Improved building filtration makes return to the office possible
Norman S. Wright Mechanical Equipment
HVAC & Data Center Solutions Since 1906
It’s been two years since so many of us started working from home. What was supposed to be a couple of weeks, maybe a month or two, tops, turned into one year, then two. But here we are, returning to what everyone is calling the new normal.
Many companies announced last month that they would be returning to the office, if not 100% then at least in some hybrid format. And while employees’ feelings are mixed about whether they are ready to be back in the office, an equally important question is whether the offices are ready for the employees.
Buildings pre-COVID
Before I look at whether buildings are ready for us to all go back to the office, let’s look at how it used to be. The HVAC system has always played an important part in the welfare of employees. As far back as the 1980s and 90s, ASHRAE has determined the acceptability of a building based on fresh air (outdoor air) and carbon dioxide levels.
Pre-pandemic, most buildings’ HVAC systems only filtered the mixed air, comprised of outside and return air, to meet ASHRAE or local codes’ minimum filtration standards. You might also see filters on the induced air inlet of fan-powered terminals if they were being used or UV on the water coils to keep the coils clean, but little else. Filtration was mostly about keeping stuff out of or off of the equipment so it could run effectively and efficiently.
A research paper titled A review of air filtration technologies for sustainable and healthy building ventilation published in Sustainable Cities and Society in July 2017 presented “a comprehensive review on the synergistic effect of different air purification technologies, air filtration theory, materials and standards”.
The researchers looked at the most prevalent filtration methods at the time, fibrous filters and electrostatic filtration, as well as nanofibre, Trombe (breathable) wall, and botanical biofiltration. The paper, now having been cited over 100 times since the pandemic started, came to the conclusion that “most research in the subject focuses on air filter efficiency improvements, pressure drop reduction, operational costs savings, etc., while the thermal comfort and acoustic effects of air filtration are generally ignored.”
To me this basically reads, filtration needs to have maximum effectiveness, but filtration will result in a cost in system efficiency and we since don’t know whether it will make the occupants more or less comfortable we should minimize the costs. With this mindset, you can see why not using high-efficiency filtration was often the best option unless it was absolutely needed.
Then just over two years later, COVID hit and filtration didn’t seem so optional anymore.?
What we learned during COVID
When COVID first hit, my company, like so many others, sent home all employees that were not essential. For the people who were still in the office, companies immediately set up cleaning and social distancing protocols. During that time, scientists didn’t know how COVID was transmitted so many precautions were being taken and Lysol, hand sanitizer, and masks were flying off the shelves.
After a lot of research, scientists have a much better understanding of COVID, how it is transmitted, and what we can do to mitigate it. The ASHRAE Epidemic Task Force worked closely with scientists and engineers in our industry to evaluate the evidence and develop detailed guidance to improve indoor environmental quality.
In a statement released on April 5, 2021, the task force said, “Airborne transmission of SARS-CoV-2 is significant and should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures."
In October 2021, ASHRAE released Core Recommendations for Reducing Airborne Infectious Aerosol Exposure which is “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”.
Section 2. Ventilation, Filtration, Air Cleaning states:
Research has shown that the particle size of the COVID virus is around 0.1 micron in size, but that it doesn’t travel through the air by itself. The virus is trapped in respiratory droplets which are predominantly 1 micron in size or larger.
ASHRAE’s recommendation of a Minimum Efficiency Reporting Value (MERV) 13 filter is at least 85% efficient at capturing particles in 1 to 3 microns size range. A MERV 14 filter is at least 90% efficient at capturing those same particles and filters with even higher MERV ratings would have higher efficiencies.
Filtration technologies for the new normal
With ASHRAE’s recommendation of a filter with a MERV of 13 or better, filtration needs to be balanced with both financial costs and equipment and operation cost associated with potential increased pressure drops.
There are several filtration and disinfection technologies available today. Some of them may be better suited for new construction, while others will allow owners to upgrade existing HVAC systems. The most common filtration and disinfection technologies available to use today are:
Let’s look at each of these technologies and how they can be used in new and existing HVAC systems.
