Ventilation is the process of exchanging indoor air with outdoor air to control moisture, dilute pollutants, and supply oxygen. In HVAC, the V stands for ventilation, but it gets the least attention of the three. Heating and cooling have obvious comfort effects. Ventilation works in the background, and problems with it show up as stuffy rooms, lingering odors, condensation on windows, or elevated CO2 levels rather than a temperature complaint.
Natural vs. Mechanical Ventilation
Natural ventilation relies on open windows, doors, and gaps in the building envelope to move air. It costs nothing to operate but depends entirely on wind and temperature differences between indoors and outdoors. On a still, humid day, natural ventilation barely functions.
Mechanical ventilation uses fans and ductwork to force air exchange on a controlled schedule. Three configurations cover most residential applications:
System Type |
How It Works |
Best Fit |
Exhaust-only |
A fan pulls stale air out; fresh air enters through envelope gaps. |
Mild, dry climates with older, leakier homes |
Supply-only |
A fan pushes filtered outdoor air in; stale air exits through gaps. |
Hot, humid climates where controlling what comes in matters |
Balanced (HRV/ERV) |
Separate intake and exhaust streams pass through a heat exchanger. Recovers 60-80% of energy from outgoing air. |
Cold or humid climates where energy loss from raw air exchange is too costly |
ASHRAE Standard 62.2 sets the baseline for residential ventilation: a 2,000 sq ft home with three bedrooms needs roughly 60-75 CFM of continuous fresh air.
Ventilation and Energy Use
Every cubic foot of outdoor air brought inside has to be heated or cooled to match the indoor setpoint. In winter, cold intake air adds heating load. In summer, hot humid intake air adds cooling and dehumidification load. This is the core tension in tight, energy-efficient homes: they seal well enough to cut infiltration losses, but that same tightness means mechanical ventilation becomes mandatory rather than optional. An ERV or HRV softens the penalty by pre-conditioning incoming air with energy captured from outgoing air, cutting the ventilation-related load by 20-30% in extreme climates.