What should I do if a musty smell is coming from HVAC vents?

What this smell usually points to

A true musty smell coming from HVAC vents usually means moisture is lingering somewhere inside the air path: on the cooling coil, in the condensate pan, or in duct materials that stayed wet long enough to grow mildew or biofilm. This symptom is different from a generally stale house with no distinct odor, and it is also different from black soot or dust around registers. If the smell is specifically damp, earthy, or like a basement after rain, technicians usually start by looking for a wet source inside the air handler and supply ducts.

How to tell the likely causes apart

If the smell shows up most during air conditioning season and is strongest in the first few minutes after the system starts, the evaporator coil or condensate area is a common culprit. If only one or two rooms smell noticeably worse than the rest, that points more toward a localized duct problem such as wet flex duct, damaged duct liner, or insulation near that branch. If the entire house smells musty whenever the blower runs and indoor air also feels sticky, high humidity and poor moisture removal are more likely. If the odor is hot, sharp, electrical, or like melting plastic rather than damp, treat it as a different and more urgent problem than mildew.

What to do based on what you notice

A damp or moldy vent smell is usually a schedule-soon issue, especially if it has lasted more than a day or two, comes back repeatedly, or is getting stronger. You can note whether it happens only in cooling mode, from all vents or only certain rooms, and whether there is any water near the indoor unit, but cleaning coils, opening equipment, and correcting drainage problems are jobs for a qualified technician. If you see soot or black residue near heating equipment, or if the odor smells scorched or electrical, stop using the system and arrange prompt service because that points away from ordinary mildew and toward a possible combustion or electrical fault.

What a service visit usually includes

For this symptom, a technician typically inspects the evaporator coil surface, blower compartment, condensate pan, trap, and drain line for slime, standing water, overflow marks, and restricted drainage. They may check temperature split and indoor humidity to see whether the AC is dehumidifying properly, and they will look for crushed, disconnected, or water-damaged duct sections if the odor is isolated to certain vents. If the smell resembles burning plastic, they are more likely to inspect blower motor amperage, wiring connections, control boards, and any overheated components before restarting the system.

What affects the repair cost

Cost depends on whether the fix is basic cleaning and drainage work or whether damp materials have to be replaced. In many markets in 2025-2026, clearing a condensate drain and treating minor buildup is often around $150-$400, while evaporator coil cleaning commonly runs about $200-$700 depending on access and severity. Replacing wet duct sections or contaminated duct liner can move into the several-hundred to low-thousand-dollar range, and correcting humidity-control issues such as airflow, sizing, or adding dedicated dehumidification can cost more. If the source turns out to be an electrical overheating problem rather than moisture, pricing depends on the failed part and labor involved.