What is ASHRAE 62.2 Equation 4.1a?

It defines the whole-house mechanical ventilation rate as a coefficient times floor area plus 7.5 cfm times the number of bedrooms plus one. The bedroom term is a proxy for occupancy, and the area term accounts for emissions from the building and its contents.

Why are there two coefficients, 0.03 and 0.01?

The current ASHRAE 62.2-2022 edition uses 0.03 cfm per square foot, while the older 2010 edition used 0.01. The standard increased the area term over time, so the tool lets you select the edition your jurisdiction has adopted to get the matching rate.

What is the infiltration credit?

A leaky home already gets some fresh air through its cracks, so the standard lets you subtract a measured infiltration rate from the required fan airflow. Tight new construction has almost no infiltration, so the credit is near zero and the fan must supply the full rate.

Does this replace local exhaust fans?

No. The whole-house rate is separate from and in addition to local exhaust in kitchens and bathrooms, which 62.2 also requires. You need both: continuous or intermittent whole-house ventilation plus spot exhaust at the moisture and odor sources.

Should the system be continuous or intermittent?

A continuous low-speed ERV or HRV is the simplest path to compliance and stays quiet. Intermittent operation is allowed if the higher airflow multiplied by the fractional run time and divided by the effectiveness still meets the whole-house rate, but it requires careful controls.

Residential Mechanical Ventilation Calculator (ASHRAE 62.2)

Tight, well-insulated homes trap moisture and indoor pollutants, so modern energy codes require mechanical ventilation to bring in measured fresh air. This calculator applies ASHRAE 62.2 to your home’s floor area and bedroom count to get the whole-house ventilation rate, then recommends an ERV or HRV size so the system is both code-compliant and comfortable.

How it works

The whole-house rate comes straight from ASHRAE 62.2 Equation 4.1a, with the infiltration credit subtracted to get the mechanical fan duty:

Qtot   = coeff × floor area + 7.5 × (bedrooms + 1)     [cfm]
  coeff = 0.03 (62.2-2022)  or  0.01 (legacy 62.2-2010)
fan    = Qtot − infiltration credit

The bedroom term assumes occupancy of bedrooms plus one, and the area term captures emissions from the structure and furnishings. For tight new construction the infiltration credit is essentially zero, so the balanced ventilation unit must deliver the full Qtot continuously.

Example and notes

A 2,000 square foot, three-bedroom home under the 2022 edition needs 0.03 × 2000 + 7.5 × 4 = 90 cfm of whole-house ventilation, which points to a roughly 90 to 100 cfm nominal ERV or HRV running continuously. Two reminders apply to every result: this whole-house rate is in addition to the kitchen and bathroom local exhaust that 62.2 also requires, and you should confirm which edition of the standard your jurisdiction’s energy code references, since IECC 2021 points at 62.2 but local amendments vary.