FCU Fan Control Question

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My building has individual 4-pipe FCU's attached to a central plant. I am
seeking clarification as to how eQuest interprets an undefined 'fan
schedule' versus creating a fan schedule.

It is my understanding that, per the help topics, if a fan schedule is left
'undefined' the "program assumes the fans are always available when needed."
This seemed to be a viable option when designing a baseline building. Since
I know the fan schedule I created one; on during occupied hours, off during
unoccupied with night-cycling enabled (per ASHRAE section G). This had a
significant impact on steam consumption, changing from 415 MBTU in the
'undefined option' versus 228 MBTU in the 'scheduled option'. Since I am
forcing the fans to be on in the scheduled option one would think it would
result in higher energy consumption values.

Could someone clarify why adding a fan schedule would impact energy
consumption so significantly? Could it be left as 'undefined' simply
allowing the fans to operate when they're needed? Does this schedule,
listed as a 'cooling' fan schedule only operate cooling and if 'yes', is the
heating schedule defined under the 'Heating' --> 'Coil Cap/Control'
'Availability Sch' (which I have as 'undefined'?

Thank you all in advance for your assistance.

Robert Des Rosiers, LEED-AP

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Joined: 2011-09-30
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Does anyone have any thoughts regarding the question posted below?

Robert Des Rosiers's picture
Joined: 2011-09-30
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You could determine the difference in fan operation between undefined
and scheduled by creating a new hourly report block with variable type
"HVAC System", and selecting "Total system supply air flow rate". The
"undefined" fan is probably running more often, or all the time. If you
are modeling per Appendix G, you should be using the occupied/unoccupied
schedule that you created regardless.

The cooling fan schedule covers both cooling and heating. You would only
enter a heating fan schedule for the hot deck of a multizone or dual-fan
dual-duct system (per the DOE-2 help description).


William Bishop, PE, BEMP, LEED(r) AP

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Joined: 2011-09-30
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