When the holiday meal is smoking, residential occupants open the windows to air things out. In commercial structures like hospitals and schools, however, opening windows can be a challenge. First off, some of them just do not open. Offices, prisons, and hospitals rarely have access to crack a window. Secondly, because commercial HVAC systems are balanced to run without opening windows, there is an equipment efficiency and cost factor to consider when allowing more air into the building. The discussion of this balance continues to be addressed by ASHRAE’s Epidemic Task Force (ETF). Assembled to help maintain communication with members, industry partners, building owners, facility operators, government agencies, and the public, ETF serves as a clearinghouse to review all technical questions and requests for technical guidance submitted to ASHRAE concerning the COVID-19 pandemic.
In August, the ETF updated its “Building Readiness” guidance for HVAC systems to help mitigate the transmission of SARS-CoV-2.
“The Building Readiness Guide includes additional information and clarifications so that owners can avoid operating their HVAC systems 24/7,” said Wade Conlan, ASHRAE ETF Building Readiness Team lead. “By rolling out this updated guidance, we are hoping to provide a more robust structure for building owners to complete the objectives of their Building Readiness Plan and anticipate the needs of building occupants.”
Specific updated recommendations to the building readiness guidance included pre- and post-flushing strategies to ensure the ventilation assists in removing bioburden during, before, and after occupancy of the building. According to the document, the building is flushed for a duration sufficient to reduce concentration of airborne infectious particles by 95%. For a well-mixed space, this would require three air changes of building volume based on outside air cfm (or three equivalent air changes including the effect of filtration and air cleaners) as detailed in the calculation methodology. There is also some guidance on methods to potentially increase the quantity of outdoor air introduced by systems.
Health Care Guidelines
Within the health care section from ASHRAE is guidance on recirculation and increased outside air fraction. Here, the committee explained that eliminating recirculation, reducing recirculation, or increasing the outside air fraction are all alternative and optional strategies for this sector.
“Evaluate recirculation or increasing outside air fraction from design levels up to 100% based on specific surge plan,” states the document. “Where surge plan has increased exhaust air quantities from spaces, increasing the amount of outside air conditioned by the air handler may be necessary to confirm balanced system airflows.”
This section suggests that technicians or building managers verify room pressures as appropriate. It also cautions that increasing outside air percentage might exceed system capacity seasonally. The consequences could require supplementing system capacity as well as increased energy usage.
Here is a list of seven other strategies to help keep a health care building biologically more secure, according to the task force.
- Protect equipment from freezing — water coils, DX.
- Watch for condensation or loss of humidity control.
- Consider relaxing temperature set points indoors.
- Consider future conditions when weather becomes more extreme before surge has passed.
- Watch for operational stability of the fan(s) and adjust to achieve desired airflow
- Adjust frequency on VFD or perform sheave change on belt driven fans.
- Take care not to allow the fan motor power input to exceed its rated capacity.
In a different section of the guidelines, schools were directly addressed by the ETF.
“This guidance has been formulated to help designers retrofit and plan for the improvement of indoor air quality and to slow the transmission of viruses via the HVAC systems,” states the document. “This guidance should be applied to each unique climate zone, unique school building, and HVAC system. All retrofits and modifications must not contradict ASHRAE 62.1 guidelines and must continue to meet or exceed applicable codes and standards. The designer needs to work closely with the local school system to work in conjunctions with new operational protocols and school operations.”
HEALTHY ALTERNATIVES: The ASHRAE committee explained that eliminating recirculation, reducing recirculation, or increasing the outside air fraction are all alternative and optional strategies for the healthcare sector.
Guidelines for humidity placed winter classrooms in the 40-50% humidity range at 72°F. Summer classrooms are to be designed to meet a 50-60% humidity range at 75°F. For ventilation, ASHRAE 62 or local ventilation standards for minimum outside air are the measurements to be used.
“For retrofitting or replacing an existing AHU, increase outside air to maximum allowable per AHU without compromising indoor thermal comfort for learning environment (due to severe thermal outdoor air conditions) or space IAQ due to poor outdoor ambient conditions (pollution),” stated the school-specific guidelines. “For dedicated outdoor air systems (DOAS) that are being replaced, size the unit considering increasing capacities beyond code minimum for ventilation. Also, consider impact to operation and equipment sizing associated with failure of energy recovery systems or filter loading.”
The guidelines suggest that during the pandemic, demand control ventilation should be disabled and schools should introduce the maximum possible outside airflow 24/7 until further notice. It is also suggested that they apply and utilize outdoor air quality sensors or reliable web-based data for outdoor pollution information as part of the new ventilation operation.
CEO enVerid Systems
For commercial buildings, the ASHRAE EFT instructs that HVAC systems be verified and commissioned to ensure at least minimum outside air is delivered to each space per ASHRAE Standards 62.1. If there are significant energy impacts, it points out, then minimum outside air as required by 62.1 should be used in tandem with a MERV 13 filter minimum.
The documents also ask designers and technicians to consider UV-C light as an enhancement where spaces require additional measures — e.g. spaces that serve vulnerable occupants — or where MERV 13 filters or 100% outside air are not possible.
Christian Weeks is CEO of enVerid Systems.
“EnVerid is pleased that ASHRAE’s updated guidance takes into account the challenges of pursuing a maximum outside air ventilation approach, particularly the substantial increase in energy expenditure it requires,” he said. “Running a building has become significantly more expensive as owners and tenants put in place myriad upgrades to address COVID-19. With this new guidance, ASHRAE offers an alternative HVAC strategy with similar efficacy that will not add substantial costs to a building’s already constrained operational budget.”
The company released a white paper, titled “HVAC Design in Commercial Buildings to Mitigate COVID-19: Improve Filtration, Don’t Increase Ventilation,” on the heels of ASHRAE’s updates.
“While scientists continue to investigate the possibility that SARS-CoV-2 can be transmitted through HVAC systems, building owners and HVAC engineers should design new mechanical systems with robust filtration
(MERV 13 or higher for commercial buildings) to minimize the risk of airborne transmission of SARS-CoV-2,” the 20-page document concludes. “Increasing ventilation in addition to improved filtration does not materially reduce the risk of airborne transmission of viruses and adds significant cost. In fact, increasing ventilation may actually be counterproductive to reducing the risk of airborne transmission of viruses such as SARS-CoV-2 due to challenges controlling humidity and outside pollution with higher OA. The best strategy is to design for improved filtration and use ASHRAE’s Indoor Air Quality Procedure (IAQP) with sorbent-based air cleaning to reduce OA requirements and offset the cost of improved filtration.”