Ventilation for a solar heated building with a large thermosyphon air heater


David Delaney
October 28, 2003
ddelaney@sympatico.ca

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.

The drawing shows two openings in the glazing.  The high opening extracts stale air, the low opening introduces fresh air. The openings would be small enough that they would not greatly disturb the pattern of air flow in the air heater. The openings would be baffled to prevent gusts of wind from destabilizing the flow patterns in the air heater during the day and the horizontal cold-warm air interface between the air heater and the heat store at night.

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.

Details, October 30, 2003

The two ventilation openings might consist of  horizontal slots just above and just below the glazing, baffled as follows:

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.
 

Addition. November 3, 2003.

Several advantages could be gained by replacing the ventilation openings at the top and bottom of the glazing by well baffled large openings low in the east and west walls of the air heater. Warm stale air would exit the upper half of each opening.  Cold fresh air would enter the lower half of each opening, forming a pool on the bottom of the air heater. The hot absorber would pump fresh air into the building circulation by heating some of the air in the pool. See the following drawing.
Ventilation openings in the east and west walls would much simpler to implement than slots in the top and bottom of the glazing. The glazing framing would be greatly simplified by the absence of the ventilation slots. The wall openings would be much more flexible than the slots, being easier to tune for optimum performance by partly covering the opening.  The wall openings would be easier to screen against insects. Stale air would be exhausted only after having been given an opportunity to heat the glazing, slightly increasing the efficiency of the air heater. The openings could be well baffled against gusts of wind by covering them with a suficient number of layers of furnace filter. Additional baffling could be provided by external shrouds.