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Here's what I'd do: (link to the test model)

Create an AirLoopHVAC to serve the Data Center Zone with:

1. Coil:Cooling:Water
2. AirLoopHVACUnitarySystem (with WaterToAir HeatPump)
3. SetpointManager:SingleZoneReheat
4. AirTerminal:SingleDuct:Uncontrolled

Create a PlantLoop to act as the heat rejection loop with:

1. Sizing:Plant (click dotted line in middle of loop and scroll down)
   • Loop Type = Condenser
2. Pump:VariableSpeed or Pump:ConstantSpeed
   • Control Type = Intermittent
3. FluidCooler:SingleSpeed or FluidCooler:TwoSpeed
4. SetpointManager:FollowOutdoorAirTemperature
   • Reference Temperature Type = OutdoorAirDryBulb
5. HeatExchanger:FluidToFluid
   • Control Type = CoolingSetpointOnOff
   • Heat Transfer Metering End Use Type = FreeCooling
6. Connect the Heat Pump Heating and Cooling Coils from the AirLoopHVACUnitarySystem to this loop

Create a PlantLoop to act as the free cooling loop with:

1. Sizing:Plant (click dotted line in middle of loop and scroll down)
   • Loop Type = Cooling
   • Design Loop Exit Temperature = 60F
2. Pump:VariableSpeed or Pump:ConstantSpeed
   • Control Type = Intermittent
3. Connect the HeatExchanger:FluidToFluid (from My Model)
4. SetpointManager:Scheduled
   • Make a 60F schedule and assign it here
5. Connect the Coil:Cooling:Water (from My Model)

Here is the cooling rate before adding the chilled water coil and free cooling loop:

Here is the cooling rate after. Notice that the chilled water coil (blue) is picking up part of the load, and the heat pump coil (orange) is picking up the rest of the load.

Here's what I'd do: (link to the test model)

Create an AirLoopHVAC to serve the Data Center Zone with:

1. Coil:Cooling:Water
2. AirLoopHVACUnitarySystem (with WaterToAir HeatPump)
3. SetpointManager:SingleZoneReheat
4. AirTerminal:SingleDuct:Uncontrolled

Create a PlantLoop to act as the heat rejection loop with:

1. Sizing:Plant (click dotted line in middle of loop and scroll down)
   • Loop Type = Condenser
2. Pump:VariableSpeed or Pump:ConstantSpeed
   • Control Type = Intermittent
3. FluidCooler:SingleSpeed or FluidCooler:TwoSpeed
4. SetpointManager:FollowOutdoorAirTemperature
   • Reference Temperature Type = OutdoorAirDryBulb
5. HeatExchanger:FluidToFluid
   • Control Type = CoolingSetpointOnOff
   • Heat Transfer Metering End Use Type = FreeCooling
6. Connect the Heat Pump Heating and Cooling Coils from the AirLoopHVACUnitarySystem to this loop

Create a PlantLoop to act as the free cooling loop with:

1. Sizing:Plant (click dotted line in middle of loop and scroll down)
   • Loop Type = Cooling
   • Design Loop Exit Temperature = 60F
2. Pump:VariableSpeed or Pump:ConstantSpeed
   • Control Type = Intermittent
3. Connect the HeatExchanger:FluidToFluid (from My Model)
4. SetpointManager:Scheduled
   • Make a 60F schedule and assign it here
5. Connect the Coil:Cooling:Water (from My Model)

Here is the cooling rate before adding the chilled water coil and free cooling loop:

Here is the cooling rate after. Notice that the chilled water coil (blue) is picking up part of the load, and the heat pump coil (orange) is picking up the rest of the load.