Conditions in HVAC systems can encourage the growth of biofilms containing bacteria and mould on moist or wet surfaces such as cooling coils, drain pans, plenum walls, humidifiers, fans, energy recovery sections and filters. Locations inside and below the cooling coil section are particularly vulnerable due to moisture condensation and transport from the coil fins. Contamination of cooling surfaces by biofilms can increase pressure drop and reduce airflow and heat recovery efficiency.
Filters capture bacteria, mould and dust that can lead to microbial growth in the humid filter environment. As growth increases, a filter’s resistance to airflow may increase. This can result in more frequent filter changes and increased exposure to germs for maintenance workers and workers. As airflow and heat recovery performance decrease, so does the air quality in the relevant areas (Kowalski 2006).
Chemical and mechanical cleaning, traditional methods of maintaining air handling unit components, can be costly, difficult to perform and dangerous for maintenance personnel. Vapours from cleaning agents cause poor air quality and chemical runoff causes groundwater contamination. In some cases, mechanical cleaning can also shorten component life. Also, shortly after cleaning, system performance may begin to degrade again as microbial growth reappears or reactivates. (Ch.62,ASHARE,2019)
UV Coil Disinfection units can be applied to HVAC systems, typically air handling units, to complement traditional system maintenance procedures (Bahnfleth 2011). The use of UVC is effective in reducing air pressure drop and increasing the heat transfer coefficient in cooling exchangers. (Bahnfleth 2017). The potential advantages of UVC surface treatment include keeping surfaces constantly clean rather than periodically cleaning dirty surfaces, eliminating the need for chemicals, lower maintenance costs and potentially better HVAC system performance.
UV lamps can be installed to target problematic components such as cooling heat exchangers, condensation pans or filters, or they can be applied to provide a wide distribution of UVC energy over an entire enclosure that may have microbial activity. Like duct air cleaning.
When used in conjunction with other forced air treatment modes, UVC provides an incremental benefit. For example, if a particulate filter removes 85% of a given agent in an incoming air stream and a UVC system with a single pass efficiency of 85% for the same pollutant is installed in series with it, the combined filter/UVC system combined single pass capture efficiency is approximately 98%. (i.e. the incremental benefit of adding an 85% efficient device is 13%). Conditions involving ventilation, filtration and UVC can be evaluated quantitatively by analyzing the entire system. (Ch.62,ASHARE,2019)
Under certain conditions, virus particles in the extract air can re-enter the building. Heat recovery devices can carry viruses attached to particles from the exhaust air side to the supply air side through leaks. In rotary heat exchangers (including enthalpy wheels) particles accumulate on the return air side of the heat exchanger surface and may then be mixed back into the air stream when the heat exchanger returns to the supply air. Therefore, it is recommended to (temporarily) turn off rotary heat exchangers during the SARS-CoV-2 phases. (Rehva,2020)
If leaks are suspected in the heat recovery sections, pressure adjustment or bypass may be an option to avoid a situation where higher pressure on the exhaust side would cause air leaks on the supply side. (Rehva,2020)
The spread of virus particles through heat recovery devices is not a problem when equipped with an HVAC system, double coil unit or other heat recovery device that guarantees 100% air separation between the return and supply side. (Rehva,2020)