[Equest-users] eQuest Infiltration Modeling as Airflow (CFM/ft2)

Mon Dec 28 19:13:48 PST 2020

Well stated Brian!

An extra anecdote/caution:

Brian’s sentiment expressed here – that pressures typical of blower door testing for envelope leakage are atypical for those found caused naturally through wind pressures and other forces for infiltration/exfiltration, is correct.  Most of the time.

If however you are tuning a model expressly to evaluate measures specifically affecting infiltration, be mindful that nature likes to play outside the box sometimes.  I was tuning a specific laboratory building with extremely leaky glazing (partly because the building was a mechanical vacuum cleaner, mostly because the glazing was original to the building, nearly 70 years old), and was utilizing AMY weather to assist with a particularly weather-sensitive and touchy calibration process.  That historical weather period coincidentally proved invaluable for this exercise, in part because the calibration period it contained some fairly extreme weather events… Some wacky looking windspeeds in the Spring caused me to google for climactic (?) events in this locality (Illinois), and I found just cause for the resulting extreme external building pressurization/infiltration: tornadoes!

Quick takeaway:

  1.  While blower door testing is indeed pretty darn extreme in the context of “normal” conditions – nothing is more extreme than mother nature =).
  2.  This also adds a little perspective on the concerns I hear raised occasionally on the idea that blower door testing with existing/aged facilities can harm with respect to building envelope performance… even if true, it’s nothing in comparison with Midwest weather events of this nature!

~Nick

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From: Equest-users <equest-users-bounces at lists.onebuilding.org> On Behalf Of Brian Fountain via Equest-users
Sent: Thursday, December 24, 2020 8:29 AM
To: Omer Moltay <omoltay at mimtarch.com>
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] eQuest Infiltration Modeling as Airflow (CFM/ft2)

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50 Pa is too high a pressure difference to use for entry into eQUEST – regardless of whether you use ACH or cfm/area.  That is a testing pressure differential rather than the ambient pressure differential created by the reference wind speed of 10 mph (4.47 m/s).  Ambient wind corresponds to a pressure differential of ~5 Pa.  Using the n50 flow rate would considerably overstate your infiltration load.

[cid:image005.png at 01D6D959.0CBDCB50]
(from eQUEST/DOE-2.2 help)

To convert the tested leakage rate (n50) at 50 Pa to the ambient 5 Pa use the formula below …
[cid:image006.png at 01D6D959.0CBDCB50]
Formula taken from an excellent white paper on modelling infiltration by RWDI available here: https://sbcanada.org/wp-content/uploads/2017/04/Air-tightness-Energy-Modelling-for-Part-3-Buildings.pdf<https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsbcanada.org%2Fwp-content%2Fuploads%2F2017%2F04%2FAir-tightness-Energy-Modelling-for-Part-3-Buildings.pdf&data=04%7C01%7CNicholas.Caton%40se.com%7Cd28d7a6799bb44a0242508d8a79a8c2d%7C6e51e1adc54b4b39b5980ffe9ae68fef%7C0%7C0%7C637443629515267558%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=yfhNqxEK1TFs9BEcpDWhRH2SIooG8uDkCV1loWkV2ew%3D&reserved=0>

Regards,

Brian

Footprint

Brian Fountain

From: Equest-users <equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org>> On Behalf Of Mark Stout via Equest-users
Sent: December 23, 2020 4:03 PM
To: Omer Moltay <omoltay at mimtarch.com<mailto:omoltay at mimtarch.com>>
Cc: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Spam released by Allowed List] Re: [Equest-users] eQuest Infiltration Modeling as Airflow (CFM/ft2)

CAUTION: EXTERNAL SENDER
I would use ACH and base it on -50 Pa and the standards the AHJ has set.  For example, IECC sets some zones to 3 ACH, but the AHJ may/can/do override that and may up it to 5 ACH.  Then you just work backwards to arrive at ACH, e.g.  the formula for CFM is (5 ACH  ÷ 60 Minutes) x Volume = CFM50. Now you know what your max infiltration.

On Wed, Dec 23, 2020 at 12:50 PM Omer Moltay via Equest-users <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>> wrote:

Dear All,

eQuest allows inputing of infiltration airflow as CFM/ft2 (of indoor space in detailed mode) for every zone.

I have calculated CFM/ft2 values for each zone in my model, but this calculation has been done for an outside - inside pressure difference of 50 Pa. Or I can do the calculation for any arbitrary pressure difference.

The question is, since infiltration is very much pressure dependent, what pressure difference does eQuest assume when the infiltration airflow input is utilized? I know that it will not take into account the dynamic changes in pressure dif. during the simulation due to wind effect or other effects, but I just want to be able to input a reasonable value since a calculated infiltration at 50 Pa is very different from say, at 5 Pa. The answer is of course, at whatever pressure difference the zone is anticipated to be operating, so what would you recommend for low-rise commercial buildings? Or any other guidance would be appreciated.

Or should I convert my calculated values to ACH and use that input, as that takes into account wind speed changes? Again, what is the reference pressure dif. to use there?

Thank you,

Ömer Moltay, LEED AP BD+C, ASHRAE BEMP, ASHRAE CPMP
Mimta EcoYapi, LEED Proven Provider
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