Wildfire Smoke and Your HVAC System

Nevada County sees significant wildfire smoke every summer and fall. Smoke events are different from ordinary air quality problems — the particle size is smaller, the duration is longer, and the decisions you make about your HVAC system during a smoke event affect both indoor air quality and equipment longevity. Here is what we know from forty-five years of servicing HVAC equipment in this specific area.

At a glance

What smoke actually does to your HVAC equipment

Wildfire smoke particles are extremely fine — PM2.5 and smaller — and they pass through lower-rated filters freely. A MERV 8 filter (common in builder-grade installs) does almost nothing to block fine smoke particles. A MERV 13 filter removes roughly 85-90% of particles in the 1-3 micron range, which includes the bulk of wildfire smoke particulates. Beyond the filter, heavy smoke events deposit fine soot on evaporator coils and blower wheels. This accumulation is slow during a single event but builds over multiple smoke seasons. A dirty evaporator coil loses heat-transfer efficiency and can eventually develop mold issues. We see significantly more coil cleaning calls in the two seasons following heavy fire years in Nevada County.

Filter choices during smoke season

MERV 13 is the practical ceiling for most residential systems without modifications. A MERV 16 filter (HEPA-equivalent) restricts airflow to the point where standard residential systems can overheat the heat exchanger, freeze the evaporator coil, or strain the blower motor. If you want true HEPA filtration for your whole home, the system needs to be designed around it — it is not a filter swap. For most homeowners, MERV 13 filters changed every 30-45 days during active smoke season is the right approach. Check the filter monthly during fire season — a smoke-saturated filter loads up fast and a clogged filter causes more equipment problems than the smoke itself. Activated carbon filters can help with smoke odors but do not meaningfully capture particles; if odor is the concern, carbon-plus-MERV-13 combination filters are available.

Fresh-air dampers and economizers

If your system has a fresh-air damper or economizer — common in systems installed after 2010 in California — close it manually during active smoke events. An economizer that is stuck open or not programmed correctly will pull smoke-laden outside air directly into your duct system regardless of how good your filter is. This is a common source of smoke smell indoors even when doors and windows are shut. If you are not sure whether your system has a fresh-air damper, check the plenum at the air handler for a motorized damper or a manual lever. We check damper operation as part of every tune-up call.

When smoke causes equipment damage

Most smoke events do not cause immediate equipment damage, but the cumulative effect on coils and ductwork is real. If you notice a smoke or burning smell coming from vents after a fire event, shut the system down — a burning smell can indicate soot ignition in a clogged filter, near a heat exchanger, or in ductwork with accumulated debris. This is different from the normal outdoor smoke smell that can seep in through gaps. A burning smell from the system itself means stop the system and call us. Post-smoke inspection is worth scheduling if your area experienced heavy smoke accumulation over multiple weeks — we clean the evaporator coil, check the heat exchanger for soot deposits, and replace the filter.

Indoor air quality upgrades worth considering

For homeowners in smoke-prone areas, three upgrades consistently improve indoor air quality during smoke events. Whole-home media filters (MERV 13, 4-5 inch thick media cabinet) hold more surface area than standard 1-inch filters, change less frequently, and maintain better airflow than thin high-MERV filters. UV germicidal lights on the evaporator coil do not capture smoke particles but do prevent mold growth on smoke-fouled coils, which is a secondary issue after heavy smoke seasons. Portable HEPA room purifiers (run in bedrooms during sleep) are inexpensive and highly effective for the spaces where you spend the most time — they require no HVAC modification. We can assess your existing system and recommend what will actually move the needle for your specific setup.