Air Exchanges Per Hour: Natural Infiltration and Exfiltration Rates
A blower door test can go a long way towards identify how “leaky” a house is, but since it runs at a standardized 50 Pa, it’s more about providing a number for comparison than showing how much air infiltration/exfiltration is actually happening at any given time. That is, if the house is at 3 ACH50, then one might assume that all the air in the house is exchanged three times every hour — but that would only be true if the house had a constant 20mph wind blowing on it (give or take). If there is no wind, then it’s almost certainly not exchanging air at anywhere near that rate.
Is there, then a “natural” ACH for a house, either independent of what’s happening outside the house or, barring that, is tied to the average pressure differential in that specific area? And if there is such a natural number, how is it determined?
FWIW, I got to thinking about this when reading up on Conservation Technology’s exhaust ventilation solution: http://conservationtechnology.com/building_ventilation.html. They claim that if a house’s “natural infiltration/exfiltration rate” is 80cfm, then the math shows that their exhaust fan is MORE energy efficient than a corresponding HRV/ERV. How would you calculate/determine that 80cfm in the first place, though?
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ACH natural is usually calculated by dividing ACH@50 by something (eg 18) to account for long term average pressure differential usually being lower than 50. This is quite inaccurate - but people rely on it anyway. It's even less accurate when misused as "ACH design-day".
Thanks, Jon. Does the 18 come from anything or is it entirely arbitrary? That is, would the offset differ by person; by location; or by something else?
It varies - more info here:
https://www.greenbuildingadvisor.com/question/what-is-n-factor
Thank you! That's exactly what I was looking for.
I read their whole spiel, and frankly it's extremely stupid. They rely on humidity sensing and occupancy sensing in the bathrooms and kitchen. They think that if you're not in the kitchen or bathroom, you don't need to breathe. I'm not facetiously putting words in their mouths, they literally say that ventilation is not required when the kitchen and bathrooms are unoccupied. I guess instead of thinking you don't need to breathe, maybe they don't understand the main purpose of ventilation. Using this system would result in zero ventilation all night when the house is fully occupied but everyone's asleep. Even during the day, many people could be in the house for several hours and the exhaust will only run for a few minutes when someone uses the bathroom.
The humidity dependent ventilation doesn't make much sense either, especially for the kitchen. Smoke and particulates can rise significantly without humidity being affected. And what if the outdoor humidity is higher than indoor? How does increasing ventilation based on humidity make any sense then?
This is just rambling at this point, but working through some scenarios converting ACH50 to N-factor CFM shows me some very unexpected numbers.
First, let's look at my current main house which is old and incredibly leaky. It's 2600 sq ft with 3 bedrooms. The recommended natural ventilation rate for this house according to the BSC would be 56cfm and the updated ASHRAE code suggests 108cfm.
Well, we had an "energy audit" (read: excuse to try and upsell us on a bunch off pointless upgrades) a few years ago with a blower door showing 2896 cfm. Work through the math and this corresponds to 8.35 ACH50. Pretty bad, but notably better than the 12+ ACH50 when we first moved in..
But the interesting part is factoring in the N-factor. According to the charts, that would be between 20 and 23, here in Phoenix. Math math math and that comes out to between 126cfm and 145cfm.
Conclusion: our current super-leaky house is so leaky that it NATURALLY infiltrates and exfiltrates MORE air than is recommended by either standard... hence, any kind of manual ventilation would be utterly pointless.
But let's then look at the 800sq ft / 1 bdrm guest house I'm building. BSC says 23cfm and ASHRAE recommends 39. Let's say I did no better than the code-minimum 3 ACH50. The "natural" air flow in that case would be between 14cfm and 16cfm. Turns out that even at code-minimum, I'd still want manual ventilation in the guest house.
That example in the original post listed 80cfm as an average natural airflow rate. Just for fun, what would the ACH50 have to be in the guest house to naturally have that kind of leakage? Try 17ACH50!!
Lesson learned: ACH (both ACH50 and N-factor corrected) is much more wildly dynamic with smaller houses than larger ones.
Fun stuff!
A very good explanation of converting from blower door numbers to natural air change (and the uncertainty, and perils of the various assumptions): Infiltration: Just ACH50 Divided by 20? (Home Energy Magazine Online January/February 1994, http://www.homeenergy.org/show/article/nav/blowerdoor/id/1015)
There is no simple way to accurately convert a single pressure-test of a building to an average infiltration rate, because many building and climate-dependent factors affect true infiltration. Long-term tracer gas measurements are the only reliable way to obtain average infiltration rates. However, tracer gas measurements are impractical for retrofitters, and even most conservation researchers. A simplified rule of thumb to let the retrofitter quickly translate a pressure-test to an infiltration rate is clearly attractive.
Persily and Kronvall developed a crude conversion technique, ACH50/20, that provides reasonable results. On the other hand, it was impossible to customize the relationship of ACH50/20 to local conditions. What are the components of the magic number, 20?
The use of natural CO2 concentration decay when a house goes unoccupied may be an interesting exception to "impractical". There are some energy savings to be had if one can more accurately model ACH-natural. Maybe even something as simple as "don't run the ERV when wind speed is above X".
> Conclusion: our current super-leaky house is so leaky that it NATURALLY infiltrates and exfiltrates MORE air than is recommended by either standard... hence, any kind of manual ventilation would be utterly pointless.
You need to add "on average". Which means that under some conditions, you are going to be breathing excessively polluted air and mechanical ventilation would improve it. Even so, at high (maybe > 5) ACH50, common practice is "no ERV/HRV needed".