Every August and September the Sacramento Valley fills with smoke from somewhere, and every year we get the same calls: which filter, should the fan run, is the house safer with the AC on or off. Most of the advice online is written for the whole country. This is what actually works in a Sacramento-area house, with its specific equipment: central systems in attics and closets, whole-house fans, and in the older neighborhoods, no ducts at all.
The short version
- Put a MERV 13 filter in the central system, if the system can take one. Most can. See below for the ones that cannot.
- Set the thermostat fan to ON during smoke events so air passes through the filter continuously, not only when the AC is cooling.
- Do not run the whole-house fan, the attic fan, or an evaporative cooler. They pull outdoor air in by design.
- If your system has a fresh-air intake, close it or set the ventilation to recirculate.
- Run a portable HEPA purifier in the bedroom, sized for the room, with the door closed.
- Change the filter early. Heavy smoke loads a MERV 13 in 30 to 60 days, not 90.
Why MERV 13, and why not higher
Wildfire smoke is mostly fine particulate, the PM2.5 that air quality alerts refer to. A standard one-inch fiberglass filter, the kind that comes with the house, catches almost none of it. A MERV 13 pleated filter catches most of it; the EPA’s guidance for home systems is MERV 13 as the target, and it describes that rating as the highest most residential systems can use safely.
The reason not to go higher is airflow. A denser filter is harder to pull air through. On a system with a generous return and a modern variable-speed blower, a MERV 13 is fine. On a 1960s ranch home in Carmichael or Citrus Heights with a single small return grille and a one-inch filter slot, a MERV 13 in a one-inch frame can starve the system of air: the coil freezes in summer, the furnace trips its limit switch in winter, and the blower motor works harder than it was designed to. We see this every fall.
The fix for those homes is not a lower-grade filter. It is a media cabinet: a four- or five-inch deep filter housing installed at the return, which holds a MERV 13 filter with several times the surface area of a one-inch one. Airflow stays where it should be, the filter lasts six months to a year in normal conditions, and the house is protected during smoke season without hurting the equipment. It is a one-visit installation on most closet and attic systems. See our indoor air quality page for what is involved.
Fan ON vs AUTO
On AUTO the blower only runs when the system is heating or cooling, so on a mild smoky evening in October the filter may not see any air for hours. On ON the blower runs continuously and the whole house’s air passes through the filter several times an hour. During a smoke event, ON is the right setting. It costs some electricity, and it is why we tell people to switch back to AUTO when the air clears. Our article on why a heating bill is so high explains what a blower running around the clock costs over a season.
The equipment that makes things worse
- Whole-house fans. Enormously popular in the Sacramento area because they work so well on summer nights, and exactly the wrong thing during smoke. They pull outdoor air through every open window and push it out through the attic. Keep it off and the windows closed until the AQI drops.
- Evaporative coolers. They cool by pulling outdoor air through a wet pad. The pad catches some smoke; most comes through. Switch to the air conditioner if you have both.
- Fresh-air intakes and mechanical ventilation. Homes built under recent Title 24 rules have a ventilation system that brings in outdoor air on purpose. Most have a switch or a controller; set it to off or recirculate for the duration. If you do not know whether your house has one, a newer tract home in Natomas, West Roseville, Folsom Ranch or Elk Grove probably does.
- Window units on “vent” or “fresh air.” Set them to recirculate.
Homes with no central system
A surprising number of homes in Land Park, East Sacramento, Oak Park and the older parts of Citrus Heights have a wall heater and window air conditioners, or a floor furnace and nothing else. There is no filter to upgrade. For those homes, portable HEPA purifiers are the whole answer: one in the room where people sleep, sized to the room’s square footage, doors and windows closed. A ductless heat pump, if one is on the horizon anyway, comes with its own filtration and turns the room it serves into a clean-air room during smoke season.
After the smoke clears
- Replace the filter even if it is not due. It has done a season’s work in a few weeks.
- Check the outdoor unit. Ash settles on the condenser coil and reduces its ability to shed heat. Rinsing it gently from the inside out with a garden hose, with the power off, is a reasonable homeowner job. Pressure washers bend the fins; do not use one.
- Switch the fan back to AUTO.
- If the house still smells of smoke after a filter change and a few days of normal running, the ducts may be holding it, particularly leaky ones that pulled smoky attic air in. That is a duct sealing question, not a filter one.
What we do
Media cabinet installation, filter sizing for systems that cannot take a one-inch MERV 13, ventilation controls on newer homes, and the fall tune-up that catches a smoke-loaded filter before it becomes a frozen coil or a tripped furnace. Same-day service across Sacramento, Roseville, Rocklin, Folsom, Citrus Heights and Elk Grove. Diagnostic is a flat $69, credited toward any work.
Frequently asked questions
What filter should I use for wildfire smoke?
MERV 13, which the EPA describes as the target for home systems and the highest rating most residential systems can use safely. If your system has a single small return and a one-inch filter slot, a one-inch MERV 13 can restrict airflow; a four- or five-inch media cabinet solves that.
Should I run my AC during wildfire smoke?
Yes, with the fan set to ON so air passes through the filter continuously, and with any fresh-air intake closed. Do not run a whole-house fan or an evaporative cooler; both pull outdoor air in.
Can I use a HEPA filter in my central HVAC?
Not in a standard residential system. True HEPA filters are too dense for a home blower to pull air through. Use MERV 13 in the central system and a portable HEPA purifier in the bedroom.
How often should I change the filter during smoke season?
Check it every two weeks and replace it when the media looks dark. Heavy smoke loads a MERV 13 in 30 to 60 days rather than the usual 90.
Why does my AC struggle after a smoke event?
Two common reasons: the filter loaded up and is restricting airflow, or ash settled on the outdoor coil and it cannot shed heat. Replace the filter and rinse the coil gently with a hose, power off. If it still struggles, call.