By Steve Dacus PE, LEED AP
Over the last five years, Washington and Oregon have fought large wildfires and the resulting smoke created by them. Wildfire smoke can cause difficulty breathing and can present health issues for building occupants. Heating, ventilating, and air conditioning (HVAC) systems are designed to bring in fresh, clean air. When smoke is present outside, our HVAC systems draw in that smoke and create hazardous breathing conditions when they were designed to do the opposite.
Many HVAC designs use variable refrigerant flow (VRF) or variable refrigerant volume (VRV) systems of indoor fan coils connected to outdoor units via refrigerant piping. Because these systems are sensitive to the temperature at the inlet of the fan coils, designers will frequently provide dedicated outside air systems (DOAS) to precondition the outside air before it is ducted to the inlet of the fan coils or discharged directly to occupied spaces.
The DOAS has filters on the incoming outside air side of the heat wheel. However, the filters are only rated at MERV-8. This is a relatively efficient filter that balances cost with particle size captured and resistance to airflow. Wildfire smoke is made up of very small molecules. In order to filter those particles out of the air, High Efficiency Particulate Air (HEPA) filters in combination with charcoal filters are the best way to eliminate the smoke and prevent occupants from detecting smoke odors inside the building. This requires custom air handling systems that are not typically installed in most buildings.
The next best option are MERV-13 filters that are 85 percent efficient at eliminating smoke particles. These filters come in different thicknesses, so the size of the DOAS unit filter rack must be considered. Such high efficiency filters come with an increase in airflow resistance and may cause the DOAS unit to provide reduced airflow than using the original lower efficiency filter. It will also capture a great deal more debris and require changing more often. Outside air will still be able to be brought into the building, although with some smoke smell and harmful particulate present in the airstream.
Another option to consider when wildfire smoke is prevalent outside is to disable the DOAS unit temporarily, since the outside air quality would be much worse than the indoor air with the resulting increased carbon dioxide levels. As shown in the schematic diagram above, the outside air will no longer be delivered to the inlet of each fan coil or to the occupied space. Shutdown of the DOAS system will also have another side effect.
Most DOAS designs use the exhaust from the system for restroom, shower room, or other exhaust. If that system is off, then there is no exhaust occurring. However, if there are dryers or other exhaust systems operating, the exhaust air will have to be made up through leaks in the exterior envelope, allowing contaminated air to enter the building. This could be addressed by the installation of a small make-up air supply unit with HEPA or carbon filters, providing outside air in a “sidestream” application. The VRF system can continue to operate normally -- heating, cooling, or recirculating air through the occupied spaces.
A variable air volume (VAV) system is another common design. This can be a custom or semi-custom air handler within a building mechanical room or on the roof with heating and chilled water coils. It may consist of a packaged rooftop unit that has a heating water coil and direct expansion (DX) refrigerant cooling coil. It could also be a rooftop packaged unit with a natural gas heat exchanger and electric reheat coils at the terminal units. To illustrate the wildfire smoke challenges for this system, reference the second system diagram below that shows the packaged VAV system with heating water.
Outside air is brought into the building through the outside air damper on the VAV unit. This air is mixed with the return air from the space within the air handler. When wildfire smoke is present, the smoke is pulled into this mixed air plenum and distributed to the occupied spaces served by the air handler. The only way to eliminate smoke from getting into the building is to close off the outside air damper temporarily through programming of the control system. This allows carbon dioxide levels to increase inside the building but keeps most of the smoke from entering the building through the air handling system. We must say “most” since all dampers have some amount of leakage. Shutting the outside air damper will not eliminate all smoke and violates ventilation codes, but is better than the alternative in the short term.
If possible, adding HEPA and carbon filters to the VAV unit will allow the unit to function normally with a higher pressure drop through the filter sections and the ensuing energy penalty while bringing in outside air. It is unlikely, however, that these filters can be retrofit into existing units due to their size and added air resistance. Lower efficiency filters such as MERV-13 will not eliminate the smoke particles entirely, but could help, and will have a smaller pressure drop on the fan system to allow retrofit within an existing air handler.
There is no doubt that wildfires are destructive and cause great harm to those in close proximity as well as those miles away. We have addressed a few strategies that could alleviate the impacts of HVAC systems bringing smoke-laden outside air into inhabited buildings. If you have additional questions about combating wildfire smoke, please contact us.