You need an ERV or HRV if your bedrooms miss the goal of 800 ppm or less, on average, while you sleep on a typical night with the doors shut, and nothing you already have gets them there in a way you can keep up every night. You don't need one just because the house is new, or tight, or because someone recommended it. A CO2 monitor and about a week of ordinary nights will tell you.
What an ERV actually fixes
An ERV (energy recovery ventilator) and an HRV (heat recovery ventilator) are both balanced ventilation systems. Two fans bring outdoor air in and send stale indoor air out at about the same rate, and a core between the two airstreams recovers much of the heat, so you aren't heating or cooling all that fresh air from scratch. An ERV also moves some moisture across. HRV vs ERV covers which suits your climate.
The job is outdoor air, delivered where people need it. An ERV isn't a humidifier, a dehumidifier or an air conditioner: Building Science Corporation notes that ERVs don't dry out a home in summer or cool it. It isn't an air purifier either, though it usually filters the air it brings in.
Signs worth checking
These are reasons to measure, not proof you need anything. Each can have other causes.
- The bedrooms feel stuffy by morning with the doors shut, even though the house is quiet and draft-free.
- Condensation on the inside of windows in cold weather. The EPA notes it can be a sign of high humidity, and the Building America Solution Center points out that newer, tighter homes tend to hold more moisture from cooking, bathing and laundry.
- Cooking smells or bathroom steam that hang around long after you're done.
- The house was built recently or air sealed. Tight homes lose the accidental fresh air that leaky ones get. Tight homes need to breathe.
"It has ventilation" isn't the same as "the bedrooms get enough"
Many tight homes already have some planned ventilation, often a bathroom fan on a timer. That can meet the code for the whole house and still leave a closed bedroom short, for two reasons.
The standard is for the whole house. ASHRAE 62.2, the main US standard for homes, sets one rate for the whole dwelling. For a 2,000 sq ft, three-bedroom house, that's about 90 cfm, shared across every room. The 2025 review behind the bedroom goal found it takes about 17 cfm of outdoor air for each person sleeping in the room, so 34 cfm for a couple, just for their bedroom. Getting your bedroom to 800 shows the math.
The bedroom may not be supplied at all. An exhaust fan pulls air out of the bathroom and lets replacement air leak in wherever it can; with the door shut, the bedroom may see little of it. Building Science Corporation also warns that in tight houses, exhaust-only suction can pull air in from attached garages or the soil. And the EPA notes that most forced-air heating and cooling systems "do not mechanically bring fresh air into the house."
The test: about a week
1. Measure your typical night. Three ordinary nights in the main bedroom, door and window the way you usually have them, the same people, the monitor in the same spot. If you can, check the other bedrooms too. Run a fair three-night check.
2. Try what you already have. One change per night, compared with your typical night:
- Your existing ventilation on boost, or running all night.
- The bedroom door open a few inches.
- A window cracked, if weather, outdoor air, noise and security allow.
Free fixes to try first has the details.
3. Read the result.
| What you found | What it means |
|---|---|
| Your typical night is already 800 or less | You don't need a system for the bedrooms |
| Over 800, but a change you can keep doing every night gets it to 800 or less | Probably not. Keep doing it, and recheck in the opposite season |
| Over 800, and nothing gets there, or the fix that works isn't one you can keep (a window in January, a fan too loud to sleep with) | A balanced system with a fresh-air supply to each bedroom is worth pricing |
Seasons matter. A window that works in October may not in January. If you're close to the line, measure again when the weather turns.
If the answer is yes
Two things matter more than the brand:
- A fresh-air supply into each bedroom, not just near it, and not just an exhaust somewhere else in the house.
- Enough air to each bedroom at the setting it will actually run on, about 17 cfm per sleeper, quietly enough that nobody turns it down.
Put your numbers in the request so every installer answers the same question: "Our main bedroom has two sleepers and averages [your number] overnight with the door shut. We'd like it at 800 or less. How many cfm will it get on the normal setting, and will you measure it after install?" The HouseExhale app writes that request from your nights.
Then compare quotes on the same job, item by item. The ductwork, access and repairs usually shape the price more than the unit itself. How to compare ventilation quotes and what drives the cost go through it.
Tonight: Start the ERV test
- Put a true (NDIR) CO2 monitor in the main bedroom at about head height, at least 2 ft (60 cm) from anyone's face and away from the window, door and vents.
- Sleep the way you usually do: door, window and any ventilation set as normal. Note who's in the room.
- Repeat for two more ordinary nights. That's your typical night.
- Then try your existing ventilation on boost or running all night for one night, and compare it with your typical night and with the 800 ppm goal.
Sources
- Akimoto et al. (2025), ASHRAE 1837-RP: bedroom ventilation and sleep quality (800 ppm and about 8 L/s per sleeper)
- Building Science Corporation: Info-611: Balanced ventilation systems (HRVs and ERVs)
- Building Science Corporation: BSI-012: Balancing act, exhaust-only ventilation does not work
- Building America Solution Center (PNNL): Whole-house ventilation strategies for new homes
- US EPA: Improving indoor air quality (most forced-air systems don't bring in outdoor air)
- US EPA: A brief guide to mold, moisture and your home
- ACCA: The 2016 update to ASHRAE 62.2 (whole-dwelling ventilation formula)