Trouble controlling temperatures with VRF System.

asked 2015-04-23 22:16:03 -0600

eklein12
373 ●2 ●7

Hello,

I am modeling a VRF System with no reheat that has an outdoor unit connected to two indoor units that heat one thermal zone. I believe I have modeled the system correctly to my knowledge, but the temperatures of the thermal zone throughout the year are not regulated well.(picture attached) image description

I have the cooling thermostat schedule set to 80 degrees F and the heating thermostat schedule set to set to 68. The system has the availability schedule always on discrete.

This is my first project with open studio/ energy plus so I am not sure where to start to try and fix this issue.

Thank you

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Comments

@eklein12, Did you try running the model with only one terminal serving the zone to see whether that improves the results? If room temperature gets better controlled with one terminal unit then you may try to split the zone into two, each served by separate terminal unit and apply cross mixing.

Chandan Sharma ( 2015-04-24 06:26:49 -0600 )edit

Thank you for replying. It unfortunately did not improve the results. I have been adjusting the input values for the VRF system and nothing seems to be helping.

eklein12 ( 2015-04-26 15:52:02 -0600 )edit

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answered 2015-07-20 11:53:05 -0600

rraustad

13834 ●79 ●12 http://www.fsec.ucf.edu/

updated 2015-07-20 11:58:23 -0600

From the figure, the cooling (26.6) and heating (20) set point temperatures are not maintained. This would indicate undersized equipment. check the size of the cooling/heating coils with respect to the zone loads. You can report the coil part-load ratio and determine if the coil is running at full capacity. Also report the zone sensible load and compare to the maximum coil capacity at times when the zone temperature set point is not met.

Add these report variables to your input file and run the simulation again. Then review the results in the variables (csv) file. Use the wildcard asterisk (*) to report all data in the simulation. Add a key name (e.g., "zone name" for zone air system sensible heating rate or "coil name" for cooling coil runtime fraction) to limit the reports to specific data or systems.

Actual energy transfer rate to the zone through the zone inlet nodes:

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

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

Actual zone predicted loads where the zone temperature set point is met IF this load is met by the HVAC system:

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

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

Runtime fractions of the coils:

Output:Variable,*,Cooling Coil Runtime Fraction,timestep; !- HVAC Average []

Output:Variable,*,Heating Coil Total Heating Rate,timestep; !- HVAC Average [W]

Output:Variable,*,Heating Coil Total Heating Energy,timestep; !- HVAC Sum [J]

Output:Variable,*,Heating Coil Runtime Fraction,timestep; !- HVAC Average []

Part-load ratio and runtime fraction of the outdoor condensing unit:

Output:Variable,*,VRF Heat Pump Part Load Ratio,timestep; !- HVAC Average []

Output:Variable,*,VRF Heat Pump Runtime Fraction,timestep; !- HVAC Average []

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Comments

@rraustad I have similar situation however, following your output variables in OpenStudio: I cant find the output variables you listed for Runtime fractions of the coils. I it because I have VRF system and VRF terminal unit. Where am i missing the variables.

rkbest ( 2015-07-24 16:09:20 -0600 )edit

You should be able to find all valid report variables in the report data dictionary (rdd) file after the simulation is executed. You will need to add Output:VariableDictionary,IDF; to your input file to create the rdd file.