[bldg-sim] how much cold air infiltration can I really expect?

Jeff Haberl jeffhaberl at tees.tamus.edu
Fri Feb 2 19:53:29 PST 2007


Glenn. 

That's a good question.

 Normally, when one uses DOE-2 one shuts off the infiltration in LOADS when the AHU is running (assuming there is some CFM//FT2 fresh air specified in SYSTEMS). This is what is used in the SAMP1.INP file that comes with DOE-2. I believe the prevaling wisdom was that this would serve as a proxy for the pressurization of the building, although DOE-2 always balances the intake vs exhaust. 

In a city like Chicago, which SAMP1 assumes, one sees the infiltration "on" in the winter and "off" in the summer, which seems strange, but is probably not a bad assumption for a building heated with baseboards and cooled with a package AC that is shut off in the winter, with ventilation supplied only by infiltration. 

In reality, most olderr buildings have the AHU running 24-7-365, but then, this would say that infiltration was "off" all year, which is probably not what happens in reality. Most likely buildings that are positively pressurized also experience infiltration, driven by combined stack and wind forces. Such a coupled model is more complex than what is happening in DOE-2 and probably most other programs short of CFD or coupled nodal programs. 

Jeff

BB 8=!  8=)  :=)  8=)  ;=)  8=)  8=(  8=)  :=')  8=)  8=)  8=? BB

Jeff S. Haberl, Ph.D., P.E.............................jhaberl at esl.tamu.edu

Professor......................................................Office Ph: 979-845-6507

Department of Architecture.......................Lab Ph: 979-845-6065 

Energy Systems Laboratory.......................FAX: 979-862-2457 

Texas A&M University..............................77843-3581

College Station, Texas, USA.......................URL: www-esl.tamu.edu

BB 8=/  8=)  :=)  8=)  ;=)  8=)  8=()  8=) 8=?  8=)  8=)  8= BB
 

----- Original Message -----
From: Glenn Haynes <glenn.haynes at rlw.com>
To: Jeff Haberl; bldg-sim at gard.com <bldg-sim at gard.com>
Sent: Fri Feb 02 09:25:04 2007
Subject: RE: [bldg-sim] how much cold air infiltration can I really expect?

Jeff,

I agree with you, especially for residential applications that typically
do not use outside air intake to the AHU.

I'm not certain, however, how various amounts of forced ventilation
affect infiltration (except to reduce it, of course) for commercial
applications.  Have you seen any studies on this?

Thanks,
Glenn

-----Original Message-----
From: Jeff Haberl [mailto:jeffhaberl at tees.tamus.edu] 
Sent: Thursday, February 01, 2007 10:14 PM
To: Glenn Haynes; bldg-sim at gard.com
Subject: Re: [bldg-sim] how much cold air infiltration can I really
expect?

It also helps to use the "S-G" infiltration method (which stands for
Sherman-Grimsrud). This seems to yield good results for wind driven
infiltration when tested against field data. 

Jeff
BB 8=!  8=)  :=)  8=)  ;=)  8=)  8=(  8=)  :=')  8=)  8=)  8=? BB

Jeff S. Haberl, Ph.D.,
P.E.............................jhaberl at esl.tamu.edu

Professor......................................................Office
Ph: 979-845-6507

Department of Architecture.......................Lab Ph: 979-845-6065 

Energy Systems Laboratory.......................FAX: 979-862-2457 

Texas A&M University..............................77843-3581

College Station, Texas, USA.......................URL: www-esl.tamu.edu

BB 8=/  8=)  :=)  8=)  ;=)  8=)  8=()  8=) 8=?  8=)  8=)  8= BB
 

----- Original Message -----
From: bldg-sim at gard.com <bldg-sim at gard.com>
To: bldg-sim at gard.com <bldg-sim at gard.com>
Cc: bldg-sim at gard.com <bldg-sim at gard.com>
Sent: Thu Feb 01 09:32:33 2007
Subject: [bldg-sim] how much cold air infiltration can I really expect?

Aaron,

 

It is a fact that infiltration or outside air can dominate the heating
loads in buildings with significant internal heat gains, high mass,
insulated shells and/or good or few windows.  It is always a high
contributor per CFM in cold weather, and IAQ concerns can mandate
significant quantities of ventilation air.  Also, bear in mind that
infiltration is always proportional to wind speed, but it can be reduced
significantly when the building is pressurized with outside air drawn in
through air handlers.

 

If you really need to estimate heat losses due to infiltration, you
should apply a method that accounts for the wind speed.  A problem that
you might encounter, however, is the somewhat random nature of this
variable in the TMY and TMY2 files.  I have found that very high wind
speeds can occur during times of relatively mild temperatures, and can
set the peak heating loads.  These wind speeds are relatively random
compared to the dry bulb temperatures in these files because they were
assigned a much lower weight in the statistical selection of the TMY
monthly data.  You can check the wind speed whenever the peak heating
load is occurring at a strange time, and just disregard this peak, or
you can alter the wind speeds in the TMY file you are using.

 

Temperature data in the TMY files are fairly well behaved because they
typically vary much slower than wind speed.  In a truly average season,
the heating loads will be strongly correlated to temperature, but when
you introduce high wind speeds with significant infiltration effects
into a model, a few high hourly wind speeds can dominate your heating
loads.  This is counter-intuitive, and should not be occurring during an
average weather season (although it can truly occur in any single
weather season).  More often than not, we intend to model typical (or
even average) conditions in a building energy and demand simulation.  If
you do not somehow adjust (chop the extremes, average over time, etc.)
the hourly wind speeds in the "typical" weather file, you can obtain
strange hourly heating results due to infiltration.  My personal
preference is to estimate average wind speeds and average peak speeds,
and alter the TMY file to reflect these averages throughout the typical
year.  This way, the average seasonal energy effects of infiltration are
not altered, and the peak wind speeds are kept in place hourly but
buffered (to reflect more typical values).  You can do this or something
similar using the DOE2 weather utilities and a spreadsheet.

 

This might be more than you asked for, but I hope it will help you or
some other modelers on this list to answer some questions you may still
have regarding the effects of infiltration.

 

Glenn Haynes, PE

Senior Consulting Engineer

RLW Analytics, Inc.

860-346-5001x202

 

________________________________

From: bldg-sim at gard.com [mailto:bldg-sim at gard.com] On Behalf Of
Dahlstrom, Aaron
Sent: Wednesday, January 31, 2007 9:29 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] how much cold air infiltration can I really expect?

 

I have noticed that infiltration makes up most of the winter heating
load for several high-bay single-story commercial buildings I'm
modeling. (big-box retailer, light manufacturing facility)

 

I've used a variety of methods - ACH estimates, CFM / so of facade (per
ASHRAE F27.10), and Area(crack) x Velocity(winter wind) - to estimate
infiltration.

 

Have other people stuck with these methods? Or does anyone know of
better ways to estimate infiltration?

 

Aaron Dahlstrom

AKF Engineers, LLP
1500 Walnut Street, Suite 1400
Philadelphia, PA 19102 
Phone:   (215) 282-2169
Fax:        (215) 735-6706
www.akf-engineers.com <http://www.akf-engineers.com/> 

 
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