Many facilities remain lightly occupied or empty because of shelter-in-place orders across the country to try and flatten the transmission of the SARS- CoV-2 virus that causes COVID-19. We seem to be getting closer to the point where the restrictions will be lifted and the country will slowly transition to a new normal. How can buildings be better prepared for the next epidemic—or even another pandemic?
The ASHRAE Epidemic Task Force (ETF) is building guidance on how HVAC systems in buildings should operate during this pandemic, restarting unoccupied buildings for post-epidemic conditions and preparing systems today to be able to respond to future epidemics. A key thing to remember is the ability to mitigate the transmission of an infectious aerosol is not solved by one strategy, and the HVAC system must play its part in the facility preparedness plan.
The ASHRAE Position Document on Infectious Aerosols discusses much of the research and information for the strategies that can help reduce the bio-burden in the space. In addition, the ETF’s Building Readiness Team (BRT), which I lead, is focused on providing practical guidance to meet the building guidance recommendations.
The intent is to try to future-proof your HVAC system for the next epidemic.
Outside Air
The most common recommendation is to increase the quantity of outside air (OA) being delivered to the space but, in reality, the first decision should be to identify the acceptable space comfort conditions during an epidemic. Most buildings are controlled to a cooling setpoint of 74 F with an upper relative humidity (RH) limit of 60 percent. They typically use a heating setpoint of 69 F but do not control the RH in the space. It is recommended to look at the potential to increase cooling setpoints to be 78 F at 60 percent.
Regardless of the space setpoint, the existing cooling coil must be evaluated for the impact of increased OA. The table below shows the impact of additional OA on a typical air-handling unit coil.
The increase of OA from 20 to 90 percent requires twice as much chilled-water flow and cooling capacity from the building chiller and three times the water-pressure drop for the pump to overcome. It is unrealistic to think the coil could perform in the middle of the summer at 90 percent OA. Any new coils being designed should be evaluated to determine the realistically achievable percentage of OA and noted in the design documents.
Keep in mind the ability to control the flow at minimum and maximum. Chilled water is much easier to control than a direct-expansion (DX) coil unless the DX unit has variable-speed compressors. It essentially becomes an on-off machine if you have considered increased OA for an epidemic in your design standards. The designer might want to consider adding dehumidification strategies, such as a wrap-around heat pipe, plate heat exchangers or an energy wheel.
There is an option to increase the ventilation air of the system via the building automation system (BAS) to dynamically respond to the space conditions. The concept is to have the outside air dampers and return air dampers respond to the space temperature and RH to increase outside air percentage in lieu of using a supply air temperature reset strategy. It is also assumed that in Epidemic Mode, the BAS would prioritize this ventilation control method over other optimization strategies, such as static pressure and supply temperature reset.
Filtering Air
The next evaluation for your system is to determine if a more stringent filter can be used. The MERV rating of a mechanical filter is determined by filter manufacturers in accordance with ASHRAE Standard 52.2: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. The image above shows the difference in diameters from visible (PM10, inhalable particles that are 10 micrometers) down to the aerosolized size of the coronavirus of 0.1 micron.
To put the size of SARS-CoV-2 into perspective, you would need 7,000 viral particles to equal the size of a fine-tip ballpoint pen. The epidemic guidance recommends MERV 13 or 14 filters over the typical MERV 8 used in most designs.
The figure above shows the performance of MERV 6 to MERV 16 filters at different particle sizes. The MERV 13 filter is clearly better at removing 0.1-micron-sized particulate than the MERV 8.
The owner should utilize a certified commissioning provider and testing, adjusting and balancing (TAB) firm to determine the potential impact of each system’s performance using MERV 13 in lieu of MERV 8 filters. (Learn more about TAB firms in “In Challenging and Uncertain Times, Turn to the ‘ Authorities in Building Performance’.”) The TAB firm should take a flow profile of the unit to determine the current conditions, then increase the pressure drop across the filter section until it reaches a pressure drop that is approximately 0.5 inches H20 greater than the design total fan pressure. The TAB company should check the new fan profile for a drop in airflow, acceptable fan-motor amperage and system stability. At this time, the information can be evaluated to determine if the fan sheave must be changed, direct drive variable-frequency drive maximum speed must be increased or airflow impact is acceptable. It is important to keep in mind that packaged equipment could have issues with airflow that is reduced more than a chilled-water coil.
Water Flow
Another concern is the water flow in the building for chilled water, heating hot water and domestic water systems if the building will be mainly unoccupied during an epidemic. The concern is over the system not being exposed for the proper duration of water treatment. These systems can foul quickly. The image on this page shows the fouling in the chiller tubes and header that affect the heat-transfer capability of the chiller and overall energy for the systems.
This one-year-old chiller, which had zero waterflow for a little less than two months, experienced bundle fouling. PHOTO: Wade H. Conlan
The owner must develop sequences of operation to ensure the systems that are not continuously operating will be exercised enough hours per day to ensure the water treatment and water quality is maintained.
BAS Evaluation
The team should also evaluate the BAS and its capabilities to control the overall systems and if it can be monitored remotely. A retro-commissioning effort could identify the shortfalls of the systems’ ability to be controlled to achieve the virus transmission mitigation strategies used during the current pandemic. The process would result in a list of facility improvement measures (FIMs) to increase the building’s epidemic mode of operation effectiveness.
