Abstract: A control apparatus for a fuel fired central heating boiler has a first temperature control circuit including a first thermistor for sensing external ambient temperature and a second thermistor for sensing the temperature of the circulating water produced by the boiler. A second temperature control circuit has a thermistor for sensing the temperature in the space to be heated. A relay is operated by the outputs of the first and second control circuits and itself controls the boiler. The first and second thermistors are connected in series and have different resistance/temperature characteristics so that a given change in the external ambient temperature corresponds to a greater change in the temperature of the circulating water.