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Re: [EnergyPlus_Support] Re: Forcing system behavior
If the unmet demand is zero, then the plant has enough capacity to meet the
request from the chilled water coil. If the zone setpoint is still not met,
then the chilled water coil is not delivering enough cooling. Either the
airflow rate is too low, or the supply air temperature is too warm. This could
be a control problem, an airflow sizing problem, or a coil sizing problem.
Mike
On 4 Dec 2004, at 0:40, sm278424 wrote:
>
>
> Thanks Mike,
>
> Can I ask one further question re: zone temperatures and unmet demand.
>
> If the unmet demand is zero, then I should have enough capacity in
> the plant loop.
>
> I have a fixed zone cooling setpoint of 23C. But in some of our
> simulations we see zone air temperatures that are several degrees
> above our setpoint. If the capacity is there, why would my zone air
> temp be off the setpoint?
>
> Thanks again,
> Shaun Martin
>
>
>
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Michael J. Witte"
> <mjwitte@g...> wrote:
> > 1. If the air loop outlet setpoint is not being met, then the coil
> capacity or
> > chilled water flow rate or chiller capacity is not adequate. Which
> cooling
> > coil model are you using? Use COIL:Water:Cooling or
> > COIL:Water:DetailedFlatCooling. COIL:Water:SimpleCooling does not
> always
> > deliver adequate capacity under certain conditions (this model will
> not be
> > available in the next release).
> >
> > 2. It is very difficult to get a hybrid forced air/radiant system
> to behave as
> > desired with current EnergyPlus control options. In your case, if
> you want the
> > radiant system to modulate, then why use a VAV air system? Perhaps
> a constant
> > volume system would be better. Also, you should set the air system
> to run as
> > the first cooling priority, so the radiant system can pick up the
> remaining
> > load. Alternatively, perhaps the VAV system is oversized? If it
> is sized
> > smaller so that it cannot meet the entire cooling load, then it
> will open to
> > full flow, thus supplying a constant volume of air.
> >
> > 3. With cool surfaces for radiant slabs, it would be expected that
> comfort is
> > maintained at higher dry bulb temperatures. Check all of the
> comfort inputs to
> > make sure thay are appropriate (clothing, activity level, air
> velocity).
> >
> > 4. These unmet demands are why #1 is happening. The chiller plant
> is not able
> > to meet the load, so the chilled water supply temperature rises,
> and then the
> > coil cannot meet the supply air setpoint. Everything is
> simultaneous in
> > EnergyPlus, so unmet demands result in unmet setpoints.
> >
> > 5. Zone/Sys Sensible Heating (and Cooling) do not include radiant
> slab system
> > loads. (I see we need to clarify the definition of this, or change
> the way it
> > is reported.) Also, if there is outside air, that will place a
> load on the
> > cooling coil which is not included in the load delivered to the
> zone.
> >
> > Mike
> >
> >
> > On 1 Dec 2004, at 20:48, sm278424 wrote:
> >
> > >
> > >
> > > Hi, again,
> > >
> > > I'm modeling a single zone with a VAV system and a radiant panel.
> I
> > > control the mixed air temp, HC water inlet and CC water inlet
> with
> > > the AIR LOOP OUTLET node, which is controlled by a SETPOINT
> MANAGER
> > > to a setpoint of 12.2C.
> > >
> > > What happens at peak cooling load is both the radiant panel and
> the
> > > VAV box go to full flow, and E+ allows the AIR LOOP OUTLET
> > > temperature to modulate.
> > >
> > >
> > > My questions:
> > >
> > > 1. Is there any way to lock in the AIR LOOP OUTLET temp in order
> to
> > > allow the supply volume to vary?
> > >
> > > 2. Can the supply temp and volume both be locked at specific
> values,
> > > so that the radiant panel modulates more fully? Can you suggest
> > > appropriate radiant panel schedules for this?
> > >
> > > 3. In our models we've seen the FANGER PMV produce PPD values of
> > > between 6-10% when zone mean air temperatures are relatively high
> eg
> > > 77-79F (the PPD formula is from the Engineering doc). Does this
> make
> > > sense to you? The comfort band seems too wide, in most buildings
> > > complaints start flowing whenever zone DB temperatures exceed 75F.
> > >
> > > 4. When we get UNMET DEMAND on our plant loops in these
> simulations,
> > > the values are very high, much more than the total zone loads
> would
> > > ever be. We can't explain the values.
> > >
> > > 5. With this model the total zone load is the rad panel cooling
> rate
> > > + the CC rate, but the Zone/Sys Cooling Rate values are all over
> the
> > > place (but usually quite low). Why should this be?
> > >
> > >
> > >
> > > Thanks
> > > Shaun Martin
> > >
> > >
> > >
> > >
> > >
> > >
> > > 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
> appropriate folder
> > > in the Files area of the Support Web Site.
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
> > ========================================================
> > Michael J. Witte, GARD Analytics, Inc.
> > EnergyPlus Testing and Support
> > EnergyPlus-Support@g...
>
>
>
>
>
>
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>
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