Quick answer
- During smoke events, run the system fan (ON, not AUTO) with the best filter your system can handle.
- EPA guidance points to MERV 13 filtration where the system supports it — verify your blower can handle the added resistance first.
- Close outdoor-air intakes; set ventilation systems to recirculate.
- Portable HEPA units are excellent supplements in the rooms you occupy most.
Why smoke is an HVAC problem
Wildfire smoke is dominated by fine particulate matter (PM2.5) — particles small enough to slip deep into lungs and standard fiberglass filters alike. San Luis Obispo County gets smoke both from local fires and drift from fires hundreds of miles away, and inland valleys can hold it for days. Your HVAC system, set up correctly, becomes a whole-house filtration machine; set up wrong, it pulls smoke inside.
During a smoke event
- Close windows and doors; wait out the worst of it indoors.
- Switch the thermostat fan from AUTO to ON so air passes the filter continuously.
- Install the highest-MERV filter your system is rated to take — EPA guidance recommends MERV 13 where supported. If in doubt, ask; a too-restrictive filter can strain the blower.
- Close any fresh-air intake or set your ventilator to recirculate.
- Run portable HEPA purifiers in bedrooms and main living spaces.
- Skip candles, frying, and anything else that adds indoor particulates.
Upgrades that earn their cost here
- Media filter cabinet (MERV 13): the single best value in whole-home filtration for smoke-prone regions.
- Duct sealing: leaky return ducts pull smoky attic or crawlspace air straight into the system.
- Fresh-air dampers with control: ventilation when the air is clean, isolation when it isn't.
- For sensitive households: dedicated whole-home HEPA bypass or high-end electronic air cleaners.
After any long smoke event, replace the filter — it did its job and is now loaded — and consider a coil inspection if the system ran hard throughout.
When you're ready