Dirty coils can totally disrupt the ability of an HVAC system to perform as it should, both in terms of the quality of the air it provides occupants, as well as operational costs. Cleaning coils with probiotic cleaners represents a “next generation” approach to HVAC coil maintenance that can help facility managers ensure the following for their facilities: better quality of air, improved performance, energy savings, and extended equipment life cycles.
Why Coils Matter
HVAC systems have been described as the “lungs” of a building; its purpose is to deliver quality air to all parts of the facility and to promote the best possible indoor environment for the occupants. For this reason, properly maintaining coils and filters are important to consider to maintain the indoor air quality (IAQ) of a building. In order to do this, it is important to select the right filters and replace them regularly within manufacturers’ recommended specifications. Also, it is necessary to ensure that the filter racks are properly installed and sealed to reduce filter bypass.
Coils are all too often an overlooked, neglected component of the HVAC IAQ equation. If the filters and the coils are not well-maintained and properly operated, performance, occupant comfort and energy issues may arise.
While IAQ is usually the core focus when it comes to HVAC systems, energy considerations are also of critical importance. Adoption of LED lighting, sealing the building envelope, motion sensors, and other measures designed to improve the energy efficiency of a building, have resulted in HVAC systems becoming the largest “energy hog”. Consequently, HVAC systems are increasingly the “next frontier” in lowering facility energy consumption.
The following are competing approaches to finding the right balance between IAQ and energy costs:
- Equipment Replacement: Favored by mechanical companies, this method is the most expensive.
- Adjustment of Automated Building Systems: This approach has the potential to reduce run times and maximize moisture control.
- Equipment Maintenance: This method is known to be favored by service providers.
These competing approaches often land on-site facility managers in the middle of budget battles over issues regarding “replacing or repairing” or “building comfort”.
If equipment maintenance costs can improve performance and can be covered through energy savings, then expensive equipment replacement can be deferred, often for many years. Plus, the building can be operated at a comfortable temperature and humidity level.
Coils are the key to an effective and efficient indoor environment.
Coils are the key to an effective and efficient indoor environment, as they have the ability to: maintain temperature, control humidity levels, and positively affect the air supply. If the coils are “dirty”, all three of these functions will be impeded and performance will decline, which in turn causes costs to rise and equipment life cycles to be impacted. In this way, coil cleaning becomes a crucial part of the merging of equipment maintenance and energy management. This makes it important to then take note of the two types of contamination that most impact an HVAC coil’s performance: biofilm and organic matter.
Coil cleaning becomes a crucial part of the merging of equipment maintenance and energy management.
The Role of Biofilm
Of these two types of contamination, biofilm is the most overlooked and, yet, is also the most important. Biofilm is defined as any group of microorganisms in which cells stick to each other on a surface. These adherent cells are frequently embedded within the self-produced matrix of an extracellular polymeric substance (EPS). In a lab, biofilm is formed by taking a piece of sterilized metal and dripping water onto it. Free floating planktonic bacteria in the air then start to find their way to this surface and attach themselves.
In a lab, a biofilm is formed by taking a piece of sterilized metal and dripping water onto it. Free floating planktonic bacteria in the air then start to find their way to this surface and attach themselves.
The importance of biofilm is based both in terms of thermal transfer efficiency and energy consumption. In a coil bank, moisture can be stripped out of the air that is being forced through the coil and onto the coil tubing. The moisture then flows down over these surfaces to be captured into a drain pan for removal. In other words, coils are a perfect biofilm reactor/generator. For this reason, biofilm is the greatest impediment to the thermal transfer of the coil and can greatly impact its energy usage.
The lower the number, the lower the thermal conductivity as measured in Watts per Meter Kelvin.
In this way, biofilm has the ability to negatively impact the thermal conductivity of a coil two to three times more than mineral deposits that are often seen on the coil. The California Society of Healthcare Engineers back in 2008 determined the extent to which even a thin layer of biofilm can seriously impact the energy consumption in HVAC systems (see table for results).
In the image above, 100 microns approximately equates to the thickness of a human hair.
So not only is biofilm a serious issue both in terms of thermal transfer and energy usage, but equally important is the understanding that biofilm is a “sticky substance” and, thus, acts as a major reason other contaminants— and particles that manage to get through the filters—can adhere to a coil. These other contaminants can then lead to significant restriction of the air flow through the coil, and thus further impede performance and energy efficiency.
