Heating systems have a diverter valve that directs the heated water to the radiators or the taps (hot water storage tank) according to the demand(s) from the timer programmer.
Sometimes the valve gets stuck and the heated water (or some of it) goes to the wrong place.
Your boiler engineer should be able to solve it relatively quickly - but not necessarily cheaply.
I think Combi boilers have the valve inside the boiler case and other types usually have it close to the hot water tank - not unlike this one...
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