why is my central heating on constantly when timer off?

My radiators come on when the hot water timer comes on although the heating timer is off. All the radiators get very hot and stay hot all the time the water timer is on. I have a new condenser boiler and timer switch (1 year old). It all worked fine until I used the central heating for the first time this year, now when the timers on for water, i cant get the radiators off!!

Donna
October 2007