Mechanical System Operation and Enhancement to Reduce Pandemic Risks
While the physical spacing of occupants, the wearing of masks, and surface disinfection can reduce the potential for infectious diseases to spread indoors, such measures do not eliminate the risk. Within the indoor environment, infectious aerosols can remain airborne for distances much larger than the 6 feet recommended for protection from droplets from potentially infectious persons. HVAC systems can play an important role in reducing the risk from infectious aerosols.
The following HVAC strategies can reduce the potential for indoor infectious disease spread during the current pandemic risk. We have categorized the strategies into 3 levels, with each level having increasing risk management potential. In general, the higher level strategies are generally more difficult, and sometimes infeasible to implement on existing HVAC systems and buildings. The strategies chosen by a facility manager should consider the level of risk at the facility, the acuteness of risk avoidance at the facility, as well as the feasibility of implementation.
Level 1 Strategies: Utilize air filters with at least a Minimum Efficiency Reporting Value (MERV)-13 efficiency, increase ventilation rates, ventilate 24/7, and flush the building with outside air pre- and post-occupancy each day.
Level 2 Strategies: Utilize air filters1 with a minimum MERV-14 efficiency, and bring in as much outside air as possible while maintaining indoor comfort.
Level 3 Strategies: Utilize HEPA air filters, humidify to 40% relative humidity (RH) if possible, install UVGI2, and install bipolar ionization.
For buildings that have had an extended dormant period, plumbing systems strategies are required to reduce the risk of spreading legionella bacteria3 and the associated illnesses, and for containing sewer gases.
These HVAC and plumbing strategies are discussed in more detail later in this article.
Level 1 HVAC Strategies to Reduce Pandemic Risk
1. Air filters should be upgraded to minimum MERV-13 as a Level 1 strategy, within the limitations of the filter rack physical size and the fan capability. It should be noted that increasing filter efficiency will likely require more frequent filter changeout. Also, for systems that operate without recirculating air from the occupied space, air filter efficiency will not affect pandemic risk.
2. Demand-controlled ventilation4 (DCV) strategies should be temporarily overridden while the pandemic risk exists. The intent is to increase ventilation to the maximum achievable rate while maintaining space comfort. A potential method of overriding the DCV strategy would be to lower the space carbon dioxide (CO2) set point to a value less than background outdoor CO2 level. Of course, overriding a DCV strategy will likely increase energy use.
3. Modify the operation of HVAC units as follows:
- Operate the units at 100% outside air for at least 4 hours prior to the initial reoccupation of the building, to “flush” the space. If the control system for the units cannot be adjusted to provide this temporary 100% outside air flush, or if the system is not capable of bringing in 100% outside air, then start the HVAC system in continuous occupied mode 1 to 2 days prior to initial reoccupation of the building.
- Operate the HVAC units 24/7 in occupied mode, including at least normal occupied mode outside air ventilation rates.
- If possible, within the capability of the unit control systems, program the units to operate at 100% outside for air 2 hours pre-occupancy and 2 hours post-occupancy each day. After the post-occupancy flush, the units can return to normal occupied mode outside air ventilation rates during the remainder of the unoccupied period. After the pre-occupancy flush, the units should return to at least normal occupied mode outside air ventilation rates during the occupied period (a recommendation for increased outside air ventilation during occupied periods is discussed under higher level strategies below).
4. Open operable windows, if any, to the extent that outdoor conditions permit acceptable indoor comfort conditions while doing so. Windows should be opened prior to occupancy and remain open for 2 hours post-occupancy each day, if possible. It should be noted that seasonal allergens might need to moderate this practice depending on the sensitivities of occupants. Also, be watchful for potential sources of outside air pollutants near the windows.
5. On variable air volume (VAV) systems, increase minimum airflow set points at VAV boxes to allow for more air to pass through the central unit air filters. The minimum airflow set point may need to be moderated by the available tempering coil capacity at each VAV box, so that space comfort can be maintained.
6. Wear personal protective equipment5 when replacing air filters or handling condensate during HVAC maintenance activities. De-energize the air handler while replacing air filters.
Level 2 HVAC Strategies to Reduce Pandemic Risk
In addition to the Level 1 strategies previously outlined:
1. Consider upgrading air filters to minimum MERV-14 efficiency (minimum MERV-13 efficiency is recommended as a Level 1 strategy), within the limitations of the filter rack physical size and the fan capability. It should be noted that increasing filter efficiency will likely require more frequent filter change out. Also, for systems that operate without recirculating air from the occupied space, air filter efficiency will not affect pandemic risk.
