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[EnergyPlus_Support] unit ventilator woes
I've created a simple one-zone model, in which the single zone is
conditioned in the following manner:
-a constant volume of ventilation air is supplied during occupied
hours from the central AHU (via a DIRECT AIR object)
-there is a unit heater to heat the space during unoccupied hours, and
make up any additional heating load not covered by the central AHU
during occupied hours
-there is an exhaust fan that runs in the middle of occupied days,
removing a volume of air from the room in excess of the air supplied
from the central AHU
-there is a makeup air unit to supply a volume of air to the zone
equal to that removed from the zone by the exhaust fan when the
exhaust fan is running. I've attempted to simulate this with a UNIT
VENTILATOR object, where I've set the maximum air flow rate, the
maximum OA air flow rate, and the minimum OA air flow rate to a volume
equal to that specified in the ZONE EXHAUST FAN, chosen the FIXED
AMOUNT outside air control type, and assigned the object the same
component availability schedule as the exhaust fan it is meant to be
compensating for.
I'm having a couple of problems with this system.
1. I'm getting an air imbalance when the exhaust fan/MUA combo is
running. As I understand it, EnergyPlus calculates a zone air balance
strictly by subtracting the sum of the flow through the zone exhaust
nodes from the sum of the flow through the zone inlet nodes and
assigning the difference, if any, to flow through the zone return
node. However, I'm running into a problem where the sum of the flow
through the exhaust nodes exceeds the sum of the flow through the
inlet nodes.
Looking into this further, I saw that the problem seems to lie with
the UNIT VENTILATOR. I've set the unit's fan volume flow rate to an
amount equal to the minimum OA flowrate specified in the UNIT
VENTILATOR description, so presumably when the unit runs, it should be
drawing air entirely from its OA node. However, what actually happens
is that it draws an amount equal to it's fan volume flow rate from
both the OA node AND the zone exhaust node to which it is connected.
As well, it outputs air into its zone inlet node at this same rate --
meaning that a volume of air equal to the unit fan's volume flow rate
is basically disappearing.
I guess I'm wondering if I've specified a part of the file wrong, or
if my understanding of how these components work together is somehow
flawed.
2. The UNIT VENTILATOR I've specified has no cooling coil, but in
situations where there is a cooling load, I'd like it add to the zone
a volume of outside air at the outside ambient temperature to make up
for the air leaving the zone via the zone's exhaust fan. According to
the Input/Output manual, when a UNIT VENTILATOR is in a COOLING/NO
COIL/FIXED AMOUNT scenario (which I've presumably described), "The
unit ventilator fixes the outside air flow rate as defined by the user
and sets the maximum and minimum outside air flow rate to be equal in
order to avoid the variation of outside air flow rate between the
maximum and minimum values. Since a coil is not present to further
condition the supply air, the zone simply receives whatever
temperature air results from the outside air controls."
That is pretty much what I want to happen, assuming that the maximum
and minimum outside air flowrates, when made equal, are set to the
amounts that I've entered in the UNIT VENTILATOR description (which
are equal). Unfortunately, this is not happening; when there isn't a
heating load, the UNIT VENTILATOR doesn't run. Again, I'm wondering if
anyone knows what I might be doing wrong.
Sorry for the amazingly long post, and thanks for any help I may receive.
Josh
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