Two terms, one for the house and one for the people
The arithmetic in wide use adds a rate proportional to floor area to a rate proportional to occupancy. The area term stands in for everything the building itself emits — finishes, furnishings, materials — which scales roughly with how much house there is. The occupancy term covers what people emit, which scales with how many of them there are. A 2,000 square foot house with three bedrooms comes out at 0.03 times 2,000, which is 60 CFM, plus 7.5 times four occupants, which is 30, for 90 CFM total.
Occupancy is normally taken as bedrooms plus one rather than as who actually lives there, because ventilation is a property of the house and outlives its current occupants. The override field exists for cases where you know better, such as a house that will be occupied well above that assumption.
What rate applies to your project is decided by the code your area has adopted and by the manufacturer instructions for the equipment, with the AHJ as the arbiter. The numbers defaulted here are the common ones, offered as arithmetic rather than as a determination.
Tight houses did not create the need, they revealed it
A leaky old house exchanges a lot of air, but it does so at rates that depend on wind and on the temperature difference, so it over-ventilates on a cold windy night, when you are paying to heat every cubic foot of it, and barely ventilates at all on a still spring afternoon. The exchange is also unfiltered and it comes from wherever the leaks are: a crawlspace, an attached garage, a wall cavity.
Air sealing removes that accidental exchange, which is the point, and it makes the deliberate exchange necessary. Mechanical ventilation is not a penalty for building tight; it is what replaces something unreliable with something you can size, filter and control. The rate here is small compared with what a leaky house does on a bad day: 90 CFM in a 17,000 cubic foot house is about 0.32 air changes per hour, which is in the same range as the infiltration figure people plug into a heat loss calculation for an average existing house.
Three ways to move the air, and what each costs you
| Approach | House pressure | Main trade |
|---|---|---|
| Exhaust only | Slightly negative | Cheapest to install; makeup air arrives uncontrolled and unfiltered |
| Supply only | Slightly positive | Incoming air can be filtered; pushes indoor moisture into assemblies in cold climates |
| Balanced HRV | Neutral | Recovers heat; no moisture transfer; two duct systems to install |
| Balanced ERV | Neutral | Recovers heat and some moisture; most expensive; frost control in cold climates |
The safety point sits with exhaust-only and it is worth stating plainly rather than burying in a footnote. Pulling air out of a house lowers its pressure relative to outside, and the makeup has to come from somewhere. If the easiest path runs past a naturally vented flue — a water heater, a furnace, a fireplace — the house can pull combustion products back down that flue into living space. Carbon monoxide is odourless and the failure mode is fatal. Any work involving a fuel-burning appliance, its venting or its combustion air supply is licensed work. Name the hazard, and hand it to someone qualified.
The energy penalty, and what recovery does about it
Ventilation air has to be brought to indoor temperature, and the sensible cost is the familiar 1.08 times CFM times temperature difference. Ninety CFM at a 60 degree difference is about 5,830 BTU per hour, which is a real line item on a design day and, in a well-insulated house, can be a noticeable fraction of the whole heating load.
Heat recovery attacks that directly by passing the outgoing and incoming streams through an exchanger. Seventy percent sensible recovery turns that 5,830 into about 1,750. The caveats are worth knowing: recovery ratings are measured at particular airflows and conditions and drop away from them, cold-climate frost control periodically interrupts or preheats and costs some of the recovery back, and the fan power to run two air streams is a continuous electrical load that partially offsets the heating saving. For the running cost side, the heating cost guide and the heating cost savings calculator take a BTU figure to money, and the home energy audit guide explains where to spend first.
Run time, and why a straight ratio understates it
If a fan moves more than the target, it does not have to run continuously, and the naive answer is to run it for the fraction of the hour that delivers the same volume: 90 CFM wanted from a 120 CFM fan is 75 percent, or 45 minutes an hour. That arithmetic is what this page reports, and it is not the whole story. Contaminants accumulate during the off period, so a scheme that runs hard for ten minutes an hour is not equivalent to one that runs steadily at the same average. Standards that permit intermittent operation apply a multiplier that grows with the length of the off cycle, and the multiplier belongs to whichever standard your area has adopted.
Also note that the fan airflow to use is what the fan delivers at the static pressure of its installed duct, not the free-air number on the box. A long flex run with a couple of elbows and an exterior hood can take a meaningful bite out of it; the duct static pressure calculator covers how that resistance adds up, and the duct size calculator handles sizing the run. Whole-house ventilation is also separate from local exhaust: kitchen and bathroom fans handle point sources of moisture and odour and their requirements are their own, as is attic ventilation, which serves the roof assembly rather than the living space.
Questions people ask
How much ventilation does my house need?
The arithmetic in common use adds a floor area term to an occupancy term: around 0.03 CFM per square foot of conditioned area plus around 7.5 CFM per occupant, with occupancy counted as bedrooms plus one. A 2,000 square foot three-bedroom house lands near 90 CFM continuous on that basis. Whether that is what you are required to provide depends on the code adopted where you build and on the equipment manufacturer instructions, and the AHJ decides. Treat the number here as the calculation, not the ruling.
ERV or HRV?
The difference is moisture. An HRV transfers heat between the two air streams and nothing else, so in winter it brings in dry outdoor air and can leave a house drier than you want. An ERV transfers some moisture as well, so it retains indoor humidity in winter and rejects some incoming humidity in summer. In hot humid climates the summer behaviour is the strong argument for an ERV; in very cold dry climates the winter behaviour usually is too. HRVs are cheaper and simpler and there are climates and houses where they are the better fit. Neither is a dehumidifier, and neither will fix a humidity problem that has a source.
Can I just crack a window?
It ventilates, but not in any way you can size or rely on. Flow through an open window depends on wind speed, wind direction and the temperature difference, so it varies by an order of magnitude across a week and goes to nearly nothing on a still mild day, which is often when the house feels stuffiest. It is also unfiltered, uncontrolled in winter, and a security consideration. The case for mechanical ventilation is that it delivers a known rate continuously, at a cost you can calculate in advance, and can recover most of the heat if you pay for the equipment that does so.
Does the bathroom fan count towards the whole-house rate?
It can, in some schemes, if it is a fan rated for continuous duty, quiet enough to leave running, and controlled to run on the required schedule. Ordinary intermittent bath fans are a different device serving a different purpose: they remove a burst of moisture at the source, quickly, and then stop. Using one as the whole-house ventilator means it runs for hours a day, so sone rating and rated life stop being marketing details and start being the specification. Local exhaust requirements for bathrooms and kitchens exist alongside the whole-house rate rather than instead of it.
Will ventilation make my house too dry in winter?
It can, and this is the honest weakness of ventilating a tight house in a cold dry climate. Outdoor air at low temperature holds very little moisture, so bringing it in and warming it produces low indoor relative humidity even though you have added no drying equipment. An ERV mitigates it by returning some moisture from the outgoing stream. If the result is still too dry, the answer is a humidifier sized to the actual moisture deficit rather than reducing the ventilation rate, and the humidifier sizing calculator works out what that deficit is in gallons per day.