During wildfire smoke events, run the HVAC system in a configuration that maximizes filtered air circulation while sealing the home against outside air. California Air Resources Board (CARB) and Sacramento Metropolitan Air Quality Management District guidance all converge on the same set of actions.
Close fresh-air intakes and dampers. If your system has a whole-house ventilator, an economizer, or a fresh-air duct feeding the return, close or disable it during active smoke. The outside air it pulls in has not been filtered by your HVAC filter and reaches indoor spaces unfiltered. Confirm dampers are in the closed position — not just the thermostat setting.
Switch the thermostat fan to ON rather than AUTO. ON runs the blower continuously, which means your HVAC filter is continuously filtering the indoor air even when the AC compressor or furnace isn't calling for heating or cooling. This is the single most effective HVAC action you can take during a smoke event.
Upgrade the filter. Standard 1-inch fiberglass or MERV 4 filters do almost nothing for PM2.5 (the fine wildfire-smoke particulate). Move to MERV 13 minimum; MERV 16 is better if your system can handle the pressure drop without cycling on high-limit. Keep a month's supply on hand during fire season — filters load fast in heavy smoke and need changing every 30 days or sooner.
Consider a portable HEPA unit for the bedroom. HVAC filtration whole-house is powerful, but a portable HEPA unit sized for the bedroom adds a second layer of filtration in the space where you spend the most continuous time. PM2.5 exposure during sleep is the highest individual risk during smoke events.
After the smoke clears, change the HVAC filter, wipe down the return grille, and schedule a professional inspection if the system ran heavily smoke-laden air for more than a week — soot buildup on the evaporator coil reduces efficiency and can create the musty-smell problem described in our AC-smell answer.