Heating Mode

Heating mode, Function

Shown On Models 215, 220:

Operating procedure
1 The start of the heater operation is completed:
- The green or red indicator lamp in the STH switch (N72s12 or S46) is illuminated.
- The coolant circulation pump (A6m1) delivers the coolant via the heat exchanger (2) and the heating and cooling circuits. Combustion is steady, the coolant is heated up. The cooling blower delivers, actuated by the control and operating module (N18, N22 or N22/7), preheated air into the interior (coolant temperature > 30 degrees C).

2 Coolant temperature < 72 degrees C, a high heat requirement is given:
- The STH heater unit or heater booster continue to operate in full load mode, the combustion air blower (A6m2) and the STH fuel metering pump or heater booster (Y23) deliver at their maximum rotational speed and output.

3 Coolant temperature reached in full load mode 72 degrees C. Heating requirement < than instantaneous heat output:
The heat output can be reduced:
- The STH or heater booster control module (A6n1) initiates a load change from full load to partial load operation.
- The STH fuel metering pump or heater booster (Y23) and the combustion air blower (A6m2) are switched within 50 s continuously to partial load operation and then deliver at their lowest output and rotational speed.

The coolant temperature remains constant or drops depending upon the heat requirements.

4 Coolant temperature drops to partial load mode under 65 degrees C.
Heat requirement > than the momentary available heating output:
- Heat output is too low, it has to be raised again.
- The STH or heater booster control module (A6n1) initiates a load change from partial load to full load operation.
- The STH fuel metering pump or heater booster (Y23) and the combustion air blower (A6m2) are switched within 50 s continuously to full load operation and then deliver at their maximum output and rotational speed.

The coolant temperature increases again.

NOTE: The change between full load and partial load mode continues throughout the entire heating period unless the heat requirement changes.

5 Coolant temperature reached in partial load mode 77 degrees C. No heat requirements:
- Because the instantaneously available heating output of the STH heater or heater booster currently operating in partial load mode is no longer required, the STH or heater booster control module (A6n1) initiates the heater afterrun process which serves to cool down the STH heater or heater booster and purge the combustion chamber for approx. 160 s:
- The STH fuel metering pump or heater booster (Y23) is switched off.
- The heater afterrun is terminated:
The combustion air blower (A6m2) is switched off.
- The STH heater unit or heater booster now enters a combustion pause:
- The circulation pump (A6m1) and vehicle blower continue to operate, the coolant temperature drops.

NOTE: Heater operation is continued when the coolant temperature drops below 72 degrees C during the combustion pause.