2. Bring in as much outside air as possible while maintaining space comfort, including the avoidance of space humidity levels above 60% RH, and being watchful for the potential of coil freeze-up within the HVAC unit:
- If possible, within the capability of the HVAC unit control systems, program the units to bring in 100% outside air and to automatically moderate outside air intake, if required, to the maximum level; while also maintaining indoor comfort set points.
- If it is not possible to program the control system to automatically moderate maximum outside air intake to maintain indoor comfort conditions, increase minimum outside air intake set points to the maximum capability of the system. These increased minimum outside air set points could be adjusted by:
- Operator input to the HVAC control system at daily or hourly intervals, based on weather and feedback from space conditions; or
- Seasonally adjusted, by determining the maximum outside air intake at which each system would be expected to maintain indoor comfort.
Level 3 HVAC Strategies to Reduce Pandemic Risk
1. Humidification: Studies have shown a large decrease in infectious spread if indoor humidity levels are maintained in the 40% to 60% RH range. The theory is that low humidity levels can cause viral droplets to more quickly dry out and spread as an aerosol, with the viral remnant rehydrating if inhaled by an occupant. This is compared to increased humidity helping droplets to remain hydrated and drop out of the breathing zone. Low humidity levels can also lessen the body’s natural infection resistance. In a typical office environment without humidification in a northern climate, indoor RH levels can fall well below 20% in the winter. However, the recommended low limit of 40% RH is typically only maintained in very special indoor environments, such as museums or certain test laboratories. Raising humidity levels needs to be done with very careful consideration of the capability of the building envelope to withstand increased moisture levels in the winter. Predictive vapor migration calculations for the building envelope, coupled with the implementation of an automated strategy to adjust humidity levels based on outside air temperature, can maximize achievable indoor humidity. The addition of humidification to a building can provide additional benefits beyond the present pandemic response, such as mitigating the spread of seasonal colds and flu, and reducing static
2. HEPA filters: Consider upgrading air filters with HEPA filters as an enhanced strategy, within the limitations of the filter rack physical size and the fan capability. Many existing systems would not be capable of accommodating HEPA filters. The use of HEPA filters will require more frequent filter change out, and the use of lower efficiency and lower cost pre-filters should be considered. Also, for systems that operate without recirculating air from the occupied space, air filter efficiency will not affect pandemic risk.
3. UVGI: This technology exists for direct disinfection of surfaces, for indirect/upper room disinfection of indoor air, and for disinfection of interior HVAC unit coils and airstream. Protection of occupants and maintenance personnel from potentially harmful UV irradiation needs to be carefully considered. For HVAC units that operate without recirculating air from the occupied space, UVGI placed within the HVAC supply airstream will not affect pandemic risk.
4. Bipolar air ionization: With bipolar ionization, molecules of air are ionized and given a positive or negative charge, allowing them to attract contaminants of the opposite charge in the air. The particles combine and become large and heavy enough to be caught by the filter or drop out of the breathing zone. Virus particles can be inactivated by this process as well. In the case of application within HVAC units, the relatively low-energy ionization units are placed within or near the air-handling equipment, and the HVAC’s air distribution system is used to disperse the ionized molecules with the spaces served. While this technology can be used in conjunction with increased outside air ventilation (including 100% outside air systems), it can also be used on systems where outside air intake capacity is limited, where the energy use associated with outside air intake is a concern, or where outside air quality itself is poor. This technology is gaining a lot of traction in the industry, although ASHRAE is still considering it a technology that requires study. Besides the potential to reduce airborne infectious spread, this system is believed to reduce other airborne contaminants, which in a post-pandemic world, would make it useful to reduce energy use by reducing outside air quantities.
Plumbing System Recommendations
After an extended dormant period, the following plumbing strategies should be considered:
1. Flush domestic water distribution systems to remove potential legionella. Flow each fixture prior to general occupancy of the building and with HVAC systems operating to remove potential airborne pathogens.
2. If domestic hot water system temperatures were lowered below 140°F, or if recirculation systems were stopped, consider a high-temperature flush.
3. Make sure sanitary drainage system traps are wet.