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

Jeff Haberl jeffhaberl at tees.tamus.edu
Thu Feb 1 19:14:06 PST 2007


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

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

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----- 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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