I've calculated the hourly part-load fraction (PLF) as a function of part-load ratio (PLR) of a multistage gas furnace in an EnergyPlus model. The result has many PLF values for a given PLR at low loads which does not seem right. It looks like something is happening with staging because there are 4 (maybe 5) discrete sloped lines. I've read through the Engineering Reference but still can't see how this is correct. Any help is appreciated.
Here is the curve:
Here are the outputs I used and how I calculated the values for every timestep using those outputs:
Output:Variable,PSZ-AC:2_Unitary_Package_HeatCoil,Heating Coil Heating Rate,Timestep;
Output:Variable,PSZ-AC:2_Unitary_Package_HeatCoil,Heating Coil Gas Rate,Timestep;
PLR = PSZ-AC:2_Unitary_Package_HeatCoil,Heating Coil Heating Rate / Design Size Stage 4 Nominal Capacity
PLF = PSZ-AC:2_Unitary_Package_HeatCoil,Heating Coil Heating Rate / (PSZ-AC:2_Unitary_Package_HeatCoil,Heating Coil Gas Rate * Nominal Efficiency)
Here are the relevant objects:
AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed,
Core Zone Multi Stage Gas Furnace, !- Name
ALWAYS_ON, !- Availability Schedule Name
PSZ-AC:2_OA-PSZ-AC:2_Unitary_PackageNode, !- Air Inlet Node Name
PSZ-AC:2 Supply Equipment Outlet Node, !- Air Outlet Node Name
Core_Retail, !- Controlling Zone or Thermostat Location
Fan:ConstantVolume, !- Supply Air Fan Object Type
PSZ-AC:2_Unitary_Package_fan, !- Supply Air Fan Name
BlowThrough, !- Supply Air Fan Placement
ALWAYS_ON, !- Supply Air Fan Operating Mode Schedule Name
Coil:Heating:Gas:MultiStage, !- Heating Coil Object Type
PSZ-AC:2_Unitary_Package_HeatCoil, !- Heating Coil Name
-100, !- Minimum Outdoor Dry-Bulb Temperature for Compressor Operation {C}
Coil:Cooling:DX:MultiSpeed, !- Cooling Coil Object Type
PSZ-AC:2_Unitary_Package_CoolCoil, !- Cooling Coil Name
Coil:Heating:Fuel, !- Supplemental Heating Coil Object Type
PSZ-AC:2_SupplementaryHeatingCoil, !- Supplemental Heating Coil Name
0, !- Maximum Supply Air Temperature from Supplemental Heater {C}
-100, !- Maximum Outdoor Dry-Bulb Temperature for Supplemental Heater Operation {C}
, !- Auxiliary On-Cycle Electric Power {W}
, !- Auxiliary Off-Cycle Electric Power {W}
, !- Design Heat Recovery Water Flow Rate {m3/s}
80, !- Maximum Temperature for Heat Recovery {C}
, !- Heat Recovery Water Inlet Node Name
, !- Heat Recovery Water Outlet Node Name
autosize, !- No Load Supply Air Flow Rate {m3/s}
4, !- Number of Speeds for Heating
2, !- Number of Speeds for Cooling
8.592, !- Heating Speed 1 Supply Air Flow Rate {m3/s}
8.592, !- Heating Speed 2 Supply Air Flow Rate {m3/s}
8.592, !- Heating Speed 3 Supply Air Flow Rate {m3/s}
8.592, !- Heating Speed 4 Supply Air Flow Rate {m3/s}
8.592, !- Cooling Speed 1 Supply Air Flow Rate {m3/s}
8.592, !- Cooling Speed 2 Supply Air Flow Rate {m3/s}
8.592; !- Cooling Speed 3 Supply Air Flow Rate {m3/s}
Coil:Heating:Gas:MultiStage,
PSZ-AC:2_Unitary_Package_HeatCoil, !- Name
ALWAYS_ON, !- Availability Schedule Name
PSZ-AC:2_Unitary_PackageHeatCoil air inlet, !- Air Inlet Node Name
PSZ-AC:2_Unitary_PackageSuppHeatCoil air inlet, !- Air Outlet Node Name
PSZ-AC:2_Unitary_PackageCoolCoil air inlet, !- Temperature Setpoint Node Name
Single- of Multi-stage PLF, !- Part Load Fraction Correlation Curve Name
, !- Parasitic Gas Load {W}
4, !- Number of Stages
0.744, !- Stage 1 Gas Burner Efficiency {W/W}
autosize, !- Stage 1 Nominal Capacity {W}
, !- Stage 1 Parasitic Electric Load {W}
0.744, !- Stage 2 Gas Burner Efficiency {W/W}
autosize, !- Stage 2 Nominal Capacity {W}
, !- Stage 2 Parasitic Electric Load {W}
0.744, !- Stage 3 Gas Burner Efficiency {W/W}
autosize, !- Stage 3 Nominal Capacity {W}
, !- Stage 3 Parasitic Electric Load {W}
0.744, !- Stage 4 Gas Burner Efficiency {W/W}
autosize; !- Stage 4 Nominal Capacity {W}
Curve:Quadratic,
Single- of Multi-stage PLF, !- Name
0.8, !- Coefficient1 Constant
0.2, !- Coefficient2 x
0.0, !- Coefficient3 x**2
0.0, !- Minimum Value of x
1.0; !- Maximum Value of x
