A wildfire smoke plan for your heating and cooling system before the next fire

Anyone who lived in Pasadena, Altadena or Sierra Madre in January 2025 remembers the smoke from the Eaton Fire, and many families are still rebuilding. Even fires far away can send smoke across the San Gabriel Valley for days. Smoke is made of very fine particles that drift through small gaps, and the HVAC system can either help keep them out of the house or pull them in, depending on how it is set up.

The time to make a plan is before the next fire, not during it, when filters sell out and every technician is busy. This post walks through what to set up now, what to do on a smoke day, and what to check once the air clears. None of it is complicated, and most of it costs little. It applies to central systems, ductless mini-splits and older homes heated by a wall furnace alike.

Most central systems in older Pasadena homes simply recirculate indoor air. They do not pull in outdoor air on purpose, but they can pull it in through leaks in the return ducts and the cabinet. Some newer homes and ADUs have a fresh air intake, an economizer or a ventilation fan that brings in outside air on a schedule. Ductless mini-splits recirculate room air and generally do not bring in outside air.

Find out which type you have. If there is a fresh air damper or a ventilation fan, learn where its control is and how to close it or switch it off. If you have a whole house fan, it should never run on a smoke day, since its whole job is to pull outdoor air through the house. A technician can show you these controls during a routine maintenance visit.

A filter only cleans the air that passes through it. Smoke that leaks in through the return duct, the attic or gaps around the air handler cabinet skips the filter entirely. Attic return leaks are especially common in postwar ranch homes and older bungalows with ducts added decades ago. Sealing return joints, the return plenum and the cabinet seams keeps the system from drawing in attic and outdoor air.

Around the house, simple fixes help too. Close the fireplace damper, check weatherstripping on exterior doors, and seal obvious gaps where pipes and line sets pass through walls. None of this makes a house airtight, but it shifts more of the air the system moves toward the filter instead of around it. A duct leakage test shows how much the ducts are leaking before and after sealing.

Once the air is clean again, replace the filter even if it still looks usable, because it has caught fine particles that can release odor. Look at the outdoor unit for a layer of ash on the coil and rinse it gently with a hose if you see one. Run the system and notice whether a smoky smell returns. A lingering smell after a new filter can mean ash settled in ducts or on the indoor coil.

For homes close to a burn area, ash in the attic and ducts is a bigger concern. Duct cleaning, a coil cleaning and a check of the air handler cabinet remove what settled during the fire. For families rebuilding after the Eaton Fire, a new system is a chance to design in a proper filter cabinet, sealed ducts and good return paths from the start. We handle the permit where the city requires one.