Alyse Falconer, PE, LEED AP BD+C, is an associate principal at Point Energy Innovations and a member of ULI San Francisco and serves on the board of the Golden Gate Chapter of ASHRAE. This article reflects an expert opinion, but not that of the Urban Land Institute or its membership.

The ongoing threat of infection from the spread of COVID-19 is raising questions from building occupants and owners about how best to operate HVAC systems to reduce the risk of exposure. Offices, multifamily, and large retail type tenants are all hoping to return to some sense of normalcy, which means people returning to buildings. How do we reopen larger buildings as safely as possible? Who should we be listening to for guidance?

In the near term, both the U.S. Centers for Disease Control and Prevention (CDC) and the Building Owners & Managers Association (BOMA) look to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) for guidance on air handling system modifications as related to COVID-19. ASHRAE stresses increasing outside air within buildings as well as upgrading air filters to minimum MERV-13 rating, at air handling units as a top priority in reducing the spread of aerosolized particles. But the feasibility of increasing the percentage of outside air must be evaluated at a building-by-building level, since it is dependent upon building-specific heating and cooling plant-level capacities.

Other measures, such as germicidal ultraviolet light for disinfecting system components and maintaining relative humidity within the space, may be applied, but there has been overwhelming and continued support for upgrading filters and increasing outside air as critical components of a layered virus control approach, which also includes maintaining social distancing, masks, hand washing, and other strategies.

Over the longer haul and as the pandemic’s spread continues, owners are recognizing the importance of good indoor air quality for occupants and installing more effective air filters. Office tenants are learning about different filter efficiencies and types, and in turn asking property managers and landlords what type of filter is installed at the buildings they occupy. One way to think about it: the higher the Minimum Efficiency Reporting Value (MERV) rating a filter has, the higher the filtration efficiency.

COVID-19 is thought to spread primarily through airborne transmission by both small and large infected droplets. The smaller salvia droplets containing virus particles, called droplet nuclei, are about 0.3 to microns (10 μm) in size. The chart below shows the effectiveness of each filter’s MERV rating as it relates to particle sizes. ASHRAE and the CDC recommend a minimum MERV filter rating of MERV-13, but many building owners are considering MERV-14 or better.COVID aside, the increasing frequency and size of wildfires on the West Coast are putting a greater importance on filter types. During periods of intense wildfires, in older buildings without good filtration, some owners have completely closed outside air dampers at buildings, acknowledging that bringing in smokey, dirty air will only be a detriment to indoor air quality.

However, what happens this fall if a second wave of COVID happens at the same time as wildfires wreak havoc in states like California, Oregon, and Washington? Recently, the air quality index (AQI) on the West Coast hit an all-time high of particulate matter concentration, due to unprecedented fires. Are buildings completely shutting critical ventilation to buildings, realizing that old, leaky filters do not do their job?

Another source of localized pollution is vehicular emissions within cities. Leadership in Energy and Environmental Design (LEED) and other certification programs encourage site selection for new construction in downtown corridors where air quality at certain times of day may not be optimal.  COVID-19 could exacerbate vehicular pollution, since many people are afraid to ride public transit, especially during rush hour, when subways and buses are tightly packaged with riders.

Over the past few years, building industry designers and engineers have viewed MERV-13, with an efficiency of 90 percent, as the base filter rating type for new construction. When MERV-13 is paired with carbon filters, the combination is highly effective in eliminating odor and filtering out ultrafine particulates and gaseous molecules, created from smoke and combustion.

A Case Study: Converting the System of a 1960s San Francisco High-Rise Office Building

Built in 1964 by SOM, 650 California Street in San Francisco is owned by Columbia Property Trust.

Is it easy to install new filters within existing air handling systems? Yes and no. Some buildings may be able to easily change filter types, from a MERV-13 filter to a more robust filter, such as a MERV-15 or MERV-16, without much modification to the system. Other buildings require a deeper retrofit of the air handling system.

At 650 California Street in San Francisco, built in 1964 by SOM, owners Columbia Property Trust decided to replace existing filters with new, better filters. The existing filters were housed on what is called a Roll-O-Matic system, basically a blanket-type filter, equivalent to MERV-7 or MERV-8, on a roll that is changed automatically by a small motor when the filter gets dirty. The company was considering installing new filters, equivalent to a MERV-16 filter, with new housing racks, to replace the existing filters. The renovation project has taken roughly four months.

