October 28, 2003
A large thermosyphon solar air heater provides a convenient location for the exchange of internal and external air. For the principles of the air heater design assumed here, see Solar air heater with flow organizer baffle array.... The scheme proposed here depends critically on the house being fairly air tight, which may make it impractical in some circumstances.
A rate of air exchange with the exterior that would represent only a small fraction of the rate of air flow through the air heater would provide a great deal of fresh air and humidity reduction. The air exchange rate would depend on the heating rate--the greater the throughput of the air heater, the greater the exchange rate. When the air heater became cold, the introduction of fresh air to the building via the air heater would stop, or slow down greatly.
Although the openings are shown in the glazing, they could well be in the south extremity of the east and west walls of the air heater.
Although no heat is recovered from the exhausted air, locating air exchange in the air heater is more efficient than allowing leaks between the inhabited part of the building and the exterior, which should be designed tight. Provided the house is sufficiently tight, the house system loses energy by the upper opening in the glazing only when the air heater is warm enough to transfer energy to the heat store. Any energy lost in the exhaust air is therefore more than made up simultaneously by newly collected solar energy. The slight reduction of the average temperature of the air heater partly makes up for the heat lost to exhausted air by improving heat collection efficiency of the air heater. The impact of this ventilation arrangement on the operation of the house system can be viewed as a copious flow of fresh air paid for by a small reduction in the overall collection efficiency of the air heater.
Although this ventilation arrangement cannot provide fresh air at night, it provides copious fresh air while the air heater is operating. Alone, it might provide sufficient ventilation for a house in normal conditions, or might provide a valuable backup for failed mechanical ventilation during extended power outages.
The ventilation openings will provide some reduction of overheating in the event that the air heater overloads the heat store while the building is unoccupied.
The slots should be further baffled by dividing sections running north-south
to block east-west flow. Such divisions are necessary in any case to support
the glazing frame. The baffles might be constructed of sheet metal, preferably
integrated into the flashing. The baffles must be screened against insects.