[Equest-users] Infiltration inputs

Wed Mar 30 14:56:25 PDT 2016

Hi Mike,

Marrying typical infiltration metrics for envelope testing context, for load calculations, and for hourly energy simulations, is tricky at best. 

For layman’s context, 50Pa equates approximately to a “brisk” 20 mph head-on wind (https://www.youtube.com/watch?v=Jf5mCS6HYaI), but pressurizing a building/space for ACH50 blower testing isn’t quite apples-to-apples with “natural” wind-driven conditions/infiltration.  

Here’s an idea for a “hack” that may be good enough to avoid a more sophisticated calculation/study – I’d welcome any discussion or suggested improvements:

0.       Open ASHRAE Fundamentals to find the page with this Sherman-Grimsrud model equation & associated tables in the ventilation & infiltration chapter (I found it on pg. 16.23 for Fundamentals-2009).  To wit: this is originally intended for approximating low-rise residential whole-building infiltration:

1.       Consider 0.6 ACH against your whole-building volume to come up with a whole-building infiltration flow rate (CFM) at those pressurization conditions (50Pa).  

2.       Take 50Pa to equate to a 20mph average windspeed condition (U).  You can go further if you like at this link: http://www.cactus2000.de/uk/unit/masswsp.shtml

3.       With this “at testing conditions” infiltration/exfiltration rate (Q), leverage the S-G model equation to solve for “whole-building effective air leakage area” (AL).  You’ll have to make a few assumptions regarding indoor/outdoor temps at the time of testing (or assume temperature-driven infiltration is negligible at such high pressurization & zero that out) & also use the associated tables to look up appropriate coefficients for your building’s height / sheltering factors.

4.       Plug that leakage area (AL) back into the equation with a 10mph wind (U) to determine whole-building infiltration CFM (Q) at 10mph.  

5.       Now for your spaces of interest, apportion how much infiltration is occurring relative to the whole building by considering exposed area of envelope for those spaces in proportion to the total building envelope area.  I’d isolate these areas to vertical envelope wall surfaces, and ignore roofs/floors for this apportioning effort as roof/floors would in a typical commercial case would be pretty airtight, hopefully.  QSPACE = QBLDG * (AREA OF SPACE ENVELOPE / AREA OF BLDG ENVELOPE)

6.       Convert QSPACE to ACH @ 10mph for your SPACE inputs, and don’t specify (default out) INF-FLOW/AREA.  This will allow your calculated infiltration to track during the simulation with hourly windspeed.

This “hack” procedure ignores that spaces with more doors & operable windows are going to be relative “weak points” for infiltration in the big picture, but I reckon if applied uniformly should approximate relative infiltration rates on a building-wide level for different tested thresholds.  This is probably “wrong” on other levels, but for your purposes might be good enough for relative before/after inputs to approximate energy savings.  

Depending on context, you might want to re-tool this suggestion to isolate to only individual space volumes, without stepping up/back from whole-building measurements.  This however hinges on how the ACH50 requirement would be applied/measured/isolated to individual spaces of a building, an area I lack experience in… Perhaps someone can fill me/us in further on that front?

~Nick

------------------------------------------------------------------------------------------------------------------------------

Nick Caton, P.E.

  Senior Energy Engineer
  Energy and Sustainability Services
  North America Operations
  Schneider Electric

D  913.564.6361 
M  785.410.3317 
E  nicholas.caton at schneider-electric.com <mailto:nicholas.caton at schneider-electric.com> 
F  913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Michael Campbell
Sent: Wednesday, March 30, 2016 11:01 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Infiltration inputs

Hello,

Hoping I can get a quick answer on this - I need to input 0.6 ACH50 into my energy model for certain spaces. What would be the appropriate way of inputting this into equest?  I understand that infiltration in equest is based on a reference wind speed of 10MPH but how does that compare to a pressure differential of 50Pa?

Thanks,

Mike

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