Clearing the Air – Infection Control and the Future of Office Ventilation
In office settings, code compliance, energy savings, and comfort control were always among the top priorities. However, due to the COVID-19 pandemic, stakeholders are asking a few additional questions related to ventilation.
- “More people are working remotely or in hotel spaces – what does that mean for our mechanical design and right-sizing?”
- “What about infection control while working in shared office environments? Do our offices share the same air?”
- “Where does the air travel when it leaves this conference room?”
- “What questions should I ask my design or facilities engineer?”
Recently, construction and functionality of office space has evolved into a more open work area where large groups of people gather to work and collaborate. This is a great idea for real-time think tanks and collaboration. However, given the current climate, we are we now looking at a very different approach to how we do business and function in our own ecosystems. Will the ongoing desire to collaborate, share, and grow in one common office space continue to be the norm?
Infectious diseases can travel on airborne particles, and subsequent particle size, velocity, and environmental airflow all have an impact on the spreading behavior. We are entering an era of conscious social distancing that may never return to the casual nature of the past. Existing health care facility and lab design considerations may begin to apply to common commercial space layouts and designs. While we may not see sensors, such as FLIR cameras, in building lobbies looking for bodies with elevated temperatures right away, there are several measures we can take and questions we can ask to facilitate an industry-wide reevaluation of stakeholder concerns, client education, design approach, and control sequences.
One immediate reaction is to simply “throw more air at it,” or teleworking/reduced attendance lead to more airflow per person present in the office (to some degree). But, beyond this, what does it mean for sub-micrometer particles if we increase airflow while keeping the same design? Will our load-driven designs become high-ventilation-driven? Grosskopf and Mousavi found that “…higher ventilation rates may not be effective in significantly reducing respiratory aerosol concentrations within patient rooms but rather may mobilize and disperse aerosols within and between… rooms…” (Grosskopf, 2014)
Another immediate concern is increased energy usage and a decreased user comfort from higher air velocities from grilles and registers. This could put occupants into a non-optimal comfort zone as outlined in ASHRAE 55. If a building engineer simply increases maximum flow per zone, there will be an increase in the maintenance of the system in addition to more air noise as well (no rebalance of reselection of outlets). We have seen negative impacts to conference call noise from airflow in recent projects, especially with modern ceiling-mounted condenser microphones for corporate systems. This results in noticeable meeting attendee background noise and a wind-tunnel effect for everyone on the call.
What has been shown effective in mitigating airborne infection is pressure relationships, which cause a higher pressure in one room to flow air to another room at a lower pressure. According to ASHRAE/ASHE, 2017, “…airborne infection isolation rooms must maintain a minimum 0.01 inWC (2.5 Pa) negative pressure relationship to adjacent spaces.”
While common in hospitals and laboratories, these design parameters are much less common in commercial office designs. Keeping in mind COVID-19 particles are 0.1-0.2µm diameter as reported by the medical community, Grosskopf and Mousavi found that “0.5[µm] particles appeared to diffuse randomly and uniformly in the environment… In summary, the results of this study suggest that current health care ventilation standards can be effective in containing respiratory aerosol transport when proper air pressure relationships and door positions between patient rooms and corridors are maintained.”
These pressure differences are nearly imperceptible to occupants and present a significant opportunity for minimizing and controlling airborne contamination.
This strategy of negative 0.01 inWC differential to an adjacency is used for “anterooms” and is common practice in medical design and construction. One key difference to address in design methodology is that with health care facilities we can assign the location of a sick individual (such as in an exam room), but what if the location of the sick individual is random? We may have to further revise system design and layout to differentiate spaces where occupants spend most of their time, versus “transitional” spaces, such as hallways and breakrooms.
Can we change filters or add additional filters? A filter is, in effect, an orifice plate in your airstream, meaning there is an added pressure drop at the same flow rate when additional filtration is added. For a system to achieve the same duct static pressure set point (assuming a conventional VAV system), the fans will need to work harder (read: use more energy) for the same design flow due to the additional resistance added by the updated filter(s). At best, this means additional energy cost; at worst, this results in a severe decrease in airflow and the ability to control comfort cooling as the sun moves across the sky and envelope loads change (thermal gain). If a high-pressure-drop high-filtration filter, such as a HEPA filter, which can remove particles down to 0.03 µm, is added to a simple system, such as a household furnace, the system would experience a significant loss of airflow and other issues will arise such as overheating furnace elements that were designed, assuming a normal airflow/normal filter.
