Index: firmware/App/Modes/ModeGenIdle.c
===================================================================
diff -u -r6f434374a54fb71ab32421aad2a17e9a848e0d8c -r6952b2201dff87a6a20282c759c890580e3fd1db
--- firmware/App/Modes/ModeGenIdle.c	(.../ModeGenIdle.c)	(revision 6f434374a54fb71ab32421aad2a17e9a848e0d8c)
+++ firmware/App/Modes/ModeGenIdle.c	(.../ModeGenIdle.c)	(revision 6952b2201dff87a6a20282c759c890580e3fd1db)
@@ -68,6 +68,7 @@
                                                          BAD_FILL_SUBSTATES_PUB_INTERVAL,
                                                          0, 0 };                               ///< Interval (in ms) at which to publish bad fill sub-states to CAN bus.
 static F32 initialReservoirWeight;                                                             ///< Initial weight of inactive reservoir in Gen Idle mode.
+static DG_RESERVOIR_ID_T inactiveReservoir;                                                    ///< Inactive reservoir
 
 // ********** private function prototypes **********
 
@@ -142,7 +143,8 @@
 
     setCPLDCleanLEDColor( CPLD_CLEAN_LED_OFF );
 
-    initialReservoirWeight = getReservoirWeight( getInactiveReservoir() );
+    inactiveReservoir = getInactiveReservoir();
+    initialReservoirWeight = getReservoirWeight( inactiveReservoir );
 
     return genIdleState;
 }
@@ -316,6 +318,13 @@
         startFillCmd( getTargetFillVolumeML(), getTargetFillFlowRateLPM() );
     }
 
+    if ( getInactiveReservoir() != inactiveReservoir )
+    {
+        // Inactive Reservoir changed, update the start value
+        inactiveReservoir = getInactiveReservoir();
+        initialReservoirWeight = getReservoirWeight( inactiveReservoir );
+    }
+
     // Check for unwanted filling
     if ( reservoirWeight > ( initialReservoirWeight +  MAX_IDLE_RSVR_WEIGHT_GAIN_ML ) )
     {