FCU with outside air fan

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Hi All,
I am trying to model a residential unit served by a Fan coil unit (FCU). It has a separate outside air fan and exhaust fan operating 24/7. The fan coil unit is available to operate for all hours but should operate intermittently depending on the room load.

I tried modeling this by selecting a 4-pipe FCU and specified the exhaust and outside air at the thermal zone level. I made the FCU to operate intermittently by making the ?Fan on mode? to be intermittent. By doing this, what I observed in the hourly report was:
1. FCU unit cycles intermittently as intended.
2. Exhaust fan operates continuously (24/7) (with the specified cfm) as intended.
3. However, the outside air cycles per the FCU.
?If the FCU is off, then OA cfm is zero
?If FCU supply air cfm < the OSA cfm specified at the zone level, then hourly OSA cfm matches the FCU supply air cfm ?If FCU supply air cfm > the OSA cfm specified at the zone level, then hourly OSA cfm matches the cfm specified at the zone level.

As mentioned before, I want my outside air to operate continuously with constant flow-rate. Can anyone suggest me a way to model this system?

Thanks in advance for your help!

Yuvaraj Saravanan's picture
Joined: 2011-09-30
Reputation: 0

Try a 4-pipe induction unit. I believe that can have primary and
secondary coils. Ignore that you can't model the fan coil fan. It is

I used to do this to model a guest room fan coil with supply and exhaust
air to the guest room. Don't do every room. Do building facing

John R. Aulbach, PE, CEM

Aulbach, John's picture
Joined: 2011-09-30
Reputation: 0

I have observed the same. I can think of 2 approaches. First, is to set
the fan to continuous for the heating energy calculation and to
intermittent for the fan energy calculation and then to derate the fan
power accordingly in the continuous model.

Second method is to create the dummy zone above the FCU zones with a
100% OA system and use the OA-FROM-SYSTEM keyword in the fan coil
system. This works well until you apply heat recovery as the recovered
heat is from the dummy zone, not the FCU zones.

bfountain's picture
Joined: 2011-09-30
Reputation: 201