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How to properly model this 2-pipe FCU system in eQuest 3.65?

asked 2014-12-05 11:09:52 -0500

amitpaul527 gravatar image

updated 2015-07-10 20:20:11 -0500

Hi everyone I'm trying to model an existing building in eQuest 3.65.

The building has a chilled water loop and a hot water loop (the same pumps changeover to pump HW in winter and CHW in summer).

All apartments have a non-ducted 2-pipe Fan Coil Unit. Nothing controls the coil (there's no 2-way valve), there's just a fan selector (LOW-SPEED/HI-SPEED/OFF), and there is no outside air being supplied to the apartments.

I'm struggling to understand how to properly model this in eQuest:

  • Should I create one HVAC SYSTEM (type FCU) per zone? Or one FCU system serving all of my zones? (Both seem to work, but I'm guessing there could be a drawback to using one versus the other)

  • What is the easiest way to ensure the outside air is zero?

  • How should I configure the FCU to mimic as closely as possible the (lack of) controls I have?

Thanks for any insight you can provide!

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answered 2014-12-05 11:20:34 -0500

updated 2014-12-05 11:22:22 -0500

Definitely you want to model it as a system per zone, using the Fan Coil system type (available in the detailed mode if not available out of the Wizard).

Fan Coils in eQuest don't bring in outdoor air, so by selecting this system type you should not have to worry about specifying zero OA. This also means that if you have any kind of energy recovery, it also won't work with a fan coil unit (because of no OA). If you wanted heat recovery you'd model each system as a Single Zone Reheat system.

The heat source for your fan coils is obviously your hot water loop, and cooling from your chilled water loop, and you'd specify the efficiency/COP at the plant level. Your fans be modeled as two speed fans under Fan Control. If you explicitly want to make heating only available for some months, and cooling for the other months, you'd put an availability schedule on the equipment for the relevant months.

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answered 2014-12-05 14:36:09 -0500

You will probably get a whole pile of unmet hours by changing the availability, which for an existing building is probably realistic, but will be a problem if this is a LEED model. You can address that by changing your heat/cool setpoints to reflect the availability of each system (i.e., change the heating setpoint to 50F during your cooling season).

For what it's worth, I had a dorm project with 2-pipe fan coil that I modeled both as a fan coil system per zone and a system for the whole building, just for grins, to see the difference (easy to play with right out of the wizard) -- and there really wasn't a difference in results. It worked well both ways. I ended up doing one big system just because it is easier. Just be sure you calculate the fan power correctly either way. Set OA to zero at zone level (exhaust at zero too) and it should not be a problem.

Good luck!

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Asked: 2014-12-05 11:09:52 -0500

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Last updated: Dec 05 '14