FM Articles

Mitigating Mold in School & Hospital HVAC Systems


Just as the lungs in our bodies feed our systems with life-giving air, the HVAC systems in hospital or school facilities feed air through the buildings to our most vulnerable occupants – the ill and the young.

The CDC states that people who are most at risk for health problems associated with exposure to mold include those with immune suppression or underlying lung disease people who are often found in hospital populations. The Institute of Medicine also found evidence linking indoor mold exposure and respiratory illness in otherwise healthy children. (See CDC “Facts about Stachybotrys chartarum and Other Molds“).

Facility managers at hospitals and schools must use due diligence to prevent mold growing in the HVAC system and winding up in the air occupants breathe.

Mold Growth in HVAC Systems is Inevitable
Molds are part of the natural environment. Molds are fungi that can be found anywhere – inside or outside – throughout the year. According to OSHA, about 1,000 species of mold can be found in the United States, with more than 100,000 known species worldwide

The dark and damp interiors of HVAC systems provide the perfect conditions for mold growth. Mold normally enters into the air handler as “spores” – microscopic particles below the limit of human visibility. These biological spores easily enter the air conditioning unit on a daily basis. The air handling unit usually already has a reservoir of   fungal and mold growth because most owners do not specify periodic sanitizing of the unit. This can become quite serious as seen in the photo (below, to the left, editor to specify) that was taken in a major hospital.

To compound the problem in hospitals, tests have found pathogens and biological spores on cooling coils from germs of sick patients coughing or sneezing that traveled in the airstream back to the air handler.

Mold, fungus, and other living organisms rapidly build up in the HVAC system, even new systems. Although filters can do much to stop the entrance of spores into the air handler, spores are very small, on the order of a few microns, and can easily bypass the filters, traveling around them.

Once inside the air handler, the spores encounter a moist environment. This results in the spores growing on all interior surfaces of the air handler, as well as within any insulation on interior walls. Microbials also adhere to cooling coil surfaces where high velocity air passes.

Microbials such as fungi and bacteria interlace and form biofilms. Biofilms are of varying thickness, ranging from below human visibility, and often might not be seen but are still present. Biofilms can also get very thick and become easily seen deposits. Biofilms grow continuously on cooling coils and other surfaces of the HVAC system. During winter months when the cooling is absent and heating coils are on, bacteria and fungi will dry out back into spore form. At this point, the microbes are very lightweight, and can easily be transported into the air duct system and into the rooms.

When microbials are growing heavily inside an air handler, an individual may notice a distinct odor caused by the organisms. That particularly unpleasant “dirty sock” smell that sometimes occurs, known as Dirty Sock Syndrome, is the result of mold growth and biofilms on the coils and other surfaces of the HVAC unit. Biological odors are a clue that proper sanitization should be done.

Handling Mold and Biofilms on HVAC Coils
Dirt and microbes on coils need to be cleaned using an effective coil cleaner. Alkaline cleaners are generally selected as they can remove the accumulated oils or greases that act as nutrients for microbial growth. Alkaline cleaners are quite effective on dirt and biofilms as well.

Coil cleaning should be thoroughly done according to best practices. Always wash off any coil cleaner after using. Cleaning residue, such as dirt or bacteria, remains on the coils if the cleaner is not washed off. Cooling coils do not have enough condensate velocity to adequately remove residues. Generally, rinsing the coils after cleaning only takes a few minutes and solves a variety of problems in the future.

A major issue with highly alkaline coil cleaners is that they dissolve away system metals. Cleaners will corrode the aluminum fins and copper tubes of the coil. Residual coil cleaner not rinsed out of condensate drain pans will corrode the condensate pans causing premature and expensive replacement.

Effective Preventative Maintenance (PM)
After cleaning, a preventive maintenance program should be put in place to help prevent the rapid growth of microbes within the system. This also helps to reduce the monthly electrical bills.

According to ASHRAE studies, owners of HVAC systems are paying up to 40% in additional electrical costs for units fouled with dirt and fungi. Biofilms on the surface of the cooling coils are like wearing a sweater when trying to cool down.

In order to address mold and biofilms, the PM program should utilize an EPA-registered antimicrobial sanitizer for use on coils and other internal air handler surfaces. A good sanitizer should attack microbes actively growing in the air handler coils and other interior surfaces. Odor reduction will be noticed quickly.

The owner and his service team will see a reduction in the necessity of coil cleaning and odors by use of one of the newer in-house applied coil coatings. This will result in less man-hours invested in cleaning as well as energy savings.

The coating should be very thin, under a micron or two. Coatings that are thicker can actually impede heat transfer and increase monthly energy consumption as a result. Improving energy efficiency by mitigating mold in the HVAC system should be high priority for any facility manager working with a tight school or hospital budget.

Mold Growth and Condensate Drip Pans
In addition to the buildup of mold and fungus on coils and HVAC surfaces, mold growth and debris can stop or plug up the drains of the condensate drip pans. When this occurs, water overflows the pan and onto the floor or rooms below. Water overflows can cause extensive mold problems in the areas surrounding the HVAC system, requiring expensive, time-consuming clean up. Mold growth between floors cannot easily be addressed and will be a continuing source of odors that are preventable by maintaining a clean condensate pan environment.

New technologies are available to help the owner and service technicians maintain a clean condensate pan with clear water. These new solutions are timed-released so that cleaning agents are continuously released without maintenance attention or work. They will perform for 3 to 6 months, and usually have a strong guarantee.

Effects of Poor Water Drainage and Microbial Growth
Hills and valleys in condensate drip pan surfaces are often overlooked by the owner and service technicians. Water accumulates in these stagnant areas, providing static areas where microbes can grow.

An owner can get rid of these hot spots of microbial growth easily by using new self-leveling pan coating technology. Applied easily in-house, coatings not only self-level pans and remove potential areas of growth, but also contribute markedly to the lifespan of the air handler by eliminating corrosion occurring from coil cleaners or time-related corrosion.

HVAC Maintenance – Best Practices to Prevent Mold Growth
Preventing and eliminating mold in your HVAC system is important to the health of the vulnerable occupants in school and hospital facilities. Implementing some key preventative and routine maintenance actions will go a long way toward mitigating mold problems in these facilities:

  1. A program requiring coil cleaning as a minimum once per year in the absence of an effective coil coating.
  2. Sanitize the coils and internal surfaces twice per year with an EPA registered anti-microbial sanitizer.
  3. Be wary of odors from the air handler and address as needed.
  4. The use of a thin coil coating is recommended to reduce odors and the frequency of coil cleaning needed ideally down to once every one or two years.
  5. Protect the condensate drip pan from overflows and odor-causing microbial growth with a timed or controlled release cleaner.
  6. For additional protection and air handler longevity, use a self-leveling coating with an antimicrobial that guards against the growth of odors from microbial causes. The coating will level the hills and valleys of the condensate drip pan to eliminate water pooling.

Lynn Burkhart is the President and Founder of Controlled Release Technologies (CRT), a creator and manufacturer of advanced HVAC Hygiene techniques since 1985. The CRT Research and Development laboratory creates technologically advanced solutions for low-cost and effective HVAC maintenance. For more information, visit to learn more. or call 800.766.9057.