Index: firmware/App/Controllers/Heaters.c
===================================================================
diff -u -r56ba1b163b0cbf6953638065b2108f745b17ec8f -r885c00adcbedd23bea67e495d6f2ee291f878b37
--- firmware/App/Controllers/Heaters.c	(.../Heaters.c)	(revision 56ba1b163b0cbf6953638065b2108f745b17ec8f)
+++ firmware/App/Controllers/Heaters.c	(.../Heaters.c)	(revision 885c00adcbedd23bea67e495d6f2ee291f878b37)
@@ -779,10 +779,12 @@
 {
     // Duty cycle = ( 69.73 * flow rate * deltaT / primary heater maximum power ) ^ 1/2
     // Multiply the duty cycle to the heater efficiency
-    F32 dutyCycle = sqrt( ( WATER_SPECIFIC_HEAT_DIVIDED_BY_MINUTES * fabs( targetTemperature - currentTemperature ) * flow ) / PRIMARY_HEATERS_MAXIMUM_POWER_WATTS );
-    dutyCycle    *= heatersStatus[ DG_PRIMARY_HEATER ].heaterEstGain;
-    dutyCycle     = MIN( dutyCycle, HEATERS_MAX_DUTY_CYCLE );
-    dutyCycle     = MAX( dutyCycle, HEATERS_MIN_DUTY_CYCLE );
+    F32 deltaTempC    = targetTemperature - currentTemperature;
+    F32 capDeltaTempC = MAX( deltaTempC, 0.0F );
+    F32 dutyCycle     = sqrt( ( WATER_SPECIFIC_HEAT_DIVIDED_BY_MINUTES * capDeltaTempC * flow ) / PRIMARY_HEATERS_MAXIMUM_POWER_WATTS );
+    dutyCycle        *= heatersStatus[ DG_PRIMARY_HEATER ].heaterEstGain;
+    dutyCycle         = MIN( dutyCycle, HEATERS_MAX_DUTY_CYCLE );
+    dutyCycle         = MAX( dutyCycle, HEATERS_MIN_DUTY_CYCLE );
 
     return dutyCycle;
 }