What it is

Sizing ventilation means figuring out how many CFM of outdoor air a house needs continuously, then picking and setting up equipment that actually delivers it. Too little and the house stays stuffy and humid — you built a ventilator that doesn't ventilate. Too much and you waste energy and can over-dry (or over-humidify) the place and drive up the heating/cooling bill. The target is a specific number you can calculate, not a vibe.

The residential ventilation standard ties the rate to two things: the size of the house (more square footage, more air) and the number of people in it (more occupants, more CO2 and moisture to dilute). You add those two pieces together to get the whole-house continuous ventilation rate.

How it works

The math is an area term plus an occupancy term:

  • Area term: a small fixed airflow per square foot of conditioned floor area. The standard uses roughly 0.03 CFM per square foot.
  • Occupancy term: a fixed airflow per person, about 7.5 CFM per occupant. Since you don't know how many people will live there, occupancy is conventionally estimated as number of bedrooms + 1 (a 3-bedroom house assumes 4 people).

Add them: total continuous ventilation CFM = (0.03 × floor area in ft²) + (7.5 × [bedrooms + 1]).

That gives the continuous rate. If the system doesn't run 100% of the time — say it cycles on a timer, or runs only when the furnace blower runs — you have to increase the airflow so the average over time still hits the target. Run it half the time and you roughly need to double the airflow while it's on. That run-time adjustment is part of sizing, not an afterthought.

There's also a credit concept: in a leakier house some of the fresh air arrives by infiltration, so the standard lets you subtract a portion of that natural infiltration from the required mechanical rate. In a tight house that credit is small to nothing, which is exactly why tight houses need close to the full mechanical rate.

In the field

  1. Measure or confirm floor area and bedroom count. Conditioned floor area, not including the garage. Bedrooms drive the occupancy assumption.
  2. Run the calc: (0.03 × ft²) + (7.5 × (bedrooms + 1)) = continuous CFM target. Write it down.
  3. Decide the run schedule. Continuous operation lets you size the equipment right at the target. Intermittent operation (timer cycling, or tied to blower runtime) means you size the airflow UP so the time-averaged flow still meets the target.
  4. Select equipment with headroom. Pick an ERV/HRV or fan whose rated airflow comfortably covers the target at the real-world static pressure of the duct system — not its free-air rating. Ductwork robs airflow; a unit rated 120 CFM in free air may deliver far less through restrictive ducting.
  5. Add spot/local exhaust on top. Whole-house ventilation does not replace bath and kitchen exhaust. Those are separate, sized for their rooms, vented outdoors.
  6. Commission it. After install, MEASURE the actual delivered CFM at the grilles/ports with a flow hood or anemometer. Balance the supply and exhaust on an ERV/HRV. Adjust speed/damper to hit the calculated number. The rating on the box is a starting point, not proof.

Normal values & targets

  • Whole-house continuous rate: (0.03 CFM/ft²) + (7.5 CFM × (bedrooms + 1)). Example: a 2,000 ft², 3-bedroom house → (0.03 × 2,000) + (7.5 × 4) = 60 + 30 = 90 CFM continuous.
  • Intermittent run-time bump: running ~50% of the time roughly doubles the required airflow during run; run-time multipliers come from the standard's table — the less it runs, the more it must move while on.
  • Local/spot exhaust (separate from whole-house): bathrooms commonly 50 CFM continuous or ~80–100 CFM intermittent; kitchen range hoods 100+ CFM (often much higher for big gas ranges). Vented outdoors.
  • Don't size off free-air ratings. Equipment performance is published against external static pressure. At a realistic 0.2–0.4+ in. w.c. of duct resistance, delivered CFM is well below the free-air number. Read the performance curve.
  • Air-change sanity check: the calculated rate usually lands the whole house near ~0.3 ACH of outdoor air — a reasonable IAQ target.

Common faults & what they mean

  • House still stuffy/humid after a "ventilator" was installed: undersized for the duct restriction, or running too little of the time, or never commissioned. Measure actual CFM — it's almost always low.
  • Energy bills jumped and the house feels over-dried (winter) or clammy (summer): oversized ventilation moving more outdoor air than needed. Dial it back to the calculated rate.
  • Calculated rate met on paper but air feels stale in bedrooms: distribution problem. The whole-house number is right but the fresh air isn't reaching the rooms that need it. Check duct routing and grille placement.
  • Range hood or big exhaust starves the house: a powerful kitchen exhaust can depressurize a tight house and even pull the ventilator backward or backdraft an appliance. Makeup air may be required.

Tech tips & gotchas

  • The number is calculable — calculate it. Floor area plus bedrooms-plus-one. Don't pick equipment by "that looks about right for this size house."
  • Free-air CFM is marketing; installed CFM is reality. The single biggest sizing miss is ignoring the duct static pressure and the unit's performance curve. A short run of smooth duct delivers far more than a long run of flex with elbows. Size for the actual system.
  • Run-time changes everything. A unit that only runs when the furnace blower runs may average way below target on a mild day when the furnace barely cycles. If you ventilate intermittently, size the airflow up and/or use a fan-cycler control to force minimum runtime.
  • Commission with a flow hood, every time. "It's running, the fan spins" is not commissioning. The job isn't done until the measured CFM matches the calculated target and the ERV/HRV streams are balanced.
  • Whole-house and spot exhaust are additive, separate jobs. Don't count the bath fan toward the whole-house rate unless it's specifically configured and controlled as the whole-house exhaust.

Safety / code notes

  • The whole-house ventilation rate, the run-time/intermittent multipliers, the infiltration credit, and local exhaust requirements all come from ASHRAE 62.2 (referenced by IRC/IMC). Size to its method by section; don't reproduce its tables — use them.
  • A large kitchen exhaust in a tight house can trigger makeup-air requirements (IMC/IRC provisions). Check whether makeup air is required so you don't depressurize the house and backdraft combustion appliances.
  • All exhaust terminates outdoors with proper clearances per IMC — never into attics, soffits, or crawlspaces.