COVID-19 PREPARING ENVIRONMENTS FOR A HEALTHY RETURN HVAC Quick Reference Guidee
HVAC CONSIDERATIONS FOR HEALTHY ENVIRONMENTS
While HVAC-related transmission is not considered the primary mode of transmission of infection, it is still considered a risk. More importantly, a well ventilated, temperature and humidity-controlled environment can boost the immune system of individuals naturally protecting them against infection.
Currently, there are three main ways infection can spread:
1
redominantly, infection is spread through large droplet transfers between people in a fairly P close range. Coughing and sneezing produce large droplets that can fall onto surfaces or directly into another’s eyes, nose or mouth. This is where the six feet guideline originates.
2
ext, transmission can occur if those large droplets land on surfaces and someone else N touches that surface who then touches their eyes, nose or mouth; or if the particles on the surface are disturbed and re-entrained in the air.
3
astly, transmission can be airborne. Some strains can potentially stay airborne and infectious L for up to three hours1 and are small enough to pass through most commercial and residential HVAC filters. Studies have shown that particulates have increased infectivity in dry air2. There is a chance that the particles could be transmitted through the HVAC system this way.
The first two methods of transmission are combatted through social distancing, hand washing and surface disinfection.
To prevent the spread of infection through the HVAC system, the following are recommended: aintain recommended temperature (per ASHRAE Standard 55-2019) and 40%-60% relative M humidity for the building.3 • It is important to keep the building at a comfortable temperature. Extreme temperatures can cause stress to the body which can weaken its response to infection. • Relative humidity between 40%-60% has been shown to improve human resistance to infection. Some studies have also shown that at this humidity range, infectious particles settle out of the air quicker and onto surfaces where they can be cleaned off. • Humidification is not typically installed in commercial buildings, outside of healthcare. Thus, 40% to 60% relative humidity may not be attainable during colder weather in most buildings without modifying the HVAC system. Increase the amount of outside air to the building. • At air handling equipment, raise minimum outside air to maximum point where the above temperature and humidity ranges can be maintained and building pressurization is controlled. However, prioritize increasing the outside air over maintaining the humidity range. • Disable demand-controlled ventilation which is designed to reduce outside air during periods of lower occupancy. • Consider “flush periods” of maximum outside air of two hours before and after occupancy.
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HVAC CONSIDERATIONS FOR HEALTHY ENVIRONMENTS
Increase exhaust from the building but keep the building (and individual floors) positively pressurized. • Toilet exhaust systems shall run 24/7. • Garage exhaust systems shall run 24/7 or two hours before and after occupancy. • Carbon Monoxide monitoring systems that control exhaust may need to be disabled. • Other exhaust systems shall run as normal but start two hours before, and for two hours after occupancy. • Consider adding exhaust to rooms where people can congregate such as pantries and conference rooms. heck HVAC system for proper operation. Avoid failures during this time where parts and labor may be C delayed, or unavailable and HVAC operation is critical. • Make sure systems are functioning and in good working order. • Verify controls are responsive, calibrate temperature and humidity sensors. • Inspect duct systems to make sure no blockages in the airstream and rooms are getting the airflow needed (i.e. closed fire damper or leaky duct fitting). • Check outdoor air intakes to the system to make sure no sources of contaminants are nearby and can be brought into the building. • Similarly, confirm exhaust discharge locations (including any added temporarily) cannot harm pedestrians or be pulled back into the building. Identify with signs areas of concern to prevent gathering near the discharge location. Minimum clearance is 10 feet, best practice is 25 feet distance. Address the filtration in the HVAC system. • Make sure the filter frame in the unit is sealed to the casing so no air can bypass the filters. • Typical commercial filters are MERV (Minimum Efficiency Reporting Value) 8. To capture any of the largest pathogen particulates, a MERV 13 is required (MERV 14 filters are preferred). HEPA filters are most effective, but not realistic for most commercial HVAC systems due to the added pressure drop. In-room recirculating HEPA filters can be installed to any critical rooms. • You can consider changing filters less frequently to avoid exposure due to replacing them. Watch pressure drops across filters to make sure airflows are not impacted. • Used filters can be sanitized with 10% bleach mixture or just bagged and put in regular trash (sanitized or not). • Service contractors shall wear PPE per latest requirements.
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HVAC CONSIDERATIONS FOR HEALTHY ENVIRONMENTS
Other HVAC equipment: • Weigh the costs and benefits of the operation of energy recovery systems (such as heat wheels or enthalpy wheels) where cross contamination between exhaust and outside air streams can occur. Turning off these systems may make design ventilation rates and maintaining temperature and humidity ranges impossible. However, if the exhaust airstreams are contaminated, it could contaminate the supply airstream through the wheel. • Increase terminal unit (VAV boxes) minimum airflow rates higher on VAV boxes for greater airflow (higher air changes) to the space. • Cooling coils, heating coils, and drain pans within air handling units can be contaminated. Consider adding UVGI (Ultraviolet Germicidal Irradiation) within the unit for cleaning of these surfaces between in-person disinfection. • If areas of the building are unoccupied (by floor or tenant suite) and served by 100% outside air units, close supply to those spaces to push air to occupied spaces. • Keep areas positively pressurized to the outside but keep floors neutral to each other. • If a floor has people that are infected, shut off the return from that floor and exhaust all the air. Specific areas of the building: • Conference rooms – Keep doors open to promote good ventilation where possible. Consider portable air filters or install exhaust fans ducted directly to outside if doors need to remain closed. • Pantries or storage rooms – Keep doors open to promote good ventilation where possible. Consider portable air filters or install exhaust fans ducted directly to outside especially if appliances such as refrigerators are present. • Assembly spaces – Consider air treatment at air handler like UVGI lamps. • Atriums – It is important that spaces that communicate to the atriums are treated similarly to be effective (same pressurization, filtration, social distancing, etc.). • Toilet rooms –Droplets and droplet residues can be generated from the plume produced from the flush and could contribute to the transmission of pathogens. Keep toilet room doors closed, and if there is a toilet lid, keep it closed. Keep toilet rooms at negative pressure 24/7.
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HVAC CONSIDERATIONS FOR HEALTHY ENVIRONMENTS
About the Author Maureen Kozel, PE Senior Mechanical Engineer kozelm@progressiveae.com With more than 15 years experience in the industry, Maureen provides in-depth HVAC consulting, from site selection and lease negotiations to design and construction administration. Maureen enjoys the design challenge of meeting client goals with the appropriate mechanical systems. She provides leadership to Progressive AE’s mechanical engineering discipline and is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and of the National Council of Examiners for Engineering and Surveying.
ONLINE RESOURCES 1. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1 2. Indoor Humidity Regulations Will Reduce Burden of COVID-19 3. ASHRAE COVID-19 Resources
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