Additional filter pressure drop may impact fan sizing for projects currently in design. Some systems fall short of design flow when their existing filters are fully loaded. Perhaps a design engineer may select a fan larger than the one that was previously scheduled. Is this increase in initial cost warranted? I suggest an investment in a modern variable frequency drive (VFD)-controlled commercial system might be worthwhile. VFD controls can be modulated up for higher flow or down for energy savings and control. Fan size is more of an ordeal to retrofit, especially in fan-wall air-handling units.
Filters are typically rated on a MERV scale with the higher number meaning tighter filtration. Common high-performance filters — such as MERV-13 filters, which are common in LEED projects — through MERV-16, which are less common, only function down to ~0.3µm. The virus can also be a component of a water or mucus droplet, which can increase the likelihood of capture on more conventional filters, though the coronavirus particle itself is typically smaller. MERV-17 through MERV-20 filters exist and could be solutions; however, installing one in a system where fans aren’t sized for that resistance and flow will introduce more issues than it solves. A typical issue one might see is the inability to meet a duct static pressure set point, which would result in the most hydraulically remote zone to be starved for airflow. This may result in supplemental measures, such as an added direct-expansion system or variable refrigerant volume system to cool or heat those spaces. Redundant filtration banks at facilities such as airports with volatile organic compound (VOC) filtration and treatment already have fans and ductwork sized for this enhanced pressure drop. While HEPA filter retrofits would help eliminate low-micron-sized particles, the immense pressure drop makes the solution impractical.
Does this mean enhanced filter sizing methodology could be applied to the commercial office environment? As sub-micron filtration systems and accessories are further developed — an inevitable result of our current crisis — it is likely you will want your fan system currently in design to have the option for more “available fan curve” to address additional pressure drop. There will also be retrofits when these systems come to market. Along with any enhanced filtration comes the facility’s responsibility for enhanced maintenance intervals. Filters left unmaintained result in filter bypass as a result of pressure behind the filter. This negates the benefits of such a system.
One system showing promise is a UL- and Office of Statewide Health Planning and Development (OSHPD)-listed ultraviolet light/titanium dioxide system. A system like this uses ultraviolet light and oxide reactions to efficiently render virus-sized particles harmless. When installed in series and downstream of a MERV-13 filter, the rate of biologicals removal is 98% or higher on a single pass. A system like this does not create ozone and is ideal for retrofit use since it only requires 120-V power with a relatively low pressure drop of ~0.05 inWC, meaning existing fans could possibly remain. Specified during design or retrofit into existing spaces, systems like this could be utilized at the zone, such as a large conference room, or system level, such as a main supply air duct. When a system such as this is installed in a supply duct, more air recirculates from a normal return air sequence of operations, driven by energy conservation, so the total body of air in the building is being retreated additionally with each pass.
Test and Balance
Test and balance (TAB) is the process of setting and verifying working fluid flow against designed operational conditions for a system. In commercial offices, this working fluid is typically hydronic water systems for air conditioning coils but also includes airflow settings at air inlets and outlets. It is conducted near the conclusion of building out a space when finishes and doors are in place. A typical TAB tolerance in a project specification is +/-10% for any specific air outlet flow cfm. This means a measurement of 270 cfm or 330 cfm on an air outlet of design flow 300 cfm will be accepted. In certain hospital and lab environments the tolerance can be tighter and offset differently such it could be +/-5%, or +10%/-0%, or +0%/-5%. Zero percent means that any reading in that direction from design will be rejected.
Should we increase the TAB accuracy requirements in our specifications? During conceptual design, additional attention should be paid to zonal pressure relationships in office spaces going forward. Questions such as these should be considered: “Is my office pulling in air from the hallway zone or pushing air out?” “Is that conference room full of people sharing air with my open-office area on the other side of the wall?” Maybe it’s time to see a negative/positive pressure relationship zone matrix included for commercial office construction, which would result in new design criteria to achieve new pressure zones with room layouts.
Higher humidity, to a point, can help systems reduce the spread of infectious disease. ASHRAE tells us, “…many airborne viruses that transmit infections are originally submicron…in air they are often attached to larger aerosol [such as water or mucus drops]…which may be more easily filtered from the airstream.” and “Pathogen transmission through the air is greater when the air is dry, and infectious particles travel deeper into the lungs when they are small. Cilia in the respiratory system, which are responsible for clearing particulates out of the bronchial tubes, have reduced function in dry conditions.” (ASHRAE, 2009)
Typically, an air-handling unit will cool outside (or mixed air) below its psychrometric saturation curve prior to entering a building’s ductwork, which results in a cooling coil “wringing the water out” from the supply air stream to reduce humidity levels. The relatively dry air enters the office space and provides greater human comfort from skin evaporation and reduced condensation. Depending on the climate zone and locale, design engineers can review the possibility of resetting the air-handling unit supply air temperature, so the coil does not dehumidify the supply air stream as much, which results in a higher relative humidity inside the office. This approach will likely require additional studies and careful calibration. Note that systems like the titanium dioxide system mentioned earlier prefer a mid-range relative humidity because its ability to remove and deactivate tiny particulate is enhanced.
