Climate and comfort · independent publisher, not a contractor
Wildfire smoke filtration on a new AC
EPA advises a high-efficiency HVAC filter, such as MERV 13, or the highest rating your system fan and filter slot can accommodate, plus recirculation and running the fan "On" during smoke events. A new installation is the best time to fit a filter rack and return that can take MERV 13 without starving airflow.
Reviewed · 6 named sources · No named reviewer yet; see our editorial policy.
What EPA recommends
- "Choose a high-efficiency HVAC filter, such as one with a Minimum Efficiency Reporting Value (MERV) 13 rating, or as high a rating as your system fan and filter slot can accommodate."
- Make sure the filter fits snugly; do not bend or crush it.
- If the system has a fresh-air intake, close it or set recirculate.
- With a high-efficiency filter, set the fan to "On" instead of "Auto" while air quality is poor.
- Replace filters more often during heavy smoke.
EPA, Wildfires and Indoor Air Quality, read 2026-10-20.
What MERV 13 actually catches
MERV is a test rating from 1 to 16. EPA's air cleaner guide says MERV 13 and higher filters must show at least 50 percent removal of the smallest particles in the test, the range that includes much of the fine smoke particle mass. Higher ratings remove more at most particle sizes. A single pass is not the point: with the fan on, the house air goes through the filter many times an hour.
| Filter | Smoke use |
|---|---|
| MERV 8 (typical builder filter) | Catches dust and lint; limited on fine smoke |
| MERV 11 | Better; still below EPA's suggested level |
| MERV 13 | EPA and South Coast AQMD's suggested minimum for smoke |
| MERV 14–16 | More capture, more resistance; only if the system is designed for it |
EPA, Guide to Air Cleaners in the Home, read 2026-10-26.
Why it matters at installation
A denser filter adds resistance to airflow. EPA notes you "may need to consult a professional HVAC technician" to find the highest filter your system can take. Too much resistance and the fan moves less air, the coil can run too cold, and the system loses capacity on exactly the hot, smoky days you need it.
The fix is surface area. A 1-inch MERV 13 filter jammed into a small return grille restricts far more than a 4- or 5-inch media filter in a cabinet, or two return grilles instead of one. On a new system the installer can size the return and filter rack for that, then measure total external static pressure at start-up and compare it with the fan's rating.
Ask for it in the quote
"Filter rack sized for MERV 13; static pressure measured and recorded at start-up." The Title 24 airflow check, where it applies, is a second reason to get this right (testing guide).
By system type
| System | Smoke set-up |
|---|---|
| Central AC or ducted heat pump | MERV 13 media filter if the fan and slot allow; recirculate; fan On during events |
| Ductless mini-split | Heads recirculate room air through their own filters; add a portable air cleaner sized for the room |
| Window AC | Close the outdoor air damper; if it cannot close, EPA says not to use it during smoke |
| Evaporative cooler | EPA: cover intakes with 4-inch MERV 13 filters or use sparingly; AQMD: avoid if you can stay cool without it |
Portable and in-duct air cleaners
South Coast AQMD suggests a portable high-efficiency or HEPA air cleaner in one room with doors and windows closed, usually a bedroom. That room becomes the clean-air refuge if the central system cannot keep up.
California adds a rule. Electronic air cleaners sold in the state must be certified by CARB for electrical safety and ozone emissions of 0.050 ppm or less, and since October 2020 that includes electronic in-duct devices an HVAC contractor might offer. CARB says certification does not measure how well a device cleans. EPA warns that ozone generators sold as air cleaners are a lung irritant and no federal agency has approved them for occupied spaces. If a quote adds an ionizer or "air purifier" to the ductwork, ask for its CARB listing.
CARB, certified air cleaning devices and South Coast AQMD smoke tips, read 2026-10-26.
Knowing when to switch modes
South Coast AQMD issues smoke advisories for large fires in or near the basin, posts them at its advisories page, and sends them by email or through its app. Its AQI map and AirNow give current readings. Set up the alert before fire season, then treat an advisory as the cue to close windows, set recirculate and switch the fan to On.
- Sign up for South Coast AQMD air alerts or install its app.
- Keep two spare MERV 13 filters in the right size before summer.
- During an advisory: windows shut, fresh-air intake closed, fan On, portable cleaner running in the bedroom.
- After the event: check the filter and replace it if it is grey.
Filter care afterwards
Change filters when they look dirty, which in smoke can be far sooner than the maker's interval. A clogged filter raises static pressure just like an undersized one. Routine care is in the new AC maintenance schedule.
Sources
- US EPA, "Wildfires and Indoor Air Quality" (retrieved 2026-10-20)
- US EPA, "Guide to Air Cleaners in the Home" (retrieved 2026-10-26)
- South Coast AQMD, "Wildfire Smoke & Ash Health & Safety Tips" (retrieved 2026-10-26)
- South Coast AQMD, "Sign Up for Air Alerts" (retrieved 2026-10-26)
- California Air Resources Board, "List of CARB-Certified Air Cleaning Devices" (retrieved 2026-10-26)
- Energy Code Ace (CEC-supported), Single-Family Compliance Manual ch. 9, § 150.2(b)1E–F, 2022 code text; confirm against the 2025 code in force since 1 January 2026 (retrieved 2026-09-29) — verify before relying on it
Questions
Can any AC take a MERV 13 filter?
Not necessarily. It depends on the fan and filter slot. EPA suggests asking an HVAC technician; a new installation can be designed for it with a deeper filter or a larger return.
Should the fan run all the time during smoke?
EPA says to set the fan to On instead of Auto while air quality is poor, if a high-efficiency filter is installed.
Does a mini-split filter smoke?
Its heads recirculate room air through their own filters. EPA recommends portable air cleaners sized for the room as an addition.
Is a thicker filter better than a 1-inch one?
For the same MERV, a deeper filter has more media area and usually less resistance, so it can filter well without choking airflow.
Are in-duct ionizers allowed in California?
Electronic in-duct air cleaners sold in California must be CARB-certified for ozone emissions and electrical safety. Certification does not prove they clean the air well.
Next step
Free quote request
Get a quote to check against these numbers
- Tell us about the job. Service, ZIP, property and timing. The form says right away if your ZIP is outside the area.
- We check the license first. Requests go only to contractors whose CSLB C-20 license we have matched on the state register. If none covers your ZIP yet, we email you and say so.
- Compare what comes back. Line the written quote up against the published ranges and the quote checklist.
No cost and no obligation. We are a publisher, not a contractor; contractors pay us for introductions. How that works.
Request AC installation quotes
Licensed Los Angeles HVAC contractors. Free, no obligation.