[EnergyPlus_Support] UNIT VENTILATOR coil control

["josh_miarch" <jeemang@xxxxxxxxx>]

I'm simulating a kitchen zone (within a larger building) that is
conditioned with a fan coil supplied with air from a central AHU
(which I've simulated with a combination of a DIRECT AIR object and a
UNIT HEATER). This zone also has a large exhaust fan with a makeup air
unit that comes on for a few hours when the zone is occupied. I've
simulated the MUA/exhaust fan combo with a UNIT VENTILATOR object
(that has a gas heating coil). 

The actual operation scheme is more-or-less as follows:

-during occupied hours air is delivered from the central AHU at 20 C
-the fan coil meets any additional heating loads to get the zone to
its occupied setpoint of 22 C or unoccupied setpoint of 18 C
-the makeup air unit, when it and the exhaust fan are on, delivers air
to the zone at 20 C. 

I couldn't figure out a way to simulate this exactly, so I just put
the objects in the following order with respect to heating priority:
DIRECT AIR, UNIT VENTILATOR, UNIT HEATER, and figured this would be
close enough. 

I'm finding that in heating season, when the UNIT VENTILATOR turns on
it delivers air to zone at a temperature (~13 C) far below where it
would need to be to meet heating loads, even though the loads are well
below the heating capacity of the UNIT VENTILATOR's coil. This results
in the zone mean air temperature going well below its setpoint.

I looked at everything I could think of, and finally found that
reducing the high internal gains in this zone (the combined peak
lighting and receptacle loads are ~6000 W, and the zone area is about
70 m²) fixed the problem -- that is, after I reduced them to about
1000 W peak, the UNIT VENTILATOR started delivering air to the zone at
an appropriate temperature (~23 C) and the zone stayed at its setpoint.

Obviously I would like to model this with the proper lighting and
receptacle gains; can anyone offer up any advice with regard to what
is going wrong, and/or what I could possibly to do fix it?

Many thanks,

Josh




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