HI, i'm guessing here but I suspect you have an 'S' plan system with two or more zone valves? one of the valves will be faulty. If you are good with electrics check for 240v on the orange wires from the valves to see which one it is. Be carefull as you will have to disconnect and seperate them to check the voltage. If unsure get a qualified engineer to do it!
Paul
September 2006