Issue overview
Configuration 1
The Air-to-Water Heat Pump fails to correctly modulate its water flow rate in variable-flow loops when compared to an equivalently sized air-cooled chiller. Despite having identical sizing, variable-flow loop placement, and temperature setpoints, the heat pump consistently operates at or near its maximum rated flow rate instead of scaling down appropriately during part-load conditions.
While both systems consume the same pumping energy with constant-flow pumps, introducing variable-flow pumps causes the chiller pump energy to drop as expected, whereas the heat pump pump energy unexpectedly increases. Hourly data confirms that while both systems successfully meet the required temperature setpoints and provide similar cooling rates, the heat pump relies on significantly higher flow rates to achieve this. During winter part-load conditions, the heat pump requires more than double the flow rate of the chiller, and it continues to demand noticeably higher flow rates even during summer full-load conditions. This excessively high flow behavior occurs on both hot and chilled water loops. The user mentions that while the heat pump modulates flow slightly better on condenser water loops, it still ultimately consumes more pumping energy than a traditional baseline cooling tower and boiler configuration.
Configuration 2
The second configuration examines variable-flow primary-secondary loops connected by a fluid-to-fluid heat exchanger. In this scenario, it is discovered that the air-cooled chiller and the heat pump consume the exact same amount of annual pumping energy. Hourly outputs reveal that the chiller begins to mimic the irregular behavior of the heat pump from the primary-only configuration, utilizing excessively high flow rates even during part-load conditions. Investigation is needed regarding why the pump behavior of the chiller and the heat pump becomes identical in a primary-secondary setup while diverging significantly in a primary-only setup.
Configuration 3
The final configuration involves the heat pump serving a condenser water loop for both cooling and heating, compared against a baseline cooling tower and boiler system. While switching to a variable-flow pump does reduce the heat pump's pumping energy, the reduction is notably smaller than the savings seen in the baseline system. Upon reviewing timestep outputs, it is identified that two specific operational errors including periods where the pump operates but all water passes through a parallel bypass branch with zero flow through the heat pump, and instances where water flows through both the hot and cold water connections simultaneously even though the unit is exclusively operating in cooling mode. Explanation/investigation needed regarding this bypass-only flow behavior and the anomalous flow through the inactive heating side during cooling operation.
Operating System (Multiple choices)
MacOS
Operating System Version
13.7.8
Version of EnergyPlus
25.2
Unmethours link or helpdesk ticket number
N/A
Defect file
Configuration 1
1b_EWYT_Var_54F-Return.idf.txt
1a_AGZ_Var_56F-Return.idf.txt
USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw.txt
issue_1.xlsx
configuration 2
1a_AGZ_VarP-S_56F-Return.idf.txt
1b_EWYT_VarP-S_56F-Return.idf.txt
Configuration 3
1b_EWYT_Var.idf.txt
Issue overview
Configuration 1
The Air-to-Water Heat Pump fails to correctly modulate its water flow rate in variable-flow loops when compared to an equivalently sized air-cooled chiller. Despite having identical sizing, variable-flow loop placement, and temperature setpoints, the heat pump consistently operates at or near its maximum rated flow rate instead of scaling down appropriately during part-load conditions.
While both systems consume the same pumping energy with constant-flow pumps, introducing variable-flow pumps causes the chiller pump energy to drop as expected, whereas the heat pump pump energy unexpectedly increases. Hourly data confirms that while both systems successfully meet the required temperature setpoints and provide similar cooling rates, the heat pump relies on significantly higher flow rates to achieve this. During winter part-load conditions, the heat pump requires more than double the flow rate of the chiller, and it continues to demand noticeably higher flow rates even during summer full-load conditions. This excessively high flow behavior occurs on both hot and chilled water loops. The user mentions that while the heat pump modulates flow slightly better on condenser water loops, it still ultimately consumes more pumping energy than a traditional baseline cooling tower and boiler configuration.
Configuration 2
The second configuration examines variable-flow primary-secondary loops connected by a fluid-to-fluid heat exchanger. In this scenario, it is discovered that the air-cooled chiller and the heat pump consume the exact same amount of annual pumping energy. Hourly outputs reveal that the chiller begins to mimic the irregular behavior of the heat pump from the primary-only configuration, utilizing excessively high flow rates even during part-load conditions. Investigation is needed regarding why the pump behavior of the chiller and the heat pump becomes identical in a primary-secondary setup while diverging significantly in a primary-only setup.
Configuration 3
The final configuration involves the heat pump serving a condenser water loop for both cooling and heating, compared against a baseline cooling tower and boiler system. While switching to a variable-flow pump does reduce the heat pump's pumping energy, the reduction is notably smaller than the savings seen in the baseline system. Upon reviewing timestep outputs, it is identified that two specific operational errors including periods where the pump operates but all water passes through a parallel bypass branch with zero flow through the heat pump, and instances where water flows through both the hot and cold water connections simultaneously even though the unit is exclusively operating in cooling mode. Explanation/investigation needed regarding this bypass-only flow behavior and the anomalous flow through the inactive heating side during cooling operation.
Operating System (Multiple choices)
MacOS
Operating System Version
13.7.8
Version of EnergyPlus
25.2
Unmethours link or helpdesk ticket number
N/A
Defect file
Configuration 1
1b_EWYT_Var_54F-Return.idf.txt
1a_AGZ_Var_56F-Return.idf.txt
USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw.txt
issue_1.xlsx
configuration 2
1a_AGZ_VarP-S_56F-Return.idf.txt
1b_EWYT_VarP-S_56F-Return.idf.txt
Configuration 3
1b_EWYT_Var.idf.txt