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, PE

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

I have noticed this too. The constant volume exhaust fan merely means that
the fan runs constantly, but may not draw air through the space (read the
help file on this, it's more eloquent than I am).

What I have done to remedy this issue using other system types is specify
that the system fan operates as a variable volume fan, with the fan curve
being linear. This will allow the fan to modulate down to the required
exhaust airflow. If you look at the help files for "SUPPLY-KW/FLOW" it
shows the fan curves for variable speed fans, cycling fans, discharge, etc.
So this is the way that the program models cycling fans already, and you can
use it to your advantage with the exhaust airflow piece.

Hope this works!


No Username provide's picture
Joined: 2011-09-30
Reputation: 200