Constant Volume Ventilation Is Not a Safe Choice

Recently a client was comparing Aircuity to a constant volume 6 air changes per hour (ACH) design for a new lab building. As we know from our 20 years of experience in analyzing building ventilation performance, a constant volume approach doesn’t account for the number of indoor air quality (IAQ) events that occur even on a daily basis.

In order to assist the customer with this comparison, Aircuity’s Client Success team analyzed the prior week’s data for the 187-room project1. They found that, during the one-week period, 198 events occurred where the Aircuity system detected an IAQ issue and drove the air change rate over 6 ACH. On an annual basis that’s a total of 10,296 events and 55 IAQ events per room per year. If a constant volume design of 6 ACH was used in the lab, there would have been many periods during which the air wasn’t at safe and healthy levels.

(Figure 1)

Figure 1 shows the Aircuity system’s Demand Control Ventilation (DCV) command values, in air changes per hour, for the project. The red horizontal line gives a visual reference for 6 ACH. The figure illustrates both the amount of time where Aircuity allowed ventilation rates below 6 ACH for energy savings as well as instances where the Aircuity system sensed IAQ events and demanded more than 6 ACH to provide cleaner, healthier air.

When looking closer at the IAQ events, it is also evident that Aircuity was effective at quickly recovering the space to [safe/clean] IAQ levels. In fact, recent analysis has shown that clearance times are cut in half from 6 ACH (90 minutes) to 12 ACH (45 minutes), meaning lab occupants were exposed to significantly less TVOCs. Aircuity’s ability to dynamically match ACH to air quality events creates a healthier environment for lab occupants. In the case of this client, a set level of 6 ACH would have been a significant shortfall and caused occupants to be exposed the unhealthy air for a much longer period of time.

(Figure 2)

Figure 2 focuses in on the DCV command values for one day of the weeklong period displayed in Figure 1. The narrow width of the elevated DCV commands shows a rapid recovery to safe, healthy air quality levels after an IAQ event.