Could be a very long answer, it depends on your type of central heating and the configuration, if it is a pumped system that has motorised valves then they could be letting water by, at the wrong times by sticking, it could be natural gravity letting a certain amount of hot water by, if it is a combi system the changover valve is probably sticking, try looking at the following website for more info http://www.plumbingpages.com/featurepages/HWhome.cfm
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