Mold in the HVAC: turn it off, this one is not a DIY
The EPA says do not run the HVAC system if you know or suspect it is contaminated with mold, because it could spread mold throughout the building. Every hour it runs seeds every room.
What is probably happening
- Condensation inside the air handler, a clogged condensate drain, or a wet evaporator coil pan.
- Duct insulation or fiberglass duct board that got wet. The EPA says wet or moldy duct insulation cannot be cleaned and must be replaced.
- A musty smell only when the system runs is the classic tell, even with nothing visible at the vents.
Do this in the next five minutes
- Leave the system off until someone has looked inside it.
- Photograph what you can see at the registers and around the air handler.
- Check for standing water under or beside the air handler and around the condensate line.
- Call an HVAC contractor to find the moisture fault. Call a mold remediation contractor for the cleaning if the growth is substantial.
- Replace the filter once the system is cleared, not before.
Do not
- Do not spray anything into the ducts. Biocides in ductwork are not a routine EPA recommendation, and you cannot reach the source.
- Do not buy a duct cleaning as the fix. The EPA suggests duct cleaning when there is substantial visible mold inside hard surface ducts, and only after the cause of the moisture is corrected, or it will recur.
- Do not accept a lab report of 'mold' as proof. The EPA notes that for about $50 a lab can tell you whether a sample on sticky tape is mold or something that looks like it.
Who to call
An HVAC contractor first, a mold remediation contractor for substantial growth
The HVAC contractor finds why it is wet (coil, drain, duct leak, insulation). The remediator removes what grew. A duct cleaning company alone fixes neither cause.
BestHonestPro is a free comparison resource, not a contractor. There is nothing to sign up for and we do not sell your details. When we begin connecting callers to contractors, we will be paid a referral fee by the contractor, never by you, and this line will say so.
What it should cost
$3,000 to $10,000
HVAC system mold remediation. Duct cleaning on its own typically runs $450 to $1,000 per system (EPA)
Source: Angi, mold remediation cost, July 2026, and US EPA duct cleaning guide. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- What is wet, and why? Coil, drain pan, duct leak, insulation?
- Is any of the ductwork lined or fiberglass board, and will you replace it rather than clean it?
- Will the moisture fault be fixed before the cleaning, and is that on the same invoice?
- What is your EPA Section 608 certification type?
End the call if
- A duct cleaning quote with no diagnosis of why the system is wet.
- Chemical fogging of the ducts as the main service.
- A quote for a new system on the first visit with no diagnostic readings.