How an HRV or ERV should be sized, and why a Manual J doesn't do it

An HRV or ERV is sized by airflow: how many cubic feet a minute (cfm) of outdoor air it brings in, where that air goes, and what a meter reads once it's installed. A Manual J doesn't work out any of that. It's the right tool for a furnace or heat pump, and it matters for ventilation in a few places, but a quote that sizes the ventilator "from the Manual J" has skipped the steps that actually size it.

Why a Manual J doesn't size an HRV or ERV

Manual J is the Air Conditioning Contractors of America's method for working out a home's heating and cooling loads: how much heat, in Btu per hour, the house loses on a cold design day and gains on a hot one. Manual S then picks equipment to match, and Manual D designs the ducts that carry the heated or cooled air. The IRC requires heating and cooling equipment to be sized that way (section M1401.3). It's good practice. For ventilation, it misses on six counts.

1. It answers a different question. A Manual J measures heat; a ventilator moves air. How much outdoor air a home needs depends on the people breathing in it, not on the weather.
2. Ventilation goes into it; it doesn't come out. A Manual J needs to know how much outdoor air comes in, so it can count the heating and cooling that air adds. One load program we checked asks the designer for that number, and offers ASHRAE 62.2's as a suggestion. ACCA's own design educator says Manual J's fresh-air figure goes back to the building codes or ASHRAE 62.2. Someone still has to choose the airflow.
3. Its room numbers are a different kind of air. Manual J's room-by-room loads feed Manual D, which sizes the heating and cooling airflow to each room. That's indoor air going round and round. At ACCA's usual 350 to 450 cfm per ton, a 2½-ton heat pump moves about 875 to 1,125 cfm of it. A bedroom can get 100 cfm of warm air from its register and no outdoor air at all. Outdoor air is a far smaller, separate number: 60 to 120 cfm for a whole 3,000 sq ft, three-bedroom house, depending on the rule.
4. It counts people as heat, not breath. Manual J assumes one person per bedroom and two in the main bedroom, partly for the heat and moisture they give off and partly to set its fresh-air figure. It doesn't ask who actually sleeps where, or whether the door is shut. That's what decides whether a bedroom stays at the 800 ppm goal: about 17 cfm of outdoor air for each person sleeping there.
5. It's sized for the extremes; ventilation runs every hour. Manual J designs for the temperature that only 1% of the year's hours are colder than, and the one only 1% are hotter than. A ventilator runs around the clock at a steady rate, with a boost.
6. Bigger is a problem for a heat pump and a plus for a ventilator. Manual S keeps heating and cooling equipment close to the calculated load, because an oversized air conditioner hurts comfort and humidity control. For a ventilator, Energy Vanguard looks for one that can move about twice the continuous rate.

The same goes for the apps. Scan-to-Manual-J apps turn a phone or iPad scan into floor plans and loads, not a ventilation layout: our tools page lists which do what.

Where a Manual J does matter

• A new heat pump or furnace at the same time. Outdoor air is part of the load, and an HRV or ERV takes much of the heat back out of the stale air before it leaves. Ask that the Manual J use the real ventilation airflow and the unit you're actually getting, not a default.
• Humid summers. An ERV passes some moisture back, but it isn't a dehumidifier. The Manual J's moisture load shows whether the cooling equipment can handle what's left. HRV vs ERV has more.
• Sharing the furnace ducts. If the ventilator connects to the heating ducts, the duct design and the furnace fan come into it. Ask whether the furnace fan has to run whenever the ventilator does, and how much outdoor air each bedroom gets with its door shut. Do I need an ERV? sets out the choices.

What actually sizes an HRV or ERV

A good design works through six steps, in this order. Each one ends in a number you can ask to see.

1. The whole-home minimum

Codes and standards set one continuous airflow for the whole home, from its floor area and bedrooms:

• The IRC table (2024 IRC, Table M1505.4.3(1)): a 1,501 to 3,000 sq ft home with two or three bedrooms needs 60 cfm, for example.
• ASHRAE 62.2: 3 cfm for every 100 sq ft, plus 7.5 cfm per person, counting one more person than there are bedrooms. The current edition is 62.2-2025, and ASHRAE's list of its changes doesn't change this formula.

Which one applies depends on where you live and who's paying: a building department enforces its adopted code, and the federal weatherization program works to ASHRAE 62.2-2016. Ask the installer which rule and edition they design to. Neither number says how much reaches a bedroom: Why tight and new homes need a planned way to breathe explains why that matters.

2. A blower-door test, if the home has one

A blower door pulls air out of the house until it's 50 pascals below the outdoors and measures how much leaks back in: CFM50. Multiplied by 60 and divided by the home's volume in cubic feet, that gives ACH50, the air changes an hour at that pressure. It's not how much fresh air the house gets on an ordinary day.

ASHRAE 62.2 lets a tested home subtract an estimate of the outdoor air that already leaks in, its infiltration credit. The estimate comes from CFM50, the home's height and a local weather factor from the standard, and in a detached house with a balanced system, like an HRV or ERV, the whole estimate counts. So an energy audit's 62.2 number can be lower than the table above.

