Cool down times (Heating and ventilation)

Coolant temperature control When the ECU first enters the active mode, it initiates a start to full load combustion. Full load combustion continues until the heat exchanger casing temperature reaches 60 °C (140 °F), when the ECU decreases the speed of the FBH fuel pump and the combustion air fan to half speed, to produce part load combustion. The ECU maintains part load combustion while the heat exchanger casing temperature remains between 54 and 65 °C (129 and 149 °F). If the heat exchanger casing temperature decreases to 54 °C (129 °F), the ECU switches the system to full load combustion again. If the heat exchanger casing temperature increases to 65 °C (149 operation.

°F), the ECU enters a control idle phase of

On entering the control idle phase, the ECU immediately switches the FBH fuel pump off, to stop combustion, and starts a timer for the combustion air fan. After a 2 minute cooldown period, the ECU switches the combustion air fan off and then remains in the control idle phase while the heat exchanger casing temperature remains above 59 °C (138 °F). If the heat exchanger casing temperature decreases to 59 °C (138 °F), within 15 minutes of the ECU entering the control idle phase, the ECU initiates a start to part load combustion. If more than 15 minutes elapse before the heat exchanger casing temperature decreases to 59 °C (138 °F), the ECU initiates a start to full load combustion.

In order to limit the build-up of carbon deposits on the glow plug/flame sensor, the ECU also enters the control idle phase if the continuous part and/or full load combustion time exceeds 72 minutes. After the cooldown period, if the heat exchanger casing is still in the temperature range that requires additional heat, the ECU initiates an immediate restart to part or full load combustion, as appropriate.

Shutdown The FBH system is de-activated when the alternator power supply to the FBH unit is disconnected, either by the engine stopping or, if the ambient temperature increases to 5 °C (41 °F) or above, by the contacts in the air temperature sensor opening. If the system is active when the alternator power supply is disconnected, the ECU deenergises the FBH fuel pump to stop combustion, but continues operation of the combustion air fan and the circulation pump to cool down the FBH unit. The cool down time depends on the combustion load at the time the alternator power input is disconnected.

Cool down times

Diagnostics The ECU in the FBH unit monitors the system for faults. Any faults detected are stored in a volatile memory in the the ECU, which can be interrogated by Testbook. A maximum of three faults and associated freeze frame data can be stored at any one time. If a further fault is detected, the oldest fault is overwritten by the new fault.

The ECU also incorporates an error lockout mode of operation that inhibits system operation to prevent serious faults from causing further damage to the system. In the error lockout mode, the ECU immediately stops the FBH fuel pump, and stops the combustion air fan and circulation pump after a cool down time of approximately 2 minutes. Error lockout occurs for start sequence failures and/or combustion flameouts, heat exchanger casing overheat and out of limit input voltage. The error lockout mode can be cleared using Testbook, or by disconnecting the battery power supply for a minimum of 10 seconds.

Start failure/flameout. If a start sequence fails to establish combustion, or a flameout occurs after combustion is established, the ECU immediately initiates another start sequence. The start failure or flameout is also recorded by an event timer in the ECU. The event timer is increased by one after each start failure or flameout, and decreased by one if a subsequent start is successful. If the event timer increases to three (over any number of drive cycles), the ECU enters the error lockout mode.

Heat exchanger casing overheat. To protect the system from excessive temperatures, the ECU enters the error lockout mode if the heat exchanger casing temperature exceeds 105 °C (221 °F).

Out of limit voltage. The ECU enters the error lockout mode if the battery or alternator power input is less than 10.5 ± 0.3 V for more than 20 seconds, or more than 15.5 ± 0.5 V for more than 6 seconds.