STH heater unit component description - GF83.70-P-4202SX

MODEL 221.0 /1, 216.3 with CODE (228) Stationary heater up to Model Year 8 

Shown on model 221.0 

Location 

The heater unit is located in the rear area of the right wheelhouse.

Design 

The heater unit contains the following components required to generate and transfer heat:

• Temperature sensor (A6b1)
• Thermal fuse (A6f1)
• Combustion air blower (A6m2)
• Glow pin unit flame monitor (A6r4)
• Fuel preheating system (A6r5) (diesel engine)
• STH or HB control unit (A6n1) (model 221) or STH control unit (A6n1) (model 216)

Function 

The start, regulation and ending of heater operation is controlled by the integrated STH control unit.

Start heater operation 

The combustion air blower is actuated. It operates approximately at its maximum speed.

The glow pin of the glow pin unit flame monitor is switched on. The preheating time of approx. 30 s starts.

After the preglow time, the combustion air blower is briefly switched off and the STH or AUH fuel metering pump (Y23) on model 221 or STH fuel metering pump (Y23) on model 216 starts the supply of fuel.

The combustion air blower is started up again. The combustion air blower and the fuel metering pump increase their speed and output to full load within the next approx. 60 s. The fuel and the combustion air form the fuel/air mixture in the combustor housing. Once the increase in the flow of combustion air and fuel delivery has been completed, the glow pin of the glow pin unit flame monitor is switched off, and the safety period to check the flame starts for approx. 45 s.

Once the safety period has ended, the glow pin unit flame monitor reports in its function as flame monitor:

• "hot" sufficient combustion heater operation continues.
• "cold" no, or insufficient, flame formation, the fuel metering pump is switched off and a non-recurring second start is initialized. If there is still no flame formation, the fuel supply is terminated by switching off the fuel metering pump and a fault shutoff is performed with combustion air blower run-on.

Heating mode 

Heater operation is controlled automatically and is dependent on the coolant temperature. Possible statuses:

• Full load operation
• Partial load operation
• Control interval

Flame formation is monitored during heater operation by the glow pin unit flame monitor. If there is a flame-out, a non-recurring second start is initialized.

If there is still no flame formation, the fuel supply is terminated by switching off the fuel metering pump and a fault shutoff is performed with combustion air blower run-on.

The STH switchover valve (Y16) is closed at coolant temperatures less than 60 °C (small coolant circuit). As of 60 °C it is actuated by pulses and the large coolant circuit is also heated.

Overheating protection 

The internal temperature sensing of the STH control unit detects overheating and actuates the heater interrupt run-on with subsequent fault interlock for approx. 120 s.

Terminate heater operation 

The combustion is ended by switching the fuel metering pump off. The combustion air blower run-on time starts.

The run-on time and speed of the combustion air blower is dependent on the operating condition in which the heater unit was switched off.