Smoke season here usually means August and September. In September 2022, for example, smoke from the Bolt Creek Fire near Index and Skykomish drifted across the region, and the Puget Sound Clean Air Agency forecast moderate to unhealthy-for-sensitive-groups air in King, Pierce and Snohomish counties. A forced-air furnace acts as a large air filter whenever its fan runs, so a smoky week can leave a lot behind in that filter.
Then fall arrives, the heat comes on, and the same filter may still be in the slot. If you did not change it after the last smoke event, November is a good time to do it.
No ducted system? Wall heaters and baseboards have no furnace filter to change, and a ductless mini-split usually has a washable screen rather than a MERV-rated filter. For those houses, our guide to setting up a clean room and box-fan filter is the better place to start.
When to change it
- During the smoke. EPA says to plan on replacing the filter more often than the manufacturer recommends and to change it when it looks dirty. The state's Department of Health guidance says to check it at least monthly in heavy use, and more often in heavy or prolonged smoke.
- As soon as the outdoor air clears. Swap in a clean filter while you air out the house.
- Before the first cold snap, if the filter has been in since summer.
- A few days later, and again in a month. A filter keeps catching what settled in the house and the ducts. How fast it darkens tells you a lot.
Filters can be hard to find during a smoke event, so buy spares now and keep them in a dry closet.
Which filter to buy
- Rating. EPA and the Department of Health both point to MERV 13, or as high as your system can accommodate. Some stores use a different scale, where FPR 10 is the rough equivalent.
- Size. Read the numbers printed on the old filter's frame, and check the depth as well as the width and height. EPA says the filter should fit snugly and should never be bent or crushed to fit.
- Airflow. Higher ratings can restrict air, especially in a 1-inch slot. If you are not sure, ask a technician. A deeper media cabinet may be the better long-term answer.
- Carbon. A carbon layer can help with some gases, but it does not replace a good particle rating.
Step by step
- Turn the system off at the thermostat, then at the switch on or near the furnace.
- Put on an N95 respirator and gloves, and open a trash bag where you can reach it. The old filter holds fine smoke particles and sometimes ash.
- Find the filter. Look at the furnace cabinet, which in many homes here is in a garage, closet or basement, or behind a large return grille on a wall or ceiling. Some houses have more than one.
- Note the airflow arrow and the size before you move it.
- Slide it out slowly, keeping the dirty side up so it sheds as little as possible.
- Photograph both faces with a note showing the date, then bag and seal it. If you may file a claim, label it and keep it. The insurance commissioner's office says not to throw anything away until an adjuster has completed the inspection, and the filter shows how far the smoke traveled.
- Wipe the slot with a damp cloth. Do not reach into the blower.
- Slide in the new filter with the arrow pointing toward the furnace, and close the panel fully.
- Restore power and write the date on the frame.
- Listen and feel. Whistling, weak airflow at the registers or a furnace that shuts off early can mean the filter is too restrictive. Go down a rating and call a technician.
While the air is still smoky, EPA says to set the fan to On, not Auto, and to close any fresh-air intake or set the system to recirculate. Many newer houses have a ventilation setting that pulls in outside air, so learn where yours is. When the air clears, reverse it: open the intake, open the windows and run the kitchen and bath fans to air the house out.
What a grey filter tells you
Filter color is a rough clue, not a test.
- Tan or light grey with lint and pet hair is ordinary household dust.
- An even, darker grey, sometimes with a smoky smell, at the end of a smoke event is what you would expect. The system moved smoky air and the filter did its job. On its own, it does not tell you whether surfaces in the house hold residue.
- Dark streaks around the edges, or a dirty clean side, suggest air is slipping around the filter. Check the size and the fit.
- A new filter going grey within days after the air has cleared is the one that matters. Something in the house or the ducts is still shedding particles.
- Black, oily soot is not wildfire smoke. A furnace or stove problem can cause it, so read about puff-back and furnace soot and have the appliance serviced before it runs again.
Typical costs around Puget Sound
These are typical Puget Sound ranges, not quotes, and they vary by store, size and season.
- 1-inch MERV 13 filter: roughly $15 to $35 each.
- 4- or 5-inch media filter: roughly $30 to $80 each.
- Filter with a carbon layer: roughly $25 to $70 each.
- HVAC technician to check airflow: commonly $100 to $250.
- Adding a deeper media cabinet: commonly $300 to $900 installed.
- Duct cleaning: commonly $300 to $1,000, depending on the system.
- Professional cleanup after a bad smoke week: roughly $1,000 to $5,000, as our smoke damage cost guide explains.
Some policies respond to wildfire smoke damage and some claims turn on the details, so ask your agent before assuming either way. If you file a covered claim and use a restoration contractor, it is $0 upfront on covered claims - insurer billed directly - deductible applies.
When to call a professional
Call an HVAC technician when the furnace struggles with a higher-rated filter or you are not sure what your system can take. Call a restoration crew when:
- A new filter turns grey within days after the outdoor air has cleared.
- Ash or residue is at the registers, on window sills or on floors.
- The smoke smell comes back every time the heat runs, or stays more than a day or two after you air out the house. See odor removal.
- Buildings burned nearby, so the residue is more than wood smoke. See smoke and soot cleanup.
- Someone in the household has asthma or a heart or lung condition and the house still smells.
- Old ducts are wrapped in white paper-like tape that is torn or crumbling. It may contain asbestos, so leave it alone and have it looked at.
Wildfire smoke cleanup explains how ducts, surfaces and textiles are handled and how the work is documented. Dispatch covers King and Snohomish counties, along with Pierce and Kitsap, any hour at (425) 842-3483, or text (844) 543-5757.
