EnergyPlus/OpenStudio VAV w/ Reheat Throttling Range Adjustment?

asked 2025-05-20 08:14:54 -0500

mattkoch
1201 ●11 ●10

updated 2025-05-20 09:41:48 -0500

14430 ●91 ●25 http://bigladdersoftware.com/

This IES article describes unmet hours troubleshooting. For some reason I cannot upload the image here, but it has a diagram in it to explain how a VAV terminal unit with reheat changes its SA temperature and flow based on RA temperature.

Of particular interest in this diagram is the +/- 2 R throttling range around the heating setpoint and the +/- 2 R throttling range around the cooling setpoint. If I am not too mistaken, the diagram suggests that if the heating setpoint where, say, 70 F and the cooling setpoint were, say, 74 F, then there would no longer be a deadband between cooling and heating.

And, what seems to be more, without adjusting the throttling ranges, if the heating setpoint were to increase above 70 F, and/or the cooling setpoint were to decrease below 74 F, then it may no longer be obvious whether to do cooling or heating for a given RA temperature. This brings me to my question on how EnergyPlus/OpenStudio determines the throttling range.

The ANSI/ASHRAE/IES Standard 90.1-2010 Performance Rating Method Reference in Section 3.5.1 appears to equate throttling range with reporting tolerance, presumably referring to OutputControl:ReportingTolerances, but as far as I can tell, these parameters only determine at what temperature deviation hours are counted as unmet hours.

Thus, is there a way to independently set throttling range in EnergyPlus/OpenStudio, and if so, where would I do that? This question arose, because one of my clients insists on a 70 F heating setpoint and a 72 F cooling setpoint, which at this point I cannot be sure won't cause some sort of trouble.

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answered 2025-05-21 08:02:18 -0500

rraustad

13974 ●84 ●13 http://www.fsec.ucf.edu/

In EnergyPlus, the VAV box will try to meet the zone load, cooling or heating required to meet the thermostat set point temperature, by adjusting the air flow. If the supply air entering the VAV box is at the air loop set point, the VAV box air flow will be proportional to the load. There should be no control issues with using 70F (21.11C) for heating and 72F (22.22C) for cooling. Throttling range does not play a role in determining what load the VAV box will meet, the zone thermostat calculates the load. If there were a 2-stage HVAC where 2nd stage turned on at some higher temperature than the base set point temperature then the ZoneControl:Thermostat:StagedDualSetpoint would do that, but this thermostat is not meant for VAV systems.

You can see the Tstat temperatures using:

Output:Variable,*,Zone Thermostat Heating Setpoint Temperature,hourly; !- HVAC Average [C]
Output:Variable,*,Zone Thermostat Cooling Setpoint Temperature,hourly; !- HVAC Average [C]

and the predicted zone loads based on thermostat temperature using:

Output:Variable,*,Zone Predicted Sensible Load to Setpoint Heat Transfer Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate,hourly; !- HVAC Average [W]

and the resulting energy to each zone using:

Output:Variable,*,Zone Air System Sensible Heating Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Air System Sensible Cooling Rate,hourly; !- HVAC Average [W]

and various VAV damper results using:

Output:Variable,*,Zone Air Terminal VAV Damper Position,hourly; !- HVAC Average []
Output:Variable,*,Zone Air Terminal Minimum Air Flow Fraction,hourly; !- HVAC Average []
Output:Variable,*,Zone Air Terminal Outdoor Air Volume Flow Rate,hourly; !- HVAC Average [m3/s]

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OK, so I appreciate all the diagrams and trial runs by all, but is it correct to state that I basically don't have to worry about throttling range because EnergyPlus takes care of things automatically? As long as my heating setpoint is below my cooling setpoint, I will be fine and there will be no simultaneous cooling and heating attempts at the zone level (zone to zone maybe, but not within a zone)?

mattkoch ( 2025-05-22 07:08:49 -0500 )edit

I think that's a fair statement. You should be OK to simulate cooling and heating set points closer than you would expect in the field (without adjustment to deadband). Field dampers respond to a change in thermostat sensed temperature, E+ simulated dampers respond to a change in load which happens AT the set point.

rraustad ( 2025-05-22 08:07:10 -0500 )edit

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answered 2025-05-20 14:23:35 -0500

Keigo
4969 ●1 ●54 ●31 https://www.linkedin.c...

