Wildfire smoke days: can your HVAC keep the smoke out of the house?

Smoke from grass fires in the Hill Country, or from fires hundreds of miles away, shows up as a haze and a smell that is suddenly inside the house too. The particles are small enough to get through an ordinary filter and to ride in through every gap in the return ducts. Here is what actually helps on those days.

What the filter can do

A pleated MERV 13 filter captures a large share of smoke particles. A cheap fiberglass filter captures almost none. Swap to the better filter before the season and keep a spare, because on smoky days it loads up fast.

How to run the system

• Fan on, not auto, so the air keeps passing through the filter even when the system is not cooling.
• Close the fresh-air intake if your system has one.
• Keep windows and doors shut, and skip the range hood and bathroom fans, which pull outside air in through every gap.

When the ducts are part of the problem

Return ducts and plenums that leak in the attic pull unfiltered attic air into the system. On a smoke day that is smoke. If the house smells of smoke with the windows shut and a good filter installed, the returns are leaking and the fix is sealing them, not a better filter. Ducts that are already coated in dust hold the smoke smell for days afterward.

After the smoke clears

Change the filter, run the fan for a few hours, and if the smell lingers, the ducts and the coil are holding it. A cleaning fixes that; a fragrance does not.