Warming Ventilation Air with a Sun-Facing Wall

How does solar air heating work?

A transpired collector is a sheet of dark, finely perforated metal mounted a short distance off a south-facing wall. A fan draws outdoor air through the holes. The sheet warms in the sun and heats the air as it passes through, then the air collects in the gap and is ducted into the building's ventilation intake. There is no liquid, no glass and no freezing risk.

Because the air is only preheated, the building's regular heating makes up the difference, so on a clear winter day the intake air can arrive well above outdoor temperature.

Where does it make sense?

Solar air heating suits buildings that bring in a lot of fresh air and need heating through the cold season: workshops, warehouses, barns, schools, multi-unit buildings. A home needs little ventilation air compared with a factory, so the payoff is smaller, though air collectors can be built for houses and cabins. Ventilation recovery is the competing approach for homes, and the heat recovery ventilator guide explains it. An HRV recovers heat from exhaust air, while a solar air collector adds heat from the sun. The two can even be combined.

What are the limits?

The collector needs sun, so output follows the weather and the day. On a cloudy evening it gives nothing and the fan should shut off. It heats air, not water, so it does not help with a hot-water bill. And it works best on a south-facing wall with few obstructions. Check shading and snow drifts at ground level. The geometry principles in the direction guide carry over.

The vertical angle is useful in winter. A low winter sun strikes a vertical wall almost square-on, and snow on the ground reflects extra light up to it, which helps in January when a flat roof collector would catch less.

Maintenance is mostly a fan, a thermostat or temperature sensor and occasional checks of the dampers. The perforated sheet has no moving parts and typically lasts for decades, though its coating may fade. A bypass damper is useful in summer so the preheated air does not enter the building when cooling is needed. Ask whether the design includes a summer bypass and how it is controlled, since forgetting it can raise cooling bills in a sealed, warm building.

What does it cost and save?

Typical installed costs depend on wall area, building size, ductwork and whether new wall cladding is being installed anyway. Savings depend on the volume of ventilation air and how many heating hours the sun covers. Prices vary by region and year, and any saving figure should come from the installer with its assumptions spelled out. Ask for a performance estimate based on weather data for your location and compare it with gas costs, as covered in home heating costs in Alberta.

How does it compare with liquid-based solar heating?

Air systems are simpler and cheaper to maintain, and do not need storage. They also cannot store heat, so every gain has to be used the same hour. Liquid systems can store heat in tanks. At Drake Landing in Okotoks, flat-plate liquid collectors on garage roofs fed storage so that summer sun could heat homes in winter, and the solar space heating guide describes why storage mattered. For a look at the field itself, see the solar collection page.

Air collectors share one more limit with every solar heating option in this cluster: they follow the sun, not the thermostat. In a northern climate the deepest cold often comes with clear, still skies, which is good for the collector, and the worst heating days on overcast, windy weeks give little. A building with a steady daytime schedule, such as a school or workshop, matches this pattern better than a house occupied mostly at night.

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