Trace-users Digest, Vol 4, Issue 1

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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.

Tony Kriel's picture
Joined: 2011-09-30
Reputation: 0

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.

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Julia Beabout's picture
Joined: 2011-10-01
Reputation: 3