Standard filter upgrades
The most obvious option to increase the filtration efficiency for a new or existing building would be to use MERV 13 or higher filters throughout the building. One thing to consider before deciding on this option is how many filters are in the building. If a building has filters installed throughout the ductwork, in every plenum, and in multiple air handlers, this can quickly become a very expensive option. Filters also need to be replaced, so maintenance requirements should also be considered.
There will also be considerations around the physical size of the new filters. A MERV 13 filter will likely have a larger depth than the existing filters in a retrofit application. Also, do the existing filters slide into a track from the side or do they press into a grid/frame, are they held in place with clips. Will the new MERV 13 filters fit or will modifications be needed?
The MERV 13 filters will have a higher pressure drop than what may be in place today. This increased pressure drop will change the airflow in the system and can possibly push the air handling equipment outside of its performance range.
The table below shows the price range and general size and pressure drop of different levels of 24”x24” MERV filters.
Note: All the costs in this article are estimates for comparison and will vary by manufacturer, specifications, and quantity.
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Needlepoint bipolar ionization
Needlepoint bipolar ionization (NPBI), is a method of charging particles to clean the air. NPBI systems, often installed directly into existing HVAC systems, are compact devices that use electrostatically charged plates to produce negatively or positively charged ions. These ions then attract and attach to airborne particulates and pathogens, forming larger and heavier clusters that are captured more effectively by MERV-rated air filters.?
A standard air ionizer uses high voltage electricity to electronically charge air molecules, turning particles into negative and positive particles, which causes them seek out and form bonds with particles in the air through a process called agglomeration. Particles begin to cluster together and the larger a cluster of particles becomes, the easier it is for the system to filter it out of the air.
Bipolar ionization units can kill mold, viruses and bacteria, disassociate volatile organic compounds (VOCs) into less harmful gases found in our atmosphere (oxygen, nitrogen, water vapor and carbon dioxide), and eliminate certain odors. Bipolar ionization systems have a relatively small footprint, so they can easily be retrofitted into an existing HVAC system.
A NPBI system, before installation, wiring, etc, for an air handler costs about $0.40 per cfm, making it a cost-effective filtration technology.
UVC
Ultraviolet germicidal irradiation (UVGI) uses ultraviolet-C (UVC) lamps to disinfect air, water, and nonporous surfaces. UV-C ranges from 200nm - 280nm and is naturally germicidal by damaging the DNA of microbes. UVC is not a new technology and has been used in building HVAC systems for years.
ASHRAE states that the effectiveness of a UV-C system to inactivate microorganisms in the air and/or on surfaces has been amply demonstrated. UVC systems are quite effective at maintaining cleanliness of HVAC coils, drain pans and other wetted surfaces and properly designed systems can be quite effective at on-the-fly inactivation of microorganisms in moving airstreams.
UVC is typically used in HVAC for surface irradiation of water coils to prevent growth on the surface of the coil which increases pressure drop and reduces efficiency. It can also be used in the ductwork for “on the fly” irradiation of air as it passes past the UVC lamp. And most recently, we are seeing manufacturers use UVC on top of HVLS fans for upper room UVGI.
The design and sizing of effective UVC systems can be complex because of the need to determine the dose delivered to a moving air stream or to an irradiated region of a room. UVC systems have very low pressure drop since there is no media for the airflow to pass through, making it a good solution for new and retrofit applications. However, UVC is damaging to HVAC components such as seals, gaskets, door handles, air tubing, and wiring, so care should be taken in retrofit applications to ensure existing components are not exposed to the UVC lamps.
Direct exposure to UVC is damaging to people as well. Caution needs to be taken to ensure that the light from UVC lamps is not visible from the occupied space and that the lamps are deactivated during any maintenance. These concerns are easily managed with good system design.
