Our Search for ACH: Determining Infiltration with a Blower Door Test
Whether you’ve followed parts one, two, and three of our ventilation series, or attended our recent webinar where Energy Circle CEO Peter Troast got into the specifics about the ventilation equipment that Kevin Brenner utilizes for his New York-based home performance business, Healthy Home Energy & Consulting, there is a lot of information to absorb. But we still wanted to know more about our workspace as we look toward a new normal, and a future where we can work in our own office again.
In our three-part series on ventilation, we started on a journey to find our total air changes per hour (ACH), which is an accumulation of our active ventilation + natural + infiltration + filtration. So far, our ventilation stat lines are as follows:
Ventilation: .3 ACH from our mechanical ventilation
Natural: 0 (In our everyday office habits, we keep all windows closed)
Filtration: 1.56 (more on this later)
The only way to find the missing puzzle piece, our infiltration from air leakage, was to schedule a blower door test. We contacted a local, independent energy auditor and rater here in Maine, Bill Winkel, and scheduled a blower door test for our office.
How Leaky Is Our Office?
We have always known that there were some major leaks throughout our workspace. From the drafts throughout the hallways, to the constant condensation in some of the windows, we were sure we would find some form of leakage to contribute to our total ACH.
The leaky nature of our office was confirmed almost immediately, when Bill informed us that his single-fan Minneapolis Blower Door could not reach 50 pascals when depressurizing our space. The DG-1000 pressure gauge accounted for this, and gave us an estimated total ACH of 4.33 from air leakage at 50 pascals—a projection that Bill anecdotally says is accurate to within 10%.
In order to calculate the approximate ACH at natural pressures, or ACH(nat), we can take that ACH50 4.33 and divide it by 20 (the simple approach according to our friend Allison Bailes at Energy Vanguard):
4.33/20 = .21 ACH(nat)
Where We Felt It the Most
While the fan was running, we took a walk with the infrared thermal camera to try and locate areas of egregious leakage. And while this may be the only time I ever take this position, it was unfortunate that it was such a nice day here in Maine! The medium temperatures inside as well as outside meant there was not enough delta-T to notice anything from the infrared images.
We could, however, walk around to each individual area and feel the drafts. Nearly every room had some form of leakage, but the two areas we noticed it the most were in Peter’s office (with a window on the front facade of the building where there apparently isn’t much integrity between the brick and the framing) and the ventilated elevator shaft.
What We Learned from Our Blower Door Test
Now that we have the official results in, and have combined them with what we learned about our state of the art HRV, we can calculate our estimated combined ACH and ACH(nat):
Estimated Combined ACH for our 103,416 cubic feet: 0.51 ACH
The Next Step--Filtration
The next step in our journey is to account for any of the “bonus” ACH we can claim from our three portable air purifiers. On September 28, we turned on 3 Levoit Core 300 HEPA air purifiers, each with a Clean Air Delivery Rate (CADR) of 135 CFM. This translates into an additional 1.68 ACH. In the coming weeks, we may also add additional filtration from the infamous “Hobo HEPA” --a MERV 13 furnace filter duck taped to a box fan. (This DIY ventilation device could be straight out of an episode of the Red and Green Show, and has been made popular in homes dealing with excessive wildfire smoke up and down the west coast).
So, accounting for all the types of air exchange, we can now calculate ACH for the office.
.3 ACH (From the HRV) + .21 ACHnat + 1.68 ACH* (from air purifiers)= 2.19 ACH
*While the Harvard School of Public Health calculator clearly says that filtered air is additive, we recognize that there isn’t full agreement on this within the building science community. It does seem questionable to us that 100% of ACH from filtration and none from any other source is an equivalently healthy environment.
How Much Ventilation Do We Need?
The ongoing theme of this autobiographical series is that even with the best equipment, and elevated leakage numbers, we are still a long ways from the recommended 5 ACH that the Harvard School for Health is currently suggesting for American schools, or 3 ACH that’s been batted around for offices, amidst the Covid-19 pandemic. Based on the expert industry friends we’ve consulted while doing this, we are feeling confident that the air exchange rate in the office is at a level that would mitigate risk should the virus get into our space, particularly in light of most of the Energy Circle team still working from home.
We may not be the medical experts when it comes to keeping the built environment safe, but we are paying close attention to what the medical experts are recommending. In the coming weeks, we will continue monitoring our IAQ devices to track the effectiveness of our collective indoor environment, while also staying abreast of the evolving science around keeping our workplaces, schools, and homes as safe as possible.