Revised based on using DesHeatVolFlowMax as limit

Author: rraustad

src/EnergyPlus/SingleDuct.cc

@@ -2877,9 +2877,14 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)
 
             CheckZoneSizing(state, this->sysType, this->SysName);
 
-            Real64 heatingMaxFlow = (this->DamperHeatingAction == Action::ReverseWithLimits)
-                                        ? state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax
-                                        : state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
+            Real64 heatingMaxFlow;
+            if (this->DamperHeatingAction == Action::ReverseWithLimits &&
+                state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow >
+                    state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax) {
+                heatingMaxFlow = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
+            } else {
+                heatingMaxFlow = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
+            }
             MaxAirVolFlowRateDes = max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesCoolVolFlow, heatingMaxFlow);
 
             if (MaxAirVolFlowRateDes < SmallAirVolFlow) {
@@ -2929,7 +2934,9 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)
             }
         } else {
             CheckZoneSizing(state, this->sysType, this->SysName);
-            if (this->DamperHeatingAction == Action::ReverseWithLimits) {
+            if (this->DamperHeatingAction == Action::ReverseWithLimits &&
+                state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow >
+                    state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax) {
                 MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
             } else {
                 MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
@@ -3221,7 +3228,13 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)
         if (state.dataSize->ZoneSizingRunDone) {
             if (state.dataSize->CurTermUnitSizingNum > 0) {
                 // if zone sizing run done, set the design max reheat air flow to the value from the design calcs
-                MaxAirVolFlowRateDuringReheatDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
+                if (state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow >
+                    state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax) {
+                    MaxAirVolFlowRateDuringReheatDes =
+                        state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
+                } else {
+                    MaxAirVolFlowRateDuringReheatDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
+                }
             }
         } else {
             // if no design calc use 0.002032 [m3/s-m2] times floor area. That's .40 cfm/ft2

tst/EnergyPlus/unit/SingleDuct.unit.cc

@@ -2765,11 +2765,27 @@ TEST_F(EnergyPlusFixture, VAVReheatTerminal_SizeMinFrac)
     state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.6;
     thisSys.SizeSys(*state);
     Real64 expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
+    // DesHeatVolFlowMax is limiting flow rate
     Real64 expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax / thisSys.MaxAirVolFlowRate;
     EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
     EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);
     EXPECT_EQ(expectedMaxAirVolFractionDuringReheat, thisSys.MaxAirVolFractionDuringReheat);
 
+    // switch magnitude of DesHeatVolFlow and DesHeatVolFlowMax, still heating dominated
+    thisSys.MaxAirVolFlowRate = DataSizing::AutoSize;
+    thisSys.ZoneMinAirFracDes = DataSizing::AutoSize;
+    thisSys.MaxAirVolFlowRateDuringReheat = DataSizing::AutoSize;
+    thisSys.MaxAirVolFractionDuringReheat = DataSizing::AutoSize;
+    state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow = 1.6;
+    state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.7;
+    thisSys.SizeSys(*state);
+    expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
+    // DesHeatVolFlowMax is NOT limiting flow rate
+    expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow / thisSys.MaxAirVolFlowRate;
+    EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
+    EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);
+    EXPECT_EQ(expectedMaxAirVolFractionDuringReheat, thisSys.MaxAirVolFractionDuringReheat);
+
     // test Maximum Flow Fraction During Reheat for cooling dominated
     thisSys.MaxAirVolFlowRate = DataSizing::AutoSize;
     thisSys.ZoneMinAirFracDes = DataSizing::AutoSize;
@@ -2779,6 +2795,7 @@ TEST_F(EnergyPlusFixture, VAVReheatTerminal_SizeMinFrac)
     state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.3;
     thisSys.SizeSys(*state);
     expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
+    // DesHeatVolFlowMax is limiting flow rate
     expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax / thisSys.MaxAirVolFlowRate;
     EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
     EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);