Mold in air ducts: signs, causes and what to do

Is it really mold?

Most of what people see at their registers is not mold.

• Dark streaks on the ceiling or wall around a vent are usually dust and soot. Air leaving the register carries fine particles that stick where it meets the cooler surface.
• Grey fluff inside the duct is ordinary dust and lint.
• Black dust on the grille in a house with candles, a fireplace or a smoker is soot.

Mold needs water. Sheet metal ducts carrying dry, heated air in winter are a poor place for it to grow. Where there is moisture in the system, it is a different matter.

Where mold actually grows in the system

• The cooling coil and drain pan. In summer the coil is cold and wet by design. Dust on a wet coil, or a pan that does not drain, grows slime and mold.
• The condensate drain line, when it is blocked and overflows.
• The humidifier. Furnace-mounted humidifiers with a dirty pad, standing water or a setting that is too high wet the duct downstream.
• Internal duct insulation. Some ducts and air handlers are lined with fibreglass on the inside. Once that lining gets wet and dirty, mold grows in it and cannot be cleaned out.
• Flexible duct, which has a thin plastic liner over insulation and holds moisture if water gets in.
• Ducts in cold spaces. Uninsulated ducts running through a crawl space, attic or garage sweat in summer when carrying cold air.
• Ducts that were flooded. Floor ducts in a slab, or basement ducts reached by a flood or sewer backup.
• Return air drawing from a damp place, such as a moldy basement or crawl space. The system then spreads the smell and the spores, even though the growth is elsewhere.

Signs worth taking seriously

• A musty smell that starts when the furnace fan or air conditioning comes on
• Visible fuzzy or slimy growth on the coil, in the drain pan or on duct lining
• Water stains or rust on ducts and around the furnace
• A drain pan that is full, or water on the floor by the furnace in summer
• Allergy-type symptoms that are worse when the system runs
• A history of flooding or a leak that reached the ducts

How to check

1. Take off a register and look inside with a flashlight and a phone camera.
2. Look at the furnace filter. A damp or moldy filter is a clear sign of a moisture problem.
3. Have a heating and cooling technician open the coil compartment and check the coil, pan and drain.
4. Check the humidifier pad and its water supply.
5. Look at exposed ducts in the basement and crawl space for rust, staining and condensation.

Testing is rarely needed where growth is visible. Where there is a smell and nothing visible, an inspection of the house for dampness is usually more useful than air sampling, because the source is often outside the ducts.

What to do about it

Fix the moisture first. Clear the condensate drain. Repair or replace the humidifier, or turn it down. Insulate ducts in cold spaces. Dry out the basement or crawl space that the return is drawing from. Without this step, anything else is temporary.

Bare sheet metal ducts can be cleaned. Proper duct cleaning uses a powerful vacuum that puts the whole system under suction, with brushes or air tools to loosen debris, and the registers and the furnace cabinet are included.

The coil, pan and blower are cleaned by a heating and cooling technician. A badly fouled coil may need to be removed for cleaning.

Insulated duct lining and flexible duct that have mold in them are replaced. Porous materials cannot be cleaned reliably.

Ducts flooded by sewage or outside flood water need thorough cleaning and disinfection of metal sections and replacement of anything porous. Ducts cast into a concrete slab that have flooded are a known problem and sometimes have to be abandoned and replaced with a new system overhead.

The filter is replaced at the end, and again a few weeks later.

Sprays, foggers and coatings

Be cautious with offers to fog the ducts with a biocide or coat them with a sealant.

• Killing mold does not remove it. Dead growth and the dirt it was living on are still there.
• Chemicals sprayed into ducts are then blown into every room. Any product used must be approved for that use and applied as its label directs.
• A coating over dirty or damp lining traps the problem.

Health Canada’s guidance on mold is consistent: remove it and fix the moisture. Disinfecting is not a substitute.

Does routine duct cleaning prevent mold?

Duct cleaning removes dust and debris, which is worthwhile after a renovation, a fire, a flood or a pest problem, or when ducts are visibly dirty. It does not, by itself, prevent mold. Dry ducts do that. Be wary of very cheap whole-house offers and of anyone who finds “toxic mold” in every system they look at.

Running the system during mold work elsewhere

If mold is being removed from another part of the house, the heating and cooling system serving that area is normally shut off and its registers sealed while the work is done, so that spores and dust are not drawn in and spread. The filter is changed afterwards.

Prevention

• Change the furnace filter regularly
• Have the air conditioning coil and condensate drain checked each spring
• Clean or replace the humidifier pad each autumn, and turn the humidifier off in summer
• Keep indoor humidity moderate: around 30 percent in cold weather and no more than about 50 percent in summer
• Insulate and seal ducts that run through cold or humid spaces
• Keep the basement and crawl space dry
• After any flood, have the ducts checked

More: mold removal and remediation.