[Trace-users] Demand Controlled Ventilation

Tue Aug 31 05:25:26 PDT 2010

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
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 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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