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American Society of Heating and Air-Conditioning Engineers (ASHAE)


General Guidance Applicable to All Homes

Because of the variation of home construction, HVAC system and climate, some of the recommendations below may not be meaningful or practical in a given specific situation:

     • Maintain normal thermal comfort conditions: The HVAC system should be operated and maintained consistent with its instructions, and to provide acceptable indoor thermal conditions, which is normally in the range of 68- 78oF (20-25oC) and 40-60% relative humidity (RH).

     • Increase ventilation rate: The home’s ventilation system should be operated at least to provide the flow rate intended at all times. If mechanical ventilation systems are not available in single-family homes, opening multiple windows is an acceptable alternative, provided the open windows do not cause excessive draft. Whole-house “Summer Cooling” fans may also be used when weather permits. Ventilation should only be increased above required minimums if normal indoor temperature and humidity conditions are maintained. General Guidance Applicable to All Homes (cont’d)

     • Operate Restroom Exhaust Fans: Exhaust fans in bathrooms, toilets and lavatories should be operated whenever the facilities are in use. If possible, they should be operated continuously. Toilet lids should be closed as much as possible.

     • Operate Stand-Alone Air Cleaners: Stand-alone air cleaners (often called air purifiers) with particle filters should be operated continuously, if available. If there is only one, it should be placed in the area where the most vulnerable people in the household spend their time. If purchasing new devices, select ones that use high-efficiency media filters (such as HEPA) and have a high Clean Air Delivery Rate (CADR).

     • Increase Room Air Motion: Increased air motion within a room, such as from a ceiling fan, may be used to augment any of the strategies above.

Additional Guidance for Homes with Forced-Air Systems 

Many homes have forced-air HVAC systems that recirculate air throughout the home through ducts. The recommendations below are specific to those systems.

     • Install High Efficiency Media Filters: Most air handlers have a slot for replaceable media/paper filters. Such filters should be upgraded to high-efficiency filters (such as MERV 13) or higher, when the system allows it. Personal Protective Equipment (PPE) is advised when changing filters that may contain virus-laden particles.

     • Operate System Continuously. Forced air systems should be run as much as possible, such as by using a “FAN ON” setting. More complex control devices, such as smart thermostats, may allow programmable operation. Continuous operation at low speed should provide substantial filtration without undue draft.

     • Operate UltraViolet Germicidal Irradiation: If present, UVGI systems should be maximally operated according to manufacturer instructions. Refer to ASHRAE guidance on Filtration and Disinfection for more information.

     • Operate Economizers: If present and weather permits, economizers should be operated consistent with maintenance of normal indoor temperature and humidity conditions. Additional Guidance for Multi-Family Homes In multifamily buildings extra care should be taken to reduce the risk of infection in one dwelling unit (e.g. apartment) from spreading to another:

     • Make Sure Water is in the Plumbing Traps: The U-shaped trap in all pluming drains should not be allowed to go dry. The easiest action to take is to make sure every sink, shower, bathtub and floor drain is used at least once a day. 30 seconds of flow is sufficient.

     • Maintain Pressurization: Use of exhaust systems is recommended to keep the home below the pressure of any adjacent common space, such as a corridor. Open windows should be minimized unless it’s necessary to meet minimum ventilation requirements or maintain acceptable indoor temperature and humidity levels.

     • Seal Chases and Bypasses: Any large openings that might allow air to flow to the dwelling unit should be sealed with caulk, foam, plastic or similar materials. Example of such openings include plumbing or other utility penetrations.

1. ashrae-residential-c19-guidance.pdf




Core Recommendations for Reducing Airborne Infectious Aerosol Exposure


The following recommendations are the basis for the detailed guidance issued by ASHRAE Epidemic Task Force. They are based on the concept that within limits ventilation, filtration and air cleaners can be deployed flexibly to achieve exposure reduction goals subject to constraints that may include comfort, energy use, and costs. This is done by setting targets for equivalent clean air supply rate and expressing the performance of filters, air cleaners, and other removal mechanisms in these terms.

1. Public Health Guidance - Follow all regulatory and statutory requirements and recommendations for social distancing, wearing of masks and other PPE, administrative measures, circulation of occupants, reduced occupancy, hygiene, and sanitation.

2. Ventilation, Filtration, Air Cleaning

     2.1 Provide and maintain at least required minimum outdoor airflow rates for ventilation as specified by      applicable codes and standards.

     2.2 Use combinations of filters and air cleaners that achieve MERV 13 or better levels of performance for air recirculated by HVAC systems.

     2.3 Only use air cleaners for which evidence of effectiveness and safety is clear.

     2.4 Select control options, including standalone filters and air cleaners, that provide desired exposure reduction while minimizing associated energy penalties.

3. Air Distribution - Where directional airflow is not specifically required, or not recommended as the result of risk assessment, promote mixing of space air without causing strong air currents that increase direct transmission from person to person.

4. HVAC System Operation

     4.1 Maintain temperature and humidity design set points.

     4.2 Maintain equivalent clean air supply required for design occupancy whenever anyone is present in the space served by a system.

     4.3 When necessary to flush spaces  between occupied periods, operate systems for a time required to achieve three air changes of equivalent clean air supply.

     4.4 Limit re-entry of contaminated air that may re-enter the building from energy recovery devices, outside air intakes, and other sources to acceptable levels.

5. System Commissioning - Verify that HVAC systems are functioning as designed.

1 core-recommendations-for-reducing-airborne-infectious-aerosol-exposure.pdf (

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