HHW Plant Loop Overheating

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I am encountering an issue where, in the absence of heating demand, my heating hot water plant temperature continues to climb. In the picture below, I have plotted boiler output (blue), supply pump flow (red), and HHW temperature (green). At times when the boiler is off due to a lack of demand, the temperature increases and the pump (sometimes) continues to cycle. When I look at individual heating coils I observe that the coil output is zero whenever the boiler's is, and that the coil temperature matches that of the plant loop. Is there a clear reason for this and are there any associated implications for my model's energy usage?

Thanks