Affects plant-loop connected heat pump water heaters (HPWH) but not HPWHs with scheduled draws (Use Flow Rate Fraction Schedule). Shows up in 8.2, but not 8.1. We confirmed that this was a byproduct of the fix to #4455. Might be related to updating of tank source side inlet/outlet temperatures.
The HeatPumpWaterHeater.idf example file has five HPWHs, in different configurations:
- Zone 5 - HPWH using mixer/splitter nodes to blend outdoor air with zone air
- Zone 4 - HPWH without mixer/splitter nodes using only zone air
- Outdoors - HPWH located outdoors and not connected to a plant loop
- Scheduled - HPWH using scheduled ambient conditions
- Plant - HPWH located outdoors and connected to a plant loop which serves zone 1-5 reheat coils and the main heating coil in the air loop.
Only (5) shows a difference in energy usage between E+ v8.1 and v8.2. I believe this is because it is the only HPWH connected to a loop via Use Side Inlet/Outlet Nodes, as opposed to a “Use Flow Rate Fraction Schedule.” Because we use a DHW loop in BEopt (as would most models doing detailed DHW simulations), this bug affects our HPWH results:
• ~50% increase in heat pump coil heating energy
• ~50% increase in heat pump coil electricity use
• Minor change in heat pump COP (as expected due to lower tank temperatures)
• ~50% lower system COP
OpenStudio measures written by PSD use DHW plant loops, so this would affect OpenStudio projects as well.
To confirm that this bug affects plant-loop-connected HPWHs, I made a copy of the Zone4HeatPumpWaterHeater from the example file (“Zone4bHeatPumpWaterHeater”) and connected it to a DHW loop with the exact same draw profile as the Zone4HeatPumpWaterHeater (see attached HeatPumpWaterHeater_V810.idf and HeatPumpWaterHeater_V820.idf). Confirming the hot water draws are identical:
Results for the scheduled Zone4HPWH are identical between 8.1 and 8.2:
But in v8.2, the plant-loop-connected Zone4bHPWH shows sagging tank temperatures and much more water heating energy:
Here is a table summarizing totals for the four days in the example file. As you can see, the results for the scheduled Zone4HPWH are consistent from 8.1 to 8.2, but the plant-loop-connected Zone4bHPWH actually has negative net Source Side Heat Transfer for the four days.
The negative source side heat transfer rate in the 8.2 Zone4bHPWH run can be seen below. Manually calculating the source side heat transfer rate using mCpΔT agrees with the reported value, more or less. It looks like the problem is that with a plant-loop-connected HPWH, the coil removes a lot of heat from the tank at the beginning of each cycle, before recovering with either the element and heat pump, or heat pump alone.
Affects plant-loop connected heat pump water heaters (HPWH) but not HPWHs with scheduled draws (Use Flow Rate Fraction Schedule). Shows up in 8.2, but not 8.1. We confirmed that this was a byproduct of the fix to #4455. Might be related to updating of tank source side inlet/outlet temperatures.
The HeatPumpWaterHeater.idf example file has five HPWHs, in different configurations:
Only (5) shows a difference in energy usage between E+ v8.1 and v8.2. I believe this is because it is the only HPWH connected to a loop via Use Side Inlet/Outlet Nodes, as opposed to a “Use Flow Rate Fraction Schedule.” Because we use a DHW loop in BEopt (as would most models doing detailed DHW simulations), this bug affects our HPWH results:
• ~50% increase in heat pump coil heating energy
• ~50% increase in heat pump coil electricity use
• Minor change in heat pump COP (as expected due to lower tank temperatures)
• ~50% lower system COP
OpenStudio measures written by PSD use DHW plant loops, so this would affect OpenStudio projects as well.
To confirm that this bug affects plant-loop-connected HPWHs, I made a copy of the Zone4HeatPumpWaterHeater from the example file (“Zone4bHeatPumpWaterHeater”) and connected it to a DHW loop with the exact same draw profile as the Zone4HeatPumpWaterHeater (see attached HeatPumpWaterHeater_V810.idf and HeatPumpWaterHeater_V820.idf). Confirming the hot water draws are identical:
Results for the scheduled Zone4HPWH are identical between 8.1 and 8.2:
But in v8.2, the plant-loop-connected Zone4bHPWH shows sagging tank temperatures and much more water heating energy:
Here is a table summarizing totals for the four days in the example file. As you can see, the results for the scheduled Zone4HPWH are consistent from 8.1 to 8.2, but the plant-loop-connected Zone4bHPWH actually has negative net Source Side Heat Transfer for the four days.
The negative source side heat transfer rate in the 8.2 Zone4bHPWH run can be seen below. Manually calculating the source side heat transfer rate using mCpΔT agrees with the reported value, more or less. It looks like the problem is that with a plant-loop-connected HPWH, the coil removes a lot of heat from the tank at the beginning of each cycle, before recovering with either the element and heat pump, or heat pump alone.
