Sources of Contamination in HVAC Systems

Posted on 04/29/2022

To perform effective duct cleaning, it’s important to know what gets into a system and why. Contaminants can enter an HVAC system in a variety of ways, and can not only cause mold growth and unwanted odors in the system, but possibly affect air quality as well. As a professional air duct cleaner, you need to know where contaminants might enter the system, how to identify any contaminants present, and what to do to prevent them from infiltrating the system in the future.

Types of Contaminants

“Basically, anything and everything that can become airborne is a potential contaminant within an HVAC system,” said Michael J. McDavid, ASCS, CVI, general manager at Professional Abatement and Remediation Technologies (PART) and member of the NADCA’s Board of Directors.

In any type of building, outdoor contaminants that might find their way into a system can include everything from mold, pollen, and dirt to carbon, soot, and manufacturing exhaust. Another common source of contamination is an interior or exterior water leak. 

Indoor contaminants may find their way into a system as well. For residential buildings specifically, a significant source is dust. “Most of your household dust is skin cells that have been shed,” McDavid said. But in commercial buildings, possible indoor contaminants depend on the facility’s purpose. In food production facilities, for example, mixing ingredients like flour and sugar in large vats creates a tremendous amount of air contamination.

“Overall, I would group contaminants in the categories of bioaerosols, soils, dirt and debris,” McDavid said.

Allowing contaminants to linger can create real problems in the system’s evaporator coils, which collect condensation and therefore provide a robust environment for issues like mold growth. “In the system, mold has the perfect temperature and humidity, and it’s got a food source,” McDavid said. “Introduce a spore and it takes off. It’s like planting a flower.” Mold growth can prevent the air from moving across and through the fins properly and can insulate them, inhibiting thermal transfer. This can block the system from dehumidifying the air, which can then exaggerate any contaminants already in the system, turning them into a food source for microbial growth.

Signs Contaminants Are Present

Any time you clean an HVAC system, you should thoroughly inspect its entirety, and if any issues with contamination are apparent, you’ll be able to locate and address them as you work. However, there may also be external signs of contaminants that will alert you to specific problems you need to resolve, such as:

  • Significant dust levels within a building. “A good tip-off that you need your ductwork cleaned is if you can’t get your dust levels under control,” McDavid said. If regular cleaning can’t keep up with the accumulation of dust, debris has likely built up within the system.
  • Dirty vents. Clients often contact a cleaner because they’ve noticed the vents are dirty, not realizing this means there is likely an issue deeper in the ductwork. You may need to conduct a particle profile to determine the source of the dirt.
  • Dirt trails on filters. If you can trace the direct path the dirt has made around the filter, it indicates that an ill-fitting filter is allowing dirt into the system. Many times, you will discover that the debris is coming from around registers that weren’t properly installed rather than from within the duct system (attic air is a good example of this). Certain areas like copy rooms have dirty vents due to toner in the air rather than from contaminants recirculating or discharging from the system.
  • Unpleasant odors. When a client requests a microbial inspection because there’s an unpleasant smell in the building, you are likely to find contaminants in the evaporator coils and in the return line. “Air takes everything it can carry with it, and it always takes the path of least resistance,” said McDavid. “This means that return air gets caught in the filter bank and collects in the evaporator coils.”

Even if a client has contacted you because they’ve noticed something is amiss, they typically don’t know what it is that is causing the problem, so it’s up to you to locate the source of contamination.

How to Mitigate the Contamination

Thorough cleaning and inspection alone are not enough to maintain an HVAC system — you need to take steps to prevent, lessen or regularly address further contamination:

  • Outdoor contaminants. It’s extremely important to ensure that the building envelope is airtight in order to keep debris out of the building. You need to check for water leaks, and you also need to examine not just the seals around windows and doors, but the whole of the building to be certain there are no infiltrations. “It doesn’t matter what you do to the system if you’ve got a hole in the side of the house,” McDavid said.
  • Indoor contaminants. In any type of building, housekeeping is key. “If you’re not doing a good job with housekeeping, the system is going to get filthy,” McDavid said. Whether someone employs a cleaning service or does their own cleaning, maintaining a regular housekeeping schedule will reduce both the amount of dust in the system and the likelihood of microbial growth. Additionally, microbial growth is more likely if humidity levels are above 65%.
  • Production-related contaminants. In the earlier example of a food production facility that fills the air with flour and sugar, if creating contamination in a work facility is part of doing business, you can’t correct that particular source of contamination, so it’s important to have regularly scheduled HVAC system cleanings in addition to ensuring there are no additional sources of contamination.
  • Contaminants from leaky filters. “Filters are not in place to protect the occupants; filters are in place to protect the components,” said McDavid. If a filter in the HVAC system is leaky, has streaks of dirt, or any gaps around it, efforts should be made to correct the issue. Filter bypass is typically caused by a filter that either isn’t the correct size or isn’t installed correctly, so you may need to take steps to either replace the filter with the correct size or seal the frame.
  • Contaminants from leaky ducts. Contaminants can be drawn in post-filtration through leaks and seams in the ducts. Fortunately, there are a lot of products designed to seal duct seams both internally and externally; meaning once you locate any leaks, you can rectify them quickly.
  • Contaminants from exhaust vents. Exhaust systems, which are designed to pump air from the inside of a building to the outside of a building, can be a source of contaminants if the exhaust system’s ductwork isn’t sealed. In that case, the materials can escape their designated path out of the building and get drawn into the HVAC system instead. As with leaky ducts in the HVAC system, the solution to this is sealing the seams of the ductwork.

HVAC systems can be infiltrated by contaminants in other ways as well. Lack of regular maintenance can result in a buildup of debris — and possibly mold — over time. This can be prevented by implementing a consistent maintenance schedule. You may not be able to protect a system from damage inflicted by mechanical failures, fires, floods, or other unexpected events, but if any such event takes place, you will need to clear the system of contaminants and repair any damage.

Thinking Outside the Duct

On most projects, the contaminants you find will be run-of-the-mill instances of dust, debris and mold spores, but you may occasionally come across less common types of pollutants. You should always keep an eye out for the possibility of foreign matter like:

  • Asbestos. “What would tip me off that there’s asbestos is the age of the building. Basically, anything built before 1980 is suspect,” McDavid said. “I would expect there to be some asbestos and lead throughout the building.”
  • Insects or animals. There might be fruit flies in a system, or possibly rodents. If there are animals in the system, they likely won’t be alive, and you’ll know to look for them because the customer complaint will mention noises or rank smells.
  • Motor vehicles. In a building next to a bus depot, you might find that a system is full of black soot. If you’re working in a hospital, there might be a process in place to briefly shut down any roof vents whenever a helicopter lands on the helipad so the air and debris swirling around the helicopter don’t get sucked into the system.

If you aren’t sure what something is, you may need to conduct a particle profile to identify it or consult an Indoor Environmental Professional (IEP). If you aren’t qualified to conduct sampling, team up with a contractor or make a referral to a qualified person.

These are just a few examples of contaminants you may find while cleaning an HVAC system. But whether you come across the contaminants mentioned here, or any of the myriad of other possible pollutants, if the source of contamination isn’t discovered in one of the usual places, you need to think outside the duct and survey the local environment to determine where it’s coming from.

Ensuring Cleanliness for Health and Safety

Keeping an HVAC system clean and clear of contaminants helps provide a safe and pleasant environment in both residential and commercial buildings. With any job you undertake, make sure you survey the environment for any areas that might be allowing contaminants access to the system so that you can make the appropriate repairs to maintain a well-functioning system.

This article originally appeared in the January/February 2022 edition of DucTales.

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