It will probably have a diverter valve - likely in your airing cupboard - the valve sends the water to either the hot water tank or to the radiators and sometimes to both.
My guess is that your valve is stuck in the mid position and not switching to the water only position.
https://www.youtube.com/watch?v=Yyn_fBEH90U
Good Luck...
February 2017
I think it is gravity fed??
Jeff
February 2017
Possibly an issue with the 3-port valve (Assuming the hot water is a pumped system and not a gravity one).
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