HVAC Systems and Mold

HVAC systems and mold and fresh air are topics we’ll look at here.


What type of HVAC (heating, air conditioning and ventilation) system do you have? forced warm air? radiators? baseboard? electric heat?

Types of HVAC systems and a few observations:

  • Forced warm air
    • By definition, you have ductwork, which is either metal or flexduct (flexible tubular, lined with plastic over fiberglass insulation). Metal ductwork can be cleaned, though expect to replace lined metal ductwork. Flexduct often cannot be cleaned effectively and should be replaced.
    • Forced warm air can be drying, and sometimes humidifiers are attached to the system. Adding water to ductwork can result in mold growth. Use a steam, UV-light, or other least-toxic germ-free technology room humidifier if needed.
    • On the other hand, if the system (including ductwork) is brand new and there is a superior (2 3/4″) pleated media filter (April Air, SpaceGuard, etc.), there would be less risk for having a dehumidifier attached, though it still might not be the best idea.
    • Use only a pleated media filter. Fiberglass filters let too much dust through. Electronic and electrostatic filters work until they get coated with dust, and then their efficiency goes down.

Baseboard and radiator, hot water systems

  • Assuming you don’t have central air, with the hot water systems there is no ductwork. This is good, in that it eliminates the complications that come with ductwork, at least if the ductwork gets dirty or mold-contaminated. On the other hand, you have no options for introducing fresh air (through ductwork).

Electric heat

  • Some individuals are sensitive to fossil fuels and would have trouble living in a house with oil or gas heat. Electric is a good alternative for them. Electric heat can also result in greater exposure to electromagnetic fields (which can be troublesome, too). If you have electric heat, get a gaussmeter to measure AC magnetic field exposure, and also measure body voltage – see the EMF tabs.

HVAC systems inspection limitations

How do you know if your HVAC system has mold contamination?

  • Monitor symptoms that may get better or worse when the HVAC system is in use.
  • Do you have central air? There is more of a risk of mold growth with central air than just with forced warm air heat. Central air has AC coils, which are sites of condensation. If the coils are dirty (because of using an inferior fiberglass filter, for example), dirty plus wet can equal mold growth.
  • If the AC system has had a quality pleated media filter (like an accordion pleat) since installation, or since previous cleaning, it may be ok. From a health perspective, the purpose of a good filter is to protect the coils from dust. If the coils are wet and clean, no problem. If they are wet and dirty, mold could grow.
  • That said, it’s a challenge for me, as a mold inspector who is not trained and certified to access AC coils, to determine if a system may be moldy. I take culture plate air samples in the air stream at supply ducts, and sometimes results confirm contamination – but you can also get false negatives. Taking off the supply vent cover (diffuser) and reaching inside for a sample of the interior of the vent sometimes shows mold growth.
  • What I usually do, if there is reason to suspect mold contamination (symptoms, inferior filter, age of system), is to refer the clients to www.nadca.com, North American Duct Cleaners Association, and have them click on the Residential tab. There’s good information there for what to look for in duct cleaners and what to expect from the job.
  • Tip:  Don’t assume that the system is contaminated just because some mold is found on a diffuser (supply vent cover). Black mold on a diffuser is typically just some common Cladosporium that likes to grow in areas of condensation. Plenty of clean systems could still have mold growth on diffusers. Just wipe off the diffuser. This is a homeowner’s maintenance task, not a sign that the whole system is contaminated. Mold on a diffuser or a contaminated system are two different issues!
  • Tip: Some duct cleaners would point to dirty fiberglass insulation as evidence that the system is contaminated. Wrong! Dirty fiberglass may be nothing more than dirty fiberglass insulation.
  • Avoid “blow and go” guys who charge lower prices and do a substandard job of cleaning the system, maybe only cleaning the ductwork and not the rest of the system.
  • Systems with known significant mold contamination should be cleaned by mold remediators under containment and negative pressure. Otherwise, you risk the spreading of mold particulates throughout the area.
  • Culture plate air testing in airflow through vents may reveal mold contamination. However, false negatives often result, especially during winter testing when spores may be dried out by heat. Spore trap testing may be better in the winter at ductwork.
  • At one house, air test results from testing at supply vents were fine. Still, the family wanted to replace aged ductwork and the AC system as a precaution. When the AC ductwork was accessed, a lot of old construction dust and other debris were found in the ductwork. They were glad they had made the replacement.

A story

“Kevin” called my office after finding my website with these words: “You should know my story.” He had worked as an energy rater, and at some homes, duct tightness testing was carried out. This type of testing requires work right at the vents, sealing them off, unsealing them, taking measurements, whatever.

