To size an ERV (energy recovery ventilator) for your home, you need to calculate the airflow rate in cubic feet per minute (CFM) that will deliver at least 0.35 air changes per hour. For most homes, this means an ERV rated between 70 and 200 CFM, depending on square footage, ceiling height, and how many people live there.
The Basic Sizing Formula
The standard approach from the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is straightforward. Take your home’s total square footage, including any finished basement, and multiply it by your ceiling height to get the cubic volume. Divide that number by 60 (to convert from cubic feet per hour to cubic feet per minute), then multiply by 0.35.
For a 2,000-square-foot home with 8-foot ceilings, that looks like this:
- Volume: 2,000 × 8 = 16,000 cubic feet
- CFM needed: 16,000 ÷ 60 × 0.35 = 93 CFM
A 2,500-square-foot home with 9-foot ceilings would need about 131 CFM. A smaller 1,200-square-foot home with standard 8-foot ceilings needs only 56 CFM. Run the math with your own numbers and you’ll have a solid starting point.
The Occupancy Rule That Can Override the Formula
ASHRAE’s residential ventilation standard (62.2) sets a floor: your home should receive no less than 15 CFM per person, regardless of what the square footage formula produces. In a typical three-bedroom house with two or three occupants, the volume-based calculation almost always exceeds the per-person minimum. But if you have a smaller home with more people, say a 1,000-square-foot apartment with five residents, the per-person requirement (75 CFM) would be higher than the volume formula (about 47 CFM), and you’d size to the larger number.
For counting occupants, the standard convention is to assume the number of bedrooms plus one. A three-bedroom home counts as four people. If your actual household is larger than that, use your real headcount.
Why You Should Oversize Slightly for Ductwork Losses
ERV manufacturers rate their units at specific CFM values under controlled conditions, but your actual installation will create resistance. Every foot of ductwork, every elbow, every filter, and every vent cap adds static pressure that reduces the air the unit can actually push through. The airflow you get at the vent farthest from the ERV will be noticeably less than what the unit produces at its outlet.
A common rule of thumb is to select an ERV rated 10 to 20 percent above your calculated CFM need. If your formula says 93 CFM, look at units rated for 100 to 120 CFM. This buffer accounts for real-world duct resistance and ensures you actually hit your ventilation target once everything is installed. Most ERVs have adjustable fan speeds, so you can dial the unit back if needed.
What Happens If You Size Too Large
Bigger is not better here. An oversized ERV pulls in more outdoor air than your HVAC system can condition, which creates problems that vary by season. In summer, the extra humid air overwhelms your air conditioner’s ability to remove moisture. The AC cools the air quickly but doesn’t run long enough to dehumidify properly, leaving your home feeling damp and clammy. Over time, this excess moisture can encourage mold growth. In winter, an oversized unit brings in more cold, dry air than necessary, making your heating system work harder and potentially drying out your home beyond comfortable levels.
The goal is to bring in exactly enough fresh air to maintain good indoor air quality without making your HVAC system fight to keep up.
Climate Matters for Choosing the Right ERV
The size (CFM) of the ERV is one decision. The type and efficiency rating is another, and your climate plays a big role. ERVs transfer both heat and moisture between the incoming and outgoing air streams, which gives them an advantage over HRVs (heat recovery ventilators) in climates where humidity control matters.
If you live in a hot, humid climate, you want an ERV with high latent (moisture) recovery efficiency so it removes humidity from incoming outdoor air before it enters your home. In cold climates, that same moisture transfer works in reverse: the ERV recaptures moisture from stale indoor air and passes it to the dry incoming stream, keeping your home from getting too dry in winter. One practical benefit in cold climates is that ERVs are less prone to frost buildup than HRVs, because the moisture transfer keeps the exhaust side warmer.
Look for units with total recovery efficiency (sometimes called enthalpy efficiency) of 70 percent or higher. This number tells you how much of the energy in the outgoing air the unit recaptures, both heat and moisture combined.
Putting It All Together
Here’s a quick sizing walkthrough:
- Step 1: Multiply your home’s total conditioned square footage by ceiling height to get cubic volume.
- Step 2: Divide by 60, then multiply by 0.35 to get your target CFM.
- Step 3: Check the per-person minimum. Multiply the number of occupants (bedrooms plus one, or actual headcount if higher) by 15 CFM. Use whichever number is larger.
- Step 4: Add 10 to 20 percent to account for ductwork losses.
- Step 5: Select an ERV rated at or just above that final number.
For a 2,000-square-foot, three-bedroom home with 8-foot ceilings and four assumed occupants: the volume formula gives 93 CFM, the per-person check gives 60 CFM, so 93 CFM governs. Adding a 15 percent buffer puts you at about 107 CFM. An ERV rated at 110 to 120 CFM would be a good fit.
If your home has unusually long duct runs, uses flex duct instead of rigid metal, or has the ERV located far from the main living areas, lean toward the higher end of that buffer. An HVAC contractor performing a Manual J load calculation can also factor the ERV’s ventilation air into your overall heating and cooling load, ensuring everything works together as a system rather than competing against each other.