IRC 2024 Whole-House Mechanical Ventilation: Continuous vs Intermittent Operation

Quick Answer

IRC 2024 Section M1507.3 requires whole-house mechanical ventilation in all new dwellings. The required airflow rate is calculated from ASHRAE 62.2 — 0.01 CFM per square foot of conditioned floor area plus 7.5 CFM per occupant, where each bedroom counts as one additional occupant above a baseline of one. The system may operate continuously at the full rate or intermittently using a timer, provided the equivalent total air volume is achieved within each hour.

Under IRC 2024, controls must be labeled and accessible to the occupant. This requirement applies to all new residential construction regardless of blower door test results.

What IRC 2024 Actually Requires

Section M1507.3 mandates that every new dwelling have a whole-house mechanical ventilation system capable of providing outdoor air to the occupied space at the rate required by ASHRAE 62.2-2019 (or the edition adopted by the jurisdiction). The formula for the continuous ventilation rate (Q_fan) is:

Q_fan = 0.01 × A_floor + 7.5 × (N_br + 1)

Where A_floor is the conditioned floor area in square feet and N_br is the number of bedrooms. The “+1” counts the primary occupant in addition to one occupant per bedroom. For example, a 2,000-square-foot three-bedroom home requires 0.01 × 2,000 + 7.5 × (3 + 1) = 20 + 30 = 50 CFM continuous.

Intermittent operation is permitted if the system is controlled by a programmable timer or occupancy-based control. When running intermittently, the airflow rate must be increased proportionally so that the total air volume delivered per hour equals what continuous operation at the base rate would deliver. If the base rate is 50 CFM and the system runs only 50 percent of each hour, it must deliver 100 CFM during its “on” cycles to achieve the equivalent hourly volume.

Section M1507.3 also requires that the system controls be labeled to explain the ventilation function and that the controls be accessible to the occupant. A whole-house fan wired to run only when the HVAC system calls for heating or cooling does not meet this requirement because occupant accessibility and labelability are compromised when the control is inside the HVAC controller’s logic rather than at a labeled switch.

The system may consist of a dedicated exhaust fan, a supply fan, a balanced energy-recovery ventilator (ERV) or heat-recovery ventilator (HRV), or a combined system where the HVAC air handler draws outdoor air through a duct equipped with a motorized damper and flow-measurement device. Each approach has different commissioning requirements to verify delivered airflow.

Why This Rule Exists

Modern residential construction targets airtight envelopes to minimize energy loss. Spray foam insulation, advanced framing, taped rigid insulation, and attention to air-sealing details routinely produce homes below 3 ACH50 and frequently below 1 ACH50 in high-performance construction. At these leakage rates, the natural infiltration that older homes relied upon for ventilation is essentially eliminated. Without a mechanical substitute, indoor air quality rapidly deteriorates: CO₂ from occupant respiration accumulates, volatile organic compounds (VOCs) off-gassing from building materials and furnishings build up, radon penetrating through the slab concentrates, and humidity from cooking, bathing, and occupants has no dilution pathway.

ASHRAE 62.2 establishes the minimum ventilation rate needed to maintain acceptable indoor air quality in typical residential occupancy. IRC 2024 adopts this standard by reference to ensure that code-compliant new homes provide a habitable environment without depending on unpredictable infiltration. The shift in IRC 2024 from a conditional requirement (ventilation only if the home is below a certain leakage threshold) to a universal requirement reflects accumulated evidence that even moderately tight homes do not provide adequate natural ventilation in most climate conditions.

What the Inspector Checks at Rough and Final

At rough-in, the inspector verifies that the whole-house ventilation duct or the outdoor air intake duct is installed and stubbed to the appropriate connection point — whether to the exhaust fan, the HRV/ERV unit, or the HVAC air handler’s outdoor air port. They confirm the duct material is appropriate and that the penetration through the envelope is blocked only by a listed motorized damper or a listed passive inlet, not by a plugged hole.

At final inspection, the inspector looks for the labeled control and verifies it is accessible. For timer-based intermittent systems, they check that the timer is programmed to meet the equivalent hourly airflow requirement. For HRV and ERV systems, the unit must be balanced (supply and exhaust within 10 percent of each other per ASHRAE 62.2) and the outdoor and exhaust termination caps must be installed per the manufacturer’s instructions with proper clearances. In some jurisdictions, the installer must provide a commissioning report showing measured flow rates at the unit and at all supply and exhaust grilles.

What Contractors Need to Know

Select the ventilation strategy during the design phase. The most common approaches for new single-family residential construction are: exhaust-only (a bath fan or dedicated exhaust fan runs continuously or on a timer to depressurize the house slightly, drawing outdoor air through passive inlets or gaps in the envelope); supply-only (a fan introduces outdoor air through the air handler or a dedicated duct, slightly pressurizing the house); and balanced ERV or HRV (equal supply and exhaust, with heat recovery). Each strategy has trade-offs for climate, cost, and moisture management.

