How to change Fan from VariableVolume to ConstantVolume?

asked 2018-12-26 20:36:03 -0500

51 ●3

updated 2018-12-27 20:47:33 -0500

Hi, I am trying to change my HVAC system (PAU with VRF) fan from Variable Volume to Constant Volume in exp.idf file but it did not work out well. What I did is copy all the obj from Fan:VariableVolume and pasted in Fan:ConstantVolume. From the Output:Variable, I generated hourly data of Air System Mixed Mass Flow Rate and the data are showing the system still operates as variable volume.

Does anyone experience the same thing as I did? Can anyone give me advice how to solve this problem? Thank you in advance.

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answered 2018-12-27 11:48:05 -0500

Aaron Boranian

13830 ●76 ●24 http://bigladdersoftware.com/

If you only made a copy of the Fan:VariableVolume object to create the Fan:ConstantVolume object, but left the VAV fan in the model, then the CAV fan is not connected to any systems ... yet. You need to search for the name of the VAV fan to see where the Branch of an AirLoopHVAC object references that name. Then, replace the Object Type and Object Name input fields of the Branch referencing the VAV fan with the same parameters (object type and object name) of the CAV fan that you created.

You'll also need to make sure the CAV fan is properly connected to the other components that the VAV fan was connected to (the CAV fan should have the same inlet and outlet nodes as the VAV fan).

I would recommend that you begin to use the Fan:SystemModel object. This is the "swiss army knife" of fan object in EnergyPlus -- this one object type can model CAV, VAV, and On/Off Cycling fan types, as well as allow for a different night-time ventilation flow rate and other performance parameters.

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@Aaron Boranian

Thank you for your reply. I have deleted all the object from Fan:VariableVolume after I pasted the object to Fan:ConstantVolume and also changed the* Object Type* to Fan:ConstantVolume at the Branch. The fan did not operate as Variable Volume.

I am currently using the Fan:SystemModel and I set the Speed Control Method to Discrete but it did not fixed the problem (Please refer to the updated photo). Can you kindly advise? Thank you.

Tina Lu ( 2018-12-27 20:55:20 -0500 )edit

@Tina Lu if you changed things from VAV to CAV fan, and the fan did NOT operate as VAV, isn't that what you want? Regarding the Fan:SystemModel, setting Speed Control Method to Discrete is correct, but for a CAV fan instead of an On/Off fan you should also set Number of Speeds to 1 and set Speed 1 Flow Fraction and Speed 1 Electric Power Fraction to the proper values for the fan's CAV operation. Note that these are fractions of the Design Maximum Air Flow Rate and Design Electric Power Consumption input fields.

Aaron Boranian ( 2018-12-29 16:47:11 -0500 )edit

Also, in your updated image of IDF Editor, there are orange cells that are trying to indicate an incorrect input field setting. For each Air System branch, the Component 3 Name field is being used to reference the name of the fan object. If you copied the VAV fans to create the CAV fans, then maybe you also changed the names and need to update these input fields of each Branch?

Aaron Boranian ( 2018-12-29 16:49:16 -0500 )edit

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