Description (page 5 of 6)

AIR CONDITIONING

82-30

DESCRIPTION AND OPERATION

The FBH unit consists of:

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A circulation pump

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A combustion air fan

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A burner housing

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An ECU/heat exchanger

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An air inlet hose

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An exhaust pipe

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An air inlet filter.

Circulation Pump: The circulation pump is installed at the coolant inlet to the FBH unit to assist the coolant flow 
through the FBH unit and the heater assembly. The pump runs continuously while the FBH unit is in standby or active 
operating modes. While the FBH unit is inactive, coolant flow is reliant on the engine coolant pump and the auxiliary 
coolant pump.

Combustion Air Fan: The combustion air fan regulates the flow of air into the unit to support combustion of the fuel 
supplied by the FBH pump and to purge and cool the FBH unit. A canister type filter is included in the air inlet supply 
line to prevent particulates entering and contaminating the FBH unit.

Burner Housing: The burner housing contains the burner insert and also incorporates connections for the exhaust 
pipe, the coolant inlet from the circulation pump and the coolant outlet to the heater assembly. The exhaust pipe 
directs exhaust combustion gases to atmosphere through a pipe below the FBH unit.

The burner insert incorporates the fuel combustion chamber, an evaporator and a glow plug/flame sensor. Fuel from 
the FBH fuel pump is supplied to the evaporator, where it evaporates and enters the combustion chamber to mix with 
air from the combustion air fan. The glow plug/flame sensor provides the ignition source of the fuel:air mixture and, 
once combustion is established, monitors the flame.

ECU/Heat Exchanger: The ECU controls and monitors operation of the FBH system. Ventilation of the ECU is 
provided by an internal flow of air from the combustion air fan. The heat exchanger transfers heat generated by 
combustion to the coolant. A sensor in the heat exchanger provides the ECU with an input of heat exchanger casing 
temperature, which the ECU relates to coolant temperature and uses to control system operation. The temperature 
settings in the ECU are calibrated to compensate for the difference between coolant temperature and the heat 
exchanger casing temperature detected by the sensor. Typically, as the coolant temperature increases, the coolant 
will be approximately 7 

°

C (12.6 

°

F) hotter than the temperature detected by the sensor; as the coolant temperature 

decreases, the coolant will be approximately 2 

°

C (3.6 

°

F) cooler than the temperature detected by the sensor.

Changeover Valve
The changeover valve is a normally open solenoid valve installed between the supply and return sides of the heater 
coolant circuit. The changeover valve is located in the engine compartment on the engine bulkhead. When de-
energised, the changeover valve connects the heater coolant circuit to the engine coolant circuit. When energised, 
the changeover valve isolates the heater coolant circuit from the engine coolant circuit.

The changeover valve is controlled by a power feed from the ATC ECU.

FBH Receiver
The FBH receiver translates the FBH request radio signals, relayed from the antenna receiver, into a voltage output 
to the FBH unit. When a request for parked heating is received, the FBH receiver outputs a battery power feed to the 
FBH unit. When a request to switch off parked heating is received, the FBH receiver disconnects the power feed.

The FBH receiver has a permanent power feed from the vehicle battery and is connected to the antenna receiver by 
a coaxial cable.