought backdraft damper in a round duct costs more and is appropriate on a fan system if it is light enough to open at your flow.

Materials

Item	Qty	Notes
A light flap	1 at the outlet, sometimes 1 at the inlet	Heavy enough to seal, light enough to open under the daytime flow.
A stop and a hinge	1 set	The flap should fall closed by gravity, not depend on a spring you cannot see.
Weatherstrip or felt at the seat	The closing edge	A rattling gap is a night leak.

How to place it

Put the main damper on the high outlet, the opening where warm air enters the room by day. Hinge it so daytime flow pushes it open and gravity or the reverse flow pushes it shut against a stop. If the inlet can also spill cold air, a second light flap there helps. On a fan system, a round backdraft damper in the supply duct is fine if your fan can open it. Feel it. If the fan starts and the flap barely moves, the damper is too stiff and you are heating the duct, not the room.

Do not put a damper where you cannot see or reach it. It will stick. Film flaps cloud and tear. Plan to look at them once a season.

A solid cover over the glazing at night also stops the loss, and it stops the next day's gain if you forget it. Covers are a good summer tool and a poor daily habit. The damper is the daily tool.

By day the flap is open in the direction of the warm stream. By night it has fallen shut.

The night test

Turn the fan off, or wait until the sun is gone. Stand at the outlet. You want no cold stream. A tissue should hang still. If it pulls toward the collector or blows cold into the room, the flap is open, warped, or hinged the wrong way. Fix it before you add insulation to the house and blame the walls.

Also test the daytime direction. A damper stuck shut is a stagnation event: hot collector, no heat delivered. Cycle it by hand. It should move without a scrape.

Wind can hold a light flap open. If your site is gusty, the flap needs a slightly better seat or a duct that does not face the wind. Do not solve wind by gluing the flap down.

Hinge the outlet flap so gravity closes it
Daytime flow opens it. You should be able to lift it with a finger.
Seat the edge
Felt or a soft stop. Look for a gap you can see through.
Test at night
No cold stream. Tissue still. Fan off.
Test the next sunny hour
The flap opens and the outlet is warm. If the flap stays shut on a sunny day, the damper has failed.

Difficulty

Cost estimate

Materials

How to place it

The night test

Hinge the outlet flap so gravity closes it

Seat the edge

Test at night

Test the next sunny hour

Further reading