A 2,000 ft² house with three bedrooms needs 90 cfm of fresh air running all the time: 0.03 × 2,000 + 7.5 × 4 = 60 + 30 by ASHRAE 62.2. An exhaust fan moving that air needs a 6 in duct. A bathroom fan is 50 cfm on a 4 in duct, a range hood at least 100 cfm on a 6 in duct. The calculator above works out the airflow, the duct and the fan for a house or one room.
How the calculator works
Choose the whole house or one room. For the house, enter the floor area and the bedrooms; for a room, pick its use and the people in it. Then pick the system: passive stack, exhaust fan, supply fan or a balanced ERV/HRV. The result is the airflow in cfm, the air changes per hour, the round and square duct, the air velocity, the fan power and the heat an ERV recovers.
Required airflow
| Space | Airflow | Source |
|---|---|---|
| Whole house, continuous | 0.03 cfm/ft² + 7.5 cfm × (bedrooms + 1) | ASHRAE 62.2 |
| Kitchen | 100 cfm intermittent or 25 cfm continuous | IRC M1505.4.4 |
| Bathroom, toilet | 50 cfm intermittent or 20 cfm continuous | IRC M1505.4.4 |
| Office | 5 cfm a person + 0.06 cfm/ft² | ASHRAE 62.1 |
| Gym | 20 cfm a person + 0.06 cfm/ft² | ASHRAE 62.1 |
Houses by size
| House | Airflow | Exhaust duct |
|---|---|---|
| 1,500 ft², 2 bedrooms | 68 cfm | 5 in |
| 2,000 ft², 3 bedrooms | 90 cfm | 6 in |
| 3,000 ft², 4 bedrooms | 128 cfm | 7 in |
Duct and fan
The duct is sized for the air velocity of the system — about 590 feet a minute for an exhaust fan, 790 for a supply fan, 690 for an ERV — and rounded up to a standard size. Longer runs and elbows add pressure: check the fan's curve at the duct length you have. A balanced ERV at 75 % efficiency recovers about 2,900 Btu/h of the 90 cfm when it is 40 °F colder outside.
Ventilation air is part of the heating and cooling load: see the Heating BTU Calculator and the AC BTU Calculator. Wiring the fan: Wire Size Calculator.