HVAC and duct mold: how it spreads and what cleanup costs

Why HVAC systems grow mold

An air conditioner makes water on purpose. The evaporator coil inside the air handler gets cold, house air blows across it, and moisture in the air condenses on the fins. That water runs into a drain pan and out a condensate line. The coil is wet whenever the system runs, and the air that passes over it is cool and humid.

Dust on the coil gives mold a wet surface to grow on. A clogged condensate line backs the pan up, and standing water sits in a dark cabinet with air moving over it. The insulation inside the cabinet gets damp and stays damp.

Ducts carry the problem outward. Sheet metal ducts are smooth and clean up well. Fiberglass duct board and the liner inside some metal ducts are porous and hold dust and moisture. Flex duct collects dust in its ridges. When any of these runs through a hot attic or a humid crawl space, the cool duct sweats, and leaky joints pull attic or crawl space air into the system.

A return duct with a gap at the framing draws air from the wall cavity, the crawl space, or the attic every time the blower runs, and whatever is growing there gets pulled into the air handler.

Humidity finishes the job. A house that stays humid grows mold in the ducts and everywhere else.

Signs you can check at the registers

The first sign is a musty smell that arrives when the system starts and fades when it stops. The smell comes from every register at once rather than from one room.

Then look at the supply registers. Dark spotting on the register louvers and on the ceiling or wall around them is a sign of growth in the duct behind them.

Open the access panel on the air handler, with the system off. Look at the coil fins, the drain pan, and the cabinet insulation. Standing water in the pan, a slimy film on the coil, and dark staining on the insulation are the usual findings.

Walk the ducts where you can see them in the attic or the basement. Condensation drops on the outside, sagging flex duct that holds water at the low point, and joints with gaps are the things to photograph.

A home test is limited. A lab confirmation of a suspected duct sample runs about $50, and a mold inspection with a camera through the duct tells you far more.

What happens if it waits

A moldy air handler is a distribution system for spores. Every cycle moves air across the coil and through the ducts into every room the system serves. Mold in one bedroom closet stays in the closet. Mold in the air handler reaches the whole house. EPA says not to run a system you know or suspect is contaminated, and the reason is spread.

The equipment suffers next. Growth on the coil cuts airflow, the system runs longer, and the drain problem that started it keeps backing up. Cabinet insulation and lined duct that have grown mold cannot be cleaned and get replaced.

Air quality is the main cost. Mold can irritate people who are sensitive to it, and a house where the smell starts whenever the air comes on is a house where people stop running the air.

Buyers pay for HVAC inspections, and a technician who finds growth on the coil writes it up as a system-wide problem.

What HVAC remediation involves

Shut the system off first. Then the work splits into the air handler and the ducts.

At the air handler, the crew cleans the coil and the drain pan, clears or replaces the condensate line, and removes any cabinet insulation that has grown mold. The cabinet gets HEPA vacuumed and treated with an antimicrobial labeled for HVAC use. A coil that cannot be cleaned, or an air handler that has rusted through, gets replaced by an HVAC contractor as a separate line.

In the ducts, the crew puts the system under negative pressure with a HEPA vacuum at the main trunk, agitates each run with brushes or air whips, and pulls the loosened material to the vacuum. Registers and boots get cleaned by hand. Sheet metal gets a treatment after cleaning. Porous duct that shows growth comes out and gets replaced.

Then the cause gets fixed: the drain, the return leak, the attic or crawl space moisture that the ducts run through, and the humidity in the house. Read mold in HVAC systems for the full process and for what to ask a company before they start.

What moves the HVAC price

Three bands cover this page, and the one that applies depends on what is contaminated.

Vents alone, with the ducts unaffected, run $100 to $500. That is register cleaning and the boots behind them.

Duct cleaning with mold treatment runs $500 to $2,000, from Today’s Homeowner, when the air handler is clean and the ducts are the problem. Routine whole-house duct cleaning without mold work runs $300 to $800, average about $400, and HomeWyse prices the labor and consumables of cleaning mold from ducts at $0.48 to $1.13 per square foot of finished floor area, so a larger house sits higher in the band.

Full system remediation, air handler and ducts together, runs $3,000 to $10,000 from This Old House and Fixr. That is the band shown above. It applies when the coil, the pan, and the cabinet are contaminated and the ducts need cleaning or replacement.

Duct material moves the price within a band. Sheet metal cleans. Duct board, liner, and flex get replaced by the foot, and ducts in a tight attic or crawl space cost more to reach than ducts in an open basement.

A house with more than one system has more than one job.

Run the house through the estimate tool and read the HVAC mold cleaning cost page for the line items. These are planning ranges from national data. An HVAC or remediation company confirms scope and price on site, and the two trades often share the job.

Keeping the system clean

Change the filter on the schedule the manufacturer gives, and use the best filter the system can pull air through.

Have the coil and the drain pan cleaned as part of routine service, and ask the technician to flush the condensate line.

Seal return ducts at every joint and at the framing so the system pulls air from the rooms and nowhere else.

Insulate ducts that run through unconditioned space so they stop sweating.

Keep the house humidity where EPA recommends, below 60 percent and ideally between 30 and 50 percent.

Fix the attic or the crawl space if that is where the ducts live. A clean duct in a wet crawl space gets dirty again. See preventing mold from coming back.