A fresh air intake brings outdoor air into the HVAC system to replace air that escapes through exhaust fans, combustion appliances, and infiltration. Modern tightly sealed homes can have dangerously poor air quality without a controlled fresh air supply.
ASHRAE Standard 62.2 specifies minimum ventilation rates for residential buildings based on floor area and number of bedrooms. Methods for providing fresh air include: a dedicated fresh air duct connected to the return air plenum, a heat or energy recovery ventilator (HRV/ERV) that exchanges stale indoor air for fresh outdoor air while recovering most of the energy, or timed exhaust fans with makeup air provisions.
Uncontrolled fresh air (random infiltration through cracks and gaps) is not a substitute for designed ventilation — it is unpredictable, unfiltered, and creates comfort problems. As homes become better insulated and air-sealed, mechanical ventilation becomes increasingly important.