UVC is a cost-effective filtration technology. UVC for water coil irradiation can run as little as $0.15 to $0.20 per cfm. Installing UVC lamps in-duct for on the fly irradiation, are roughly $0.30 to $0.40 per cfm. Upper-room UVC fixtures can be installed at roughly $7.00 to $8.00?per-square-foot of treated area.
Portable HEPA filtration units
We are seeing many new portable HEPA air cleaners used to combat airborne contaminants and pathogens ranging from small units intended for residential use, to larger, commercial-grade products. These units offer economical clean air solutions for many applications. They also have the added benefit of the occupant seeing and hearing the product near them in the office, which can give peace of mind that the air is being filtered, whereas many of the other equipment discussed here are “behind the scenes”.
These units typically have a MERV-8 anti-microbial prefilter to capture large pollutants and a high capacity HEPA filter to capture 99.97% of air contaminants that are 0.3 microns in size. These units draw in unfiltered room air, pass the air through the prefilter and HEPA filter, and supply it back into the space as filtered air. Some portable HEPA units are also available with optional UVC or bipolar ionization as well for added filtration.
Because the units are portable, they can be rolled into any room as needed. They typically have variable-speed EC motors and are quiet enough to be used in almost any environment. You can find units that range from 150 cfm to over 1500 cfm. The Harvard Healthy Buildings strategy recommends targeting 6 air changes per hour (ACH) for classrooms to maintain ideal air quality, so units should be sized appropriately for the space it is installed in.
Portable HEPA filtration units typically cost between $2000-5000 per unit, so they are a cost-effective filtration technology, especially as a convenient, single-room solution.
Photocatalytic oxidation (PCO)
Photocatalytic Oxidation (PCO) is an air purification technology that has the ability to destroy particles as small as 0.001 microns. PCO works by combining UV light with titanium oxide-coated filters or titanium dioxide. The photocatalytic process results in hydroxyl radicals and superoxide ions, which are highly reactive. They combine with VOCs and bacteria from the air and break them down into harmless products like carbon dioxide and water.
Because PCO removes gaseous pollution sources such as cleaning solvents, off-gassing from building materials, vehicle exhaust, paint fumes, jet fumes, manufacturing process emissions, and agriculture process emissions that cause illnesses, it has typically been used for odor control but there has been a growing interest in it since COVID. PCO has a low pressure drop, depths of about 20”, and unlike some other air purification technologies, it does not produce ozone.
PCO filtration units for air handlers costs about $1.50 per cfm, making it one of the more expensive filtration technologies.
Which method is best for your building
It may seem like there are more options for filtration than there were pre-COVID, but most of these technologies have been used in buildings for decades. They just weren’t elevated to the level of importance that they are today. As we prepare to go back into the office, our offices need to be prepared for us.
ASHRAE has created building readiness guidelines (https://www.ashrae.org/technical-resources/building-readiness ) for buildings during and after epidemics. Building owners should consider evaluating their buildings’ HVAC systems to check that it is operating per design, is capable of being modified to align with HVAC mitigation strategies, and identify deficiencies that should be repaired. ASHRAE also recommends owners improve the efficiency of the filters serving their HVAC systems.
Every building is different, so owners need to look at their particular building and HVAC system to determine the best method of filtration, especially in existing systems. Many buildings are adopting a combination of these technologies to meet their needs. There may also be local codes and standards that need to be met.
I for one am excited for the world to go back to “normal”. I enjoy being able to have in-person interaction with friends and co-workers, start traveling again, and go to trade shows, events, and vacations. I also am proud to be a part of the industry that is working to make it possible for all of the new normal to happen.
For solutions to meet your filtration and disinfection?needs in the Northern California, Nevada, Colorado, Wyoming, Utah, Hawaii, and Pacific areas, visit?normanwright.com.
Jenny Abney Sivie, LEED AP - CIO / Director of Marketing, Norman S. Wright Mechanical.
Jenny Sivie is CIO / Director of Marketing with Norman S. Wright and well known in the business as both a product and marketing expert. You can find her regular technical podcasts, covering all aspects of commercial HVAC along with much other information on her YouTube channel?NSW HVAC Academy.