Ventilation & Infiltration

The Effect of Pressure

Outside air is constantly supplied to a building through the HVAC system. There is rarely a means to relieve this excess air volume, other than via exfiltration through the envelope. What results is an imbalance: Up to 25 percent more air is supplied to all zones in a building than is returned to the air-handling unit. This imbalance causes a positive building pressure. This effect is evidenced when exterior doors opening inward are a little hard to open. (If outward- opening doors remain slightly ajar, […]

Ventilation & Infiltration

The Stack Effect

Building walls facing the prevailing wind have the highest ventilation losses per area of opening. The opposite side of the building is only slightly better. The flow of air past the structure creates a positive pressure on the leading side and a negative pressure on the trailing side, to cause infiltration and exfiltration, respectively. This is called the stack effect. (For very tall buildings there is a horizontal neutral plane, on either side of which the exfiltration and infiltration of air reverses.) This principle can […]

Ventilation & Infiltration

Ventilation and Infiltration Introduction

The ideal building has a thermal envelope that completely isolates the inside conditioned space from the outside environment. This is the target configuration for many designers and builders, as it eliminates two space-conditioning loads: (1) infiltration of unconditioned air that the HVAC system must spend extra energy to treat, and (2) exfiltration of conditioned air through the building envelope; air that must be replaced with outside air—which, in turn, must be cooled down to set- point, at significant cost—to maintain the air balance. This ideal […]