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 a 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,
?mer Moltay, LEED AP BD+C, ASHRAE BEMP, ASHRAE CPMP
Mimta EcoYapi, LEED Proven Provider
Hekimsuyu Cad. 559. Sk.
No:39 34255 Kucukkoy Istanbul Turkey
Tel: 90-212-617-2296
Fax: 90-212-617-2297
Mobile: 90-533-957-1394
www.eco-yapi.com
www.buildingenergymodels.com
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 <
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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
Regards,
Brian
Footprint
Brian Fountain
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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Nick Caton, P.E. (US), BEMP
??? ????, P.E. (US), BEMP
Senior Energy Engineer
Energy Manager, Yokota Airbase
ESS - Energy & Sustainability Services
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M US
Email
+81 . 070 . 3366 . 3317
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nicholas.caton at se.com
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