Abstract: A safety circuit 31, which compulsorily closes an on-off valve for supplying a gas to a burner when a microcomputer for performing a combustion control is in an abnormal condition, does not operate unless abnormality occurs to the microcomputer 1, and therefore even when the safety circuit 31 is out of order, an ignition operation is performed as it is. A transistor 4 is not turned off until a fixed time elapses from a start of an ignition sequence in the safety circuit 31, but the fixed time is shortened by a timer acceleration circuit 32, and if the transistor 4 is turned off by a transistor 2 being turned on, the safety circuit 31 is determined as normal, so that the normal ignition sequence is executed thereafter.

Abstract: A conventional watchdog timer circuit is arranged to judge a microcomputer to be normal as long as a pulse signal is being outputted from the microcomputer. Therefore, an abnormal condition cannot be detected when an operating frequency of the microcomputer lowers, for example. As a solution, the watchdog timer circuit is provided with a function for detecting the frequency. If the frequency of the pulse signal deviates from a predetermined frequency range, a reset signal is outputted indicating that the microcomputer is abnormal, even if the pulse signal is being outputted. A solenoid-operated gate valve is thereby closed to stop the supply of gas to a gas burner.

Abstract: A conventional watchdog timer circuit is arranged to judge a microcomputer to be normal as long as a pulse signal is being outputted from the microcomputer. Therefore, an abnormal condition cannot be detected when an operating frequency of the microcomputer lowers, for example. As a solution, the watchdog timer circuit is provided with a function for detecting the frequency. If the frequency of the pulse signal deviates from a predetermined frequency range, a reset signal is outputted indicating that the microcomputer is abnormal, even if the pulse signal is being outputted. A solenoid-operated gate valve is thereby closed to stop the supply of gas to a gas burner.

Abstract: When an error occurs to a combustion device, a microcomputer inside a control apparatus for the combustion device closes a safety valve thereby stopping gas supply to a gas burner. In a conventional control apparatus, when a power source resetting is actuated with a power switch, the safety valve is opened to supply the gas burner with gas even though the cause for the error has not been resolved. As a solution, when an error occurs, the fact of such an occurrence is stored in a nonvolatile memory, i.e., electrically erasable programmable read-only memory, EEPROM. It is so arranged that, unless the reset switch is operated, the memory in the EEPROM is not reset. After resetting, the memory in the EEPROM is read out. As long as the reset has not been made, the safety valve is not opened, but the program immediately proceeds to a state in which the operation of the reset switch is held in suspension.