[Trace-users] Air Flows & Outside air Responses

AChapin at sriregistrar.com AChapin at sriregistrar.com
Thu Jul 31 07:47:00 PDT 2014

Hi John,

I have attempted to answer your questions from the two emails copied below 
starting with the email from Tuesday.

Tuesday Email:
Yes. Note that for VAV systems it may be sufficient to set the block 
airflow in the Create Systems - Advanced window.
The VAV box will still modulate assuming the Clg VAV min in the VAV 
control section of the Create Rooms - Airflows window is set to less than 
100%, as it is in the screen shot that you provided.
Assuming the pretend building you are referring to is the ASHRAE 90.1 
Appendix G Baseline alternative, than you should not manually change these 
cfms to match the design. Instead, the Baseline airflow is required to be 
autosized based on a 20 degree delta t between the design cooling supply 
air temperature (set both the Cooling Supply Max and Min values in the 
Create Systems - Design Temperatures window) and the design cooling 
setpoint for the spaces served by the system. Note that the Main Cooling 
coil capacity units on the Create Systems - Heating and Cooling Coil 
Overrides window should be set to % of Design Cooling Capacity instead of 
% of Design Capacity by Adjusting Airflow.
Enter the scheduled pump motor values.

Wednesday Email:
I normally set the Baseline model outdoor airflow manually by setting 
Apply ASHRAE Std62.1-2004/2007 field to No and entering the average 
ventilation airflow per square foot into the Cooling and Heating 
ventilation inputs. I calculate the ventilation airflow per square foot by 
taking the total design outdoor airflow associated with each Baseline 
system and dividing by the sum of the floor area of the spaces served by 
that Baseline system. The only issue you may run into is if you have a 
system with a minority of rooms with a high percentage of OA to SA ratio 
and by averaging the OA over every room served by that system you end up 
with a lower VAV minimum for those outlier rooms, but this is 
conservative, therefore the reviewer shouldn't have a problem with it.
In response to your side note: You should make sure that your Proposed 
Case model reflects the design and that the Baseline Case model has 
constant ventilation airflow independent of the hourly supply airflow of 
the VAV system. Note that to get TRACE to model constant OA in a VAV 
system, the Fixed return / outside air dampers box located in the Advanced 
Options sub-window of the Create Systems - Options window must be 
unchecked. Also, if the design has a fixed % of OA (such as the 30% 
mentioned in your email), ensure that the minimum 62.1 required 
ventilation is still supplied during the minimum expected VAV box position 
during the course of the annual simulation to comply with Section 5.4 of 
ASHRAE 62.1-2007.
You could do that, or see my response to question #1 above.

I hope that helps and good luck!

Alex N. Chapin
Sustainability Program Engineer
SRI Green Building Certification, LLC
300 Northpointe Circle ? Suite 304
Seven Fields, PA 16046 ? USA

achapin at SRIRegistrar.com
T: (724) 934-9000 x619
F: (724) 935-6825

Building Sustainability
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Date: Tue, 29 Jul 2014 15:39:04 +0000
From: "Eurek, John S NWO" <John.S.Eurek at usace.army.mil>
To: "trace-users at lists.onebuilding.org"
                 <trace-users at lists.onebuilding.org>
Subject: [Trace-users] Air Flows (UNCLASSIFIED)
<E777FD5A41D3B04A83296213737A85D70DC51048 at EIS-MB0503CPC.eis.ds.usace.army.mil>
Content-Type: text/plain; charset="us-ascii"

Classification: UNCLASSIFIED
Caveats: NONE

I use trane trace to get the loads. Then I select equipment, but I do not 
use the cfms trace provides.

Examples from trace:
Stair well 7 cfm. (The unit I selected has only went down to 200 cfm)
Storage room 12 cfm (I provided 50 cfm)
And I round up other areas such as corridors and offices.

The leed reviewer says the values in the model and in the design do not 

Do I need to go back through my trace file and enter the air flows I used 
in my design?
The main system is an AHU with VAV boxes. If I go into each office and 
enter the airflows to match the drawings, will the program use that 
airflow at 100% or will the VAV box still modulate?

Will I need to go through the pretend building and also manually change 
the cfms to match the design? Or should I use the values trace spits out?

Also, a comment was that the pumps (and air handling unit) in the model 
did not match the model.  I calculated that there would be 40 feet of head 
in the system, but I selected (and scheduled) a pump that provides a 
minimum of 60 feet of head. Do I need to enter in the scheduled values or 
the values I calculated?

Classification: UNCLASSIFIED
Caveats: NONE

Date: Wed, 30 Jul 2014 16:37:44 +0000
From: "Eurek, John S NWO" <John.S.Eurek at usace.army.mil>
To: "trace-users at lists.onebuilding.org"
                 <trace-users at lists.onebuilding.org>
Subject: [Trace-users] Outside air (UNCLASSIFIED)
<E777FD5A41D3B04A83296213737A85D70DC561D3 at EIS-MB0501WPC.eis.ds.usace.army.mil>
Content-Type: text/plain; charset="us-ascii"

Classification: UNCLASSIFIED
Caveats: NONE

For Air Handling Units with VAV boxes Trace calculates the air flows based 
on loads and outside air requirements.

I haven't looked into the program code, but I have noticed that changing 
interior loads changes the amount of outside air in the system.

This makes sense to me since, if the AHU is operating at 30% outside air 
and a room needs more air for cooling, the total amount of outside air for 
the building would change.

When I model the pretend building, it has different insulation, causing 
different energy demands, causing different air flows, causing different 
outside air values.

How do I get the two outside air values to match?

[On a side note, the outside air values shown on the report give the worst 
case outside airflow, it wouldn't mean that the baseline and pretend 
buildings will have identical outside air flows for the total 8760.]

Do I go into the pretend building model and manual enter the outside air 
cfm for each room to make it match the proposed building?

John Eurek

Classification: UNCLASSIFIED
Caveats: NONE
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