My answer to the following post may help.

fan coil unit control with setpoint deadband

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I have AirTerminal:SingleDuct:VAVReheat only, no fan coil units, but I suspect you are saying the Temperature Difference Between Cutout And Setpoint (TDBCAS) approach is applicable nevertheless? I am having a hard time matching this to the IES image I mentioned above, though. The EP TDBCAS documentation says:

if MAT < HSP:
  HSP = HSP + DT
elif MAT > CSP:
  CSP = CSP - DT

But the VAV diagram does not call for any setpoint shifting. Are we using the same definitions of throttling range and deadband?

mattkoch ( 2025-05-21 07:59:17 -0500 )edit

TDBCAS is applicable for VAVReheat. Besides, for cooling, Setpointmanager:WarmestTemperatureFlow with FlowFirst is also necessary to reproduce the diagram in the IES article.

Keigo ( 2025-05-21 10:02:28 -0500 )edit

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answered 2025-05-20 14:28:56 -0500

kwalkerman
1562 ●11 ●10 https://ep3.solutions

updated 2025-05-21 10:02:04 -0500

I modified the 5ZoneAirCooled.idf example file that comes with EnergyPlus in order to inspect this. I created two files - one with a 70F heating setpoint and 72F cooling setpoint and teh other with a 70F heating setpoint and a 74F cooling setpoint

Both show appropriate control of heating and cooling most of the time. However, in the heating months, there are occasional times during the heating season, when the cooling setpoint is overshot. I haven't as yet pinpointed the exact cause, but it may be related to the supply air temperature setpoint during winter months.

This test model was created by modifying the EnergyPlus example file in EP3. Diagrams and data visualization are also generated with EP3.

Results Visualization1

Results Visualization2

EDIT

The times when space temperature was overshot in the above models was due to the chiller plant not being available during the winter months. Thus the AirLoopHVAC was not meeting setpoint. The VAV damper control (which is what this question is about) is adequately adjusting flow and temperature to meet Zone temperature setpoints even when there is only a 2F spread between heating and cooling setpoints (70F and 72F):

Results Visualization

This test model was created by modifying the EnergyPlus example file in EP3. Diagrams and data visualization are also generated with EP3.

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@kwalkerman you might try checking the thermostat control type in winter and make sure it's dual instead of heating only. I recall that 5ZoneAirCooled uses all the thermostat control types as a test for this example file.

Schedule:Compact,
Zone Control Type Sched, !- Name
Control Type,            !- Schedule Type Limits Name
Through: 12/31,          !- Field 1
For: SummerDesignDay,    !- Field 2
Until: 24:00,2,          !- Field 3
For: WinterDesignDay,    !- Field 5
Until: 24:00,1,          !- Field 6
For: AllOtherDays,       !- Field 8
Until: 24:00,4;

rraustad ( 2025-05-20 14:44:32 -0500 )edit

Yes, thank you. That is one of the first edits I made to the file.

kwalkerman ( 2025-05-21 03:14:48 -0500 )edit

Thank you, @kwalkerman and @rraustad, for looking into this, much appreciated. Unfortunately, OpenStudio does not allow me to import the complete .idf, only parts thereof, so I cannot view this example in the way I am used to. However, I am very familiar with the objects in both the OpenStudio .osm and EnergyPlus .idf files. In those, I still do not see provisions for adjusting throttling ranges. Are you merely saying that the results for this example come out as expected, so EnergyPlus must somehow be doing something internally to automatically adjust the throttling ranges?

mattkoch ( 2025-05-21 07:30:31 -0500 )edit

@mattkoch - if you would like to request a demo version of EP3, you can use it to view, create and inspect existing complete IDF files - these files included.

As far as deadband and throttling ranges, yes, EnergyPlus has internal logic to control air terminals and ZoneHVAC objects. & See the edit to my answer above regarding zone temperature control.

kwalkerman ( 2025-05-21 09:59:15 -0500 )edit

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EnergyPlus/OpenStudio VAV w/ Reheat Throttling Range Adjustment?

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