change FanSpdRatio calculation to use enthalpy difference

coil heating rate calculation is like the following
AirMassFlow * (OutletAirEnthalpy - InletAirEnthalpy) * PartLoadRatio

In the computation of fan speed ratio, the coil heating capacity is
FanSpdRto * Garate * 1005.0 * (Tout - Tin)

The difference between
1005.0 * (Tout - Tin) and (OutletAirEnthalpy - InletAirEnthalpy)
can cause mismatches between coil heating rate and heating demand

Here we change the fan speed ratio caculation to also use the enthalpy
difference way to compute its heating capacity to match the demand

src/EnergyPlus/DXCoils.cc

@@ -17481,8 +17481,10 @@ 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
@@ -17767,6 +17769,27 @@ 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

@@ -932,6 +932,8 @@ 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