These images are of biofilm on coils, which pose a possible health threat because of the biological entities that make it up.
These pathogens have been found in the biofilm of an HVAC coil.
Biofilm on coils also poses a possible health threat because of the biological entities that make it up. Addressing the presence of biofilm on HVAC coils is critical due to the importance of maximizing the thermal conductivity of coils, minimizing the energy consumption of the unit, and reducing health risks.
How Best to Clean Coils
In far too many instances, HVAC technicians are cleaning coils using products and processes that do not remove biofilm, and so fail to optimize thermal conductivity and significantly reduce energy consumption.
Furthermore, a majority of the coil cleaners used are corrosive. This means that they are:
- Dangerous to apply: There are numerous instances of workers’ compensation claims arising from injuries sustained from the use of corrosive cleaners.
- Potentially damaging to the coil: Corrosive cleaners pose the risk of shortening the life cycle of the coil units.
- Ineffective to remove biofilm: Corrosive cleaners may even encourage biofilm reformation by creating microscopic pitting that captures more moisture.
Corrosive coil cleaners are applied in conjunction with processes that are totally inappropriate for the task at hand.
It is thus important to recognize the extent to which these corrosive coil cleaners cause long term irreversible damage to the coil fins, tubing, metal frames and the environment. Once the directional fins and coil tubing have been exposed to corrosive cleaners, the surface of the coil actually starts to melt away. This leaves microscopic pitting on those surfaces, which then prevents the moisture from draining off the coil surface as designed, since the pits hold moisture. This moisture retention results in the following:
- The potential repopulation of biofilm forming pathogen bacteria.
- The opportunity for biofilm to help organic particulate pass through the coil and adhere to it, and so speed up the coil fouling process.
To make matters worse, these corrosive coil cleaners are applied in conjunction with processes that are totally inappropriate for the task at hand. Such processes range from merely washing down the coil with ambient temperature water delivered via a hose from a facility tap, causing an inadequate pressure to force the contamination and cleaner out of the coil, to technicians applying high pressure water that ends up bending the fins and creating an issue that is even worse.
Once biofilm has started to grow inside a coil, especially in the heart of coils that have four or more rows, these conventional coil cleaning chemicals and processes are inadequate at addressing the formation of pathogen biofilms, which are biological communities. Pathogen biofilms can replicate as quickly as every 20 minutes, and the bacteria can communicate with each other via a process known as Quorum Sensing. Once they get to a certain critical mass, the bacteria can “off gas”, which is known to building occupants on the receiving end as “Dirty Sock Syndrome”.
If the coils are neglected and not cleaned, it becomes more difficult to restore their performance through a cleaning alone.
For these reasons, it is recommended that coils be cleaned at least once every year. If the coils are neglected and not cleaned, it becomes more difficult to restore their performance through a cleaning alone. For this reason, it is important to find the right products and processes to effectively remove the impacted biofilm and particulate out of the center of the coil.
One coil cleaning method and cleaning product combination that has proven successful at safely and consistently restoring coil performance is a combination of:
- Superheated Pressurized Water (SHPW): Wet steam at between 265°F and 285°F delivered at approximately 1,500 PSI. The extreme high temperature facilitates the dissolving of the solids inside the coil, and the pressure and flow help flush the debris from the coil.
- pH neutral probiotic coil cleaner: The probiotics work to break down the biofilm. Being pH neutral, they can be applied and left on the coils days in advance of using the SHPW. This can be beneficial to do because the airflow from the fan units can drive the probiotic cleaner deeper into the coil.
The inclusion of probiotics into the coil cleaning equation produces improvements in BTU output that are meaningfully greater than the improvement in the air flow through the coil as measured in CFM. The removal of the biofilm from the coil that occurs due to the introduction of the pH neutral probiotic cleaner improves the thermal transfer efficiency of the coil and so optimizes its energy performance. This can lead to adjustments in set points, avoidance of blowing compressors, and reduced use of the system.
For these reasons, cleaning coils with both SHPW and pH neutral probiotic cleaners are an effective way to clean HVAC coils that is not only safe for the technicians conducting the cleaning, but also safe for the equipment and the environment.
By Graeme Marsh, Managing Director, Z BioScience Inc. and Greg Long, President, IAQ Consulting Services Inc.