Night Cycle Fan Control

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Hi All,

We have an issue with a model that we were hoping to get some feedback
on from the eQuest community.

For ASHRAE HVAC System #8, how would you approach the night cycle fan
control? ASHRAE 90.1 par. G3.1.2.4 doesn't really cover this for System
8 and leaves it open to have the baseline as "Cycle on Any." However
with PFP boxes, you could also argue it should be "Zone fans only" since
they are part of System 8. There is no clear direction one way or the
other in ASHRAE that we can find. Depending what you choose, it can be
a major energy factor on the baseline. What's the right answer? The
proposed design is Zone Fans only if that makes any difference.

Thanks in advance,

Gerry Hazel

Gerard Hazel2's picture
Joined: 2013-09-19
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Gerard and others,
I would say that typical actual operation of most parallel box systems and the intent of System 8 in App G is a hybrid of the two eQ control strategies that can't be modeled in eQ. That would be Heating: Zone Fans Only; Cooling: Cycle AHU fans. Has this bothered anyone else ever? Do other programs allow the user to model this normal operation?

I suppose it's debatable whether this specifically adheres to the G3.1.2.x Fan System Operation, which states "Supply and return fans... shall be cycled to meet heating and cooling loads during unoccupied hours." A strict interpretation of that would lead to the use of "Cycle on any" for "System 8" baselines also. "Cycle on any" is also what the Comnet Guidelines say quite specifically to use for all baseline cases. So much for what I think.

As far as cooling, if the actual/proposed system stays off during unoccupied cooling as you imply by noting "The proposed design is Zone Fans only", what is the unoccupied zone cooling tstat schedule value for the baseline to cycle on? I suppose you could grab the values from some default schedule.

I think the T24 ACM rules make more sense for night cycle control in this case, that is baseline = proposed. "Zone Fans Only" for both cases.

Clear as mud?
Fred Porter, BEMP, LEED(c) AP

Fred Porter's picture
Joined: 2011-09-30
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