Fix documentation of VRF Heat Pump Total Heating Rate, should equal the sum of coil heating rate, not air terminal heating rate

doc/input-output-reference/src/overview/group-variable-refrigerant-flow-equipment.tex

@@ -619,7 +619,7 @@ \subsubsection{Outputs}\label{outputs-039}
 
 \paragraph{VRF Heat Pump Total Heating Rate {[}W{]}}\label{vrf-heat-pump-total-heating-rate-w}
 
-This output field is the operating total heating capacity of the variable refrigerant flow heat pump in Watts. The capacity includes any degradation due to defrost mode. This value is calculated for each HVAC system time step being simulated, and the results are averaged for the time step being reported. This value should match the sum of the individual zone terminal unit output variables for Zone VRF Air Terminal Total Heating Rate.
+This output field is the operating total heating capacity of the variable refrigerant flow heat pump in Watts. The capacity includes any degradation due to defrost mode. This value is calculated for each HVAC system time step being simulated, and the results are averaged for the time step being reported. This value should match the sum of the individual zone terminal unit heating coil output variables for Heating Coil Heating Rate plus any piping loss.
 
 \paragraph{VRF Heat Pump Cooling Electricity Rate {[}W{]}}\label{vrf-heat-pump-cooling-electric-power-w}
 
@@ -1153,6 +1153,10 @@ \subsubsection{Outputs}
 
 Note: refer to the rdd file after a simulation for exact output variable names
 
+\paragraph{VRF Heat Pump Total Heating Rate {[}W{]}}\label{vrf-heat-pump-total-heating-rate-w-fluidTCtrl}
+
+This output field is the operating total heating capacity of the variable refrigerant flow heat pump in Watts. The capacity includes any degradation due to defrost mode. This value is calculated for each HVAC system time step being simulated, and the results are averaged for the time step being reported. This value should match the sum of the individual zone terminal unit heating coil output variables for Heating Coil Heating Rate plus any piping loss.
+
 \paragraph{VRF Heat Pump Compressor Rotating Speed {[}rev/min{]}}\label{vrf-heat-pump-compressor-rotating-speed-revmin}
 
 This output only applies for the VRF-FluidTCtrl model. This is the rotating speed of the compressor, which indicates the loading index.

src/EnergyPlus/DXCoils.cc

@@ -17253,6 +17253,7 @@ void ControlVRFIUCoil(EnergyPlusData &state,
     MaxSH = 15;
     MaxSC = 20;
     Garate = state.dataDXCoils->DXCoil(CoilIndex).RatedAirMassFlowRate(1);
+    // why always limit the minimum fan speed ratio to 0.65?
     FanSpdRatioMin = min(OAMassFlow / Garate, 1.0); // ensure that coil flow rate is higher than OA flow rate
 
     if (QCoil == 0) {
@@ -17388,12 +17389,20 @@ void ControlVRFIUCoil(EnergyPlusData &state,
         if (QCoilSenHeatingLoad > QinSenMin1) {
             // Modulate fan speed to meet room sensible load; SC is not updated
             FanSpdRatioMax = 1.0;
-            auto f = [QCoilSenHeatingLoad, Ts_1, Tin, Garate, BF](Real64 FanSpdRto) {
-                return FanSpdResidualHeat(FanSpdRto, QCoilSenHeatingLoad, Ts_1, Tin, Garate, BF);
+            Tout = Tin + (Ts_1 - Tin) * (1 - BF);
+            Real64 RatedAirMassFlowRate = state.dataDXCoils->DXCoil(CoilIndex).RatedAirMassFlowRate[0];
+            auto f = [QCoilSenHeatingLoad, RatedAirMassFlowRate, Tout, Tin, Win](Real64 FanSpdRto) {
+                return FanSpdResidualHeatUsingH(FanSpdRto, QCoilSenHeatingLoad, RatedAirMassFlowRate, Tout, Tin, Win);
             };
             General::SolveRoot(state, 1.0e-3, MaxIter, SolFla, Ratio1, f, FanSpdRatioMin, FanSpdRatioMax);
             // this will likely cause problems eventually, -1 and -2 mean different things
-            if (SolFla < 0) Ratio1 = FanSpdRatioMax; // over capacity
+            if (SolFla < 0) {
+                if (f(FanSpdRatioMin) <= 0) { // capacity <= demand
+                    Ratio1 = FanSpdRatioMax;  // over capacity
+                } else {                      // capacity > demand even for the minimum fan speed
+                    Ratio1 = FanSpdRatioMin;
+                }
+            }
             FanSpdRatio = Ratio1;
             CoilOnOffRatio = 1.0;
 
@@ -17674,6 +17683,27 @@ Real64 FanSpdResidualHeat(Real64 FanSpdRto, Real64 QCoilSenHeatingLoad, Real64 T
     return (TotCap - ZnSenLoad) / ZnSenLoad;
 }
 
