5 Hospital HVAC Design Strategies for the Post-COVID-19 World
Keep these ideas in mind as potential solutions for providing a robust HVAC system that can support future flexibility.
As the A/E/C industry helped hospitals respond with temporary retrofit solutions to meet the surge requirements of the COVID-19 pandemic, many of the interventions were focused on augmenting or modifying existing HVAC systems to create negative pressure in a standard medical/surgical patient room.
It is important that the best practices from the retrofits be applied to continuing efforts as well as the design and engineering of new hospitals.
One clear takeaway is: hospitals will need to consider investing in HVAC systems with the appropriate components and infrastructure to provide environment required to support acuity adaptable isolation spaces for airborne pathogens like COVID-19 or other viruses.
Below are five considerations and potential solutions for providing a robust HVAC system that can support future flexibility.
1. HEPA Filtration Recommendations. To minimize the chance of future viruses spreading through hospital HVAC systems, experts recommend implementing and maintaining proper particle filtration with HEPA filters. HEPA stands for high-efficiency particulate air and HEPA filtration systems are extremely effective at capturing and removing airborne particles, microorganisms, and other contaminants from a facility's indoor air. Installing a HEPA filtration system will impact air distribution, so it is critically important that your HVAC system is designed with enough fan power at the air handling units (AHU) to support their use.
Consider installing HEPA filter racks on AHUs that serve negative pressure areas. Another option is to install HEPA racks that you do not intend to use in air handlers under normal operations, and only use the HEPA filters in a pandemic situation. This would address any concerns about operation and maintenance costs related to HEPA filters. Another option would be to have a dedicated exhaust air duct to the outside that features connections for portable negative pressure machines with HEPA filters.
2. Air Handling Unit (AHU) Recommendations. Recognizing the importance of bringing outside air into hospitals in a pandemic situation when planning a new hospital, consider engineering your AHUs to handle dual conditions for heating and cooling coils: minimum outside air intake for normal conditions and 100 percent outside air for pandemic and emergency conditions. Standby heating and cooling capacity can be configured to be utilized for increased outside air for these conditions.
This would encourage more hospitals to want to include standby cooling equipment in the design and encourage the use of chilled water-cooling systems in more facilities. Air cooled, direct expansion systems are commonly used in smaller facilities. However, they generally do not have extra capacity available to increase outside air without compromising comfort.
Most building codes already require standby heating equipment. The intent would be to use available standby capacity for emergency conditions.
Current hospital code requires a minimum of 25 feet separation between exhaust source and intake source of outside air. However, this does not apply to economizer return fans. This is an area that requires additional investigation, as best practices would dictate a similar separation from intake source if return fans are used for emergency exhaust.
Consider use of high intensity UV lights or other similar air treatment systems at the cooling coils. In addition to helping to keep coils clean, they can also be effective in killing a high percentage of virus particles passing through the AHU.
3. Negative Pressure Environment Recommendations. Negative pressure rooms feature mechanical ventilation systems which maintain the pressure of the room at a slightly lower level than the pressure of the entry area so that it allows air to flow into the isolation room but not escape from the room, as air naturally flows from areas with higher pressure to areas with lower pressure, thereby preventing contaminated air from the isolation room to escape outwards.
Construction of a negative pressure room requires special attention be given to details that include sealing all wall penetrations and use of monolithic, gypboard ceilings. Door seals and sweeps are also commonly used. As a result, they are much more expensive to construct.
When converting existing standard rooms to negative pressure environments, negative pressurization may be compromised due to existing construction details that cannot be changed. Although proper directional airflow can be achieved, the measured pressure differential may be less than expected.
4. Return Air and Exhaust Air Recommendations. Not only is intake of outside air important, it is also important to address return air and exhaust air best practices in new hospital design. Some designs have discharged return or relief air into mechanical rooms. Future projects should avoid this and have all return air ducted to the outside in order to reduce the risk of infection and allow return fans to be used for emergency exhaust.
Code requires that return air grills be located in the headwall or over the patient bed in isolation rooms. Moving forward, perhaps med/surg patient rooms should also have the return grills be placed on the headwall rather than at the doorway. However, a doorway location is more appropriate for non – infected patients that are susceptible to being infected or immunosuppressed. Because these are conflicting directions, the location of the return grilles in the med/surg patient rooms should be thoroughly reviewed with the hospital during design. An option to consider would be to put two return air grilles in a patient room, one by the door and one by the bed, each sized for 100 percent airflow. That would allow the facility to isolate one of the grills depending on operation i.e. normal or pandemic mode.
In lieu of using return fans for emergency exhaust, an option to consider is installing dedicated pandemic exhaust risers with exhaust fans sized to exhaust the entire floor or other designated area.
Current code does not allow switching or changing the pressure relationship in a room from positive to negative. However, the code could be reevaluated for these situations if procedures are implemented at switchover. The main concern is that the rooms are not actively switched back and forth. Switching room pressure relationships needs to be a structured process reviewed with the hospital and code authorities.
Air valves or air terminal units may be used on the return side of each patient room to allow the airflows and room pressures to be automatically changed without requiring manual rebalancing. However, this is expensive to implement. Some jurisdictions do require this configuration.
Regardless of which design is implemented, reduce the use of flexible ducts for return and exhaust air, as it is easily clogged by lint and dust which significantly reduces airflow.
5. Ventilator Support Recommendations. One of the most challenging aspects of the COVID-19 retrofit projects was getting the appropriate levels of medical grade air and oxygen to support the ventilators required to treat the most critically ill patients.
When designing a new hospital, it is important to identify future ventilator loads accurately. Ventilators require access to medical grade air as well as oxygen, which is why it is difficult to convert traditional med/surg beds to ICU/CCU beds. In planning future hospitals, consider installing larger pipes, as they can accommodate much more medical air or oxygen at a fraction of the cost it would take to retrofit a med/surg floor, allowing med/surg floors to be easily upgraded to ICU/CCU beds.
For medical air and oxygen on patient floors, the minimum size should be ¾-inch and runouts to individual rooms should be ½-inch. Larger sizes may be appropriate if a higher ventilator load is anticipated. Another factor to consider is the patient diversity, as not all patients will need the same amount of medical air and oxygen. It is essential to discuss typical or desired ventilator settings with the hospital. Diversity is typically not considered when only planning to accommodate a limited number of ventilators, and conservative values have traditionally been used.
The opportunity. The current crisis highlights the impact of HVAC systems in keeping both patients and front line caregivers safe and has taught us the importance of designing flexibility into our future hospitals. Following World War II, President Harry Truman signed the “Hospital Survey and Construction Act” into law that ignited a healthcare construction boom in the 1950s and 1960s. In 1975, the Hill-Burton Act supported the financing of an additional 6,800 facilities. The impact of both pieces of legislation resulted in one of the most extensive and advanced health care systems in the world. The COVID-19 pandemic offers the A/E/C community and healthcare leaders an opportunity to reimagine the design of the next generation of hospitals.
Scott Winfrey, P.E., P. Eng., LEED AP, HFDP (firstname.lastname@example.org) is Associate Principal with Specialized Engineering Solutions.