[Trace-users] Demand Controlled Ventilation

[Julia Beabout]

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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