PTC Heater System (Where Fitted) (Heating and ventilation)

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.

PTC Heater System (Where Fitted)

The PTC heater system, like the FBH, is an auxiliary heating system that compensates for the relatively low coolant temperatures inherent in the diesel engine. When the heater blower is selected on (any speed) and the temperature control is selected to the 1 o'clock position or above, i.e. warm to hot, the PTC heater automatically comes on to boost the temperature of the air flowing through the heater assembly. The system consists of: l The PTC heater. l A heater switch. l Two relays. l A link harness and fuses.

PTC Heater The PTC heater is installed in the heater assembly on the downstream side of the coolant heater matrix. The PTC heater is an electrical heating element consisting of alternating layers of radiator fins and thermistor elements installed in a frame and secured with spring clips. Plastic end covers on the frame each incorporate two lugs for locating the PTC heater in the heater assembly casing. Silicone caps are installed on the lugs to prevent rattling. Three power supply leads and a common earth lead are connected to the thermistor element layers. A grommet protects the leads where they pass through the heater assembly casing.

The thermistor elements are made of a ceramic based compound which is treated with a semiconductor material to make it conductive. At low temperatures the thermistor elements offer low electrical resistance, but once a predetermined temperature is exceeded their resistance rapidly increases with increasing temperature. This characteristic provides a fast warm-up (less than 10 seconds) of the thermistor elements when they are supplied with electrical power, followed by a temperature regulating effect where the current consumption is matched to the heat output.

When the PTC heater is supplied with electrical power, the thermistor elements heat up to the predetermined temperature. The radiator fins absorb heat from the thermistor elements and transfer it to the air stream.

The PTC heater is rated at 900 W at a nominal operating voltage of 13.5 V, ambient air temperature of 0 °C (32 °F) and air flow of 5 kg/minute (11 lb/minute). The initial current draw is in the region of 55 to 75 A, depending on the ambient air temperature and the temperature of the PTC heater. The theoretical surface temperature of the PTC heater, without any airflow, is 165 °C (329 °F) maximum.