Case study: Hospital HVAC retrofit

Air handling units were retrofitted with ultraviolet germicidal irradiation to mitigate the coronavirus in a hospital

By Roger Koppenheffer and Caleb Marvin October 21, 2021
Table 1: The UV-C light schedule provides the information needed to specify the ultraviolet lights used for the project. Courtesy: Certus Consulting Engineers

A hospital in the Dallas/Fort Worth area engaged Certus Consulting Engineers to retrofit two of its existing air handling units with ultraviolet germicidal irradiation as part of its response for COVID-19 virus readiness. The facility requested a cost breakdown of two options for UVGI in the two AHUs. 

  • Option 1: Provide UVGI to prevent bacterial and fungal growth on the cooling coil and drain pan. 
  • Option 2: Provide kill “on-the-fly” UVGI to inactivate any particulates in the airstream and bacterial in addition to preventing fungal growth on the cooling coil and drain pan. 

The first step in the design of the options was determining the correct placement of the UVGI system. With both options, the facility requested that the UVGI system be provided to combat bacterial and fungal growth on the cooling coil. This required both options to be placed downstream of the cooling coil to shine on the coil and the drain pan. 

The next design consideration was to determine the average irradiation intensity needed to satisfy the different target goals of each option. The high relative humidity of the supply air coming off the cooling coil fully saturated was taken into consideration. The air velocity in the cabinet was within the recommended criteria of 500 feet/minute so it was deemed unnecessary to de-rate the efficiency of the UV lamps.  

Additional design considerations were acknowledged for client safety. The safety of the maintenance staff is a major priority when specifying the system. To prevent accidental exposure to the UVGI system when maintaining the AHUs, a door switch interlocked to the UVGI enable/disable was provided.  

AHUs with viewing windows need special UV protection on the glass. Any time the door was opened into the AHU section with the UVGI system, the door switch opened the relay disabling the lights and hence protecting maintenance staff. A lamp life meter and status connection were designated to allow the facility maintenance staff to see the UVGI system at the building automation system. Certus Consulting Engineers engaged a UV lamp manufacturer for assistance in determining the correct average irradiance and pricing for both options.  

For option 1, 10 watts/square foot was selected to prevent the growth of bacteria and fungi while 21.25 watts/square foot was determined to be able to inactivate particulates “on-the-fly” for option 2. The pricing for the kill “on-the-fly” came in roughly 30% higher than cooling coil cleaning only. 

The facility decided to move forward with the kill “on-the-fly” option to retrofit the AHUs based on the information provided. The design allowed the facility to improve the air quality delivered to patients by inactivating any particulates that passed through the air stream in the AHU with the added benefit of cleaning the cooling coil and drain pan.  

Author Bio: Roger Koppenheffer is a principal at Certus Consulting Engineers. He a founding principal and brings 26 years of experience in the mechanical, electrical and plumbing consulting field to manage and engineer a multitude of projects, specializing in health care. Caleb Marvin is an associate at Certus Consulting Engineers. He has more than five years of mechanical engineering experience in the design and construction of health care facilities. He brings technical knowledge and solution-oriented expertise to the health care engineering industry.