Abstract: In a system for controlling a number of central heating boilers of large capacity having means for switching-off each boiler when the flow temperature reaches a predetermined maximum, the temperature at which each boiler is switched on again can be automatically varied in dependence upon the outside temperature in such a manner that as the outside temperature rises the boiler is switched on at progressively lower boiler flow temperatures, thus increasing the length of the boiler cycles. This is achieved by a boiler cycling controller which receives signals from temperature detecting devices located externally of the building and in the water flow and return lines and is operable automatically to vary the point at which the boiler is switched on.