Index: firmware/App/Modes/ModeFill.c
===================================================================
diff -u -r69cab05a9f77dc834cb5f2c93feb7f6db8bd5307 -r037f0edb0b880130563058c809ba50308f2a63e9
--- firmware/App/Modes/ModeFill.c	(.../ModeFill.c)	(revision 69cab05a9f77dc834cb5f2c93feb7f6db8bd5307)
+++ firmware/App/Modes/ModeFill.c	(.../ModeFill.c)	(revision 037f0edb0b880130563058c809ba50308f2a63e9)
@@ -132,8 +132,11 @@
     turnOnUVReactor( INLET_UV_REACTOR );
     turnOnUVReactor( OUTLET_UV_REACTOR );
 
-    startPrimaryHeater();
+    // NOTE: The target flow rate should be set prior to setting the start primary heater
+    // because the initial guess in the heaters driver needs the target flow to calculate
+    // the new PWMs for the main and small primary heaters
     setROPumpTargetFlowRate( TARGET_RO_FLOW_RATE_L, TARGET_RO_PRESSURE_PSI );
+    startPrimaryHeater();
 }
 
 /*********************************************************************//**