You do not mention your boiler make & model
Boilers heat the water and a valve directs the heated water to the radiators or to the taps - storage tank on some boilers and directly to the taps on a combi boiler.
If the diverter valve is stuck in one position this may be the cause - a heating engineer will fix it for you - as soon as you can find one.
In the mean time, if you have a storage tank for hot water it may well have an immersion heater - a luxury that combination boilers lack.
Good luck...
Peccavi
January 2010
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