An important FIM should be a new BAS mode of control that would allow an Epidemic Mode of operation to be selectable by the facility staff. This would automatically increase the space-comfort setpoints, engage the dynamic increase to OA ventilation control, deactivate demand-controlled ventilation, adjust air-handling unit setpoints to accommodate the improved filters, ensure water flow in the chilled and heating hot- water systems, and apply other HVAC strategies to reduce the bio-burden on the space. This Epidemic Mode should also ensure that the BAS alarms will contact the BAS technicians working remotely.
Finally, a systems manual is needed to indicate how these systems are intended to operate in each mode of control. Commissioning providers create these when a project is completed, as suggested by ASHRAE Guideline 1.4-2019: Preparing Systems Manuals for Facilities. It is important that the document’s Epidemic Mode is clearly identified. This document is a great training tool for the facility staff and occupants for the next epidemic or pandemic.
Analyzing the building’s space comfort setpoints, examining the ability for the systems to increase ventilation air, performing retro-commissioning on the facilities to identify FIMs to better reduce the space’s bio-burden in an epidemic, modifying the BAS to allow for remote monitoring and automating the Epidemic Mode, and documenting the modes of operation in a systems manual can help building owners prepare for the next epidemic.
How about bipolar ionization to help kill the virus as well as attaching the ions to the virus atoms thus making them bigger giving the filters a better chance of filtering them out of the air.
Wade:
I am a retired engineer/business executive and working on an analysis of a passive anti-viral system for the hospitality industry. If effective it will require little to no addition of outside air. I am doing this in attempt to help the industry and I am not receiving or seeking any compensation for this work and I would like to e-mail you a copy of the details versus posting publicly. I hope to have this completed within the next week and would appreciate you sending me your e-mail. You can send directly to my personal e-mail. Thanks.
John – thanks for your comment on the article and some interesting thoughts on how to combat the virus. I will try and address each item, but I am not a medical doctor and so parts of items 2 and 3 are for the medical field, as you noted.
1. UV lights have the ability to kill the virus and you are correct that it needs enough exposure time. In an AHU, the air velocity is near 500 fpm which is acceptable for UV-C. The design needs to include enough intensity to accommodate the typical exposure time to the UV-C in that compartment. Many UV systems in AHUs were meant to just keep the coil and drain pan clean and do not have the required intensity. Here is a link to a presentation by Dr. Bill Bahnfleth as part of the ASHRAE ALI Courses that is Reducing Infectious Disease Transmission with UVGI” https://attendee.gotowebinar.com/recording/8373797400651662856.
2. There are electrostatic filters that are in the industry, but having them target this particle would be clearly a medical version. Please be aware that some of these create ozone which is not desired.
3. I just want to clarify, all filters can capture the smaller particles they are just not very good at doing that as compared to higher efficiency filters. The filters we are talking about are use to filter out the mixed air – return and outside air – prior to being delivered to the space from the AHU. So the typical system is filtering as you suggest. Also note that this SARS-CoV-2 particle is not typically observed floating on its own, it is usually attached to other others that make that overall particle bigger. There are studies that show that the particle size of clumped materials is typically 1 to 4 microns when the SARS-CoV-2 RNA is involved.
I hope this answers your concerns and questions.
Wade Conlan
Let me start by saying I am not an expert in any related field, and I will speak very directly without very technical terms. I live in Florida and have followed the number of Covid-19 cases in Florida and throughout the country closely.
Based on my own rudimentary analysis, the virus appears to be spreading in the states with warmer climates now that HVAC systems are running near continuously while more modest climates are leveling out with the number of virus cases. I know I am not stating anything that is not obvious to anyone that has a degree in the applicable areas. I am not an expert as I said before, but my entire life I have analyzed situations and been a problem solver.
I have a few ideas as to how an HVAC system could reduce the spread and want to leave them with you, the experts, to figure out the details. Let me start with the premise that the virus molecule is too small to be captured by a filter and does not survive long in high heat or under UV light. From what I have read, this is true. With this being said, I have the following ideas:
1. From what I understand, adding a UV light to a typical system does not work because the air is moving too fast to kill the virus. If the air chamber were significantly made larger in an area, the rate of flow would be decreased, and the UV light would have a greater time to kill the virus.
2. My next idea comes from Earth Science. In a stream large rocks are pushed down the middle and small particles are typically pushed to the side. This is accentuated when it is a winding stream. If the medical field could find an electro-magnetic panel that not only attracted the virus molecule but killed it upon contact would remove or reduce the threat of recirculated air.
3. My last idea really relies on the medical field. I understand the virus is too small to be captured by most air filters but in most circumstances, air filters are only installed for incoming air, not where air is being supplied to a room. If the medical field could find something that would attach itself to the virus molecule and make it larger, it could potentially be able to be captured by a filter at the return end of a HVAC unit.
I am somewhat confident none of my ideas are practical as described but I hope something I have mentioned opens up a new direction of study that may finally lead to significant improvements and the end of Covid-19.