Upgrading the Filters in the Air Conditioning System

HOW CAN OUR AIR CONDITIONER (or heating unit) help to improve safety in the sanctuary during the High Holy Days? The short-term answer is better filtration.…

NOTE:  Air conditioning filtration is one of three parallel avenues that are being pursued to reduce the risk of airborne infection in the sanctuary, for the sake of public health during the pandemic. The other two avenues are: adding fans in the sanctuary, and placing air purifiers on the bimah.

Executive Summary

As an expedient for the High Holy Days, we intend to temporarily employ a pair of higher-grade filters in the sanctuary’s air conditioning system. They are designed to remove a significant portion of the coronavirus-laden aerosols that happen to get drawn into the return ducts (near the front of the room). Cost: $83.

Expert Filtration Guidance

Most central mechanical [ventilation] systems … use filters on a … rating scale [called] minimum efficiency reporting value, or MERV, and typically use a low-grade filter [which is not designed to capture particles as small as 0.3 μm — the size of greatest interest in this pandemic.].… For infection control, buildings should upgrade to MERV 13 filters when possible, which could capture approximately 66% … of these sized particles.… Upgrading filters in mechanical systems is particularly important in buildings that use systems that recirculate air within the same room.…

Improving indoor air ventilation and filtration only accounts for far-field (ie, beyond 6 feet) aerosol transmission and does not significantly influence close contact transmission. Wearing masks is still important indoors for source control and for close contact with individuals even when high air exchange rates are achieved.

—Joseph G. Allen and Andrew M. Ibrahim, “Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission,” JAMA Insights; in Journal of the American Medical Association 325(20) [April 16, 2021]: 2112–13. doi:10.1001/jama.2021.5053

A Helpful Consultation

This morning, I consulted one of the senior technicians at the vendor who has the service contract for our heating and air-conditioning system, namely Shane. I asked about the wisdom of swapping out our existing MERV-4 filters for a MERV-13 upgrade. He cautioned that we face a trade-off: better filtration of aerosol-sized particles is achieved at the expense of the ability to cool down the room. The denser filter slows down the air circulation.

He suggested that we avoid exchanging the existing 1" filter for a 1" MERV-13, because the airflow would be unduly restricted by the dense filter. Better to use a 2" filter, which in effect offers double the filtering surface area.

When I replied that our filter chambers had a divider that seemed to restrict the filter to a 1" thickness, he explained that the middle flange was usually removable — thereby allowing a 2" model to fit. And this afternoon, I confirmed that what he said was true in our case. So I have ordered a pair of 2" MERV-13 filters for our rooftop HVAC units.

Installing the Filters

For the sanctuary, our two furnace filters also serve the air conditioning system. Each one is behind a small door, inside a chamber that is attached to one of the side-by-side furnace units, which in turn are housed inside the rooftop shed above the bimah. The dividing flanges in the filter chamber slide out. (Pull on them with pliers or vice-grips.)

One of the two filters for the sanctuary. The other is 24" x 24".
(As usual, click or tap on an image to enlarge it.)