Skip to content
DIY Solar Book, home

Part III: Solar Space HeatingChapter 4. Solar Air Heaters

Sizing Ducts, Fans, and Thermostats

How to pick airflow, duct area, and a snap-disc so the collector delivers heat instead of stagnating.

Difficulty
Intermediate
Cost
Free
published

A collector without enough airflow overheats and throws heat back out the glazing. A collector with a huge fan and a tiny duct delivers a lukewarm breeze and a disappointed builder. This page connects three numbers: how much air you want, how big the duct has to be to carry it without a fight, and when the fan should run. It assumes you already know the heat path in how collectors work.

The wall collector used about 3 cubic feet per minute per square foot as a starting target. That is the number this sketch starts with. A thermosiphon skips the fan and therefore skips this page's fan section, not the lesson about short, generous ducts.

Difficulty

Level
Intermediate
Time
An hour of arithmetic before you buy a fan
Skills
Reading a fan label without believing the free-air number

Cost estimate

Free

The sketch is free. A listed fan and a listed snap disc are part of the collector budget, often a few tens of dollars if you are not hiring an electrician for a new circuit.

Materials

ItemQtyNotes
Collector areaSquare feet of glazingNot the area of the wall around it.
A fan whose curve you can see1Free-air cfm is the flow with no duct. You will have a duct.
A listed temperature control1Snap disc or a line-voltage thermostat rated for the load. Not a plastic household thermostat inside the collector.

Airflow

Start near 2 to 4 cfm per square foot of collector. Lower in that band runs hotter and can make a weak fan look adequate. Higher moves more heat until the temperature rise gets so small the room does not feel it, and until the fan noise is the main product. Three is a middle to type into the sketch.

The number on the box is often free-air flow, measured with nothing attached. A duct, a damper, a screen absorber, and a dirty filter all add resistance. Flow falls as resistance rises. If the maker shows a curve of flow versus pressure, pick a fan that still delivers your target partway along that curve, not only at the free-air point. If there is no curve, buy a fan with free-air flow well above the target and keep the duct very short, then measure the outlet temperature and decide.

Airflow sketch

Collector area times a flow target gives cubic feet per minute. A starting target near 3 cfm per square foot is a planning number, not a lab result. The duct diameter assumes a round duct at the velocity you type. A fan's free-air rating is higher than the flow you will get once the duct is attached.

96 cfm

A round duct near 5.4 inches across hits that velocity. Pick the next stock size and keep the run short.

Ducts

The sketch also asks for a duct velocity. Something around 500 to 800 feet per minute is a quiet planning band for these short runs. Faster gets noisy. Slower means a duct so large it will not fit the wall. The diameter it prints is the round size that hits that velocity. Choose the next stock size you can actually buy, in metal. Keep the run to a few feet. Two elbows can matter more than a slightly small pipe.

Do not use a plastic dryer hose inside or downstream of a hot collector. It sags, it leaks, and it is not a heat-rated duct. Seal the metal joints. Insulate a duct that passes through a cold space, or you will spend the heat on the way to the room.

The damper goes in the supply, where it can fall shut. The fan can sit on the inlet so it is pushing room air in, which keeps the fan a little cooler. Either way, the fan motor does not belong in the stagnant collector.

Duct, fan, and controlA collector feeds a short round duct, then a fan. A sensor on the absorber decides when the fan runs. The note says a free-air fan rating is not the installed flow.CollectorFanSnap discFree-air cfm is not the flow in this duct.Keep the run short. Every elbow spends pressure the fan may not have.
Collector, a short duct, a fan, and a control that only runs when the absorber is hot. The free-air rating is not the flow in that duct.

When the fan runs

The fan should run when the collector is hotter than the room and stop when it is not. A snap disc on the absorber, set to close somewhere around 90 to 110 degrees F, is a simple version if the disc is listed for the way you wire it. A differential control that compares collector and room is kinder and costs more. Either one must fail toward off. A fan that runs all night pulls room air through a cold box if the damper also fails.

Do not put the control inside the collector unless it is rated for that temperature. Household thermostats are not.

  1. Compute a target flow

    Area times about 3 cfm per square foot. Write the number down.

  2. Size a short metal duct

    Use the velocity sketch. Avoid crushed bends and plastic hose.

  3. Pick a fan for the duct, not for the box label

    Free-air flow is the optimistic end. Leave margin.

  4. Control it from collector temperature

    Listed snap disc or a differential control. Off at night. Test that off.

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.