Currently, commercial office air-handling systems walk a fine line when mixing recirculated air with outside air. This is driven by the desire for reduced energy consumption. Commercial ventilation systems in California come with economizer damper sections that offer the ability to mix outside air with recirculating air. The intent is energy savings. Outside air is typically written in mechanical equipment schedules as a “minimum outside airflow” for offices that typically follows code-required minimum ventilation rates. Let’s say a stakeholder group instructs a building engineer to increase the minimum outside air on all air-handling units, regardless of code updates.
At 100% outside air (which is typical in labs and hospitals), energy efficiency takes a backseat over the need for new air changes in the facility. For this reason, health care facilities can use three to five times more energy than similarly sized office buildings. (ASHRAE, HVAC Design Manual for Hospitals and Clinics, Second Edition, 2013) Revised control set points could be attempted with a modern control system but would result in a decreased ability to condition the space and control comfort in the building. This is due to coil sizing — and consequently heating and cooling capacity — in the existing air-handling units that were driven by a recirculation-based analysis. This would result in the coil’s inability to keep up with the increased cfm at certain operating points and outside air conditions. This would also drive water plant energy use up. When supply air temperature is being measured as not meeting the set point, the coils in the air-handling unit will actuate to 100%. With the coil valves fully open, the water plant set point control loops will be impacted as chillers and boilers stage up to meet the differential now presented between this set point and the actual reading. Instead of 100% outside air set point adjustments in existing facilities, maybe designers and code committees should consider whether recirculation should be thought of another way: a maximum recirculation of the mixed air set point?
The proximity of the outdoor air intake to potential contamination sources can pose an increase in the risk to occupants. There should be new attention to the spacing of inlets and outlets on the roof/exterior. Perhaps this should be the same attention spent on locating a backup generator exhaust away from an air inlet? This raises more questions (All of these ideas are worth considering but come with added cost):
- “Should every commercial install have high-velocity exhaust fan outlets or extend above the roofline?”
- “Should there be tighter commercial office construction guidelines for drywall and architectural assembly gaps for external air infiltration paths?”
- “Should we filter our exhaust outlets or increase velocity (similar to lab exhaust)?”
- “Should we filter incoming outside air to a higher standard?”
Air Delivery Strategy
A foundation of effective airborne contaminate control is airflow from clean to less-clean spaces. Typical commercial office approaches include both ceiling supply and return, with an assumption that this will provide a well-mixed environment. In most commercial offices, the return air path is a “plenum return” in order to reduce first cost. This means return air back to the air handler (or exhaust path, depending) is simply taken from one location typically above a ceiling. This is done under the assumption that the air is well mixed and there is no need to return or exhaust at the point of occupancy. This avoids the first cost of return-specific ductwork, and the ongoing cost of return or exhaust fan upsizing is avoided.
For smaller installations, this may result in the need to add a return or exhaust fan where one was not specified before. Should all return/exhaust air paths be ducted going forward? Will return air paths have their own monitoring and control capabilities? The first cost and energy usage impacts of this would be significant. Gone would be the days of surplus space available above ceilings. This would be followed by potential structural constraints and the need for more complete BIM modeling of even basic office buildouts to ensure the fitment of all systems.
Studies have shown that air exhausting near an occupant is most effective at airborne infection control, “…airflow paths, induced [supply] airflow paths, and [exhaust] grille placement can be coordinated to establish effective contaminant control. Locations of the [supply] and [exhaust] openings are the most important elements that directly affect the pollutants dispersion in the room.” (Cho, 2019)
Airflow throw patterns from air outlets may now take further importance. Baseline industry designs might be reconfigured for floor supply and ceiling return or vice-versa. Airflow would then be in a continuous displacement direction instead, which could result in a rise of underfloor air delivery (or exhaust) system type implementations. The questions become:
- “Does our design approach to the orientation and spacing of air inlets and outlets need to change now?”
- “If return grilles should be located near contaminant sources, would there be one exhaust grille for every X square feet? For every so many occupants in an office area?”
- “With these new considerations for air inlet and outlet placement, how could obstructions from fixtures, furniture, and equipment be impacting?”
- “Will we require airflow supply-exhaust short-circuiting studies from these new, more numerous, exhaust air locations?”
