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Re: [EnergyPlus_Support] Re: unit ventilator woes
Thanks for your reply.
The thing is, is I want the exhaust fan to be in there. The purpose of the
UNIT VENTILATOR object is to simulate the real-life makeup air unit that
supplies the zone with air when the exhaust fan is running. So, although
removing the exhaust fan would fix the air imbalance problem within
EnergyPlus, it would also result in the physical situation I'm trying to
model not being represented accurately.
Josh
On Jan 25, 2008 11:16 AM, tjutw <tjutw@xxxxxxxxx> wrote:
> From EnergyPlus InputOutput reference, I think exhaust fan is not
> necessary in your input file.
>
> Field: outside air relief node
> This field is a node name (character string) used to identify the node
> associated with air exhausted out of the unit ventilator to the
> outdoor environment.
>
> After removing the exhaust fan, there is no warning about "air imbalance".
>
> Hope it helps,
>
> Tj Ywh
>
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx<EnergyPlus_Support%40yahoogroups.com>,
> "Josh K" <jeemang@...> wrote:
> >
> > I've uploaded the simulation file in case anyone wants to take a
> look at it.
> >
> > Many thanks,
> >
> > Josh
> >
> > On Jan 18, 2008 4:47 PM, josh_miarch <jeemang@...> wrote:
> >
> > > 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
> > >
> > >
> > >
> >
> >
> >
> > --
> > Josh Kjenner, EIT, LEED AP
> > Manasc Isaac Architects Ltd.
> > 10225 100 Avenue :: Edmonton, AB T5J 0A1
> > 780.429.3977
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>
>
--
Josh Kjenner, EIT, LEED AP
Manasc Isaac Architects Ltd.
10225 100 Avenue :: Edmonton, AB T5J 0A1
780.429.3977
[Non-text portions of this message have been removed]
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