[Equest-users] DX vs. Chilled Water: Unrealistic cooling loads

Walson, Kristy kristy.walson at tlc-eng.com
Thu Jan 12 20:17:10 PST 2012

Hi all,

I am working on a model of a building that is on a district cooling and heating loop.  My baseline building is System type #3 (Constant Volume DX, Fossil Fuel Furnace) and I am simulating the district plant in the proposed using an air-cooled chiller with the plant efficiency for cooling and a boiler with the plant efficiency for heating.

I was having trouble with the cooling electrical consumption in the proposed model being much larger (by a factor of 10) larger than the cooling in the baseline model, which doesn't make sense.  After a lot of fruitless "tweaking", I took the baseline model and just changed the system type from a "packaged single zone" (DX) to the system type "Variable Air Volume."  I didn't change anything else (except for some of the settings that reverted back to defaults when I changed the system type).  The cooling electrical consumption went from 160,000 kWh (baseline, pkgd single zone) to 1,400,000 kWh (VAV, chilled water).  The EIR for the DX equipment is about the same as the EIR for the chiller.

I don't understand why the program is doing this.  Has anyone run into this before?  If so, what did you do to bring the cooling down to a more reasonable number?  The same thing is also happening on the heating side, but I'm hoping if someone has an answer for the CHW side, then I can apply the same theory to the HW.  Thanks in advance for your help.

Kristy Walson

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