Thermo 90 S / Thermo 90 ST 2 General description
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2.3 Temperature sensor
The temperature sensor records the coolant temperature
at the coolant outlet of the heater as an electrical
resistance. This signal is fed to the control unit where it is
processed.
2.4 Temperature limiter
The overheating guard circuit (bimetallic) protects the
heater against impermissibly high operating
temperatures. The overheating guard circuit trips if the
heat exchanger temperature exceeds 127 ±7 °C and
switches off the heater.
2.5 Burner insert
The fuel is distributed over the burner cross-section in the
combustion pipe in the burner insert.
2.6 Glow plug
The mixture of fuel and air is ignited by the glow plug when
the heater is started. The glow plug takes the form of an
electrical resistor and is positioned in the burner insert on
the side away from the flame.
2.7 Flame monitor
The flame monitor is a low-ohm PTC resistor, which
changes its resistance depending on the heat emitted by
the flame. The signals are fed to the control unit where
they are processed. The flame status is monitored by the
flame monitor whenever the heater is operating.
2.8 Combustion pipe
The mixture of fuel and air burns in the combustion pipe,
thus causing the heat exchanger to become hot.
Combustion pipe
2.9 Circulating pump
The circulating pump ensures that coolant is pumped
through the vehicle and heater circuits. The pump is
switched on by the control unit and it runs throughout
operation of the heater (including in the control pause).
Circulating pump
2.10 Control unit
The control unit is the central component. It guarantees
the sequence of functions and monitoring of combustion
mode. It is flange-mounted on the combustion air fan,
although in the case of the Thermo 90 S it may be
installed externally in the vehicle.
2.11 Metering pump
The fuel supply and metering for the heater is assured by
an external metering pump. It addition, it serves as a shut-
off element when the heater is switched off.
The solenoid coil of the metering pump receives its pulses
from the microprocessor in the control unit.
Burner insert
Glow plug
Flame monitor
