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There has been apparently an autosizing problem with Air-to-Water HeatPumps in EnergyPlus, which may or may not have been resolved in latest edition. As a user I succeeded in getting around the autosizing problems by

• setting the Rated Evaporator Air Flow Rate {m3/s} to an appropriate fixed numerical value
• making sure that the associated HeatPump Fan is assigned the same Maximum Flow Rate {m3/s}, (avoiding the autosizing option, if necessary).

I also adjusted

• Rated Sensible Heat Ratio .

In order to avoid troubles, make also sure the entries for

    * `Rated Evaporator Inlet Air Dry-Bulb Temperature {C}`
    * `Rated Evaporator Inlet Air Wet-Bulb Temperature {C}`
    * `Rated Condenser Inlet Water Temperature {C}`

are correct, and the COP- and Capacity curves are normalized at the rated temperatures.

An Example of how a solution may look like is as follows:

Coil:WaterHeating:AirToWaterHeatPump,
    AWHP_Coil_DHW,           !- Name
    700,                     !- Rated Heating Capacity {W}
    3.7,                     !- Rated COP {W/W}
    0.75,                    !- Rated Sensible Heat Ratio
    7,                       !- Rated Evaporator Inlet Air Dry-Bulb Temperature {C}
    6,                       !- Rated Evaporator Inlet Air Wet-Bulb Temperature {C}
    35,                      !- Rated Condenser Inlet Water Temperature {C}
    0.1,                     !- Rated Evaporator Air Flow Rate {m3/s}
    autocalculate,           !- Rated Condenser Water Flow Rate {m3/s}
    No,                      !- Evaporator Fan Power Included in Rated COP
    No,                      !- Condenser Pump Power Included in Rated COP
    No,                      !- Condenser Pump Heat Included in Rated Heating Capacity and Rated COP
    12,                      !- Condenser Water Pump Power {W}
    0.1,                     !- Fraction of Condenser Pump Heat to Water
    Fan_AWHP_DHW_OUT,        !- Evaporator Air Inlet Node Name
    ExhaustAirDHW_OUT,       !- Evaporator Air Outlet Node Name
    DHWTankSourceOUT,        !- Condenser Water Inlet Node Name
    DHWTankSourceIN,         !- Condenser Water Outlet Node Name
    0,                       !- Crankcase Heater Capacity {W}
    5,                       !- Maximum Ambient Temperature for Crankcase Heater Operation {C}
    WetBulbTemperature,      !- Evaporator Air Temperature Type for Curve Objects
    AWHP_DHW_rCAP,           !- Heating Capacity Function of Temperature Curve Name
    ,                        !- Heating Capacity Function of Air Flow Fraction Curve Name
    ,                        !- Heating Capacity Function of Water Flow Fraction Curve Name
    AWHP_DHW_rCOP,           !- Heating COP Function of Temperature Curve Name
    ,                        !- Heating COP Function of Air Flow Fraction Curve Name
    ,                        !- Heating COP Function of Water Flow Fraction Curve Name
    HPWHPLFFPLR_DHW;         !- Part Load Fraction Correlation Curve Name

and finally:

Fan:OnOff,
    Fan_AWHP_DHW,            !- Name
    Always On Discrete,      !- Availability Schedule Name
    0.6,                     !- Fan Total Efficiency
    250,                     !- Pressure Rise {Pa}
    0.1,                     !- Maximum Flow Rate {m3/s}
    0.8,                     !- Motor Efficiency
    1,                       !- Motor In Airstream Fraction
    OutdoorAirIN_DHW,        !- Air Inlet Node Name
    Fan_AWHP_DHW_OUT,        !- Air Outlet Node Name
    ,                        !- Fan Power Ratio Function of Speed Ratio Curve Name
    ,                        !- Fan Efficiency Ratio Function of Speed Ratio Curve Name
    DHW;                     !- End-Use Subcategory