Your HVAC System After a Wildfire: What to Do Before You Run It

Of all the mistakes homeowners make during wildfire cleanup, running the HVAC system too soon is among the most common. When a contaminated system operates after a wildfire, each cycle can redistribute smoke particles and residues onto freshly cleaned surfaces, so HVAC wildfire smoke cleanup is an incredibly important step.

What Happens When HVAC Runs During a Wildfire

A forced-air HVAC system continuously cycles air through the home. The return side pulls air from living spaces through return registers, the air filter, and the air handler (blower compartment and evaporator coil) before the air enters the supply ducts that distribute conditioned air to each room.

When wildfire smoke is present, every minute the system runs, it draws smoke-laden air through this pathway. The air filter captures some particles but without a MERV rating of 13+, fine PM2.5 particles and gases will pass through. Those particles accumulate on:

Duct walls: a film of soot and smoke residue coats all interior duct surfaces.
The air handler and blower wheel: heavy accumulation reduces efficiency and becomes an ongoing source of re-contamination.
Evaporator coils: a prime location for soot accumulation that degrades cooling efficiency and holds moisture, creating potential for mold growth.
Supply registers and diffusers: soot deposits visible as dark rings around registers throughout the home.
All room surfaces: soot redistributed by the system deposits on furniture, walls, and ceilings throughout the home.

“If you can, number one, replace the filter right away. The return ducting is going to be pulling in that smoke and soot. I would suggest replacing the filter, supplementing a charcoal filter, but then looking to have those ducts cleaned, at least the return.” — Cole Fechner, VP, Dryco Restoration Services

The Correct HVAC Recovery Sequence

Recovering an HVAC system after wildfire smoke exposure follows a specific order. Skipping steps, especially running the system before ducts are cleaned, spreads contamination that was otherwise contained.

Turn off the system immediately. Do not run it while soot is present in the home, even briefly.
Visually inspect all return registers. Visible soot film or dark discoloration confirms smoke infiltration and indicates that duct cleaning is likely necessary.
Replace the air filter with a MERV-13 or higher rated filter. MERV-13 filters are significantly more effective than MERV-8 filters at capturing fine particles, including many in the 0.3–1.0-micron size range.
Add a supplemental activated charcoal or carbon filter if your system accommodates one. Carbon media adsorbs volatile organic compounds that standard filters allow through.
Schedule professional duct inspection and cleaning before resuming normal system operation.
After duct cleaning, have the duct system treated with a professional deodorizing fog before resealing any access panels.
Remove and thoroughly clean all supply and return register grilles, replacing any that cannot be fully cleaned.

HVAC Component Assessment Guide

Component	DIY Action	Professional Action Required When	Priority
Air filter	Replace with MERV-13 immediately	Any time after a serious smoke event	Immediate
Return air registers	Wipe visible soot; inspect for film	Soot film visible; dark discoloration	Immediate
Supply registers	Wipe visible soot	Soot visible; odor from registers when system runs	Before first use
Return ductwork	Visual inspection only	Any confirmed smoke or soot infiltration	Before first use
Supply ductwork	Visual inspection only	Visible soot; odor on first use	Before first use
Air handler / blower	Check filter compartment for soot accumulation	Soot on blower wheel or heat exchanger	Before first use
Evaporator coils	Visual inspection only	Soot accumulation; reduced cooling performance	Before first use
Furnace heat exchanger	Do not attempt DIY inspection	Any fire or smoke event; annual inspection recommended	Before first heat cycle

Duct Cleaning vs. Filter Replacement: What Is the Difference?

These two steps are not interchangeable. Filter replacement removes the filter media that captures particles as they pass through, but it does nothing to remove soot that has already been deposited on duct walls, blower components, and coil surfaces.

Professional duct cleaning is generally considered the best practice after HVAC wildfire smoke exposure. At minimum, the return air ducts should be inspected, and any contaminated ductwork should be professionally cleaned.

Mold Risk in HVAC Systems After Wildfire

If wildfire suppression activities or firefighting efforts introduced moisture into the home, soot accumulation combined with damp HVAC components may increase the risk of mold growth. Any HVAC wildfire smoke event where the system ran while both smoke and moisture were present should be assessed for mold growth in addition to soot accumulation.

If you find that your HVAC system’s been contaminated, either by mold or smoke damage, and you live in Duluth, don’t hesitate to contact the Dryco team.

Frequently Asked Questions

Q: My HVAC was off during the wildfire. Do I still need to inspect it?

Yes, but the urgency is lower. Even a system that was off can develop passive smoke infiltration through the ductwork if the home sat in a smoke zone for multiple days. In any HVAC wildfire smoke exposure, inspect return registers for soot film and replace the filter. A professional inspection is still recommended before the first heating season after a significant smoke event.

Q: How do I know if my duct cleaning was done properly?

A properly completed duct cleaning includes before and after photos and mechanical agitation of all duct surfaces. The NADCA (National Air Duct Cleaners Association) standard requires cleaning the full system, not just accessible sections. Ask your contractor for documentation.

Q: Can I use my HVAC to ventilate the home after a wildfire?

Most residential HVAC systems recirculate indoor air rather than bringing in large amounts of outdoor air. Unless your system is specifically designed for fresh-air ventilation and outdoor air quality has returned to safe levels, avoid using the HVAC system for ventilation after a wildfire.