is pre glowing accordingly the fuel supplied to the combustion pad 2 can reliably be vaporized by the heated combustion pad 2 during an initial stage of fuel ignition the amount of air supplied to the combustion chamber 1 is increased but not up to the full rate therefore the fuel vapor starts being stably combusted as the flames produced by the initial ignition of the fuel vapor are not disturbed by the air flow introduced into the combustion chamber 1 such a controlled air flow is also effective to suppress undesirable unburned fuel emission from the combustion heater the control unit 10 operates to stop activating the combustion heater as follows when the main switch of the air conditioning system is turned off the control unit 10 de energizes the solenoid 5a to close the solenoid operated valve 5 as shown in fig3 b the fuel supply passage 4 is closed to cut off the supply of the fuel from the fuel tank 3 to the combustion pad 2 at the same time while checking the detected signals from the flame sensors 9a 9b 9c the control unit 10 successively reduces the level of the signal s3 to rotate the fan 7 at.