“In the current environment, we felt that upgrading our buildings’ filtration was a major step to improving the overall indoor air quality for our tenants wherever feasible across our portfolio,” says Stephen Smith, senior vice president of property management for Columbia Property Trust.

In undertaking this type of project, a few key questions first asked were: How do we take on this project? What is the upfront cost? How about the projected energy cost for the new filters with a higher pressure drop? What is the operational and maintenance cost?

Tackling the Renovation—What Is Possible?

The existing system’s Roll-o-Matic filters were measured to be roughly 12 feet wide by 14 feet in height at each supply fan, with approximately two to three feet before and after the filter array. There was space for the new installation of a filter rack, which is to be arranged in two-foot-by-two-foot cell increments, at 12 inches deep. The remaining space will be blanked off with sheet metal to avoid custom filters. The filter installation must be airtight so that the risk of bypass air is reduced as much as possible. Bypass air, air which is not filtered and leaks through cracks in the array, can reduce a filter’s effectiveness dramatically.

Lastly, what is the velocity through the system? It is critical to use low air velocities—below at least 500 feet per minute (FPM)—so that air can move smoothly through the filter media. Velocities ranged from 420 to 460 FPM.

The filter upgrade project met all the above criteria. The project was a go from a physical standpoint.

Upfront Costs Are a Concern—More So Now Than Ever

Since the owner is a large real estate investment trust, money was reserved for this capital improvement project; however, given current economic conditions, everyone is tightening their budget. The owners had a general number in mind but wanted to be sure that the project could come in on budget.

A request for proposals (RFP) was put together, outlining the project requirements as an upgrade to the existing system and sent to four contractors with an invitation to bid, with three responses to the RFP. Pre-bid site walks were conducted on different days to minimize any potential risk of infection between contractors and facility engineers.

The upfront cost for this project came out to roughly $0.25 per total building square footage. This project was a very affordable upgrade for the building. The actual filter cost, without any modifications to the fan system (provided as a metric for readers who have filters that can be easily changed out) ending up being about $0.06 per cubic foot per minute of airflow or $0.04 per square foot.

What about Energy and Maintenance Costs

Since energy bills are paid directly by the tenant, ownership was concerned about an increase in utilities, especially since many tenants are not even in the building right now. The following assumptions were plugged into an energy calculation:

  1. Hours of operation, Monday through Friday, from approximately 6:00 a.m. to 6:00 p.m.;
  2. Increase in fan energy use is around 0.4 inch static pressure; and
  3. San Francisco electric energy rate is around $0.15 to $0.16 per kilowatt hour.

It was determined that the new filters would increase energy use within the fan system by less than $0.02 per square foot, an incredibly cost-effective solution to improve overall indoor air quality for tenants.

Example of v-cell filter.

As for maintenance costs, some manufacturers within the filter industry claim that v-cell filters can outlast traditional bag-style filters by six months to one year. V-cell filters are a rigid-type frame filter, which allows for a greater surface area for filtration when compared with a bag filter. The larger the surface area, a more even distribution of air flow across the filter, which means that the entire filter can be used to trap particulates, versus a bag filter, which is quite floppy and can be less effective.

As related to COVID-19, there are recommendations among building professionals that facility engineers are to have full personal protective equipment (PPE) when changing filters, since filters at air handling units may contain COVID-19 aerosolized particles. Fewer filter changes mean less time suiting up in full PPE, less risk for facility engineers, and more time available for other routine type maintenance.

It must be noted, however, that every building is different, and it is highly recommended that owners consult an expert to review buildings on a case-by-case basis. The above case study in this article is not meant to be the solution for all buildings.

Tenants are looking to landlords and building owners to create a safe environment for returning to work, school, stores, bars, and restaurants. Working with facility engineers to increase the flow of outside air into the building and installing robust filters in some combination will reduce the risk of spreading aerosolized COVID-19 particulates within the air system, as part of a suite of strategies that include practicing good personal hygiene and wearing face masks. As our case study above has shown, upgrading the filtration system can be a cost-effective strategy, both in the upfront costs and operating and maintenance costs over time.

ALYSE FALCONER, an associate principal at Point Energy Innovations, is a registered professional engineer and LEED-accredited professional responsible for designing sustainable and high-performance buildings. With over 10 years of experience, she has designed, managed, and completed multiple LEED Platinum and Gold certified buildings, net-zero energy and carbon projects, and Passive House projects. Her project expertise ranges from mixed-use offices, laboratories, and hospitals to universities and public schools.