With the increasing threat of wildfire smoke and its impact on outdoor air (OA), WellStat has seen an increase in concern on how these wildfires are altering clients indoor air quality (iAQ). Let's explore a particular case where a client of WellStat's noticed that all 6 of their buildings exceeded the PM10 threshold and were curious if there was a way to decrease the negative impact the outdoor air quality has on their indoor air.
We suggested to the client that, when you expect OA to have considerably higher PM than indoor air, you should maintain or increase the return to OA ratio on the floors that register lower PM than OA and check for return filters for those that don't. An excessive buildup of PM at night may indicate that the OA dampers are not closing properly after building shut down. It is very common that OA dampers are not seal tight as that is not their function, however it is good to check that the OA actuators are properly closing the dampers.
High PM build up may also indicate a building's overall negative pressure. This may be caused by exhaust fans left running during shut down, or a high pressure air front hitting the building. If some zones are more sensitive, the high indoor PM may be mitigated by leaving the AHUs running at night for those zones. This should be done by recirculating low PM air from within the building, closing the OA dampers 100% and also the exhaust dampers, if applicable. It's worth noting that re-routing air from the building into a sensitive zone to make it higher pressure is done at the expense of making the rest of the zones more negative pressure, and this may make the PM situation worse for those zones.
"It's remarkable how OA affects iAQ." - WellStat Client
This is also a good opportunity to check for correct sensor locations. For example, at one of this clients' locations, the OA is lower than many indoor sensors, at least before start-up, but after a while you can see that the indoor sensors drop in PM, while OA remains relatively constant. If you don't see the same behavior at some sites, it is possible the location of the OA sensor is swapped with some other interior one.