What your system can actually do about wildfire smoke

Tick what is true of your building. The plan splits into what to do while smoke is outside and what to sort out before the next season — plus the things that damage equipment and get recommended anyway.

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Build the plan

This is about what heating and ventilation equipment can and cannot do. It is not health advice — for that, follow your local public health guidance, which is written by people qualified to give it.

Tick everything that is true

The filter is a thin one, roughly an inch thick There is a deep filter cabinet, four inches or more Outside air comes in deliberately — a fresh-air duct, ERV or HRV It is a commercial rooftop unit with an economiser Ductless heads only, with washable filters The building is draughty — gaps at doors, windows or around the return Ducts or the return run through an attic, crawlspace or roof void There is at least one portable air cleaner

While the smoke is here

Do now

Tick something on the left and this fills in.

Change the filter for a fresh one of the best grade your system is known to carry, and change it again after the event — a loaded filter in smoke chokes airflow quickly. Do not jump several grades at once on a thin slot.
Fit the highest-rated cartridge the cabinet is rated for, and note the date. Deep cabinets are the one case where a high grade usually costs you little airflow.
Close the fresh-air damper, or put the ERV or HRV into recirculation, while smoke is outside. Put it back afterwards — a ventilation system left closed is a problem of its own in a tight building.
Have the economiser put to minimum outdoor air or closed, and write down that you did it and who did it. Economisers left shut after a smoke event are one of the commonest causes of a stuffy building in autumn.
Rinse and dry the head filters. Worth knowing: a ductless head recirculates room air and does not bring outside air in, so the smoke is arriving through the building, not through the unit.
Close what you can reach — trickle vents, window gaps, a door to an unconditioned garage. In a draughty building, leakage brings in far more smoke than the ventilation system does.
If the return runs through an attic or crawlspace and leaks, the system will pull air from there whenever it runs. Until that is sealed, running the fan continuously can make things worse rather than better.
Run the portable cleaner in the room people are actually using, with the door closed, on a setting you can live with. One machine sized for one room beats the same machine in a hallway.

Before next season

Sort out later

This fills in too.

Ask whether a deep filter cabinet will fit. More filter area is what lets you use a better grade without losing airflow, and it is a spring job, not an August one.
Have the static pressure measured with the cabinet loaded, so you know for certain what grade it can carry rather than assuming the highest one on the shelf.
Get a damper or a switch fitted somewhere you can actually reach. Plenty of houses have a fresh-air duct with no practical way to close it.
Have the economiser serviced and the damper proven to close fully. Agree in writing who changes the setting during an event and who puts it back.
Consider what you would do in a prolonged event. With no ducts, the realistic options are portable cleaners sized per room, or adding a small ducted or in-wall filtered supply — both are decisions to make calmly.
Seal the obvious leaks. It is the single most useful thing on this page, and it pays for itself in heating and cooling regardless of whether there is smoke.
Have the return duct sealed and tested. A leaky return in an attic undermines filtration, heating, cooling and every figure anybody quotes you.
Size it properly for the room and buy spare filters in spring. They sell out regionally within hours of a bad forecast.

Regardless of what is ticked — do not

Force a thick filter into a slot built for a thin one, or stack two filters. Both restrict airflow and neither seals properly.
Fit the densest filter on the shelf without knowing the static pressure. On some systems that drops airflow far enough to trip a furnace on its high limit.
Keep resetting equipment that is tripping out. It is telling you the airflow has changed.
Use an ozone generator or anything sold as an "air purifier" that deliberately produces ozone. Ozone is a lung irritant and this is not a close call.
Leave the economiser or fresh-air damper closed once the smoke has cleared. Somebody has to put it back.

Plainly

What this page is not

Not health adviceWe can tell you what a system can do to the air it moves. What that means for anybody's health is for public health guidance and for your doctor, and we are not going to pretend otherwise.
Not a filter recommendationA specific grade can only be recommended once somebody has measured the static pressure on your system. Anyone recommending one without that is guessing.
Not a substitute for the forecastAir quality forecasts and public guidance change through an event. This page does not.

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