Deep dive · Updated October 17, 2026

Mold in HVAC systems and ducts: what to do

Local versions: San Rafael, Novato, Mill Valley, Larkspur, Corte Madera, San Anselmo

HVAC systems can harbor mold in drip pans, on coils, in air handlers and in ductwork, especially where ducts run through damp crawlspaces, as they do in many Marin homes.

Signs

• A musty odor when the system runs
• Visible growth around vents or registers
• Moisture or standing water in the drip pan
• Mold elsewhere in the home that keeps returning

First: stop running it

The EPA advises not running an HVAC system if you know or suspect it's contaminated with mold, because doing so could spread mold throughout the building (EPA).

Professional assessment

An HVAC professional or mold provider can inspect the air handler, coils, drain pan, condensate line, and accessible ducts. The EPA has guidance on duct cleaning that discusses when cleaning may be warranted, including substantial visible mold growth.

Cleanup

• Fix the moisture cause: clogged condensate lines, condensation on ducts in a damp crawlspace, leaks
• Clean components according to industry practices
• Replace insulation lining or flex duct that can't be cleaned
• Address the crawlspace (see crawlspace mold)

Prevention

• Change filters as recommended
• Keep the condensate line clear
• Insulate and seal ducts in crawlspaces
• Control crawlspace humidity

Where mold grows in an HVAC system

• Drain pan: under the cooling coil (or heat pump coil), where water collects. If the drain is clogged or the pan is rusted, standing water grows mold.
• Evaporator coil: wet during cooling, and it can collect dust that feeds growth.
• Blower and housing: dust and moisture can build up.
• Ducts: especially ducts in damp crawlspaces, where condensation forms on or inside them, and internally insulated (lined) ducts that are hard to clean.
• Registers: dark spots on registers can be mold or dust staining. Condensation on cold registers in humid rooms can grow mold.

Many Marin homes have furnaces without air conditioning, so the cooling coil may not apply. Heat pumps, increasingly common, do have coils and drains.

What duct cleaning can and can't do

The EPA's guidance notes that duct cleaning has never been shown to actually prevent health problems, and that you should consider cleaning if there's substantial visible mold growth inside hard surface ducts or other components, if ducts are infested with vermin, or if they're clogged with excessive dust and debris (EPA duct cleaning). It also cautions that if the moisture problem isn't fixed, mold will likely return, and that insulated ducts that get wet or moldy generally can't be effectively cleaned and should be removed and replaced.

Choosing who to call

• HVAC contractor: for coils, drain pans, condensate lines, and duct replacement. Check the CSLB license (C-20 is the warm-air heating, ventilating and air-conditioning classification; see CSLB classifications).
• Mold remediation provider: for contaminated building materials around the system, and sometimes for coordinating duct work.
• Independent inspector: if you want an assessment without a sales interest.

Questions to ask

1. Where exactly is the growth: pan, coil, ducts, registers?
2. What's causing the moisture?
3. Can this duct type be cleaned, or does it need replacement?
4. How will you protect the rest of the house during cleaning?
5. Will you use any chemical treatments, and are they registered for use in HVAC systems?

Ducts in crawlspaces

Many Marin homes run ducts through crawlspaces. In damp crawlspaces, ducts can sweat, disconnected joints can pull damp crawlspace air into the system, and crushed sections trap moisture. Sealing and insulating ducts, and controlling crawlspace moisture (see crawlspace mold), are often part of the fix.

Filters

Use the filter type your system is designed for, and change it on schedule. A higher-efficiency filter catches more particles, but a filter too restrictive for your system can reduce airflow. Ask your HVAC contractor.

Maintenance schedule

• Monthly to quarterly: check and change filters as recommended
• Each season: look at the drain pan and condensate line if you have AC or a heat pump
• Yearly: professional service of the furnace or heat pump, including the blower and coil
• After crawlspace water events: inspect ducts under the house for disconnections, crushing and wet insulation

If you're replacing the system

A new furnace or heat pump is a good time to evaluate the ducts. Sealing leaky ducts and replacing old, lined or damaged sections can improve both air quality and efficiency, and it removes a common mold reservoir.

Mold smell only when heat runs?

If you notice a musty smell mainly when the furnace starts, the source may be in the ducts, the return path or the crawlspace air being drawn in through leaky duct joints. Turn the system off and have it inspected rather than masking the odor with scented filters or sprays.

Related reading

• Bathroom and HVAC mold service
• The complete mold guide

Frequently asked questions

This guide is general information, not legal, insurance or medical advice. Sources are linked in the text. Marin Mold Removal is a referral service and doesn't perform the work.