Demand Controlled Ventilation

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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

Jen Redington's picture
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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

Julia Beabout's picture
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Joined: 2011-10-01
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Jen, Julia,

I?m sorry but I strongly disagree with this e-mail. One must read and understand ASHRAE 62.1 first to understand how to use Trace for that calculation. Trace will work properly but you need to know the procedure. There is some trial and error but it still is as fast as or faster than manually performing all the ASHRAE 62.1 calculations. Critical rooms may require some manual adjustment or features like setting the max Z factor can be very helpful. There is a PDF on the Trane web site I strongly recommend you download that explains the process pretty well. You can also call the help desk for guidance.

The baseline energy usage can and often does increase when the ASHRAE 62.1 calculation is properly applied because the presence of critical zones with high occupant density, like conference rooms, will set the OA percentage for an entire AHU. All the rooms fed by that AHU will have that same OA percentage and thus some rooms will receive more OA than a simple calculation (as is done by "Sum Room OA Reqs" on the advanced systems tab). This increase in OA will increase the energy consumption. If you selectively over air the critical zones to lower the OA percentage at the AHU, which is the accepted procedure, then the supply air will increase leading to increased fan and reheat energy.

This ASHRAE 62.1 feature and the DCV calculation is one of the best advantages that Trace has over other programs like Equest that cannot do the ASHRAE 62.1 calculation properly. The Trace program can both design and model HVAC system OA requirements properly. It also can be used to properly design and model DOAS units including capturing the sensible and latent cooling provided. I have not seen any program that can model DCV or OA better than Trace if you use it properly.

I have used Trace to document DCV savings in LEED project submissions without difficulty. It just needs to be used properly.

Rick

R I C H A R D E L L I S O N
BEAP, BEP, BEMP, BESA, CBEP, CDSM, CEA,
CEM, CMVP, CTAB, DGCP, HBDP, LEED AP, PE
Manager of Energy and Simulation
Southland Industries
22340 Dresden Street, Suite 177
Dulles, VA 20166
Office: 703.834.5570
Direct: 703.

Richard-Ellison's picture
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Joined: 2011-09-30
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Rick,

Do you have a link to the PDF, or a document name?

Caleb B.

PRAIRIE ENGINEERING, P.C.

mr_bulow's picture
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Joined: 2010-10-07
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The help desk can walk you through it but here are the steps. (I?m not comfortable attaching the document due to copy write considerations)

Here?s a link: http://www.trane.com/COMMERCIAL/DesignAnalysis/DownloadCenter.aspx?i=909

Select Knowledge Base

One needs to visit the web site and log in with the 4-digit customer code.

Select Trace 700

Select Knowledge Base Categories

Select How to questions

The following question provides information and link to the pdf document

Wave #22394: How do I model demand control ventilation? How do I model CO2 sensors?

Rick

R I C H A R D E L L I S O N
BEAP, BEP, BEMP, BESA, CBEP, CDSM, CEA,
CEM, CMVP, CTAB, DGCP, HBDP, LEED AP, PE
Manager of Energy and Simulation
Southland Industries
22340 Dresden Street, Suite 177
Dulles, VA 20166
Office: 703.834.5570
Direct: 703.

Richard-Ellison's picture
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Joined: 2011-09-30
Reputation: 1

Hi Richard,
Thanks for the feedback. I think we're really saying the same thing, though.
Critical spaces can throw the calc off - which was the point I was trying to
make as well (i.e. my judgment comment) - but you stated it much more clearly.
Thank you. I was just trying to relay my personal experience that it can
sometimes take more time and work to finagle the critical spaces in Trace than
to do the calcs "by hand" (i.e. spreadsheets) and force them in Trace. I
probably should have qualified my statement by saying that I work on a lot of
labs and healthcare projects which also contain a lot of 100% exhausted spaces
which, again, in my experience, can make the ASH 62 calc even more challenging
and time consuming to manipulate within Trace.

Again, which ever way works for people. I use both approaches depending on the
project. Both are valid as long as one understands the ASHRAE 62 and DCV calc
procedures and what Trace has modeled for OA. I was just attempting to relay my
experience and a possible alternative approach.
Julia

Julia Beabout's picture
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Joined: 2011-10-01
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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

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