diff --git a/src/EnergyPlus/DualDuct.cc b/src/EnergyPlus/DualDuct.cc
index 9d1fc9ba77b..7974886b75b 100644
--- a/src/EnergyPlus/DualDuct.cc
+++ b/src/EnergyPlus/DualDuct.cc
@@ -1861,13 +1861,21 @@ namespace DualDuct {
 			Node( OutletNode ).Quality = Node( HotInletNode ).Quality;
 			Node( OutletNode ).Press = Node( HotInletNode ).Press;
 
-			if (Contaminant.CO2Simulation) {
-				Node(OutletNode).CO2 = max(Node(HotInletNode).CO2, Node(ColdInletNode).CO2);
+			if ( Contaminant.CO2Simulation ) {
+				if ( Node( OutletNode ).MassFlowRate > 0.0 ) {
+					Node( OutletNode ).CO2 = ( Node( HotInletNode ).CO2 * Node( HotInletNode ).MassFlowRate + Node( ColdInletNode ).CO2 * Node( ColdInletNode ).MassFlowRate) / Node( OutletNode ).MassFlowRate;
+				} else {
+					Node( OutletNode ).CO2 = max( Node( HotInletNode ).CO2, Node( ColdInletNode ).CO2);
+				}
 			}
-			if (Contaminant.GenericContamSimulation) {
-				Node(OutletNode).GenContam = max(Node(HotInletNode).GenContam, Node(ColdInletNode).GenContam);
+			if ( Contaminant.GenericContamSimulation ) {
+				if ( Node( OutletNode ).MassFlowRate > 0.0) {
+					Node( OutletNode ).GenContam = ( Node( HotInletNode ).GenContam * Node( HotInletNode ).MassFlowRate + Node( ColdInletNode ).GenContam * Node( ColdInletNode ).MassFlowRate ) / Node( OutletNode ).MassFlowRate;
+				}
+				else {
+					Node( OutletNode ).GenContam = max( Node( HotInletNode ).GenContam, Node( ColdInletNode ).GenContam );
+				}
 			}
-
 		} else if ( Damper( DamperNum ).DamperType == DualDuct_OutdoorAir ) {
 
 			OutletNode = Damper( DamperNum ).OutletNodeNum;
@@ -1889,26 +1897,35 @@ namespace DualDuct {
 			Node( OutletNode ).Quality = Node( OAInletNode ).Quality;
 			Node( OutletNode ).Press = Node( OAInletNode ).Press;
 
-			if (Damper(DamperNum).RecircIsUsed) {
-				if (Contaminant.CO2Simulation) {
-					Node(OutletNode).CO2 = (Node(OAInletNode).CO2*Node(OAInletNode).MassFlowRate + Node(RAInletNode).CO2*Node(RAInletNode).MassFlowRate) / Node(OutletNode).MassFlowRate;
+			if ( Damper( DamperNum ).RecircIsUsed ) {
+				if ( Node( OutletNode ).MassFlowRate > 0.0 ){
+					if ( Contaminant.CO2Simulation ) {
+						Node( OutletNode ).CO2 = ( Node( OAInletNode ).CO2 * Node( OAInletNode ).MassFlowRate + Node( RAInletNode ).CO2 * Node( RAInletNode ).MassFlowRate ) / Node( OutletNode ).MassFlowRate;
+					}
+					if ( Contaminant.GenericContamSimulation ) {
+						Node( OutletNode ).GenContam = ( Node( OAInletNode ).GenContam * Node( OAInletNode ).MassFlowRate + Node( RAInletNode ).GenContam * Node( RAInletNode ).MassFlowRate ) / Node( OutletNode ).MassFlowRate;
+					}
 				}
-				if (Contaminant.GenericContamSimulation) {
-					Node(OutletNode).GenContam = (Node(OAInletNode).GenContam*Node(OAInletNode).MassFlowRate + Node(RAInletNode).GenContam*Node(RAInletNode).MassFlowRate) / Node(OutletNode).MassFlowRate;
+				else {
+					if ( Contaminant.CO2Simulation ) {
+						Node( OutletNode ).CO2 = max( Node( OAInletNode ).CO2, Node( RAInletNode ).CO2 );
+					}
+					if ( Contaminant.GenericContamSimulation ) {
+						Node( OutletNode ).GenContam = max( Node( OAInletNode ).GenContam, Node( RAInletNode ).GenContam );
+					}
 				}
 
 			} else {
-				if (Contaminant.CO2Simulation) {
-					Node(OutletNode).CO2 = Node(OAInletNode).CO2;
+				if ( Contaminant.CO2Simulation ) {
+					Node( OutletNode ).CO2 = Node( OAInletNode ).CO2;
 				}
-				if (Contaminant.GenericContamSimulation) {
-					Node(OutletNode).GenContam = Node(OAInletNode).GenContam;
+				if ( Contaminant.GenericContamSimulation ) {
+					Node( OutletNode ).GenContam = Node( OAInletNode ).GenContam;
 				}
 			}
 
 		}
 
-
 	}
 
 	//        End of Update subroutines for the Damper Module