Chamber Fan mode 3 (#21594)

Marlin/Configuration_adv.h

 
   //#define CHAMBER_FAN               // Enable a fan on the chamber
   #if ENABLED(CHAMBER_FAN)
-    #define CHAMBER_FAN_MODE 2        // Fan control mode: 0=Static; 1=Linear increase when temp is higher than target; 2=V-shaped curve.
+    #define CHAMBER_FAN_MODE 2        // Fan control mode: 0=Static; 1=Linear increase when temp is higher than target; 2=V-shaped curve; 3=similar to 1 but fan is always on.
     #if CHAMBER_FAN_MODE == 0
       #define CHAMBER_FAN_BASE  255   // Chamber fan PWM (0-255)
     #elif CHAMBER_FAN_MODE == 1
     #elif CHAMBER_FAN_MODE == 2
       #define CHAMBER_FAN_BASE  128   // Minimum chamber fan PWM (0-255)
       #define CHAMBER_FAN_FACTOR 25   // PWM increase per °C difference from target
+    #elif CHAMBER_FAN_MODE == 3
+      #define CHAMBER_FAN_BASE  128   // Base chamber fan PWM (0-255)
+      #define CHAMBER_FAN_FACTOR 25   // PWM increase per °C above target
     #endif
   #endif
 

Marlin/src/module/temperature.cpp

             fan_chamber_pwm = (CHAMBER_FAN_BASE) + (CHAMBER_FAN_FACTOR) * ABS(temp_chamber.celsius - temp_chamber.target);
             if (temp_chamber.soft_pwm_amount)
               fan_chamber_pwm += (CHAMBER_FAN_FACTOR) * 2;
+          #elif CHAMBER_FAN_MODE == 3
+            fan_chamber_pwm = CHAMBER_FAN_BASE + _MAX((CHAMBER_FAN_FACTOR) * (temp_chamber.celsius - temp_chamber.target), 0);
           #endif
           NOMORE(fan_chamber_pwm, 225);
           set_fan_speed(2, fan_chamber_pwm); // TODO: instead of fan 2, set to chamber fan