Lesson Learned: Single Zone Packaged DX vs. Packaged VAV system types

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Hi all,

I just wanted to share a lesson learned that I ran across recently while using Equest. Here are the pertinent specs for the building:

* Proposed Case

o 65,000 sq ft of laboratory (with ac/hr requirements)

o Cooling is provided by a chilled water district plant

o Heating is provided by a district steam boiler plant

o 100% outdoor air

o Humidity control is being utilized

* Baseline

o Per G3.1.1 Exception (c), baseline system type is #3, Packaged DX, Constant Volume, Fossil Fuel Furnace

o Humidity control is included because it's in the design case (ASHRAE 90.1-2007 Table G3.1 Part 1.b)
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o 100% outdoor air, or a percentage to match the proposed

Because the ASHRAE baseline system was #3, I used the Equest system type "Packaged Single Zone" for the baseline model and then proceeded to pull my hair out for a day because the baseline model was performing better than the proposed. Then I decided to try changing the baseline system type to "Packaged VAV" and setting the fan control to "Constant" with the minimum flow ratio set to "1", in order to force it to still be a constant volume system. The first thing I noticed after only changing the system type was that the cooling energy almost doubled. The upshot is the following, for these Equest system types:

* Packaged Single Zone

o When you add reheat in order to control humidity, the software shows an increase for the heating energy, but does not show an increase in cooling energy. Cooling energy should also increase because it now has additional heat due to reheat to overcome. The place to verify humidity control is the SS-N report, which showed that humidity was not being controlled, despite the fact that I had set the maximum humidity to 60% in the inputs.

* Packaged VAV

o When you add reheat, the software shows an increase in both heating and cooling energy. This is correct. Otherwise, your baseline building will always perform better than the proposed. Again, verify that humidity is being controlled in the SS-N report.

LESSON LEARNED: Always use the Packaged VAV system type in Equest when modeling DX equipment. If you have a constant volume system, just adjust the fan control to be "Constant" and the minimum airflow ratio to be "1".

I hope this makes sense. Feel free to comment if you've had experience with this. Thanks!

Kristy M. Walson, PE, LEED AP BD+C, BEMP

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