[Trace-users] Trace-users Digest, Vol 4, Issue 1

Tue Jul 8 08:07:39 PDT 2008

I've modelled DCV in Trace both ways as well.  The method I use kind of depends on how I'm planning on controlling the outdoor with respect to changes in demand for OA (by population estimation or by direct CO2 sensing).....and on how well Trace is calcing the OA quantity needed in that particular model.
If the building and it's outdoor air drivers are pretty comples, I'm usually pretty skeptical of Trace's ability to accurately calculate the correct quantity of outdoor air using one of the ASHRAE 62 methodologies (there always seem to be one or two rogue rooms that screw up the calc...which you can fix...but, it can take a lot of extra effort with minimal reward).  So, the first thing I usually do is check to see if Trace appears to be calcing and varying the OA quantity correctly.  (Be sure to select the methodology you want using the "Systems Ventilation Flag" on the Systems/Selection/Advanced screen).  If it seems like Trace is calcing and varying the OA quantity correctly, then I stick with letting Trace determine the OA quantity.  If not, then I usually input the peak quantity of outdoor air at the unit (based upon my own ASHRAE 62 calc) on the Systems/Options/Advanced screen and use a schedule to vary the quantity based on how
 my population varies.  
Regarding Trace not changing the supply air fan cfm when it changes the outdoor air cfm:
Unless you have a 100% outdoor air unit, the control loop that determines the amount of outside air at your AHU would be a separate control loop than the one that controls your supply fan.  Each would be controlled by separate independent variables.  The outdoor air quantity would be determined by either population estimations or CO2 sensors.  The supply fan speed/cfm would be controlled by a duct static pressure sensors (which would primarily be a function of cooling or heating load).  So, a change in outdoor air quantity would not necessarily mean there should be a change in the total supply air quantity.  So, I would say Trace is correct when it doesn't change the total supply fan energy when you select DCV or utilize other outdoor air control methods in the program.  If the supply air quantity seems to be coming up short of the outdoor air quantity at certain points, you can check to see if the "Force VAV minimum always >= to
 nominal ventilation during design" on the Actiona/Change Load Parameters screen is checked.

----- Original Message ----
From: Tony Kriel <tkriel at sustaineng.com>
To: trace-users at lists.onebuilding.org
Sent: Tuesday, July 8, 2008 9:47:43 AM
Subject: Re: [Trace-users] Trace-users Digest, Vol 4, Issue 1

I prefer using the ventilation schedules that follow occupancy patterns.  It
is, "The simple" way to model DCV, and almost the same thing (I believe).  

As far as the reduction in fan energy, I'm almost certain you would reduce
the fan energy if you put a VFD on the fan type.

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Today's Topics:

  1. Demand Control Ventilation in TRACE (Eric Wilson)

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Message: 1
Date: Mon, 7 Jul 2008 14:23:21 -0500
From: "Eric Wilson" <eric at sedac.org>
Subject: [Trace-users] Demand Control Ventilation in TRACE
To: trace-users at lists.onebuilding.org
Message-ID:
    <904a636e0807071223p55864b0fk1b061f2259f7189b at mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"

There are two ways to model DCV in TRACE:

  1. Using ventilation schedules that follow occupancy patterns.
  2. Using the CO2-based DCV option on the "Systems" tab, "Advanced" button
  and setting CO2 sensors to "Room" on the Rooms tab.

I'm curious which method of modeling DCV you all have found to be more
accurate or preferable.  It appears that neither method automatically takes
fan energy savings into account--as far as I know, this must be done
separately using a fan schedule.  Using "Cycle with occupancy" doesn't quite
seem to be what we want to model DCV fan energy.  Any ideas?

*Eric Wilson*
Smart Energy Design Assistance Center
University of Illinois - Urbana-Champaign
www.sedac.org
(217) 244-4671
(800) 214-7954
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