Fix #11039: supp heat blocked by negative QToHeatSetPt

src/EnergyPlus/UnitarySystem.cc

@@ -8195,8 +8195,20 @@ namespace UnitarySystems {
 
         if (this->m_SuppCoilExists &&
             (state.dataUnitarySystems->HeatingLoad || state.dataUnitarySystems->CoolingLoad || state.dataUnitarySystems->MoistureLoad < 0.0)) {
-            if ((FullSensibleOutput < (state.dataUnitarySystems->QToHeatSetPt - HVAC::SmallLoad)) && !FirstHVACIteration) {
-                SupHeaterLoad = max(0.0, state.dataUnitarySystems->QToHeatSetPt - FullSensibleOutput);
+            Real64 suppLoadTarget = state.dataUnitarySystems->QToHeatSetPt;
+            if (state.dataUnitarySystems->HeatingLoad) {
+                if (ZoneLoad > suppLoadTarget) {
+                    suppLoadTarget = ZoneLoad;
+                }
+                // QToHeatSetPt can be negative when the predictor overestimates
+                // zone temp. Floor at 0 so supp fires when primary can't
+                // achieve neutral output. (#11039)
+                if (suppLoadTarget < 0.0) {
+                    suppLoadTarget = 0.0;
+                }
+            }
+            if ((FullSensibleOutput < (suppLoadTarget - HVAC::SmallLoad)) && !FirstHVACIteration) {
+                SupHeaterLoad = max(0.0, suppLoadTarget - FullSensibleOutput);
                 this->m_SupHeaterLoad = 0.0;
                 // what does this line even do? I know we want the supplemental heater on only if there is a dehum load,
                 // but for HP's the supp heater should also run if the heating coil can't turn on