Heat source control in a zoned heating system

A method of controlling the output of a heat source 160 to multiple zones 110, 120, 130, in which a signal of heating demand 114, 124, 134 for each zone with a heating apparatus 112, 122, 132 is received and an overall heating demand is calculated using an averaging function of the multiple signals. A duty cycle is then set based on this demand such that the output of the heat source reflects net heating demand across all zones. The averaging function may involve taking a mean average value of all demand signals and may further involve applying a scaling factor which increases the calculated overall demand. Alternatively, multiple different scaling factors may be applied to each individual demand and the scaling factors may be pre-determined. Pre-determination of the scaling factors may involve determining the heating demand for each zone and setting a scaling factor based on the magnitude of heating requirement. The heating demand signal may be a value representing the degree to which a valve connected to the heat source is open. The heat source may be a boiler. Apparatus and software to conduct the method is also claimed.