Kevin said that no one had told them in training that they should be wearing respirators when doing this testing. He believed that he had inhaled so many mold spores from exposures at moldy systems that Aspergillus started growing in his brain. Medical testing diagnosed this condition, and surgery was done to remove the Aspergillus growth, leaving him with epilepsy.

Then the mold migrated to his lungs. One anti-fungal medication after another was tried, but nothing seemed to stop the growth. Finally, a new, stronger medication, V-Fend, recently approved by the FDA, arrested the growth of the mold. The liver and kidneys have to be monitored when on these antifungal medications, because what hurts mold hurts us, too, since we are made of the same compounds as mold. I wonder how Kevin is doing, these years after his phone call…. His generosity in sharing his story reminds us all to protect our lungs by wearing P100 or N95 respirators around mold.

Who should do the duct cleaning assessment?

Persons trained and licensed in HVAC systems, such as heating and air conditioning contractors, duct cleaners, and some mold remediation specialists, can open up the system for a more in-depth evaluation. Such an assessment may be beyond the scope of a residential mold inspection. I told you above about the North American Duct Cleaners Association, with members who display the NADCA emblem.

The question might be asked by the homeowner: “Should I have the AC system cleaned as a precaution?”

  • Getting a baseline cleaning done and then protecting ductwork from there with a pleated media filter, is not a bad idea. If fiberglass filtration had been used at any time in the system, then it can be assumed that there is dust and dirt on the AC coils and in the ductwork. Cleaning may be in order.
  • On the other hand, cleaning can also result in dissemination of mold particles if improperly done. If chemicals are used to fog ductwork, homeowners could be sensitive to the chemicals. I would recommend just mechanical cleaning, without use of chemicals. Who knows if, when the active ingredient is used up, the residual material might support the growth of bacteria, mold, or viruses?
  • Tip: Benefect is a popular least-toxic cleaning and sanitizing substance – but it is NOT approved on the label for use as a fogger. Benefect is not advised for inhalation with fogging.
  • Because some individuals have had more health issues after cleaning than they had before, the EPA has recommended cleaning only if mold contamination has first been established.
  • Cleaning a contaminated system should be done by a professional mold remediator, under containment and negative pressure, to avoid the dissemination of mold particulates hither and yon.
  • Some systems are so moldy that they cannot be adequately cleaned and should be replaced.
  • Flex ductwork should be replaced, because the “mountains and valleys” inside the ductwork cannot be adequately cleaned – unless air puffing is deemed sufficient.

What are some common pleated media filters?

  • Examples of pleated media filters would be the low-end ones, such as the 3M Filtrete, and the higher-end ones, such as the 2 3/4″ filters offered by Aprilaire, SpaceGuard, etc. These filters look like accordion pleats.
  • Filters must be matched to what your system can safely tolerate without resulting in too much resistance to the working of the AC motor. Check with the manufacturer regarding filter recommendations and MERV numbers.
  • Along with a good HEPA vacuum cleaner, the filter in your HVAC system also helps to clean indoor air. Newer “smart” thermostats have cycling, lower energy use, for the fan. That is, optimal use of the fan helps to clean air and dilute particulate contaminants.

Let’s review pointers about HVAC systems and mold, specifically, about cleaning.

HVAC cleaning

  • Read through the U.S. Environmental Protection Agency (EPA) booklet on duct cleaning prior to contracting with a cleaning service. (Your mold remediation company may also clean HVAC systems.) www.epa.gov/iaq/pubs/airduct.html
  • Check out the Residential tab at the North America Duct Cleaners Association website, www.nadca.com.
  • Be aware that many “duct cleaning services” only clean ductwork. You need a service that cleans the AC coils and the entire system. As “they” say, the drainage pan should be clean enough to eat dinner off.
  • If there is flexduct, many indoor air quality professionals believe it cannot be adequately cleaned and should be replaced. The inside surfaces are not smooth and so would be difficult to clean out with mechanical brushing. However, there is a method of air blasting that can be used on flexduct that is not too dirty. Check with your HVAC or duct cleaning contractor regarding options or replacement.
  • If ductwork is lined with sound-proofing material or insulation, it cannot be cleaned and must be replaced.
  • There may be a less costly alternative to replacement in some cases. Caliwel for Ductwork has EPA registration for use in duct encapsulation. It would be a judgment call as to whether you could have Caliwel sprayed over sound-proofing material or insulation. Sometimes if replacement or remediation is too costly, “managing” contamination in place by spraying over it may be an option – not the best choice but you may have to work with your circumstances.
  • Yellow or pink insulation in older ductwork might contain asbestos. This ductwork should be removed by a certified asbestos contractor.
  • Many duct cleaning services use biocides in the air ducts, which is not recommended. Some individuals would be sensitive to the chemicals used in the air ducts. Further, it is not known if break-down chemical products may support the growth of mold later on. The alternative is essentially mechanical removal of debris, i.e., brushing, mechanically agitating, and vacuuming the inside of the air ducts. This should get most of the debris out.
  • In a study with human wiping of surfaces compared with the best mechanical methods, wiping always was more effective than brushing – but not all surfaces can be reached by wiping.
  • The protocols for cleaning a contaminated AC system can be quite detailed, including erecting containment and use of negative pressure, to avoid dissemination of mold particulates.
  • If AC coils cannot be satisfactorily cleaned in place, they should be removed for off-premises cleaning or replacement.
  • Post-cleaning testing could involve spore trap testing at several vents to ascertain that air blowing through has reasonably low levels of spores or hyphal fragments (branch-like structures of mold growth).
  • Again, if ductwork is new or just cleaned, make sure there is a good-fitting pleated media filter to protect the AC coils from dust. The coils are a site of condensation, so if you have water and dust, mold will grow. If the coils are clean and there is water, mold would have nothing on which to grow.

