Water Heating Coil UA calculation

asked 2020-03-03 12:23:15 -0600

updated 2020-03-08 11:43:41 -0600

Hi all,

I am writing to share the problem against which I spent a day of my time reaching a solution (but not satisfactory for how the problem shows up) that could help others who may run into the same problem:

1- Water Heating Coil, part of a FCU, hard sized in EnergyPlus (through DesignBuilder interface), via Nominal Capacity and the 4 temperatures (air in, air out, water in, water out) and the flow rates in the parent object ( Four Pipe Fan Coil)

2- I try to run the simulation I've FATAL ERROR "UA is zero for COIL: Heating: Water ...".

3- After many checks I've found that the fatal error disappers if:          a- Deleating (directly in the IDF file) the designBuiledr precompilated "autosize" value for the U-Factor Times Area Value field of the Coil: Heating: Water object, or        b- Putting hard sized values (any, not necessarly correct) ​​directly in DesigBuilder in "UA and Design water flow rate" performance input method, and than coing back to "Nominal capacity" performance input method in order to use proper selected input values

The unpleasant situation is double:

1) I choose to input coil data through its nominal capacity, so I should not worry about the other input path. I know DesigBuilder precompiled data aim at help and speed up setting the model, but in this unlucky case they forced me to debug the model for no reason;

2) I think there is a bug in the Energy Plus code: it takes into account values ​​on fields that should not be taken into account because of the selection for the performance input method. from InputOutputReference: "If NominalCapacity is chosen, the user must input a Rated Capacity for the coil; UA of the Coil and Max Water FlowRate of the Coil will be ignored"

I want to clarify that the fatal error shows up in the simulation run, after passing the warmup and the sizing runs

   *** EnergyPlus Warmup Error Summary. During Warmup: 0 Warning; 0 Severe Errors.    *** EnergyPlus Sizing Error Summary. During Sizing: 8 Warning; 0 Severe Errors.    *** EnergyPlus Terminated - Fatal Error Detected. 8 Warning; 0 Severe Errors; Elapsed Time = 00hr 00min 1.59sec

If anyone has some ideas about the reasons for this unexpected behavior any contribution is welcome

Best regards


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I had the same problem and used the same approach. I did not know how to validate if the random value I've used for UA was used elsewhere in the simulation though. Since the documentation says this value is not used when nominal capacity is chosen I did not bother too much with it.

Frederic Leveille gravatar image Frederic Leveille  ( 2020-03-03 13:30:49 -0600 )edit