Overcooling

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I have modeled a building with DX rooftop units. In an effort to save
time, I have zoned the building per rooftop. Am I creating a problem by
having only one zone per rooftop if I designate that rooftop as a VAV
system without reheat? I am cooling often down to 64-65 degrees when I
have a setpoint of 70.

Thanks,

David Gajewski, CEM

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It's possible that you are ok, but a lot of variables need to be aligned.

The building location (climate/exposures), use, occupancy schedule and
ventilation rates all play a big part in determining whether the shortcut is
valid. I don't usually have that much good luck.

The big variable is whether you will actually need reheat and what is
driving the need for reheat. If you've determined you will need reheat then
the simplification will usually underestimate the amount of cooling and
airflow required, since the model will only return the average cooling load
instead of the overcooling/reheat that occurs in some of the zones. The
degree of this underestimate will depend on the variables mentioned earlier
and how the space ends up being zoned.

Michael Hardy, PE, F.NSPE

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Christian,

You are correct. For a smooth submittal, modelers should demonstrate that the space qualifies as being semi-heated under ASHRAE 90.1 Section 3 (page 13): semiheated space definition along with Table 3.1. The semiheated space should have the same heating type, capacity, fan volume, and fan power in both models. Also, something that can be overlooked is that the baseline envelope should follow the semiheated column in the 5.5-X tables (just the semiheated space, of course).

Thanks,

DAKOTA KELLEY

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