Gera Tools

Residential Mechanical Ventilation Calculator (ASHRAE 62.2)

Calculate whole-house ventilation CFM per ASHRAE 62.2 and IECC 2021

Enter conditioned floor area and bedroom count to compute the whole-house mechanical ventilation rate from ASHRAE 62.2 Equation 4.1a, apply an optional infiltration credit, and get a recommended ERV or HRV size and control type. For HVAC contractors on tight new construction. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

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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.

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.

Worked example

A 2,000 square foot, three-bedroom home under the 2022 edition:

Qtot = 0.03 × 2,000 + 7.5 × (3 + 1) = 60 + 30 = 90 cfm

For tight construction with no infiltration credit, the mechanical unit must deliver 90 cfm continuously. This points to a nominally rated 90–100 cfm ERV or HRV, since nominal ratings are measured at low static pressure and real duct systems impose some resistance.

Contrast with the older 2010 standard for the same home:

Qtot = 0.01 × 2,000 + 7.5 × 4 = 20 + 30 = 50 cfm

The tripling of the area coefficient from 0.01 to 0.03 has a large effect on larger homes — a 3,000 sq ft home goes from 60 cfm (2010) to 105 cfm (2022) just from the area term.

ERV vs HRV: which to specify

Both energy recovery ventilators (ERV) and heat recovery ventilators (HRV) meet the airflow requirement, but they handle moisture differently:

  • HRV — recovers heat only; exhausts moisture with the outgoing air. Better for humid climates where you want to expel indoor humidity in winter.
  • ERV — recovers both heat and a fraction of moisture. Better for very dry cold climates where retaining indoor humidity in winter improves comfort.

The default recommendation for most mixed climates is an HRV. In very cold, dry climates an ERV prevents the building from over-drying in winter.

What this calculation does not cover

  • Local exhaust requirements — kitchen and bathroom exhaust is in addition to the whole-house rate and is also required by 62.2
  • Spot pressurization — tight construction sometimes needs a blower-door test to confirm the infiltration credit claimed is actually achieved
  • MERV filter upgrades — adding higher-efficiency filtration to the ERV/HRV increases static pressure and can reduce delivered airflow below the rated CFM; size up accordingly

Confirm which edition of 62.2 your jurisdiction’s energy code references before specifying — IECC 2021 points at 62.2 but local amendments vary.