Re: [EnergyPlus_Support] Forcing system behavior

["Michael J. Witte" <mjwitte@xxxxxxxx>]

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@xxxxxxxx




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