The room pressure may be maintained at either slightly positive or negative pressure, which is achieved by using slightly unequal supply or exhaust ventilation rates. A balanced mechanical ventilation system refers to the system where air supplies and exhausts have been tested and adjusted to meet design specifications. In a negative pressure system, the room is in negative pressure, and the room air is compensated by “sucking” air from outside. In a positive pressure system, the room is in positive pressure and the room air is leaked out through envelope leakages or other openings. For a room with locally generated pollutants, such as a bathroom, toilet or kitchen, the negative pressure system is often used. Conversely, in cold climates, exfiltration needs to be prevented to reduce interstitial condensation, and negative pressure ventilation is used. In these cases, a positive pressure mechanical ventilation system is often used. For example, in warm and humid climates, infiltration may need to be minimized or prevented to reduce interstitial condensation (which occurs when warm, moist air from inside a building penetrates a wall, roof or floor and meets a cold surface). The type of mechanical ventilation used depends on climate.

Fans can either be installed directly in windows or walls, or installed in air ducts for supplying air into, or exhausting air from, a room. Mechanical fans drive mechanical ventilation.

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