Hot water also turns radiator's on

you have valves outside of boiler usually beside cylinder,you may have a y plan (three way valve) or an s plan two zone valves,one of these valves is your problem,any plumber heating engineer can sort this for you.

Hi Paul, You're right to suspect a problem. The heating rad's should come on controlled by a timer or setting the controls to "ON". Turning on a tap should give hot water and not start the heating. This is all controlled by a simple circuit and one or two valves inside the boiler. A GAS-SAFE (used to be called CORGI) heating / plumbing engineer should be able to repair or replace the valve(s) Hope this helps. Marti

Hi guys you are both right could be the zone valves you dont tell us how old the system is because on some older systems they put on a non return valve on the return from the hot water cylinder to stop the flow of hot water going into the heating system this could be faulty or lack of one hope this helps