Index: firmware/App/Controllers/DryBiCart.c
===================================================================
diff -u -ra631e128cd6be44aa232e23b58a229bfb8fe9043 -r43ebe58702128e865210533f9e6deaf13f99d262
--- firmware/App/Controllers/DryBiCart.c	(.../DryBiCart.c)	(revision a631e128cd6be44aa232e23b58a229bfb8fe9043)
+++ firmware/App/Controllers/DryBiCart.c	(.../DryBiCart.c)	(revision 43ebe58702128e865210533f9e6deaf13f99d262)
@@ -160,6 +160,8 @@
 static BICARB_CHAMBER_FILL_EXEC_STATE_T handleBicarbChamberPressureCheckState( void );
 static BICARB_CHAMBER_FILL_EXEC_STATE_T handleBicarbChamberSupplyVentStartState(void);
 static BICARB_CHAMBER_FILL_EXEC_STATE_T handleBicarbChamberSupplyVentEndState(void);
+static BOOL isDryBicartBalChamberFillWindowOpen( void );
+static void closeD65IfBalChamberSwitchImminent( void );
 // Drain
 static DRY_BICART_DRAIN_EXEC_STATE_T handleDryBicartDrainStartState( void );
 static DRY_BICART_DRAIN_EXEC_STATE_T handleDryBicartFluidDrainState( void );
@@ -540,6 +542,44 @@
 
 /*********************************************************************//**
  * @brief
+ * The isDryBicartBalChamberFillWindowOpen function checks whether D65 may be
+ * opened for dry bicarb refill: fresh-side BC valves closed and BC switch not
+ * within the 50 ms valve-close window.
+ * @details \b Inputs: getBalChamberFreshSideValvesClosedStatus, isBalChamberSwitchImminent
+ * @details \b Outputs: none
+ * @return TRUE when the BC fresh-side fill window is open for D65.
+ *************************************************************************/
+static BOOL isDryBicartBalChamberFillWindowOpen( void )
+{
+    BOOL result = FALSE;
+
+    if ( ( TRUE == getBalChamberFreshSideValvesClosedStatus() ) &&
+         ( FALSE == isBalChamberSwitchImminent() ) )
+    {
+        result = TRUE;
+    }
+
+    return result;
+}
+
+/*********************************************************************//**
+ * @brief
+ * The closeD65IfBalChamberSwitchImminent function closes D65 when the balancing
+ * chamber is within 50 ms of switching.
+ * @details \b Inputs: isBalChamberSwitchImminent
+ * @details \b Outputs: valve states
+ * @return none
+ *************************************************************************/
+static void closeD65IfBalChamberSwitchImminent( void )
+{
+    if ( TRUE == isBalChamberSwitchImminent() )
+    {
+        setValveState( D65_VALV, VALVE_STATE_CLOSED );
+    }
+}
+
+/*********************************************************************//**
+ * @brief
  * The setBicartFillRequested function sets the bicart fill request
  * flag value to be True.
  * @details \b Inputs: bicarbChamberFillRequested, dryBiCartDrainRequested,
@@ -1060,20 +1100,25 @@
  *************************************************************************/
 static BICARB_CHAMBER_FILL_EXEC_STATE_T handleBicarbChamberCartridgeFillWaterStartState( void )
 {
-    BICARB_CHAMBER_FILL_EXEC_STATE_T state  = BICARB_CARTRIDGE_FILL_WATER_END_STATE;
+    BICARB_CHAMBER_FILL_EXEC_STATE_T state  = BICARB_CARTRIDGE_FILL_WATER_START_STATE;
 
-    // Close vent valves and descaling valve
-    setValveState( D80_VALV, VALVE_STATE_CLOSED );
-    setValveState( D81_VALV, VALVE_STATE_CLOSED );
-    setValveState( D85_VALV, VALVE_STATE_CLOSED );
+    if ( TRUE == isDryBicartBalChamberFillWindowOpen() )
+    {
+        // Close vent valves and descaling valve
+        setValveState( D80_VALV, VALVE_STATE_CLOSED );
+        setValveState( D81_VALV, VALVE_STATE_CLOSED );
+        setValveState( D85_VALV, VALVE_STATE_CLOSED );
 
