The answer to this depends on what kind of boiler you have,,, Combination or system (heat only )
Do you have a cylinder for water ??
Ste, March 2011
there is a diverter valve somewhere that changes over depending on demand.The way it is controlled will depend on makes and models which you failed to give any info!
A1, March 2011
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I can answer this question...
central heating comes on then the hot water is turned on?