Whole-House Ventilation ERV/HRV Sizing: ASHRAE 62.2 Beyond the Formula

The 2,400 sq ft tight new build with 3 ACH50 needs mechanical ventilation. The blower door confirmed the envelope is below the IECC 2024 infiltration threshold; the IAQ guy says you need an ERV; the homeowner is asking why their current 90 CFM bath fan can’t just run continuously. The honest answer is that whole house ventilation ERV/HRV sizing has two layers most contractors compress into one number: the ASHRAE 62.2 ventilation requirement, and the ERV/HRV unit’s effective delivery rate at design conditions. Skip the second and you install a unit that meets the spec on paper but moves less air than the calc says it should.

This walks through the calculation, the climate logic for ERV vs HRV, and the part where unit ratings don’t equal delivered airflow.

Whole House Ventilation: The ASHRAE 62.2 Sizing Formula

The current standard is ASHRAE 62.2, with the 2022 edition being the version most current codes reference. The whole-house ventilation rate in CFM is:

Where A_floor is conditioned floor area in sq ft and N_br is the number of bedrooms. The first term scales with floor area (3 CFM per 100 sq ft); the second term is occupancy-driven, with the +1 assuming bedrooms + 1 occupants and 7.5 CFM/person.

Worked Example — 2,400 sq ft, 3 Bedrooms

Plug in the numbers:

The whole-house ventilation requirement is 102 CFM continuous. That’s the standard’s required exhaust-equivalent airflow. For a 1,600 sq ft 2-bedroom condo:

ERV/HRV Sizing: Effective Rate vs Nominal Rate

Here’s where most calculations stop and most installations start drifting from spec. ERV and HRV units are rated at a nominal CFM at a specific external static pressure — usually 0.4″ w.g. for residential equipment. Your duct system rarely runs at the rating-test pressure. If your installed external static is 0.6″ w.g. (common in retrofits where the ventilation duct shares trunks with the supply), the unit moves less air than its sticker.

The fix is to size the unit’s nominal capacity above the calculated requirement so the operating-point delivery still meets it. For a 102 CFM requirement on a typical residential install:

• If your duct design is clean (under 0.4″ w.g. external static): a 120 CFM nominal unit covers the 102 with about 15% margin.
• If your duct design is constrained (0.5–0.7″ w.g.): step up to a 150 CFM nominal unit. The fan curve drops faster than most contractors expect at the high end of the operating range.
• Always look at the manufacturer’s fan curve, not just the rating-plate CFM. Building America Solution Center and HVI’s certified products list publish CFM at multiple static pressures for HVI-certified units.

ERV vs HRV — Climate Drives This

Both units recover sensible heat. The difference: an ERV has an enthalpy core that also transfers moisture between airstreams; an HRV transfers only sensible heat. The selection isn’t preference, it’s climate.

The blanket rule contractors used to follow — “HRV in the north, ERV everywhere else” — isn’t wrong, but the edge cases (humid northern winters, dry southern climates) get specifically inverted decisions in the cleaner climate-zone analysis.

The Infiltration Credit

ASHRAE 62.2 has a credit mechanism that lets you reduce mechanical ventilation if measured infiltration is high enough. The mechanism uses an effectiveness factor Φ (phi). For balanced ventilation systems — ERVs and HRVs both qualify — Φ = 1, which means the credit doesn’t apply directly. Your full ASHRAE 62.2 rate stands.

For unbalanced systems (exhaust-only or supply-only), Φ < 1 and you can sometimes reduce the mechanical rate based on infiltration. This is one of the reasons the standard tilts toward balanced ventilation as the envelope tightens past 3 ACH50 — you can’t lean on infiltration credit to shrink the unit.

Common Sizing Errors

Treating the Bath Fan as the Whole-House System

A continuously-running bath fan at 50–80 CFM is part of the ventilation strategy under ASHRAE 62.2’s exhaust-only path, but only when the fan is HVI-rated for continuous operation and the local infiltration math works. For a tight envelope (under 3 ACH50), this almost never penciled out before; under the 2022 standard with balanced-system assumptions, it doesn’t pencil out at all in most new construction.

Forgetting Defrost-Cycle Output

Cold-climate HRVs and ERVs run defrost cycles when the exhaust airstream drops below freezing in the core. During defrost, ventilation rate drops — sometimes to zero. The 24-hour effective delivered rate is lower than the steady-state rate. In zones 6–8, oversize the unit by 15–25% to account for defrost duty cycle, or pick a unit with electric pre-heat.

Ignoring the Below-15-CFM Threshold

ASHRAE 62.2’s minimum: if the calculated ventilation rate is below 15 CFM, no mechanical system is required. Realistic for a small studio with one bedroom. For the 2,400 sq ft house above, this clearly doesn’t apply — but contractors retrofitting tiny accessory dwellings sometimes over-spec when the standard would let them skip the ERV entirely.

Verifying the Calculation

Run the room-by-room air changes per hour calculation for the building’s actual envelope before sizing. The ASHRAE 62.2 number is whole-house, but the per-room ACH check identifies whether you need targeted exhaust at high-moisture spaces (bath, kitchen, laundry) on top of the central ERV/HRV. The air change rate calculator handles the volume-and-CFM conversion across rooms once you have the unit selected. For tight new construction where ASHRAE 62.2 sets the floor and your blower door result drives the climate-zone selection, both numbers feed the unit-selection decision.