Skip to content
DIY Solar Book, home

Part III: Solar Space HeatingChapter 4. Solar Air Heaters

Thermosiphon (No-Fan) Air Heaters

When buoyancy alone can move enough air, and the duct mistakes that stop it.

Difficulty
Intermediate
Cost
$80–200
published

Warm air is lighter than the room air it came from. If you give it a low way in and a high way out, it will rise through a collector without a fan. That is thermosiphon. The driving pressure is small, a tiny fraction of the pressure a fan can make, so the collector has to be tall, the ducts short and generous, and the absorber easy to flow across. A damper has to stop the loop from reversing after dark, when the collector is the cold part of the loop and room air falls through it.

The window heater is already close to this idea: low slot, high slot, flaps. A fan, as on the wall collector, is what you add when the box is large, the ducts turn corners, or the height is too short to trust buoyancy. Read how collectors work before you omit the fan on purpose.

Difficulty

Level
Intermediate
Time
A weekend, then a sunny day and a night to see if it reverses
Skills
Building a tall airtight box and a damper that actually closes

Cost estimate

$80–200

No fan and no control saves money. Glazing and a taller frame spend it. A damper is not optional and is not free.

Materials

ItemQtyNotes
A tall south-facing boxTaller than it is deepHeight drives the flow. A long low box is a weak thermosiphon.
A low-resistance absorberSheet or open screenA tight mesh can stall a no-fan loop. See the absorber comparison.
Short, large passagesInlet low, outlet highEvery elbow spends pressure you do not have.
A light backdraft damperOn the outletIt must open under a whisper of rising air and fall shut when flow reverses.

What buoyancy can and cannot do

The force comes from a column of warm air next to a column of cooler room air. More height and a larger temperature difference make a stronger push. The push is still small. A long duct, a crushed bend, a screen that the air can barely cross, or a damper that sticks will stall the loop. You then have a stagnating box, hot glazing, and almost no heat in the room.

Design for that limit. Put the collector on a south wall. Take air from low in the room and return it high, with the high opening well above the low one. Keep both runs only as long as the wall thickness if you can. Use a sheet or a fin absorber, or a screen you have already felt air pass through easily. Do not add a long horizontal duct and hope the sun will make up for it.

Flow will be less orderly than a fan. The outlet may be hotter and the total heat smaller. That is an acceptable trade when the site is a cabin, a shop with no convenient circuit, or a window where a fan would be louder than the heat is worth. It is a bad trade for a 4 by 8 foot collector with two elbows.

A no-fan air heaterA tall collector with a low inlet and a high outlet. Warm air rises on its own. A damper at the outlet closes when the collector is colder than the room.Cool inWarm outNo fan. The height is the motor. The damper is the night valve.
Cool air enters low. Warm air leaves high. Nothing else pushes. The damper at the outlet keeps the loop from reversing at night.

Night reversal

After sunset the absorber cools below room temperature. Unless something stops it, the loop runs backward: room air falls through the collector and returns cold. A light flap at the high outlet, hinged so it opens upward or inward with the daytime flow and drops shut against a stop, is the usual cure. The damper guide has the details. Test it. Feel the outlet after dark. A flap that rattles in the wind is not closed.

Do not rely on a check valve meant for water. Do not seal the openings with tape as the permanent plan. You will forget, or you will leave them sealed on the next sunny day.

How to know it is working

On a clear day the high opening should breathe a warm stream without a fan. If it does not, the path is too tight, too flat, or leaking around the absorber. At night the stream should stop. If the room is colder in the morning next to that wall than it used to be, the damper failed and the collector is a hole.

  1. Give the loop real height

    Low inlet, high outlet, collector tall, ducts short. If you cannot draw that, use a fan.

  2. Keep the air path easy

    Sheet or open screen, generous openings, no crushed bends.

  3. Fit a damper that falls shut

    Light, free, and tested by hand before you rely on it.

  4. Watch one sunny day and the night after

    Warm stream by day. No cold stream after dark. Fix the damper before you declare success.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

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.