In cold climates, exhaust-only systems can draw extremely cold air through unintended envelope gaps, creating comfort and moisture problems. HRV systems are preferred in cold climates because they recover heat from the outgoing exhaust air before delivering cold outdoor air. In hot-humid climates, ERVs transfer both heat and moisture, reducing the latent load on the cooling system.

Commission the system before final inspection. Measured delivered airflow is the only way to confirm that the installed system meets the calculated rate. A bath fan running continuously at a rated 50 CFM may deliver only 35 CFM at the static pressure created by 20 feet of flex duct. Use a flow hood or calibrated anemometer and document the results. HRV and ERV commissioning requires balancing supply and exhaust flows and verifying core defrost operation in cold climates.

Label the controls as required. A permanent label that reads something like “Whole-House Ventilation Fan — Run continuously for healthy indoor air quality” satisfies the labelability requirement. Labels are often missed at inspection and cause a re-inspection trip for something trivially easy to fix in advance.

What Homeowners Get Wrong

The most common misconception is that local exhaust fans in bathrooms and kitchens satisfy the whole-house ventilation requirement. They do not. Local exhaust fans address spot-source pollutants (shower moisture, cooking fumes) but do not provide a predictable, distributed supply of outdoor air to all occupied spaces. A bathroom fan that runs only during and after showers cannot deliver the equivalent of 50 CFM continuous outdoor air to the living room, bedrooms, and hallways.

Homeowners also frequently turn off whole-house ventilation fans because they believe the house is “already ventilated” by opening windows. In climates with hot summers or cold winters — which is most of the country — windows are closed for months at a time. During those periods, the mechanical ventilation system is the only source of outdoor air. Turning it off to save electricity eliminates the indoor air quality protection that the code requires and can allow CO₂, VOCs, and radon to accumulate to levels that affect health and alertness.

ERV and HRV owners often fail to replace filters on schedule. A clogged HRV core reduces both airflow and heat-recovery efficiency. Most manufacturers recommend checking the core filter every three months and replacing it every six to twelve months. A blocked filter can reduce delivered airflow by 40 to 60 percent, dropping the home well below the minimum ventilation rate.

State and Local Amendments

Many cold-climate states — including Minnesota, Wisconsin, Vermont, and Maine — have adopted HRV or ERV requirements for new construction by amendment or through state energy code, replacing the simpler exhaust-fan approach permitted by IRC 2024 base text. These states typically require balanced ventilation because unbalanced systems in very cold climates cause moisture and infiltration issues with fully air-sealed envelopes.

California Title 24 Part 6 has a parallel requirement through the California Mechanical Code and the residential energy code, requiring whole-house ventilation with specific fan efficacy limits (CFM per watt). California also adopted IAQ fan controls that require fans to be listed as low-sone quiet devices. Some California jurisdictions require continuous operation rather than timer-based intermittent operation. Washington state similarly requires HVI-certified fans with specific CFM-per-watt thresholds. Always verify local amendments before specifying the ventilation system.

When to Hire a Professional

HRV and ERV selection, sizing, and commissioning benefit significantly from an HVAC professional’s involvement. Balancing supply and exhaust flows to within 10 percent, selecting the correct core size for the climate, and properly routing intake and exhaust duct to avoid short-circuiting require experience and measuring equipment. A poorly commissioned ERV may deliver less than half the required ventilation rate despite appearing to work normally. HVAC contractors with NATE or BPI certification in building performance are the best-qualified professionals for this work. For exhaust-only systems using a bath fan, a qualified HVAC technician or licensed general contractor who understands ASHRAE 62.2 can properly size, install, and commission the system without specialized equipment beyond a flow hood.

Common Violations Found at Inspection

• No whole-house ventilation system installed — builder assumed local exhaust fans satisfy the requirement.
• Timer-based intermittent system programmed at the continuous rate without the required CFM multiplier, delivering only half the required hourly air volume.
• Ventilation control not labeled or not accessible to the occupant.
• HRV or ERV supply and exhaust flows not balanced, with one side more than 10 percent higher than the other.
• Outdoor air intake duct to air handler not equipped with a motorized damper, leaving the duct open when the system is off and allowing uncontrolled infiltration.
• Delivered CFM measured significantly below the designed rate due to undersized or excessively long duct runs.
• HRV core filter not replaced at commissioning, restricting airflow from day one of occupancy.
• Passive air inlets not installed, leaving the exhaust-only system to draw air through uncontrolled gaps in the envelope.