The relative energy use of passive stack ventilators and extract fans.

The relative energy use of PSV and extract fans has been a matter of considerable controversy, particularly in the UK. A steady state methodology is presented based on the approach of BS5250 and that of Professor Meyringer (Air Infiltration Review November 85). The ventilation, over and above background ventilation, required to remove moisture is shown to be affected by; the rate of moisture production in the dwelling, the moisture content of the outside air, the air temperature of the dwelling, the air tightness of the dwelling, the moisture absorption of the structure and furniture, the dwelling size, whether trickle vents are open or closed, the proportion of moisture removed in the kitchen or bathroom. Equations are derived for the energy used by PSV, both uncontrolled and humidity controlled and by humidity controlled extract fans. Manually controlled systems, either fans or PSV, have not been considered because their use depends on human behaviour and there is, as yet, a lack of detailed reliable data. Conditions are determined for one or the other to be the greater. The effect of varying the above variables and opening or closing the kitchen door is investigated. It is shown that PSV both controlled and uncontrolled, has an energy advantage in heavily occupied, cold and small dwellings. In average dwellings there is little difference and extract fans are more energy efficient in large, warm and lightly occupied dwellings.