These considerations require new attention to control systems and sequences of operation. Likely, stakeholders will now demand that a review of the sequence of operation for airborne contaminate countermeasures be considered in the engineering design. Expect to see interest in IAQ and air monitoring not seen before in commercial spaces. Air monitoring for CO2 concentration or particulate (possibly as an indicator or leakage) will be as important as the need to monitor pressure differentials between zones. While previously of great importance in high rises and smoke control system behavior, conventional office buildouts will now likely monitor pressure between conference rooms or private offices and hallways or entryways. This will require hardware points (sensors, wiring, devices) and result in more first and future cost from owners at the benefit of overall employees’ wellbeing. Owners, if not required by potential code revisions, will need to determine this cost benefit and value for protection against viral-sized particulates. Controls costs will also increase significantly.
So, what if a newly monitored zone pressure alarm is triggered? ASHRAE rightly suggests the behavior of adjacent zones should behave accordingly: “…exhaust should cease when supply airflow is stopped in areas otherwise maintained at positive or neutral pressure relative to adjacent spaces. Likewise, supply air should be deactivated when exhaust airflow is stopped in spaces maintained at a negative pressure.” (ASHRAE, 2009)
Added sensors may be required for movable partitions and windows, possibly to deactivate pressure zones. A sequence of operation sections specific to airborne infection control may become the new normal. This added complexity will result in a greater need for annual recommissioning of these systems compared to the original design.
For hospital systems, energy cost is the fifth design priority under performance, safety, reliability, and maintenance costs. (ASHRAE, Health Care Facilities, 2009)
While this is arguably true for health care applications, it would take significant leaps for energy to take a back seat for all commercial office space. It might be possible to utilize existing CO2 demand-control ventilation controls as an energy-saving tactic to drive down energy usage in a new way while improving airborne infection control.
One strategy to increase the outside air mix percentage is to introduce additional dilution by setting CO2 demand control ventilation zone set points to a lower ppm concentration than were previously utilized. Demand control set points are typically seen at 1,000ppm CO2 concentration relative to atmospheric. For example, a differential CO2 set point of 500 ppm (relative to ambient outdoor) would result in a lower “tolerance to occupancy” prior to the control system driving the outside air damper further open. This may be a desirable approach since it still allows the energy usage curve to fit the occupancy curve to some degree.
This article was written during COVID-19 shelter-in-place orders. While we hope for a rapid return to the previous normal, we must consider the pace of change in the industry. Building codes will have new scrutiny, but code cycle changes may be too slow to have a meaningful impact in the short term. An administrative bulletin could be a short-term solution. If it becomes clear that commercial offices should be required to have filtered exhaust, or minimum outside air percentages must change, or some of the other ideas we have considered come to fruition, the ventilation phase of the project will be an important consideration. It may be relatively easy to capture these ideas compared to designs now prior to an advanced state, like construction documents. It used to be when an HVAC designer started, the primary concerns were a combination of cost, comfort, and energy. With today’s events, it’s becoming more pressing that contaminant protection be included in the discussions through these questions:
- “How does this new approach impact energy modeling and existing design methodologies?”
- “Will building envelope designs change to add more energy use from the HVAC systems?”
- “What if the project is already in construction?”
- “Do we need new programs to study particulate migration in HVAC designs?”
- “What variances is your local authority or department of building inspection prepared to consider?”
- “How qualified will our building operating engineers need to be?”
To get started with an existing facility, you may wish to collaborate with your facility/design professionals and review as-built drawings and final test and balance reports to identify areas with a dedicated air-handling unit, and which rooms those units serve, for possible pressure differential set points.
Is this a return to constant volume systems? I would argue no. We have the controls and existing expertise to design systems that can balance the need to limit the spread of airborne infectious disease while acknowledging our collective social responsibility for energy conservation, which prevent global implications of a different kind.
“In microbiology, reservoirs allow microorganisms to survive, amplifiers allow them to proliferate, and disseminators effectively distribute bioaerosols.” (ASHRAE, Indoor Environmental Health, 2017) By working together, we can help keep commercial office spaces, and other non-health care projects, from being any of these.
ASHRAE. (2009). Health Care Facilities. In ASHRAE, HVAC Applications.
ASHRAE. (2013). HVAC Design Manual for Hospitals and Clinics 2nd Edition.
ASHRAE. (2017). Indoor Environmental Health. In ASHRAE, Fundamentals.
ASHRAE/ASHE. (2017). Standard 170. ASHRAE.
Cho. (2019). Removal of Airborne Contamination in Airborne Infectious Isolation Rooms. ASHRAE Journal.
Grosskopf. (2014). Bioaerosols in Health-Care Environments. ASHRAE Journal.