[Trace-users] Demand Controlled Ventilation

Tue Aug 31 06:14:39 PDT 2010

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

________________________________
From: Richard Ellison <REllison at southlandind.com>
To: Julia Beabout <juliabeabout at yahoo.com>; Jen Redington 
<redington at vitetta.com>; trace-users at lists.onebuilding.org
Sent: Tue, August 31, 2010 8:25:26 AM
Subject: RE: [Trace-users] Demand Controlled Ventilation

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 OAReqs" 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
PE, BEMP, CEM, CEA, CTAB, CBEP, LEED AP 
Manager Energy and Modeling
Southland Industries
22340 Dresden Street, Suite 177
Dulles, VA 20166
Office: 703.834.5570
Direct: 703.834.2438
Fax: 703.834.5572

From:trace-users-bounces at lists.onebuilding.org 
[mailto:trace-users-bounces at lists.onebuilding.org] On Behalf Of Julia Beabout
Sent: Monday, August 30, 2010 9:20 AM
To: Jen Redington; trace-users at lists.onebuilding.org
Subject: Re: [Trace-users] Demand Controlled Ventilation

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 OAcfm 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 OAcfm 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 OAcfm 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 OAcfm 
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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