Why is the zone reheat coil operating while the zone predicted sensible load is zero?

asked 2017-08-02 12:51:46 -0500

Molly Curtz
1348 ●3 ●12

updated 2017-08-02 14:53:09 -0500

I am looking at an Energy Plus model with a VAV system with AirTerminal:SingleDuct:VAV:Reheat terminal units. I am seeing that the reheat coil is operating sometimes when the output variable Zone Predicted Sensible Load to Setpoint Heat Transfer Ratehas the value zero. I looked at the zone temperature and at these times the reheat coil does need to operate to maintain the heating setpoint. Why is the predicted load zero, while zone heating from the reheat coil is needed to maintain the zone heating setpoint? And, how does the reheat coil determine what wattage to output in this case?

This is happening in my test file where I have been messing with the system controls, but I also looked at one of the example files for a VAV reheat system, and I see the same thing happening there always at the end of the day. Here are some graphs of the results over a couple of (winter) days:

First, the Heating Coil Heating Rate for the terminal unit heating coil and the Zone Predicted Sensible Load to Setpoint Heat Transfer Rate. You can see the heating coil is sometimes operating when the predicted sensible load to setpoint is zero.

image description

Next, a graph of the zone temperature, heating and cooling setpoints for the same time window. You can see that the heating coil is just maintaining the heating setpoint when it operates at zero predicted zone load. image description

Thanks in advance for any ideas.

Some context: the actual designed system I am trying to model is a single zone variable volume, variable temperature system with no terminal reheat. I would like to control my system so that all heating and cooling occurs at the AHU heating and cooling coils, and eventually I want to remove the terminal reheat assuming I can get the model to work without it.

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@rraustad, Thanks very much! Of course. That totally makes sense. I need to provide minimum flow for ventilation during occupied hours, but I will try letting my system maximum supply temp. reset float up to a temperature in the zone deadband temp range and see if that resolves it. How does the terminal reheat coil determine required output? Is there an adjustment to predicted zone load based on the zone supply flow and temp? When does this calculation occur in the simulation?

Molly Curtz ( 2017-08-02 14:04:04 -0500 )edit

The reheat coil first sets the air mass flow rate to the minimum allowed. This would be 0 or MinStop flow in the case where predicted load = 0. Then calculates SA needed based on OA needed and uses the max of these. Then checks to see if this mass flow rate at the current SAT exceeds the heating set point and turns on the heating coil if necessary. This calculation occurs after the air loop has simulated and air is available at the zone level and the SAT is known.