Exhaust, Supply or Balanced Ventilation

Exhaust only ventilation runs a fan that pulls air out of the house, and replacement air seeps back in through leaks in the shell and any intentional inlet vents, as DOE explains. The house sits slightly below outdoor pressure while it runs.

HVI describes the usual setup as one fan at a central exhaust point, and a better one as a fan ducted from rooms where pollutants start, such as bathrooms and kitchens. Under the IRC, a bathroom fan on continuous duty must move at least 20 cfm; the whole-house rate is set separately, as the ASHRAE 62.2 page explains.

Supply ventilation uses a fan to push outdoor air into the house, so the house runs slightly above outdoor pressure and stale air leaves through leaks, fan ducts and any intentional vents. HVI notes it works best feeding the rooms people use most, such as bedrooms and the living room.

The incoming air can be filtered for pollen and dust or dehumidified, and positive pressure helps keep soil gases and flue gases out. The drawback shows up in cold climates: warm, humid indoor air is driven into cold attics and walls, where it can condense and cause mold and decay.

In broad terms, exhaust only ventilation fits cold climates, supply fits hot or mixed climates, and balanced systems fit anywhere (HVI). The logic is moisture: in cold weather you don't want humid indoor air pushed into cold walls, and in warm, damp weather you don't want humid outdoor air pulled into them.

In Climate Zones 6, 7 and 8, the 2024 IRC settles it by requiring a heat or energy recovery ventilator. Elsewhere the house matters as much as the weather: an attached garage, a crawl space or an atmospherically vented water heater argues against running negative all day.

It can. Any system that pulls out more air than it brings in lowers indoor pressure, and that can backdraft a gas, oil or wood appliance that isn't sealed-combustion or power-vented, spilling flue gases, including carbon monoxide, indoors.

HVI flags the moments when bath fans, range hoods and clothes dryers, which also depressurize the home, run at the same time as an exhaust ventilation system. The Building America Solution Center adds that a tighter house reaches a deeper negative pressure for the same exhaust. Big hoods raise the same issue (makeup air for range hoods).

A good plan names the strategy, the airflow and how both will be checked. Ask the installer for:

• The strategy and why, tied to your climate, appliances and any attached garage.
• The design airflow in cfm and the method behind it, the ASHRAE 62.2 formula or the IRC table, with any intermittent-run factor shown.
• Fan ratings under HVI or AMCA test methods, at your duct's static pressure.
• Intake and exhaust locations. The IRC keeps outdoor air intakes at least 10 ft from vents, chimneys, streets and parking areas (R325.4.1), with exhaust outlets kept away from opening windows and property lines.
• Controls that allow a manual override and are labeled, as IRC M1505.4.2 requires.
• Dampers, grilles, duct sizes and how flows will be measured at startup.
• A combustion safety test before and after, if the house has any vented fuel-burning appliance.