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My radiators always heat up when my boiler is on hot water only - can anyone tell me why this could be. the thermostat seems to have no affect but i have a hot water only switch on boiler and this doesn't stop the radiators heating either so i can't see how it can be the thermostat. I have recently replaced the pump and the 3 way valve in the airing cupboard but this has not cured it - any ideas anyone? :? :?

Separate the electrical actuator from the operating shaft of the diverter valve, and operate the room temperature thermostat near the programmer, and watch the actuator, to check it motors OK;- also check the cylinder thermostat also makes it motor in the proper manner. Manually operate the diverter's shaft into the hot water only position--- radiators should get cold. If manual operation of the diverter is the only way you got cold radiators, perhaps you did not realise the actuator side-lever has an "auto" setting. Other possibilities include wrong wiring of the new diverter electrical lead into the nearby junction-box, new room-temperature thermostat needed (I've had to replace mine). I have also heard it's inadvisable to replace components with a different brand (suggests different wiring requirements).

"cez raf" had same hot rads problem after having his system upgraded. Plumbers ignored his suspicion re. the diverter valve, but ultimately this was found to be the problem. They had quoted £350 to fix it, but wasted a lot of their own time, so they will be out of pocket if "cez raf" sticks to his guns! They messed it up, so it should cost nowt. (I presume he recalled the contractor who did his upgrade work;-- might be a good idea for you to have your contractor called back-- good plumbers advertise their work qualily referring to membership of trade associations;- twist their arms!)