-    // open inlet water to bicart
-    setValveState( D65_VALV, VALVE_STATE_OPEN );
-    setValveState( D64_VALV, VALVE_STATE_OPEN );
+        // open inlet water to bicart only when fresh-side BC valves are closed
+        setValveState( D65_VALV, VALVE_STATE_OPEN );
+        setValveState( D64_VALV, VALVE_STATE_OPEN );
 
-    dryBiCarbSupplyStartTime = getMSTimerCount();
-    dryBiCartPersistenceStartTime = 0;
+        dryBiCarbSupplyStartTime = getMSTimerCount();
+        dryBiCartPersistenceStartTime = 0;
 
+        state = BICARB_CARTRIDGE_FILL_WATER_END_STATE;
+    }
+
     return state;
 }
 
@@ -1090,6 +1135,20 @@
     F32 d66Pressure                         = getFilteredPressure( D66_PRES );
     F32 fillCompletePressure                = GET_FILL_COMPLETE_PRESSURE_PSI;
 
+    closeD65IfBalChamberSwitchImminent();
+
+    if ( ( FALSE == isDryBicartBalChamberFillWindowOpen() ) &&
+         ( d66Pressure < fillCompletePressure ) )
+    {
+        setValveState( D65_VALV, VALVE_STATE_CLOSED );
+    }
+    else if ( ( TRUE == isDryBicartBalChamberFillWindowOpen() ) &&
+              ( d66Pressure < fillCompletePressure ) )
+    {
+        setValveState( D65_VALV, VALVE_STATE_OPEN );
+        setValveState( D64_VALV, VALVE_STATE_OPEN );
+    }
+
     // check D66 pressure greater than or equal to  17 PSI
     if ( d66Pressure >= fillCompletePressure )
     {
@@ -1157,6 +1216,8 @@
     F32 d66Pressure                         = getFilteredPressure( D66_PRES );
     LVL_STATE_T bicarbChamberLevel          = getBicarbChamberLevelStatus();
 
+    closeD65IfBalChamberSwitchImminent();
+
     // Once upper level reached high , close the valve, timeout is for safety in case level sensor didn't work(10 sec), or else its a time based filling (3 sec)
     if ( ( LVL_STATE_HIGH == bicarbChamberLevel ) ||
          ( TRUE == didTimeout( dryBiCarbSupplyStartTime, DRY_BICART_SUPPLY_VALVE_D80_OPEN_TIME_MS ) ) )
@@ -1192,7 +1253,8 @@
             drybicartPersistenceOnLowercartPressureStartTime = getMSTimerCount();
         }
         // 200 ms persistence on D66 pressure
-        if ( TRUE == didTimeout( drybicartPersistenceOnLowercartPressureStartTime, DRY_BICART_PERSISTENCE_DURATION_MS ) )
+        if ( ( TRUE == didTimeout( drybicartPersistenceOnLowercartPressureStartTime, DRY_BICART_PERSISTENCE_DURATION_MS ) ) &&
+             ( TRUE == isDryBicartBalChamberFillWindowOpen() ) )
         {
             setValveState( D65_VALV, VALVE_STATE_OPEN );
             drybicartPersistenceOnLowercartPressureStartTime = 0;
@@ -1608,7 +1670,7 @@
  * @details \b Outputs: dryBiCartDataPublishInterval
  * @param Override message from Dialin which includes the interval
  * (in ms) to override the DD dry bicart data publish interval to.
- * @return TRUE if override successful, FALSE if not
+ * @return TRUE if override successful, FALSE if not.
  *************************************************************************/
 BOOL testDryBiCartDataPublishIntervalOverride( MESSAGE_T *message )
 {