*This article is based on a presentation given by Robert Rizen at the National Air Duct Cleaners Association’s 2017 annual conference. 

Coil cleaning is a much-needed service for many customers, whether residential, commercial or industrial, in health care or education. Coils are everywhere from evaporators to condensers, reheat, heat recovery, heating, process, chiller, cooling towers and refrigeration.

Cleaning is an ongoing process. Coils are getting dirty during everyday use due to improper installation, inadequate prefilters, no filters, leaking filter racks and dirty operating environments. Some coils need monthly, quarterly or yearly cleaning, depending on the operating environment.

 Dirty coils can lead to:

  • Replacement of the coil sets
  • Increased run time on HVAC equipment
  • Inefficient chiller operation
  • Excess chiller wear and tear
  • Loss of compressors and fan motors
  • Increased energy use
  • Poor indoor air quality  
  • “Dirty sock” syndrome
  • Microbial and bacterial growth (humidity) hinders the water-shedding ability of the coil
  • Improper airflow to occupied spaces
  • Improper temperature delivery

HVAC is typically 50 percent or more of a facility’s monthly bills. There’s a big incentive to keep these systems running efficiently and at peak performance.

Why do clients care?

There is a direct verifiable return on investment for coil cleaning as proved by a study from the American Society of Heating, Refrigerating and Air-Conditioning Engineers:

  • A 34-story building in New York City has 1.2 million square feet of floor space, which has to be cooled from 6 a.m. to 6 p.m. using four large 30-year-old air handlers, ranging in size from 81-250 tons.
  • After testing during the first control week, two of the air handlers were taken offline for two days to perform a deep cleaning of both air conditioners. The two systems were then put back into service and tested in exactly the same manner for another week.
  • HVAC inspectors and testing, adjusting and balancing contractors continuously measured 54 different data points from the two air conditioning systems for a week before and a week after cleaning, including coil differential pressure, air and water temperatures, condensate temperature, supply air velocities, outside air temperatures, humidity, volumetric flow rates, voltage and amps.


The ASHRAE study found that cleaning each system decreased coil differential pressures by 14 percent, which produced a corresponding increase in the flow rate and overall cooling capacity of the system by the same amount. 

After cleaning, the smallest air conditioning unit of the four started punching well above its weight, adding an extra 19-22 tons of cooling capacity (an additional 67-77 kilowatts), increasing its overall capacity by 25 percent.

The thermal efficiency of the cooling coils in the cleaned systems increased by 25 percent and condensate water temperature dropped from 3-4°C before cleaning to 1-3°C after.

The inspectors estimated that 100 tons (352 kilowatts) of cooling capacity would be added to the building once all four air handlers had been cleaned and restored in this manner.

Based on year-over-year HVAC building costs, ASHRAE estimated that cleaning one of the air handlers resulted in efficiency improvements that will lead to energy savings of up to $40,000 annually.

A ‘hidden enemy’

A substance known as “biofilm,” growing deep inside the coil sets is a common unnoticed problem. It’s easy to make a coil appear clean and shiny, but what’s going on deep inside the matrices?  

Biofilm is a complex microbial matrix growing on coils and drain pans. It’s composed of different microorganisms adhering to surfaces and producing polysaccharides, proteins and nucleic acids. The substance can stick together and is often difficult to penetrate with outside agents, such as antimicrobials.

Biofilm acts as an insulator on the coil surface, affecting the temperature and airflow. It does not take much buildup to alter the operation of the coil. Byproducts of biofilm include:

  • Volatile organic compounds
  • Microbial volatile organic compounds
  • Known opportunistic organisms causing infections
  • “Sick building syndrome”
  • Occupant complaints
  • Number of employee sick days used due to allergy/coldlike symptoms

Biofilm is not a new problem. Science and research have exposed it as being a more common occurrence than previously thought.


Prevention: Have proper filtration in place before the coil sets, and fix leaks, gaps in filters and gaps in filter racks. Prefilters may be needed in some cases. Pleated filters are best. 

Cleaning: Regular cleaning from the time it is put in service can go a long way to preserve expensive coil sets. In fact, regular cleaning and inspection can solve most of the issues discussed in this article.

Treating: There are products designed for treating the drain pans and certain sprays may be applied to the coil sets. These products include something to inhibit biological growth.

Get started

It is not very expensive to get into coil cleaning, but proper training is crucial. Some coils can be cleaned with nothing more than a garden hose, some coil-cleaning chemicals and a spray wand. 

Larger, thick coil sets will need more attention, requiring higher pressures and water flow to rinse, hot water or steam, chemicals and careful water control. Time is the real factor in cleaning. Chemicals need dwell time, and rinsing to restore a coil’s performance takes a large amount of time.

Suggested tools include:

  • Foaming coil chemical application gun
  • Long coil wand/lance with a spray jet on the end. This item is great for cleaning condensers without disassembly.
  • Water containment devices
  • High flow, low pressure power washers, which allow for high water volume for rinsing.
  • Harsh chemicals commonly used years ago are now frowned upon, and research has shown that many were doing damage to the coils construction and drain pans. With many new higher efficiency coil sets being made from mixed metals, many prefer to use a metal-safe chemical.
  • There is also a probiotic cleaner and treatment being produced to address biofilm issues that are resistant to other chemicals. Science is making it easier to do a thorough job.