Mold Control on a Budget


Here's the positive side of duct cleaning, when someone says: "I feel better and my symptoms are less after we had the ducts cleaned." Of course, feeling better doesn't always happen. Sometimes there's no apparent change or symptoms even get worse after duct cleaning. Well, let's get to work with the controversial topic of duct cleaning.

Page outline:

  • How does mold get into ductwork?
  • How do you figure out if ductwork needs cleaning?
  • What's the biggest mistake people make in hiring a duct cleaning service?
  • What do the national guidelines and other sources say about duct cleaning?
  • How do you find a reputable duct cleaning company?
  • What should be included in a duct cleaning job?
  • Should you have post-remediation testing after the duct cleaning?
  • How do you keep mold from re-growing?

How do you find a reputable duct cleaning company?

  • Try the NADCA membership list.

  • Ask your HVAC contractor for a recommendation.
  • I seldom get called to check duct cleaning jobs afterwards (by spore trap testing) so haven't learned much about how effective the average duct cleaning job is. I guess homeowners assume they got their money's worth.

What should be included with a "duct cleaning" service?

How does mold get into ductwork?

  • Mold growth is pretty much a given with central air conditioning, at least if the AC coils are dusty and dirty. Water condenses on the AC coils, and if dust or dirt is on the coils, mold grows.
  • Other causes of mold contamination may exist, such as using a cheap fiberglass filter which lets dust pass through onto the AC coils, having an air intake in a moldy basement, an oversized unit, or an in-line humidifier (which is not recommended). Protect ductwork from moisture. Do not add moisture to ductwork.
  • Sometimes there is lots of debris in the ductwork from the time of construction...sawdust, trash, etc.

How do you determine if ductwork needs cleaning?

  • One way is to run the AC or the furnace for 10 minutes or so and then take a spore trap test at two vents (while the unit is operating). A spore trap test analysis includes a count of living and dead spores, as well as hyphal fragments. These results often give a pretty good idea what's happening with mold growth in the ductwork or AC unit...but sometimes they don't pick up on a problem.
  • Look at insulation inside the AC unit. If you see a lot of black, that could be Cladosporium mold. Tape test it.
  • You could also tape test the surface of AC coils, but remember that some mold is almost always found on the coils. A judgment call needs to be made between "normal" ambient mold and too much mold.
  • Also take off one or more vent covers, and tape test inside the duct work, avoiding areas that have a lot of debris.
    At one air duct, I took the tape sample inside the air duct on the top surface, and the tape was completely black with Cladosporium mold.
  • At new houses, if the builder or HVAC contractor didn't cover over ducts during construction, they may be contaminated with sawdust.
  • If you have an older house and have always used the inexpensive fiberglass filters, your ductwork and AC coils probably are pretty dirty. We call fiberglass filters "elephant filters," because they let just about everything pass through.

What is the biggest mistake people make in hiring a duct cleaning service?

  • They hire a duct cleaning service. That is, they hire a service that cleans only ductwork, not the unit as well. If the ductwork is cleaned, but the AC or furnace is still contaminated, then cleaned ductwork could be re-contaminated in short order.
  • How can a so-called professional duct cleaning service clean only ductwork and not the unit, too?
    I don't understand that, either. Ask them.
    The standards of the North American Duct Cleaners Association (NADCA),, call for cleaning both the ductwork and the unit - but not even all NADCA members clean both. Beware.
  • "Why," you may ask, "Do they call it 'duct cleaning,' if the industry really should be called 'HVAC cleaning,' or something like that?" ("HVAC" = heating, ventilation, and air conditioning)
    Answer: I don't know.

What do the national guidelines and mold publications say about duct cleaning?

  • Only clean if there is known mold contamination.
  • There's little evidence that preventive cleaning of ductwork results in health benefits.
  • Be cautious about what chemicals are used in the ducts. Some people could prove allergic. There is little research to prove effectiveness.
  • There is some speculation that biocides (cleaning chemicals) could break down and provide nutrients for mold to grow on.
  • Tests show that human wiping of ductwork cleans better than mechanical cleaning. Problem is, a good deal of ductwork is not accessible for wiping.
  • Flexduct cannot be adequately cleaned and should be replaced if contaminated.
  • Ductwork lined with fiberglass or asbestos should be replaced. Check the EPA website for an appropriate brochure, and then search on your topic. "Iaq" stands for "indoor air quality."

  • The EPA (US Environmental Protection Agency) has a useful duct cleaning brochure for homeowners. You can download this at
  • This brochure has two checklists, one for choosing a duct cleaner and one for checking a job afterwards. The latter checklist can also be found on NADCA's website,
  • NADCA says that if the AC coils cannot be thoroughly cleaned in place, they should be taken off-premises for cleaning - or replaced.

Should you have post-remediation testing after duct cleaning?

  • I'd have it before cleaning, to establish that cleaning is needed, and after cleaning, to establish that readings are improved. Discuss this with your duct cleaning service, because they may find it an unusual request.
  • I'd do spore trap testing before and after.
  • Could it add considerably to the cost to bring in an independent mold inspector? Yes. But hopefully you'd only have to go through this once, because afterwards you'd know how to maintain the ductwork.

How do you keep mold from re-growing?

  • Disconnect and remove any attached humidifier.
  • Install a pleated media filter to protect the newly cleaned (or replaced) AC coils from dust and dirt. Change the filter in accordance with manufacturer's instructions. A pleated media filter looks like an accordion pleat.
    Do NOT use the cheap fiberglass filters. They are close to useless. Do not depend on electrostatic or electronic filters. They can work fine, until they are coated with dust - which could take 3 months or 3 hours.
  • Make sure the filter fits well and that there is no air by-pass around the filter. Use duct tape if needed to seal off the filter from room air.
  • UV lights kill mold and bacteria but you need dwell time for it to happen. Some manufacturers are making tube UV lights, which get placed in ductwork.
    If the UV light is aimed at the coils, it has to be carefully positioned to reach as much of the coils as possible. It's not possible to get 100% coverage of the coils.
    The easier route may be to use the pleated media filter to keep the coils clean. If coils are clean, then even if they get wet, mold won't grow.

Whole house dehumidification

If you live in a damp area or want a controlled indoor environment, look into a whole house dehumidification system, such as by Lennox.

If you had a filtered fresh air source added in to that system, you'd be in good shape.

An even better plan might be to maintain a controlled indoor environment:

- ERV (Energy Recovery Ventilator from, Energy Foundation Incorporated)

- portable dehumidifier for the basement (from

- Room air purifier in living spaces (IQ Air or a HEPA unit from E.L. Foust)

Energy considerations

Bear in mind that dehumidification uses energy and is getting less popular in the HVAC industry for that reason. Certainly we all need to be concerned about reducing our carbon footprint.

If you need dehumidification at your house (or just to save  on energy costs and reduce your carbon footprint), do look into geothermal heating to reduce your heating/cooling bill 40+% monthly, plus solar heating. New, lower-priced versions are likely to be coming down the pike as time passes. One vision is for roofing material to have solar collection capability.

For further exploration: 

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Mold and other environmental inspections available in

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Updated 9-2011