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Part III: Solar Space HeatingChapter 4. Solar Air Heaters

Under-Window and Soffit Collectors

Low and high mounting spots that keep ducts short when a full wall is not available.

Difficulty
Intermediate
Cost
$100–300
published

Sometimes the south wall is mostly window, or the only clear sun is up at the eave. An under-window collector sits below the glass and feeds the room through a short opening at the sill. A soffit collector sits at the roof overhang and ducts into the top of the wall. Both exist to keep the air path short. Both are easy to get wrong, because the vertical rise inside the collector is small and because you are cutting near a window, a header, or a roof edge.

Use these spots when a full wall collector will not fit and a window heater would block a window you still need. If the rise is only a few inches, plan on a small fan. Buoyancy, as the thermosiphon guide describes, needs height you may not have here.

Difficulty

Level
Intermediate
Time
One or two weekends
Skills
Flashing against a wall or a soffit, and not cutting a structural header by accident

Cost estimate

$100–300

Smaller than a full wall box, but flashing, a damper, and a short metal duct are still real costs. A fan and a listed control push the high end.

Materials

ItemQtyNotes
A shallow insulated boxSized to the opening you haveDepth still has to hold insulation, an absorber, and an air gap.
Glazing that sheds rainThe faceTwinwall or tempered glass. Flash the top into the wall or the soffit.
Short ducts or sill openingsInlet and outletLow in, high out, even if both are close together.
A damper1Night reversal is worse when the collector is small and cools fast.
A small fanIf the rise is shortListed, and not buried in the hot box. See the duct and thermostat guide.

Under the window

The collector lives on the wall beneath a south window. The outlet should enter at the bottom of the window opening or through the wall just under the sill, and the inlet should be lower than that, through the wall near the bottom of the box. The window itself stays a window. Do not build a collector that you have to climb over to escape the room, and do not fasten the box in a way that blocks the sash.

Rain runs down the wall onto the top of the box. That joint needs flashing tucked up under the sill or the siding, not a bead of caulk as the only plan. The box must not become a shelf that holds water against the wall.

Height is the limit. If the absorber column is short, thermosiphon will be weak. A small fan on the inlet, running only when the box is hot, is the better design. Use the airflow sketch in the duct guide so the fan is a breeze, not a leaf blower.

At the soffit

A soffit collector is up out of the snow and out of reach of a mower, which is the attraction, and it is also up on a ladder at the edge of a roof, which is the cost. The box or the ducted absorber sits in the overhang, glazed toward the south, and a short duct drops through the top of the wall. Flash it as part of the roof edge. Do not block the soffit vents that the attic needs. A vented roof that can no longer breathe will ice-dam or rot, and the heat you gained is a poor trade.

The air path from a soffit into a room often needs a fan, because the collector itself may be nearly horizontal. Horizontal is a bad thermosiphon. Put the fan and the damper where you can reach them without removing the soffit every October.

Under a window and in a soffitLeft, a shallow collector under a window with a short rise into the room. Right, a collector at the soffit ducted through the top of the wall. Both keep the duct run short.Under the windowCollectorSoffitCollectorShort duct into the wall
Under the window, the collector stays below the glass. At the soffit, the collector stays at the eave and the duct is short. Neither one should become a rain shelf or a blocked vent.

What both still share with any air heater

Insulation behind the absorber, a dark metal surface, one glazing layer, a damper, and a way to survive stagnation. Measure the outlet on a sunny day. If you cannot feel a warm stream, the spot was convenient and the collector is not finished.

  1. Confirm sun and structure

    South, winter sun from the shade map, and a place to fasten that is not a header you do not understand.

  2. Choose fan or buoyancy on purpose

    Short rise means a fan. Write that down before you build a silent box that stagnates.

  3. Flash the top and keep existing vents

    Rain still has to run off the wall, and the attic still has to vent. The collector must not stop either one.

  4. Damper and test

    Warm air by day, nothing cold at night, and the window or the soffit vent still works.

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.