ERV/HRV Ventilation Testing & Balancing in Massachusetts

How the Blower Door Sets Your Ventilation Rate

Massachusetts calculates the minimum continuous rate as 0.03 × floor area + 7.5 × (bedrooms + 1) − 0.052 × Q50 × S × WSF. Q50 is the measured leakage from the blower door test, S is a story factor (1.00, 1.32 or 1.55) and WSF is a county factor (0.66 in Suffolk County).

Example: a 2,000 sq ft, 3-bedroom, 2-story home in Suffolk County that tests at 1,000 cfm50 needs about 45 cfm. Tightened to 425 cfm50, the same house needs about 71 cfm. The tighter the house, the more mechanical ventilation it needs.

More detail: mechanical ventilation CFM requirements.

Worked example of the Massachusetts R403.6 mechanical ventilation CFM calculation

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 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.

What We Test

Supply and exhaust airflow at the ERV/HRV, measured with a flow hood or flow grid (RESNET 380 or ACCA 5).
Supply and exhaust within balance of each other, so the house is not pressurized or depressurized.
Delivered airflow compared with the design rate.
Bath and kitchen exhaust rates.
Inlet location, dampers and controls checked against R403.6.7.

Common Problems We Find

ERV left on the factory default speed, far above or below the design rate.
Supply and exhaust out of balance because of crushed flex duct or unadjusted dampers.
Outdoor intake too close to a dryer vent or plumbing vent.
Controls set to intermittent or turned off, so the system is not continuous.

Moisture or condensation already? See moisture and mold in tight new homes.

One Visit With Your Final

We usually test ventilation during the final HERS rating inspection, along with the blower door and duct leakage tests. Need equipment sizing too? See Manual J, S and D.

FAQ

Is ventilation testing required in Massachusetts?

Yes. The installed whole-house ventilation system must be tested and verified (R403.6.3) by a HERS Rater, HERS Rating Field Inspector or BPI Certified Professional.

How does the blower door test affect ventilation?

The measured leakage (Q50) is part of the Massachusetts ventilation formula. A tighter house gets less infiltration credit and needs more mechanical ventilation.

What does balancing an ERV mean?

Adjusting supply and exhaust airflow so they match the design rate and each other, so the house is not pushed positive or negative.

Can you fix an ERV that fails the airflow test?

We find the cause (fan speed, dampers, duct problems), help your HVAC contractor adjust it, and retest.

Do existing homes need ventilation testing?

Code testing applies to new systems in new construction and covered projects. We also test existing homes with moisture or air quality problems.

Town-by-Town Permit Checklists

Local building department details, checked on each town's own website:

Sources: 225 CMR 22 (Stretch/Specialized residential) · 780 CMR 10th edition residential energy amendments. Code text current as of October 2026; DOER has proposed updates to the Stretch and Specialized codes, so confirm before you file.