Two cautions. The credit is an average over a year for the whole house, and a shut bedroom on a calm night doesn't get it. And it's no reason to choose a smaller unit (step 5). The room planner takes an auditor's credited 62.2 number if you have one, and never lets it lower what the bedrooms need. Just made your home tighter? covers what to ask an auditor for.

3. Each room's share

Neither the IRC nor ASHRAE 62.2 says how much of the whole-home number each bedroom should get. Canada's approach does something like it: Ontario's code, following the CSA F326 standard, counts a minimum capacity of 10 L/s (about 21 cfm) for the main bedroom and 5 L/s (about 11 cfm) for each other bedroom. The Building America checklist for balanced systems asks for a ducted supply or return in each occupied room with a door.

HouseExhale plans each bedroom at 17 cfm of outdoor air per sleeper, about what it takes to hold a bedroom at 800 ppm according to an ASHRAE-funded 2025 review, and takes stale air from the kitchen, bathrooms and laundry in a matching amount. Take the 2,100 sq ft house above, with a couple in one bedroom and one person in each of the other two. The bedrooms get 35, 20 and 20 cfm (each rounded up to 5), the living areas share 40 more, and the night total comes to 115 cfm. That's more than either whole-home rule (60 or 93 cfm), because a closed bedroom has to be supplied on its own. By day, the whole-home minimum is enough. Getting a bedroom to 800 explains the per-person number.

4. Ducts and duct pressure

The ducts decide how much of a unit's airflow arrives. The more the fans have to push, the less they move: HVI, which certifies HRVs and ERVs, lists each unit's airflow at several duct pressures, and one 90-cfm-class ERV in its directory delivers 100 cfm at 0.1 inch of pressure but 55 cfm at 0.4 inch.

A designer adds up each run's length and fittings, works out the pressure the fans push against at the design airflow, and checks the unit's certified airflow at that pressure. Duct methods like Manual D, or the duct tables in ASHRAE 62.2, do that part of the job. HouseExhale checks a quoted unit at both 0.4 and 0.5 inch, for headroom.

5. The unit

The unit has to do three things: deliver the night total at the design duct pressure, run steadily at the daytime minimum, and recover heat well in cold weather (HVI lists heat recovery at 32°F). Going bigger is fine as long as it turns down to the daytime setting; Energy Vanguard aims for about twice the continuous rate. The units we've reviewed show certified airflow at each duct pressure.

6. Measure it after the install

A design isn't a delivery. In a 2020 ASHRAE Journal review, preliminary results found fewer than half (48%) of new Building America homes met ASHRAE 62.2 as installed (64% in Colorado), and most shortfalls came from installation or operation mistakes.

So the airflow gets measured: total supply and exhaust, and each grille, with a flow hood, a flow grid or the unit's own test ports, and supply and exhaust balanced to within about 10% of each other (Washington's code, for one). Where the 2021 energy code is in force, the system has to be tested, and whoever runs the test signs a written report for the building department (IECC R403.6.3). ENERGY STAR raters must measure it too; they can't take the design values on trust. Then your own monitor has the last word: after the install covers the airflow sheet and the recheck nights.

The measurements, and who makes them

Seven numbers a good quote shows

1. The whole-home minimum, with the rule and edition it comes from.
2. Fresh air to each bedroom, in cfm, on the normal setting.
3. The night total and the daytime setting.
4. The design duct pressure, and the unit's HVI-certified airflow at it.
5. Any blower-door credit, with the test result it came from.
6. A signed airflow sheet after the install, room by room.
7. If the heating or cooling is changing too: a Manual J that includes the ventilation airflow and the unit's heat recovery.

How to compare quotes has the ten questions to ask alongside these, and the room planner works out your own numbers to check a quote against.

My installer says they'll run a Manual J. Isn't that enough?

It's the right check for a furnace or heat pump, and worth having if one is part of the job. For the ventilator, ask what airflow they put into the Manual J and where that number came from: the code table, ASHRAE 62.2, or the bedrooms. Then ask how much of it reaches each bedroom, and for the unit's certified airflow at the duct pressure they expect. A Manual J report answers none of those by itself.

Can I size it from the house's volume, like 0.35 air changes an hour?

That's an older approach. EPA's archived page still cites ASHRAE's 0.35 air changes an hour, but not less than 15 cfm per person. It can give a very different number from today's rules: for a 4,000 sq ft, three-bedroom house with 8 ft ceilings, 0.35 air changes an hour is about 187 cfm, ASHRAE 62.2 gives 150 and the 2024 IRC table 75. Today's codes and ASHRAE 62.2 work from floor area and bedrooms. For a bedroom, the number of people sleeping there decides it.

Is a bigger HRV or ERV a problem?

Not if it can turn down. Unlike a heat pump, a ventilator with room to spare costs little in comfort: Energy Vanguard looks for one that can move about twice the continuous rate, for a boost setting, more efficient running below full speed, and room to raise the rate later. Check that it runs steadily at the daytime setting and that its certified airflow covers the night total at the design duct pressure.

Does a blower-door test mean we can buy a smaller unit?

It can lower ASHRAE 62.2's whole-home number, through the infiltration credit. It doesn't lower what a closed bedroom needs at night: about 17 cfm of outdoor air per sleeper. The credit is a yearly average for the whole house, and a shut bedroom on a calm night doesn't get it.