Hi, have a Potterton Netaheat boiler with an EP2002 programmer.
Problem I have is that when the Hot Water only is on, powere is being fed to the white wir on the 3-way valve( Horstman-which then puts the valve onto radiators only.
Have had problems with kettling in the pipes before with just hot water on and centra; heating was fine. Have had the immersion on and heating on , but on having a closer inspection and using this site http://octaveblue.co.uk/c_heating/

when I checked with just hot water on- I had power to the white cable on 3 way valve. When I checked the programmer- there appeared to be a bit of dark residue a if something had burnt out- but not too much, so not sure if this is normal. Was wondering if the unit may have burnt out and somehow the room cylinder is still activating and powering the white cable.

Jim, December 2011