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Re: [EnergyPlus_Support] Re: Preheat Coil Operation
This file turned out to be an interesting puzzle. Thanks(?) for sending it.
This is a large building with 4 VAV air handlers, the perimeter air handler
serves many zones, and separate air handlers serve one or two core zones each.
All system have large OA fractions. The initial complaint was undersized
preheat coils.
1. Compact HVAC is always using 0.3 for "minimum system air flow ratio" in the
SYSTEM SIZING object. Apparently, this flow rate is being used to size the
preheat coils rather than the minimum outside air flow rate which is autosized
and which is significantly more that 30% of the design supply air flow rate.
Changing this value to 1.0 in the expidf file (for all 4 systems) solves the
preheat sizing problem. (This is a new bug CR.)
2. Next step - diagnose max iteration errors. Are zones controlling well?
Max iterations can occur if zones are out of control. Turned on zone
temperature and heating/cooling setpoint not met report variables. Found that
core zones 1C, 4C, and 5C are not maintaining cooling setpoint. Zone temps
exceed 30C on the summer design day in these zones. Why?
(Note, in v1.4.0, the ASHRAE comfort warning is no longer on by default as it
was in v1.3.0, so it must be enabled or zone temps must be reported to be sure
systems are working as expected.)
Looking at the *component* sizing reports in the eio file, the "Max Water Flow
Rate of Coil" is zero for OPERATING ZONE COOLING COIL (serving zone 1C). Why?
Looking at the *zone* sizing reports in the eio file, the cooling design load
for zone 1C is reported as occuring on the heating design day (zone 4C also
reports this way). These are core zones with no outside environment impact and
identical internal gains on the summer and winter design days. Not sure why
the winter day is being selected, perhaps it's just a slight difference within
control tolerance and the winter days comes out slightly larger? (This will be
a suggestion CR.) So the cooling coil is sizing for mixed air conditions on
the winter day, so it has cold air entering the coil for the sizing
calculation, thus zero cooling coil load at design.
Tried adding a tiny amount of infiltration to the core zones to force the peak
cooling load to track outdoor air conditions so the cooling coil will size in
the afternoon of the summar design day. This works. If desired, you could
schedule the core zone infiltration to be on for design days, and off for other
days, so that is is used just for sizing and not for the annual simulation.
Now all 5 core zones meet cooling setpoint. Still getting max iteration
errors.
3. In version 1.4.0, COIL:WATER:COOLING was significantly improved, but
compact hvac is still using COIL:Water:DetailedFlatCooling. Switching this
model to COIL:WATER:COOLING, max iteration errors are gone. Will probably
switch Compact HVAC back to the other coil for the next release. (Another CR.)
I will send you the latest working file in a separate e-mail.
Mike
On 15 Nov 2006 at 14:43, asheaffe wrote:
> Mike,
>
> I posted the file in the Problem Submital folder under the
> name "HospitalBest.zip" Considering the problems I am having, the
> name is uniquely ironic, I know. Also, I am having lots of issues
> with the Max Iterations being exceeded. I have tried adjusting
> tolerances down, etc. Any ideas?
>
> Thanks for your help.
>
>
>
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Michael J. Witte"
> <mjwitte@...> wrote:
> >
> > I tried to reproduce the problem by adding preheat to example file
> CompactHVAC-
> > 5ZoneVAVWaterCooled.idf, but the preheat coil puts out 7.2C, even
> with 100% OA
> > on the winter design day. Please post the file in the yahoo file
> area or send
> > zipped to energyplus-support@...
> >
> > Mike
> >
> >
> > On 14 Nov 2006 at 22:36, asheaffe wrote:
> >
> > > I am simulating a COMPACT VAV system that has a preheat coil due
> to
> > > the large amount of OA that my zones require. I have the gas
> preheat
> > > coil set to deliver 7.2C minimum to the system. However, during
> the
> > > simulation, the outlet temp of the coil regularly fluctuates
> below
> > > that temp. In fact during the design day it stays at -3.5C.
> The
> > > schedule for the coil is currently blank, meaning it should be
> > > available all the time. Any ideas on what might cause this? I
> can
> > > post the file, if there are no quick fixes.
> > >
> > > Thanks,
> > >
> > > Andrew
> > >
> > >
> > >
> > >
> > > The primary EnergyPlus web site is found at:
> > > http://www.energyplus.gov
> > >
> > > The group web site is:
> > > http://groups.yahoo.com/group/EnergyPlus_Support/
> > >
> > > Attachments are not allowed -- please post any files to the
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> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
> > ========================================================
> > Michael J. Witte, GARD Analytics, Inc.
> > EnergyPlus Testing and Support
> > EnergyPlus-Support@...
> >
>
>
>
>
>
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
>
> The group web site is:
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>
> Attachments are not allowed -- please post any files to the appropriate folder
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>
> EnergyPlus Documentation is searchable. Open EPlusMainMenu.idf and press the
> "search" button.
>
> Yahoo! Groups Links
>
>
>
>
>
>
========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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http://groups.yahoo.com/group/EnergyPlus_Support/
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