Cleaning window or wall AC units

We have addressed HVAC systems and mold. Mold is even more of an issue at window and wall units. Why? because filters may not be sufficient to keep the coils clean, and so Cladosporium grows on the coils… and between the coils. How do you get between these delicate and closely spaced coils to clean them? You get the picture.

  • Proper cleaning of the AC units would involve removing them, removing the insulation, sealing off the electrical components, steam cleaning the units, steam cleaning them a second time a day later, and putting in new insulation. Then, cover the intake vents with the allergen reduction filters.
  • Or, if you have no steam cleaner, take the AC unit to the yard, cover the electrical components, remove insulation, and spray-clean with somewhat high-pressure water from the hose. Be careful not to damage coils with the stream of water.  Wipe off and then finish drying in the sun. Replace the insulation with new.
  • If the unit is 10 years old or older or is not an EnergyStar unit, you would do well to upgrade to an EnergyStar AC unit. Money will be saved off your electric bill.
  • When purchasing a new unit, get one where the coils are accessible. With some styles, you can’t get to the coils easily.
  • Consider installation of an additional filter on the air intake of a new or recently cleaned unit, that is, on the vent where room air is drawn into the unit, not where the cooled air leaves the unit. Not all AC designs lend themselves to this optional step. The purpose of these filters is to protect the cleaned coils from dust, on which mold could grow. Filtete offers these allergy reduction filters. If these filters were installed on the cleaned units, the units may not have to be cleaned as often, but still follow the manufacturers instructions for cleaning.

What about fresh air?

  • What about it, indeed! Are you aware that in most houses (unless windows are opened), the same stale air is recirculated? Air conditioned air may feel fresher, but it’s the same stale air being conditioned. Think about it. If you keep windows closed, how does fresh air and oxygen get into your home? the little exchange when a door is opened and closed?
  • Commercial buildings have code requirements re: air exchange and ventilation, but not residential buildings, to my knowledge. What are the options for introducing fresh air into your home, especially in bedrooms at night?
    • Crack a window – but not much air exchange takes place on a still-air night.
    • Crack a window and run an exhaust fan either in a bathroom or a nearby bedroom. This will help pull fresh air into your bedroom. Depending on the layout of your bedroom, you might be able to work with an exhaust fan in one of your windows.
    • Look into an ERV (energy recovery ventilator) or HRV (heat recovery ventilator) for your home. This energy-efficient system brings in filtered fresh air and exhausts stale air. The unit can be turned off during rainy weather or if a neighbor is burning wood.
    • There are one-room window ERV units. When I locate that resource, I’ll update this point. Please email me if you need the information.
    • Some companies sell ThermaStor products, which consist of a large dehumidifier that can be attached by duct to the outside. Fresh air is brought in, filtered and dehumidified, and dumped either into the basement and/or ductwork, so that all year long you can have fresh air coming out ductwork. A representative told me that the coils have anti-microbial coating to cut down on the risk of mold. That said, if mold develops, they can be hard, if not impossible, to clean, because of their design. You might not be able to access the coils.

A story

A colleague and I were sitting in my small 10×10 office, with the door closed and the window closed. He had a data logger tracking CO2 (carbon dioxide) levels. Those levels were going up-up-up. Then, I opened the window, and the levels started falling. What if this were your bedroom instead of my office, with maybe two people sleeping in it, with no windows open and the door closed? Do you think those two people might wake up feeling a tad sluggish in the morning?

Improve your indoor air and water quality and reduce exposure to mold and electromagnetic fields