It's hot water going into the radiator isn't it - you wouldn't notice cold water - you are not heating the house but the rads are getting warm... I'll bet...
The boiler has a Diverter Valve to direct the heated water to where it's needed - On a Combi boiler it's inside the boiler case and often in the airing cupboard with a conventional boiler.
If the diverter valve gets stuck the heated water can go to where it's not wanted.
A qualified heating engineer could fix it quite easily.
Good luck...
Peccavi
June 2009
valve on each side of radiator have to be turned off, take plastic cover off other side of rad to turn it off.
Rayb
June 2009
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