+Real64 FanSpdResidualHeatUsingH(Real64 FanSpdRto, Real64 QCoilSenHeatingLoad, Real64 RatedAirMassFlowRate, Real64 Tout, Real64 Tin, Real64 Win)
+{
+
+    // FUNCTION INFORMATION:
+    //       AUTHOR         Yujie Xu (yujiex)
+    //       DATE WRITTEN   Jul 2024
+    //
+    // PURPOSE OF THIS FUNCTION:
+    //       Calculates residual function (desired zone heating load - actual heating coil capacity)
+    //       This is used to modify the fan speed to adjust the coil heating capacity to match
+    //       the zone heating load. This one uses Hin and Hout difference rather than Tin and Tout difference
+    //       like in FanSpdResidualHeat
+    //
+    Real64 ZnSenLoad = QCoilSenHeatingLoad;
+    // +-100 W minimum zone load?
+    if (std::abs(ZnSenLoad) < 100.0) ZnSenLoad = sign(100.0, ZnSenLoad);
+    Real64 Wout = Win;
+    Real64 TotCap = FanSpdRto * RatedAirMassFlowRate * (PsyHFnTdbW(Tout, Wout) - PsyHFnTdbW(Tin, Win));
+    return (TotCap - ZnSenLoad) / ZnSenLoad;
+}
+
 void SetMSHPDXCoilHeatRecoveryFlag(EnergyPlusData &state, int const DXCoilNum)
 {
 

src/EnergyPlus/DXCoils.hh

@@ -933,6 +933,8 @@ namespace DXCoils {
 
     Real64 FanSpdResidualHeat(Real64 FanSpdRto, Real64 QCoilSenHeatingLoad, Real64 Ts_1, Real64 Tin, Real64 Garate, Real64 BF);
 
+    Real64 FanSpdResidualHeatUsingH(Real64 FanSpdRto, Real64 QCoilSenHeatingLoad, Real64 RatedAirMassFlowRate, Real64 Tout, Real64 Tin, Real64 Win);
+
     void SetMSHPDXCoilHeatRecoveryFlag(EnergyPlusData &state, int const DXCoilNum); // must match coil names for the coil type
 
     void SetDXCoilAirLoopNumber(EnergyPlusData &state, std::string const &CoilName, int const AirLoopNum); // must match coil names for the coil type

src/EnergyPlus/HVACVariableRefrigerantFlow.cc

@@ -276,6 +276,9 @@ void SimulateVRF(EnergyPlusData &state,
 
         if (state.dataHVACVarRefFlow->VRF(VRFCondenser).CondenserType == DataHeatBalance::RefrigCondenserType::Water)
             UpdateVRFCondenser(state, VRFCondenser);
+        // add back piping correction to coil side
+        state.dataDXCoils->DXCoil(state.dataHVACVarRefFlow->VRFTU(VRFTUNum).HeatCoilIndex).TotalHeatingEnergyRate *=
+            state.dataHVACVarRefFlow->VRF(VRFCondenser).PipingCorrectionHeating;
     }
 }
 
@@ -11705,15 +11708,19 @@ void VRFCondenserEquipment::CalcVRFCondenser_FluidTCtrl(EnergyPlusData &state, c
         this->VRFCondCyclingRatio = CyclingRatio;
 
         Tsuction = this->EvaporatingTemp; // Outdoor unit evaporating temperature
-        this->HeatingCapacity =
-            this->CoffEvapCap * this->RatedEvapCapacity * CurveValue(state, this->OUCoolingCAPFT(NumOfCompSpdInput), Tdischarge, Tsuction) +
-            this->RatedCompPower * CurveValue(state,
-                                              this->OUCoolingPWRFT(NumOfCompSpdInput),
-                                              Tdischarge,
-                                              Tsuction); // Include the piping loss, at the highest compressor speed
+        if (FirstHVACIteration) {
+            this->HeatingCapacity =
+                this->CoffEvapCap * this->RatedEvapCapacity * CurveValue(state, this->OUCoolingCAPFT(NumOfCompSpdInput), Tdischarge, Tsuction) +
+                this->RatedCompPower * CurveValue(state,
+                                                  this->OUCoolingPWRFT(NumOfCompSpdInput),
+                                                  Tdischarge,
+                                                  Tsuction); // Include the piping loss, at the highest compressor speed
+        }
         this->PipingCorrectionHeating = TU_HeatingLoad / (TU_HeatingLoad + Pipe_Q_h);
-        state.dataHVACVarRefFlow->MaxHeatingCapacity(VRFCond) =
-            this->HeatingCapacity; // for report, maximum condensing capacity the system can provide
+        if (state.dataHVACVarRefFlow->MaxHeatingCapacity(VRFCond) == Constant::MaxCap) {
+            state.dataHVACVarRefFlow->MaxHeatingCapacity(VRFCond) =
+                this->HeatingCapacity; // for report, maximum condensing capacity the system can provide
+        }
 
         this->CoolingCapacity = 0.0; // Include the piping loss
         this->PipingCorrectionCooling = 1.0;
@@ -12064,10 +12071,10 @@ void VRFCondenserEquipment::CalcVRFCondenser_FluidTCtrl(EnergyPlusData &state, c
 
         // From the VRF_FluidTCtrl model
         TotalCondHeatingCapacity = this->HeatingCapacity;
-        TotalTUHeatingCapacity = TotalCondHeatingCapacity * this->PipingCorrectionHeating;
+        TotalTUHeatingCapacity = TotalCondHeatingCapacity;
 
         if (TotalCondHeatingCapacity > 0.0) {
-            HeatingPLR = min(1.0, (this->TUHeatingLoad / this->PipingCorrectionHeating) / TotalCondHeatingCapacity);
+            HeatingPLR = min(1.0, (this->TUHeatingLoad) / TotalCondHeatingCapacity);
             HeatingPLR += (LoadDueToDefrost * HeatingPLR) / TotalCondHeatingCapacity;
         } else {
             HeatingPLR = 0.0;