Whole-Home Ventilation
Filtering cleans the air you already have. Ventilation replaces it. If your house fills with cooking fumes, humidity and off-gassing faster than it clears them, no filter is going to solve that.
A filter cannot remove what a filter cannot catch
Filtration is excellent at particulate. It does essentially nothing about the things that accumulate in a closed house — and those are what make indoor air feel stale rather than dusty.
Carbon dioxide
People. It climbs steadily in a closed bedroom overnight and is a well-documented cause of stuffiness and poor sleep.
Cooking emissions
Gas burners in particular, which produce combustion by-products directly into the room you are standing in.
Humidity
Showers, cooking, laundry and breathing. With nowhere to go it condenses on the coldest surfaces and feeds mould.
Off-gassing
Furniture, flooring, paint, cleaning products and everything else that quietly releases compounds indoors for years.
The only thing that reduces any of these is diluting them with outdoor air. Older homes did that accidentally, through gaps. Every efficiency improvement since has closed those gaps — which is why the code now requires ventilation on purpose.
The Title 24 indoor air quality fan
California requires low-rise homes to have whole-dwelling mechanical ventilation supplying a calculated minimum of outdoor air. Most people meet this requirement without knowing what it is.
1 CFM per 100 square feet, plus 7.5 CFM per occupant
Occupants are counted as the number of bedrooms plus one — so a three-bedroom home counts as four people regardless of who actually lives there.
- The fan is intended to run continuously, not on demand.
- It must be quiet enough to leave running — rated at a maximum of one sone.
- Its switch has to be labelled, so nobody switches it off assuming it is an ordinary extractor.
- Kitchens, bathrooms and laundries need local exhaust in addition to this.
If you have ever found a bathroom fan running constantly in a newer home and turned it off wondering why it was on — that was almost certainly the ventilation system, and switching it off defeated the only deliberate fresh air supply the house had.
Three ways to do it, and what each costs you
All three meet the requirement. They differ enormously in what happens to the energy you have already paid for, and in what the incoming air passes through on the way in.
Exhaust-only ventilation
A continuously running fan pushes indoor air out. Replacement air finds its own way in through whatever gaps exist.
In its favour
The cheapest way to meet the requirement, and considerably better than nothing.
The trade-off
You throw away the energy in the air you exhaust, and the incoming air arrives unfiltered through whatever gap it finds — including on smoke days.
Balanced with heat recovery (HRV)
Supplies and extracts equal volumes, passing them through a heat exchanger so outgoing air pre-conditions incoming air. The two streams never mix.
In its favour
Recovers the large majority of the energy. Incoming air is ducted and filtered rather than pulled through gaps.
The trade-off
Higher installed cost, and it needs ducting for both supply and extract.
Balanced with energy recovery (ERV)
The same, but the exchanger transfers moisture as well as heat, using an enthalpy core.
In its favour
Generally the better fit in a dry climate, because you are not exhausting all of your indoor humidity in summer.
The trade-off
Same cost and ducting considerations as an HRV. The right choice between the two depends on the house.
What happens on a smoke day
This is the argument that matters most in Los Angeles, and it is the one that separates the cheap approach from the good one.
An exhaust-only fan works by pushing air out, which slightly depressurises the house. Replacement air is pulled in through every gap in the building — around windows, through the attic hatch, past the floor. None of it is filtered. On a bad air day that is precisely the wrong thing to be doing.
A balanced system supplies and extracts equal volumes through ducts, which means the incoming air arrives through a known path — and passes through a filter first. Typical practice is around MERV 13 on the supply side, with a coarser filter on the extract side to protect the heat exchanger.
The difference is between deliberately introducing filtered fresh air, and depressurising your house so unfiltered smoke gets pulled in through the cracks. Both meet code. Only one of them is what you want in September.
Ventilating is step five of five
Each step makes the next one work. Done out of order, you spend more and get less. See the whole process.
- 1 Identify
Measure before buying anything, so you treat the problem you actually have.
Air quality testing → - 2 Remove
Clear genuine contamination out of the ductwork, where conditions warrant it.
Duct cleaning → - 3 Seal
Close the route unfiltered attic air uses to get back into the system.
Duct sealing & replacement → - 4 Filter
Filtration that works, because the air reaching it is the air you meant to treat.
Whole-home purification → - 5 Ventilate
Dilute what filters cannot catch by replacing indoor air with outdoor air.
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Related reading
HRV vs ERV: Which One Makes Sense in a Dry Climate?
Most HRV vs ERV advice is written for Canada. The answer is different in a hot, dry climate — and so is the plumbing.
Read the article →Why Is My House So Dusty Even After Cleaning?
Dust that returns within days is usually being pulled in through leaking ducts, not settling out of the room.
Read the article →The 'Static Pressure' Secret: Why Your HVAC System is Suffocating
The most common reason an AC runs all day without cooling has nothing to do with refrigerant.
Read the article →Ventilation FAQs
What is the Title 24 indoor air quality fan?
California requires low-rise homes to have whole-dwelling mechanical ventilation delivering a calculated minimum of outdoor air. It is designed to run continuously rather than on demand, must be quiet enough to leave on, and its switch has to be labelled so nobody turns it off by mistake.
How much ventilation does Title 24 require?
The calculation is one CFM for every hundred square feet of floor area, plus seven and a half CFM per occupant. Occupants are counted as the number of bedrooms plus one, so a three-bedroom home is treated as four people regardless of who lives there.
What is the difference between an HRV and an ERV?
Both bring in fresh air while recovering energy from the air leaving. An HRV transfers heat only. An ERV also transfers moisture using an enthalpy core, which generally suits a dry climate better because you are not exhausting all of your indoor humidity.
Can I just turn the ventilation fan off?
You can, and many people do without realising what it is. That is precisely why code requires the switch to be labelled. Turning it off saves a trivial amount of electricity and removes the only deliberate fresh air supply the house has.
Does a ventilation system help with wildfire smoke?
A balanced system does, because incoming air is ducted through a filter before it enters. An exhaust-only fan does the opposite on a smoke day, since it depressurises the house and pulls unfiltered outside air in through every gap in the envelope.
Do I need ventilation if I already have a good air filter?
They solve genuinely different problems. A filter cleans the air already inside the house. It does nothing about carbon dioxide, humidity or the compounds that furniture and finishes release. Only bringing in outdoor air dilutes those, which is why filtration on its own plateaus.
Is my older home too leaky to need ventilation?
Possibly, and that is genuinely worth measuring rather than assuming either way. Older homes often did ventilate by accident. But every efficiency improvement tightens the envelope, so a house that was adequately leaky twenty years ago may well not be any longer.
How noisy is a whole-house ventilation system?
It should be barely noticeable, and code sets a limit precisely because a system people can hear is a system people switch off. A properly selected and correctly ducted unit is quiet enough that most households forget it is running.
Find out what your house actually does
We measure how much your home ventilates on its own before recommending anything. Some need a system. Some already have one nobody realised was there.
Sources
The code requirements and thresholds on this page come from the following. All are public — check them yourself.
California Energy Commission
Indoor air quality and mechanical ventilation requirementsWhole-dwelling ventilation rate, continuous operation and sound limit.
ASHRAE
Standard 62.2 — Ventilation and Acceptable Indoor Air Quality in Residential BuildingsThe standard Title 24 adopts for residential ventilation.