Yes, run it. On a smoky day, keep your windows and doors closed and run your central air conditioner or heat pump with the fan set to “On” and any fresh-air intake closed or switched to recirculate. The system pulls indoor air through its filter and sends it back, so the filter is what cleans the air.
The short version (EPA guidance for smoke events):
- Keep windows and doors closed. Open them briefly when the outdoor air improves.
- Central AC or heat pump: fan on “On,” fresh-air intake closed or on recirculate.
- Use a MERV 13 filter, or the highest rating your system can handle, and replace it sooner in heavy smoke.
- Window AC: close the outdoor-air damper and seal the unit, or don’t use it.
- Check your local air quality on AirNow before you open anything up.
What to do with a central AC or heat pump
A typical central system takes air from inside the house, runs it across the coil and through the filter, and returns it to the rooms. That is why a closed-up house with the system running is the best place to be on a bad air day. Smoke only gets in where outdoor air does: open doors and windows, leaky seals, and a fresh-air intake if your system has one.
Three settings matter:
- Fan “On,” not “Auto.” Auto stops the fan when the house reaches temperature, and the air stops being filtered. EPA recommends running the fan continuously while air quality is poor.
- Recirculate. If your system has a fresh-air intake or ventilation damper, close it or switch to recirculate for the duration of the smoke.
- Doors and windows shut. When the outdoor air improves, even for an hour, open up and air the house out.
The filter does the work: MERV 13
MERV is the rating scale for how well a filter catches particles. Smoke is mostly very fine particles, which low-rated filters let through. EPA recommends a MERV 13 filter, or the highest rating your system can accommodate, fitted snugly so air can’t slip around the edges.
“Highest your system can accommodate” is the part people skip. A denser filter pushes back harder on the blower. Some systems, especially older ones with a thin filter slot, run short of airflow with a thick high-MERV filter, and that can hurt cooling or heating. If you aren’t sure what yours takes, ask a licensed technician before you buy. Our guide to MERV and HEPA filters compares the two.
In heavy smoke a filter loads up fast. Check it more often than you normally would and replace it when it looks gray and clogged. A clogged filter cuts airflow, and that wears the equipment.
Window units, portable units and evaporative coolers
Not every way of cooling the house handles smoke the same way.
| Type | What EPA says | What it means in practice |
|---|---|---|
| Central AC or heat pump | Run with fan on, fresh-air intake closed or on recirculate | Best option when the filter is right for the system |
| Window AC | Close the outdoor-air damper and seal the unit to the window; if the damper won’t close, don’t use it | Gaps around the frame leak smoke. Use a fan or move to another room instead if you can’t seal it |
| Single-hose portable AC | Use sparingly | It exhausts indoor air through the hose, so the house pulls replacement air in from outside |
| Evaporative (“swamp”) cooler | Use sparingly | It works by bringing outside air in. Covering the intakes with MERV 13 furnace filters is one EPA suggestion |
We install and service central systems, heat pumps and ductless mini-splits, not window units, so we can’t tell you which window model to buy. What we can tell you is how the types differ, and our guide to air conditioner types covers the basics.
Portable air cleaners, and what to avoid
A portable air cleaner in the room where you spend the most time adds a second layer of cleaning. EPA says to pick one sized for the room, run it on high during smoke, and make sure it does not produce ozone. EPA also points to CARB’s list of certified devices that emit little or no ozone. Ozone generators are a different thing entirely: CARB strongly advises against using them at home because ozone harms the lungs.
A box fan with furnace filters taped to it is a workable stopgap, and EPA has evaluated common designs. A commercial unit with a known clean-air delivery rate is preferred.
Check the air before you open up
Smoke can reach Santa Rosa from fires inside the county and from fires far away; EPA notes that smoke can arrive from thousands of miles away. Look up your ZIP code on AirNow before you decide whether it is safe to air the house out, or whether to stay sealed up and keep the filter running.
After the smoke: what to check and when to call
When air quality recovers, replace the filter and open the house up. EPA says duct cleaning is only needed if the ducts are clogged with dust and debris or particles are blowing out of the registers. It does not recommend routine duct cleaning after smoke.
Call a licensed technician if the airflow from your vents is weak after a smoke event, if a new filter does not restore it, or if the system runs but won’t cool or heat. Gas furnaces, refrigerant, electrical controls and the inside of the cabinet are work for a licensed contractor; the filter and the thermostat settings are the parts you can safely handle yourself.
If you want better filtration than a standard filter can give, a whole-home air filtration system treats all the air your system moves. Our page on air filtration systems for homes and businesses covers what they do. For help with a system that is struggling, see our air conditioning and heating services.
Dale Heating Cooling & Sheetmetal has served Sonoma County since 1993 and holds California licenses C-20 and C-43 (#952649). Call (707) 526-5709 or use our contact page and we will arrange a visit to look at your filter, your airflow and your filtration options.