Water Heater Heat Pump with Outdoor Air Temperature falls below Min. Temp

asked 2017-06-13 03:29:59 -0600

Fish Lee's avatar

updated 2017-06-16 05:16:30 -0600

I am simulating a hot water plant with WaterHeater:HeatPump:PumpedCondenser. I have found the water heater is operating abnormally when the inlet air temperature is less than what it is specified.

Currently it is set at the allowable minimum (-5 degree C). When the outdoor air temperature falls below -5C on the 5th day, the water coil of heat pump somehow can operate at full capacity as the first graph below.

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The tank temperature is still dropping regardless of the heat transfer from source side. image description image description

The weirdest thing is the water heater that the heat pump is connected to is heating at negative rate!

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Has anyone experience such problem? Anything I can do about it?

I have placed the heat pump code in below.

WHPH1,                   !- Name
HVACTemplate-Always 1,   !- Availability Schedule Name
Always 44.5,             !- Compressor Setpoint Temperature Schedule Name
1.2,                     !- Dead Band Temperature Difference {deltaC}
WHPH1 LoadSide Inlet Node,  !- Condenser Water Inlet Node Name
WHPH1 LoadSide Outlet Node,  !- Condenser Water Outlet Node Name
autocalculate,           !- Condenser Water Flow Rate {m3/s}
autocalculate,           !- Evaporator Air Flow Rate {m3/s}
OutdoorAirOnly,          !- Inlet Air Configuration
,                        !- Air Inlet Node Name
,                        !- Air Outlet Node Name
HPPlantAirInletNode1,    !- Outdoor Air Node Name
HPPlantAirOutletNode1,   !- Exhaust Air Node Name
,                        !- Inlet Air Temperature Schedule Name
,                        !- Inlet Air Humidity Schedule Name
,                        !- Inlet Air Zone Name
WaterHeater:Mixed,       !- Tank Object Type
HPWHPlantTank1,          !- Tank Name
HPWH Use Inlet Node1,    !- Tank Use Side Inlet Node Name
HPWH Use Outlet Node1,   !- Tank Use Side Outlet Node Name
Coil:WaterHeating:AirToWaterHeatPump:Pumped,  !- DX Coil Object Type
HPWHPlantDXCoil1,        !- DX Coil Name
-5,                      !- Minimum Inlet Air Temperature for Compressor Operation {C}
26,                      !- Maximum Inlet Air Temperature for Compressor Operation {C}
Outdoors,                !- Compressor Location
,                        !- Compressor Ambient Temperature Schedule Name
Fan:OnOff,               !- Fan Object Type
HPWHPlantFan1,           !- Fan Name
BlowThrough,             !- Fan Placement
,                        !- On Cycle Parasitic Electric Load {W}
,                        !- Off Cycle Parasitic Electric Load {W}
;                        !- Parasitic Heat Rejection Location

HPWHPlantTank1,          !- Name
autosize,                !- Tank Volume {m3}
Always 38.3,             !- Setpoint Temperature Schedule Name
2,                       !- Deadband Temperature Difference {deltaC}
60,                      !- Maximum Temperature Limit {C}
Cycle,                   !- Heater Control Type
autosize,                !- Heater Maximum Capacity {W}
0,                       !- Heater Minimum Capacity {W}
,                        !- Heater Ignition Minimum Flow Rate {m3/s}
,                        !- Heater Ignition Delay {s}
NaturalGas,              !- Heater Fuel Type
0.98,                    !- Heater Thermal Efficiency
,                        !- Part Load Factor Curve Name
,                        !- Off Cycle Parasitic Fuel Consumption Rate {W}
,                        !- Off Cycle Parasitic Fuel Type
,                        !- Off Cycle Parasitic Heat Fraction to Tank
,                        !- On Cycle Parasitic Fuel Consumption Rate {W}
,                        !- On Cycle Parasitic Fuel Type
,                        !- On Cycle Parasitic Heat Fraction to Tank
Outdoors,                !- Ambient Temperature Indicator
,                        !- Ambient Temperature Schedule Name
,                        !- Ambient Temperature Zone Name
HPWHPlantTank OA Node,   !- Ambient Temperature Outdoor Air Node Name
0,                       !- Off Cycle Loss Coefficient to Ambient Temperature {W/K}
0,                       !- Off Cycle Loss Fraction to Zone
0,                       !- On Cycle Loss Coefficient to Ambient Temperature {W/K}
0,                       !- On Cycle Loss Fraction to Zone
,                        !- Peak Use Flow Rate {m3/s}
,                        !- Use Flow Rate Fraction Schedule Name
,                        !- Cold Water Supply Temperature Schedule Name
HPWH Use Inlet Node1,    !- Use Side Inlet Node Name
HPWH Use Outlet Node1,   !- Use ...
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@Fish Lee it would be helpful if you included the input fields for the HPWH in your question. You can click on the "1 0 1 0 1 0" button to create a code block, then paste the text of the object's input fields within that.

Aaron Boranian's avatar Aaron Boranian  ( 2017-06-13 09:02:40 -0600 )edit

Thanks for the tips @Aaron Boranian . I have inserted the code in my question.

I discover a little more about what is happening. After the heat pump shut down, the water tank is actually drawing heating from the conditioned zones! The heating rate of coils become negative afterwards.

Fish Lee's avatar Fish Lee  ( 2017-06-13 21:35:26 -0600 )edit

@Fish Lee the heating rate of the heat pump's DX coils turns negative? That doesn't seem right. Could you also paste a graph of the storage tank's water temp? My guess is that the tank water temp reaches 60C (your HPWH compressor setpoint of 55C + deadband of 5C), so there is no need for heating from the heat pump system.

Aaron Boranian's avatar Aaron Boranian  ( 2017-06-14 10:35:15 -0600 )edit

@Aaron Boranian I have tried to simplify the plant from 4 heat pumps into 1. Its behaviour was still abnormal when the temperature is below the limit. I have revised the post with more details.

Fish Lee's avatar Fish Lee  ( 2017-06-16 05:17:40 -0600 )edit

@Fish Lee very strange ... From the documentation, the water heater heating rate output variable is for the auxiliary heating element or burner, which should never go negative. I would add output variables for temp and flow rate at all nodes in the HPWH system (tank use side, tank source side, coil water side, coil air side) as well as outdoor air temp. Hopefully the node data will help to solve what's going on.

Aaron Boranian's avatar Aaron Boranian  ( 2017-06-16 09:30:32 -0600 )edit