Ventilation in the COVID Era – Where’s your return air grille?

Author: Lou Vogel, PE, LEED AP, President
Published: December 7, 2020
Last updated: December 7, 2020

You don’t have to be a ventilation engineer to have some idea of what your heating and cooling system is doing. You don’t need to know all about it, but finding your return air grille can help. It can give you an idea of where the best spaces are in your building in terms of airflow and ventilation.

In the incisive words of Bob Dylan, “You don’t need a weatherman to know which way the wind blows,” if you feel air blowing on you and there is another person between you and the source of air, relocate! Unfortunately with this virus, we have to assume that everybody is shedding virus particles, because of the asymptomatic transmission possibility.

There has been a lot of attention paid to what effectiveness level of air filter you have in your air handling system. This effectiveness rating is given using a MERV value (Minimum Efficiency Reporting Value). Higher values are better, and the numbers, (11, 13 etc.) tell us how good they are at capturing different sized particles. I can go into relevant particle sizes and the difference between the MERV values in a later post, but there are some other factors that can have a greater impact on reducing the transmission of the virus that I will cover now.

Many heating and cooling systems push air into rooms and spaces through a ducted system, with the opening into the room typically covered with grillwork (to keep people from stuffing things in there) which can direct the air into the room at different angles. The location of the supply air grill and the direction of the incoming airflow is typically easy to find, you can put your hand close to the grill and feel the air blowing when the fan is on. The air enters the room there, and then that air has to get back to the fan and filter and then back to the room (hopefully with some outside air added along the way).   

Finding your return air grill

The return air (RA) grill covers the duct opening where the air is sucked back to the fan. This can be found in a room, hallway or other thoroughfare in your building. The RA grills are often located near the thermostat because the temperature of the air returning to the fan gives a decent indication of whether the room or zone needs more or less heat (in the winter). Quite often, there is only one large RA grill for a bunch of rooms. It would be nice to have one in each room but, for cost reasons, this is often not the case.

You can tell it is a return grill by putting a piece of paper on it and see if it sticks. Once located, you can get an idea of the airflow in a space. The air gets pushed in forcefully and then gradually makes its way back to the RA grill. If the RA grill is far away in a corridor, you can’t count on it reducing the quantity of aerosolized particles quickly enough.

Also, the effectiveness of the filter in the air handling unit becomes less important because the particles just won’t be getting to it. In this situation, if you have more than one person in the room, you should be looking at providing a portable HEPA or UV-C filter that ideally is located between the occupants.

Wearing a mask, especially indoors, is our main line of defense. Masks, paired with hand-washing hygiene, are supporting efforts to reduce the spread of COVID. In addition, if you pay close attention to the airflow in your building or in a space you will occupy for prolonged periods of time, you can further prevent exposure of the virus to you and your community.