How to Keep Wildfire Smoke Out of Your House

How to Keep Wildfire Smoke Out of Your Fremont Home

To keep wildfire smoke out of a Fremont home, close windows and doors, set your HVAC fan to run continuously on recirculate, and use a MERV 13 or higher filter. Turn off any fresh-air intake or whole-house fan. A portable HEPA purifier or a DIY box fan filter in the bedroom adds a clean room for sleeping.

Before smoke season

Buy two or three MERV 13 filters in your system’s size and keep them on hand
Ask about a four-inch media filter cabinet if your system only takes one-inch filters
Seal obvious gaps around doors and windows with weatherstripping
Pick one room, usually a bedroom, to be your clean room

During a smoke event

Check the Air Quality Index at AirNow or the Bay Area Air District. When it climbs above 100, close up the house. Set the thermostat fan to “On” rather than “Auto” so air passes through the filter all day. If your system has a fresh-air damper or ERV, close it or set it to recirculate. Avoid frying food, burning candles or vacuuming without a HEPA vacuum, which all add particles indoors.

Homes in Niles, where smoke can settle into the canyon, and on the Mission San Jose hillsides often see the highest readings in town. A clean room with a HEPA purifier makes a big difference for sleep.

The DIY box fan purifier

Tape a 20-inch MERV 13 filter to the intake side of a 20-inch box fan, with the airflow arrow pointing toward the fan. It is not pretty, but studies of this setup show it removes a meaningful share of fine particles from a single room. Four filters taped into a cube around the fan work even better.

After the smoke clears

Replace the HVAC filter, even if it has only been a few weeks. Smoke loads filters quickly. If you notice lingering odor from the vents after several heavy smoke seasons, a camera inspection will show whether duct cleaning is worth it.

Questions we hear often

Does my AC bring smoke inside?

A standard central AC recirculates indoor air and does not pull in outside air. Window units and systems with fresh-air intakes can, so close those dampers.

Is MERV 13 too restrictive for my furnace?

A thin one-inch MERV 13 can be on some systems. A four-inch media filter has more surface area and works with most residential systems.

How often should I change filters during smoke season?

Check every two weeks during heavy smoke and replace when visibly gray.