Heater operation, function

P83.70-3210-09

shown on model 215, 220

1 Burner housing

2 Heat exchanger

A6m1 Circulation pump

A6m2 Combustion air blower

A6n1 STH control unit (except model 220)
STH or HB [ZUH] control unit
(model 220)

N22/7 Comfort AAC [KLA] control and operating unit (model 203 with code (581) Automatic air conditioning or code (581) Comfort automatic air conditioning, model 209 with code (581) Comfort automatic air conditioning)

N72s12 STH switch (model 203, 209, 230)

S46 STH switch (Models 215, 220)

Y23 STH fuel metering pump (except model 220)
STH or heater booster fuel metering pump (model 220)

CAN-B Controller Area Network bus class B (interior compartment) (CAN-B)

N18 HEAT [HAU] control and operating unit
(model 203 without code (580) Air conditioning (or Tempmatic for USA) or without code (580) Automatic air conditioning or without code (581) Automatic air conditioning or without code (581) Comfort automatic air conditioning)

N22 AAC [KLA] control and operating unit
(model 215, 220, 230, model 203 with code (580) Air conditioning (or Tempmatic for USA) or code (580) Automatic air conditioning, model 209 without code (581) Comfort automatic air conditioning)

Function sequence

1 The activated heating mode is indicated by the indicator lamp in the STH switch (N72s12 or S46). The 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 blower motor delivers, actuated by the control and operating unit (N18, N22 or N22/7), preheated air into the interior (coolant temperature > 30 °C).

2 At coolant temperatures below 72 °C there is a high heat requirement. The STH heater unit (except model 220) or the STH or HB [ZUH] heater unit (model 220) operates in full load mode, the combustion air blower (A6m2) and the STH fuel metering pump or the STH or HB [ZUH] fuel metering pump (Y23) operate at their maximum rotational speed and output.

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3 If in full load mode the coolant temperature reaches 72 °C, the heat requirement is less than the currently available heat output. The STH control unit or the STH or HB [ZUH] control unit (A6n1) initiate a load change from full load to partial load mode.
The STH fuel metering pump or the STH or HB [ZUH] fuel metering pump (Y23) and the combustion air blower (A6m2) are switched continuously variably to partial load mode within approx. 50 s and then operate at low output and rotational speed again. The coolant temperature remains constant or sinks, depending on the heat requirement.

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4 If in partial load mode the coolant temperature drops below 65 °C, the heat requirement is greater than the currently available heat output. The heat output must be increased again. The STH control unit or the STH or HB [ZUH] control unit (A6n1) initiates a load change from partial load to full load mode.
The STH fuel metering pump or the STH or HB [ZUH] fuel metering pump (Y23) and the combustion air blower (A6m2) are switched continuously variably to full load mode within approx. 50 s and then operate at their maximum output and rotational speed again. The coolant temperature increases again.

 The change between full load and partial load operation continues for the entire duration of heater operation as long as there are no changes in the heat requirement.

5 If the coolant temperature in partial load mode reaches 77 °C, there is no heat requirement. The STH control unit or the STH or HB [ZUH] control unit (A6n1) triggers the heater run-on period for approx. 160 s for cooling the STH heater unit or the STH or HB [ZUH] heater unit and for purging the combustion chamber. The STH fuel metering pump or the STH or HB [ZUH] fuel metering pump (Y23) is switched off. At the end of the heater run-on period the combustion air blower (A6m2) is switched off. The STH heater unit or the STH or HB [ZUH] heater unit then enters a combustion pause. The circulation pump (A6m1) and the blower motor continue to run and the coolant temperature drops.

 Heater operation is continued when the coolant temperature drops below 72 °C during the combustion pause.