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
