Most building owners and managers want to take a proactive approach when planning to open up their facilities after being shut down. Essential businesses that have had to adapt to the pandemic on the fly offer operational models that most people are familiar with. Social distancing and face coverings have become the norm for grocery stores and home improvement centers. These Big Box stores have the advantage of large interior volumes/spaces and good ventilation which makes customer shopping relatively safe when simple precautions are followed. Customers move through the facility in short time frames with minimal interaction with others.
Returning to work in an office environment is more challenging. While social distancing, face coverings and washing hands still apply, making some alterations to the HVAC Equipment and Controls is something that should be considered.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers, known as ASHRAE, in conjunction with the Center for Disease Control have come up with some basic parameters to be considered for HVAC systems modifications in response to research into the ways that the Covid-19 virus appears to be transmitted within buildings. They have encouraged controls companies to develop new operating parameters that will be known as “Epidemic Mode” that allows building owners to manage the operational changes to their HVAC equipment. For office buildings the critical thing to consider is the length of time that staff/tenants are in the building and how the air is being re-circulated or exhausted.
The research shows that it takes about 1000 virus particles to infect someone with Covid-19. Meeting someone in the corridor and talking for less than a minute it is unlikely that you could receive enough virus to be infected. However if you are sitting at your desk for 7 hours it would only take 150 particles per hour from a Covid-19 positive person in the office shedding virus at their work space to get you infected. That is if the air is being recirculated within the space. The challenge is to make sure that offices can open and be relatively sure that the space is not a contributor to Covid-19 spread.
What ASHRAE has come up with to be included in the “Epidemic Mode” is generally as follows:
- HVAC systems should be setup to purge the air in spaces 2 hours before they are occupied in the morning and 2 hours after closing at night. The air does not have to be conditioned during the purge but should be run at 100% fan speed with as much outside air that your system can provide.
- During the work day the system should be run with conditioned air on and have as much outside air as the system can provide. The higher the fan speed the better and of course the fan should be on while the spaces are occupied. The problem is that most systems can only provide the code required amount of outside air which may be only 15%
- If the system has Demand Controlled Ventilation, DCV, it should be disabled. DCV registers CO2 levels in the air and increases outside air ventilation when CO2 levels are elevated due to higher occupant loads. Outside Air, or OSA, ventilation should be set to as high as possible when in “Epidemic Mode” hence CO2 levels are irrelevant.
- Humidity levels during occupied times should be maintained between 40% and 60%. This level is best for minimizing the transfer of the Covid-19 virus. After hours lower humidity levels are best.
- Filtration parameters. The Covid virus particle size is between .06 and .14 microns. MERV 13 filters only filter around 97% of particles down to 2.5 microns in size. Not effective for the very small Covid virus particles. Higher MERV and HEPA Filters, can filter 99.97% of particles down to .3 microns in size, and have the capability to filter down to .01 at a still extraordinary level. Most office building HVAC equipment is not setup to utilize HEPA filters. However, it is important to realize that the Covid-19 Virus is not a standalone “floating in thin air” virus, but must be attached to something (ie, water droplets, bacteria, dust). To further confuse the issue the Center for Disease Control and the World Health Organization have determined that the virus is not airborne, but spread by person-to-person contact through discharge from an infected person.
So that should clear everything up, right? This simple MERV Chart may help clarify what your standard HVAC Equipment could be capable of filtering.
New Outside Air Inlet on Left
Based on the above research, Dyron Murphy Architects decided to make some physical modifications to our office’s HVAC systems. We decided that the more Outside Air that we could bring in the better and the less air we re-circulated back to the AC units was better as well. To accomplish this we were able to easily add additional outside air intake inlets at our 2 units. That increased the original outside air intake size by 160%. (Original OSA was 263 sq. in, newly added OSA is 414 sq. in. for a total OSA of 677 sq. in.)
Added MERV 13 Filter at Return Air
In addition we added MERV 13 Air Filters at the return air duct from the interior space. The MERV 13 Filters, in addition to intercepting possible virus laden air from interior spaces, added additional restriction to the return air flow, thereby encouraging units to take more outside air than inside air. Without actual measurements it is difficult to calculate exactly what the percentage of outside air will be. Rough calculations varied widely from between 47% and 73% OSA.
Building Relief Exhaust Fan
By adding this increased amount of outside air and restricting the return air, the space would, under normal conditions, become over pressurized. In our case our HVAC system is setup with a building exhaust fan controlled by our Trane DDC system and is operated based on the internal building pressure. So far the exhaust fan has done a pretty good job of equalizing pressure in the space even with the much increased OSA quantities.
The current air filters within our AC units are only MERV 8, which is standard for an office building. We have talked with our service maintenance company about an upgrade to MERV 13 Filters.
Completely clogged OSA Filters
We also had a conversation with them about why OSA Filters also need to be maintained!! If your outside air filter is completely clogged you’re not even getting your minimum 15% OSA.
With these modifications we should be able to operate the units in “Epidemic Mode” through the summer and fall. So far they have cooled the space adequately even with outside temperatures reaching 100 degrees. When we get to December and outside temperatures drop into the teens we may have to reduce the amount of outside air in order for units to keep the spaces heated adequately.
With the changes we’ve made we hope that our staff feels safe in returning to the office. We will continue to monitor the research moving forward and make adjustments as required.
As an added benefit, the changes we have made to our HVAC system operations also conforms to most of the requirements for “Healthy Buildings”. That includes higher levels of filtration that includes PM2.5, raising air flows from code minimum of 20 CFMs/per person to a minimum of 40, and higher percentage levels of outside air.
We encourage building owners and managers to reach out to their architect, mechanical engineer or controls companies to see what options are available for their specific HVAC system. Hopefully they can help you address the safe return of your tenants or staff.
Be Safe & Be Smart
Prepared By Jim Houser, Dyron Murphy Architects P.C.