Combustion control system

A combustion control system comprising:

• a combustion unit for a fuel;
• a porportional control valve for proportionally controlling the supply of the fuel to the combustion unit;
• a temperature sensor for detecting the temperature of a fluid heated with the heat of combustion produced by the combustion unit;
• and
• a control circuit comprising a bridge circuit including the temperature sensor as a component thereof, a proportional integrating differentiating circuit comprising a proportional differentiating circuit including an operation amplifier and a proportional integrating circuit including an operation amplifier and an integration capacitor, the proportional integrating differentiating circuit being adapted to receive an output from the bridge circuit representative of a deviation of the fluid temperature from a first temperature setting and to give a drive output to the proportional control valve, and a discharging circuit including a comparator for comparing an output from the differentiating circuit with a second temperature setting and adapted to cause discharge of a charge from the integration capacitor when the comparison in the comparator indicates that the fluid temperature exceeds a specified level.