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HVAC

Mold in air conditioning systems and ducts

Best practice Sources: EPA 402-K-01-001, EPA 402-K-97-002

In plain English

An AC system is a moisture machine: the cooling coil pulls water out of the air into a drain pan. When the pan or drain line clogs, or ducts sweat in a hot attic, mold has everything it needs — and the system then blows its spores through the whole house.

If you know or suspect the system is contaminated, don't run it until it has been looked at. During remediation, vents in the work area are sealed off, and filters serving the area are upgraded (at least MERV 8) and changed afterward.

Routine duct cleaning isn't recommended by EPA. It makes sense when there is substantial visible mold inside hard ducts or other components, vermin, or dust actually blowing out of the registers. Mold in insulated or lined ducts usually means replacing that section, not cleaning it. And unless the moisture cause is fixed, the mold comes back.

Key points

Don't run a system you suspect is contaminated.
Keep drain pans clean and draining; insulate ducts against condensation.
Upgrade filters to MERV 8 or better; never run without a filter.
Be wary of unsolicited duct cleaning, and of sealants or biocides sprayed in ducts.

Official text

EPA 402-K-01-001 — HVAC System p. 7 Best practice

Do not run the HVAC system if you know or suspect that it is contaminated with mold. If you suspect that it may be contaminated (it is part of an identified moisture problem, for instance, or there is mold growth near the intake to the system), consult EPA’s guide Should You Have the Air Ducts in Your Home Cleaned?[4] before taking further action (see Resources List).

[4] Although this document has a residential focus, it is applicable to other building types.

Photo caption: Photo 4B: Mold growth on air diffuser in ceiling

Photo caption: Photo 4C: Moldy air duct

Photo caption: Photo 4A: Contaminated fibrous insulation inside air handler cover

Citation: EPA 402-K-01-001, HVAC System, p. 7.

Read on the official site ↗ See it in the full EPA 402-K-01-001

EPA 402-K-01-001 — Equipment pp. 23–24 Best practice

Moisture Meters: Measure/Monitor Moisture Levels in Building Materials

Moisture meters may be helpful for measuring the moisture content in a variety of building materials following water damage. They can also be used to monitor the process of drying damaged materials. These direct reading devices have a thin probe which can be inserted into the material to be tested or can be pressed directly against the surface of the material. Moisture meters can be used on materials such as carpet, wallboard, wood, brick, and concrete.

Humidity Gauges or Meters: Monitor Moisture Levels in the Air

Humidity meters can be used to monitor humidity indoors. Inexpensive (<$50) models are available that monitor both temperature and humidity.

Humidistat: Turns on HVAC System at Specific Relative Humidity (RH)

A humidistat is a control device that can be connected to the HVAC system and adjusted so that, if the humidity level rises above a set point, the HVAC system will automatically come on.

HVAC System Filter: Filters Outdoor Air

Use high-quality filters in your HVAC system during remediation. Consult an engineer for the appropriate efficiency for your specific HVAC system and consider upgrading your filters if appropriate. Conventional HVAC filters are typically not effective in filtering particles the size of mold spores. Consider upgrading to a filter with a minimum efficiency of 50 to 60% or a rating of MERV 8, as determined by Test Standard 52.2 of the American Society of Heating, Refrigerating, and Air Conditioning Engineers. Remember to change filters regularly and change them following any remediation activities.

Photo caption: Moisture Meter

Photo caption: Photo 9: Moisture meter measuring moisture content of plywood subfloor

Citation: EPA 402-K-01-001, Equipment, pp. 23–24.