On a smoke day, your HVAC system is either the thing keeping the indoor air breathable or the thing pulling smoke inside. Which one depends on the filter, the fan setting, and whether the system has any connection to outside air. Most houses in the San Gabriel Valley and San Fernando Valley have a plain recirculating system with no fresh-air intake, and that is good news. Set it up right and it becomes a whole-house filter.
What smoke does to the air
Wildfire smoke is mostly very fine particles, the kind the air-quality index tracks as PM2.5. They are small enough to stay suspended for days and to get past a cheap filter. When the Santa Ana winds push a fire in the foothills, Pasadena, CA, Altadena, and the whole valley floor can sit under it.
The AQI at AirNow.gov tells you what is happening outside in near real time. The goal indoors is to keep outside air out and run the inside air through a filter that actually catches particles that size.
The filter is most of the answer
A MERV 8 filter, the standard pleated one, does little for smoke. It is built for dust and pollen. MERV 13 is the rating that catches a meaningful share of smoke-sized particles, and it is what the EPA points to for smoke events, with the caveat that the system has to be able to pull air through it.
That caveat matters. A 1-inch MERV 13 filter resists airflow considerably more than a 1-inch MERV 8. On a system with a small return, which describes most 1950s and 1960s ranch homes in Reseda and Northridge, that extra resistance can starve the coil and freeze it. If your system already struggled with airflow, do not add restriction during a smoke event and hope. The clean solution is a 4-inch media filter cabinet, which has enough surface area to run MERV 13 at roughly the resistance of a thin MERV 8.
Two more things:
- They load up fast. A filter that normally lasts three months can be visibly gray after a week of heavy smoke. Check it every few days and keep a spare on the shelf before fire season, not during it.
- Do not run without one. A bare coil collects the smoke residue instead, and that is a much harder cleaning job.
Fan settings
Your thermostat's fan has two settings: auto, where the blower runs only when heating or cooling, and on, where it runs continuously. During smoke, set it to on. The air in the house then passes through the filter all day, not just during cooling cycles.
Continuous fan uses more electricity and loads the filter faster. Some smart thermostats offer a circulate mode that runs the fan for a set number of minutes each hour, which is a decent compromise on lighter smoke days. Our thermostats page covers which ones do this.
Close every path for outside air
- Fresh-air intake. Newer homes and some remodels have a small duct from the return plenum to a hood on the roof or wall. If yours has a damper, close it. If it does not, tape it shut for the duration.
- Whole-house fans. Common in older San Gabriel Valley homes. Do not run one. It pulls outside air in through every open window at high volume, and the louvers in the ceiling leak attic air even when it is off. Make sure the louvers are closed.
- Evaporative coolers. They cool by moving outside air through wet pads. On a smoke day that is a smoke pump. Leave it off and close the vents.
- Window units. Most have a small lever or slider marked vent or fresh air. Set it to closed or recirculate.
- Mini splits. They recirculate room air only, so they do not bring smoke in. Their mesh filter is not fine enough to take smoke out, though. In a mini split room, add a portable HEPA cleaner.
- Rooftop package units. Common on flat-roofed homes and small commercial buildings in the San Fernando Valley. Many have an economizer, a damper that pulls in outside air when conditions look favorable. It needs to be at its minimum position or closed. If there is no manual control, that is a tech's job.
Portable air cleaners
A portable HEPA unit in the bedroom is the single most useful add-on for a smoke event, and it works alongside the central system rather than instead of it. Size it to the room using the clean-air delivery rate on the box.
Skip anything that advertises ozone, and be careful with ionizers, which can produce it. The California Air Resources Board keeps a list of certified air cleaners. Choose from it. For a longer-term answer, in-duct filtration on the central system is covered on our air purifiers page.
What not to do
- Do not open windows at night to cool the house unless the AQI has actually dropped. Smoke often settles overnight.
- Do not run the whole-house fan or the evaporative cooler.
- Do not run the system with no filter.
- Do not swap in a restrictive filter on a system with weak airflow without watching for ice on the coil or a drop in air at the registers.
- Do not use an ozone generator.
What you can check yourself
- Read the MERV rating printed on your current filter and look at it against the light. Replace it if it is gray.
- Set the thermostat fan to on.
- Look at the return plenum in the attic or closet for a small round duct heading toward the roof or an outside wall. That is a fresh-air intake.
- Check the whole-house fan louvers are closed.
- Set window units to recirculate.
- Check the AQI before opening anything.
When to call a licensed tech
- Airflow drops or the coil ices after you move to a higher-MERV filter. The system cannot handle it in that slot, and a media cabinet is the fix.
- A rooftop package unit's economizer needs to be set or checked.
- You want a 4-inch media cabinet or in-duct filtration installed before the next season.
- After a heavy smoke season. The coil and blower wheel collect residue that a filter change does not fix, and that is an HVAC maintenance visit.
- You smell smoke from the registers when the system runs on a clean day. That is a return leak pulling attic air, and it means the system was pulling smoke during the event too.
Common questions
Should I run the AC during wildfire smoke?
Yes, as long as the system is closed to outside air and has a decent filter. Running it filters the air. Turning it off just lets the house heat up while the particles inside stay put.
What is the right filter for wildfire smoke?
MERV 13 if your system can pull air through it, ideally in a 4-inch cabinet. A true HEPA filter does not belong in the furnace filter slot. It is far too restrictive for a residential blower, and it needs its own bypass unit.
Do mini splits filter smoke?
Not meaningfully. Their filters are a coarse mesh meant to protect the coil. They do keep outside air out, so pair them with a portable HEPA cleaner in the rooms you use most.
If you want your system checked and set up before the next fire season, or the coil cleaned after this one, we do a $69 diagnostic, credited toward the work. Boost Services Heating and Cooling answers at 818-277-5056 between 6am and 10pm, seven days, or contact us.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.