ERV Requirements for New Homes in Wellesley, MA

What Wellesley Requires

Item	Requirement
Energy code	Specialized (Opt-In) Code (225 CMR 22). Wellesley's Specialized (Opt-In) Energy Code took effect January 1, 2024.
System type	Continuous balanced ventilation with heat or energy recovery (ERV or HRV), R403.6.1
Efficiency	SRE of 65% or more at 32°F for units of 300 cfm or less, HVI-listed
Noise	Whole-house fans 1.0 sone or quieter (R403.6.5)
Outdoor intake	At least 10 ft from plumbing vents and similar sources; screened; no air from an unconditioned basement (R403.6.7)
Testing	Airflow verified by a HERS rater, HERS Rating Field Inspector or BPI professional (R403.6.3)
Building department	Building Department, 888 Worcester Street, Suite 160, Wellesley, MA 02482, 781-431-1019

Full code walk-through: ERV/HRV requirements in Massachusetts.

Why Proper Ventilation Matters in a Tight Wellesley Home

Today’s code-built homes are sealed tight and tested with a blower door. That saves energy, but the old leaks that used to air a house out are gone. Without working mechanical ventilation, everything made inside stays inside.

Moisture: showers, cooking, breathing and drying new framing and drywall add water vapor every day. The EPA recommends keeping indoor humidity below 60%, ideally 30–50%. Above that, windows sweat and mold can start.
Indoor air quality: CO2, cooking smoke, and chemicals off-gassing from new finishes, cabinets and flooring build up in a sealed house.
Comfort and health: steady fresh air in bedrooms and living areas, without opening windows in January.
The building itself: controlled humidity protects framing, insulation and finishes from hidden condensation.

That is why Massachusetts requires continuous, balanced ventilation with heat or energy recovery in new homes, and why it has to be tested before final.

How an ERV Works

Diagram of how an energy recovery ventilator (ERV) works: fresh outdoor air and stale indoor air pass through the core without mixing

Two fans run continuously: one brings fresh outdoor air in, the other pulls stale air out of kitchens, baths and laundry.
Both airstreams pass through the core side by side but never mix.
In winter the warm outgoing air pre-warms the cold incoming air, so you get fresh air without a big heating penalty (code requires 65%+ sensible recovery).
In summer an ERV also moves some humidity from the incoming air to the outgoing stream. An HRV recovers heat only.

Choosing between them? See ERV vs. HRV.

Why Proper Balancing Is So Important

Diagram showing balanced, negative-pressure and positive-pressure ERV settings and their effect on a tight house

In a tight house, even a small mismatch between supply and exhaust changes the pressure of the whole home:

Too much exhaust (negative pressure): the house pulls replacement air through walls, the garage and the basement, which can bring in dust, fumes and soil gases, and can backdraft fuel-burning appliances.
Too much supply (positive pressure): warm, moist indoor air is pushed into wall and attic cavities in winter, where it can condense and grow mold.
Wrong total airflow: too little leaves stale, humid air; too much wastes energy and dries the house out in winter.

We measure supply and exhaust with a calibrated flow hood or flow grid, adjust with your HVAC contractor until they match the design rate, and document it for the inspector. See ventilation testing and balancing.

How Much Airflow a Wellesley Home Needs

Massachusetts sets the minimum continuous rate from floor area, bedrooms and the blower door result: Q = 0.03 × floor area + 7.5 × (bedrooms + 1) − 0.052 × Q50 × S × WSF. The tighter the house tests, the more mechanical ventilation it needs.

See the ventilation CFM calculation and ventilation testing and balancing.

Worked example of the Massachusetts R403.6 ventilation CFM calculation

Common Wellesley ERV Problems We Find at Final

ERV left on factory speed, far off the design airflow.
Supply and exhaust out of balance, pushing the house positive or negative.
Intake too close to a dryer or plumbing vent.
System switched off or set to intermittent, so moisture builds up. See moisture and mold in tight homes.

More for Wellesley Projects

HERS rating in Wellesley
What you need for a building permit in Wellesley
Stretch code help in Wellesley
Manual J, S and D for equipment sizing
ERV vs. HRV

We serve Wellesley from our Framingham office at 68 South St.

FAQ

Is an ERV required for new homes in Wellesley?

Yes. Wellesley follows the Specialized (Opt-In) Code, which requires balanced ventilation with heat or energy recovery (an ERV or HRV) in new homes under 225 CMR 22 R403.6.1.

Can I use an HRV instead of an ERV in Wellesley?

Yes. Either meets code if the unit is HVI-listed with a sensible recovery efficiency of at least 65% at 32°F.

Who tests the ERV for the Wellesley building department?

A HERS rater, HERS Rating Field Inspector or BPI professional measures the installed airflow (R403.6.3). We usually do it during the final HERS inspection.

How do I know how many CFM my Wellesley home needs?

It depends on floor area, bedrooms and the blower door result. We calculate it from your plans and confirm it at final.

Do additions in Wellesley need an ERV?

Large additions and major renovations that trigger a HERS rating must meet the mandatory ventilation items. Small additions usually follow component rules; ask us about your project.