[Trace-users] Demand Controlled Ventilation

Tue Aug 31 07:53:06 PDT 2010

Jen -
I would agree in general with Julia's process; in multizone (e.g. VAV) systems especially TRACE's DCV algorithm is based almost entirely on the critical zone.  Though, it could be argued that if everything is set up correctly, this is how a DCV system should be operated.  But it is tricky to make sure everything is set up correctly, especially with TRACE's limited hourly reporting.

I would add one additional thought that may shed a little more light on things.  Did you change the System Ventilation Flag (under Systems - Advanced) from 'Sum Room OA' to 'ASHRAE...Vent Reset' to ALLOW for DCV?  If so, you're also turning on the critical zone based ventilation control.  This will change your OA volumes throughout the entire year, going well beyond the demand controlled hours.  If you changed that flag, check the reports to see what happened to the design ventilation rates; my guess is that they've gone up in the baseline.

Scott Hackel, P.E., LEED AP

Energy Engineer
Energy Center of Wisconsin

455 Science Drive, Suite 200
Madison, WI  53711
608.238.8276  x129
shackel at ecw.org

The Buildings Team
Design decisions through energy, economics and emissions analysis.
www.ecw.org/Buildings 

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Today's Topics:

   1. Demand Controlled Ventilation (Jen Redington)
   2. Re: Demand Controlled Ventilation (Julia Beabout)

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Message: 1
Date: Mon, 30 Aug 2010 08:15:32 -0400
From: "Jen Redington" <redington at vitetta.com>
To: <trace-users at lists.onebuilding.org>
Subject: [Trace-users] Demand Controlled Ventilation
Message-ID: <5e13a5e7-99f0-4d8f-94bb-498e099d566d at vitetta.com>
Content-Type: text/plain; charset="iso-8859-1"

I am doing an energy model for LEED and added demandcontrolled ventilation to several spaces in both the design and baselinemodels.  This doubled my energy cost savings for the project.  The spaceheating gas use went down 25% in the design model but doubled in the baselinemodel. I can?t see any reason why the baseline energy use would increase,and the savings in the design case seem a little unrealistic since DCV is onlybeing used in some spaces.  Any ideas what could be wrong? Any one else have anysimilar issues?

Thanks,

Jen Redington

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Message: 2
Date: Mon, 30 Aug 2010 06:19:54 -0700 (PDT)
From: Julia Beabout <juliabeabout at yahoo.com>
To: Jen Redington <redington at vitetta.com>,
	trace-users at lists.onebuilding.org
Subject: Re: [Trace-users] Demand Controlled Ventilation
Message-ID: <544973.43622.qm at web111410.mail.gq1.yahoo.com>
Content-Type: text/plain; charset="utf-8"

Hi Jen,
I agree.  That does sound fishy.  Generally, I do not use the DCV control 
feature within Trace to calc my energy savings from DCV.  Some LEED reviewers 
have questioned using it as well.  It's not that the algorithm is not correct 
its just that applying it to a real project can be tricky and require some level 
of judgment that the software doesn't know about.  Similarly, I rarely use the 
ASHRAE 62 calc capabilities in Trace to simulate that calc.  Except of the 
simplest of situations the calc can be thrown off very easily.  One room can 
throw off the whole calc, so some judgment and adjustment is needed ....in my 
experience, it frequently take more time to comb through the calcs to figure out 
which spaces are throwing off the calc than to estimate the savings via other 
means.  I still estimate both with in Trace so it doesn't count as an 
exceptional calculation for LEED but I don't use Trace's built in routines to do 
it.

For DCV, based on the occupancy schedules for the spaces with DCV, I calc and 
make an adjusted OA schedule for those spaces.  This is actually a lot less work 
and easier than it sounds.
Similarly, for the ASHRAE 62 calc, I do it on a spreadsheet and then force that 
cfm value at the system level in Trace (System level, Options Tab, Advanced 
options button).  If you have a VAV system that needs an ASHRAE 62 calc and you 
are using DCV in some spaces, then you will need to adjust the system level OA 
schedule to reflect the DCV.  

You can check to see that the program is bringing the correct OA cfm and adjust 
the schedule as needed by reviewing the cfms via the View Reports, Analysis tab, 
Graph Profiles and Energy button.  Once there, In the view menu, turn on the 
"settings" menu and switch to the "table" view.  Then you can navigate the 
settings on the left side of the screen to get Trace to display the cfms of the 
AHU/system(s) in question.   (You could also use this viewing feature to review 
what Trace did with the OA cfm when you used the automatic DCV and/or ASHRAE 62 
calc within Trace to help find clues as to what's going on and if indeed Trace 
is simulating those features and the OA cfm correctly).  

Again, the manual adjusted schedules and forced cfm method may sound difficult 
and like a lot of work, but it's not really.  Once you do it once, you're done.  
And, it gives you a confidence level that Trace is simulating the OA cfm 
correctly.  

Hope that helps.
Julia

________________________________
From: Jen Redington <redington at vitetta.com>
To: trace-users at lists.onebuilding.org
Sent: Mon, August 30, 2010 8:15:32 AM
Subject: [Trace-users] Demand Controlled Ventilation

I am doing an energy model for LEED and added demand controlled ventilation to 
several spaces in both the design and baseline models.  This doubled my energy 
cost savings for the project.  The space heating gas use went down 25% in the 
design model but doubled in the baseline model. I can?t see any reason why the 
baseline energy use would increase, and the savings in the design case seem a 
little unrealistic since DCV is only being used in some spaces.  Any ideas what 
could be wrong? Any one else have any similar issues?

Thanks,

Jen Redington
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