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This could be the room thermostat but thought i would ask the experts and get a second opinion first.
I will try to be as informative as possible.
I believe i have a conventional Glow Worm boiler controlled by a Landis & Staefa RWB9 controller (beside the boiler) and a Landis & Gyr room thermostat (in the hall). There is a Landis & Gyr MAV 322 3 way splitter sending the hot water to either the hot tank or radiators.
There are 3 radiators upstairs with thermostats and one without, whilst downstairs there are 4 radiators with thermostats and a further rad without control.
Usually, it is excellent and ultra quiet but recently it has started to run flat out even with the thermostat turned down or off. The only way to turn the thing off is to turn the boiler off on the controller.
Could it be as simple as the room thermostat??
The entire system is around 6 years old.

does everything else work ok,

CH on HW off - heating only work ?
HW on CH off - water only works ?

etc etc ?

If eveything else works ok then i'd say the fault would be with the boiler themostat rather than the room stat. Even if the room stat was on full and the boiler stat down low, the boiler stat would cut the boiler off.

hi

i cant understand why you have a room stat and trv's remove one or the others it wouldnt cause the problem you are having it does sound to me like the room stat is faulty hopefully it isnt the pcb try just disconecting the wires from the back of the room stat and see if it stops this would illiminate the stat not knowing if its a combi or open vented system could help be carefull that it is not a 240v stat you will know this if there is a neutral wire in there if theres only 2 wires it will be fine otherwise turn the poer off to the boiler

get back to me

sorry its an open vented system probably be a 240v stat turn boiler off before you disconnect the stat at the wall

Hi Andy, it could be the thermostat but it is unlikely as they are usually very reliable, it is more likely to be the 3port motorised valve stuck.
A quick check is to disconnect a wire at the stat. Switch off supply to system, look at wiring at stat, probably got 3 core & E cable with red live in, yel as sw. line back to valve & blue as Neutral for inbuilt 'Accelerator' resistor. disconnect yellow and make safe in connector block, turn on system.select CH only, does boiler fire? if so it is not the thermostat, or programmer ,so most likely is valve, if this is suspected then will need confirming by heating eng. who can fit replacement,
regards SPARX

you can come and wire me up ive never been good with electrics hope this helps the guy out go for it

respect sparkx

Thanks for the replies, i will check out what you mention and report back.
To be honest, i have not checked the controller as you mention regards
CH on HW off - heating only work ? - HW on CH off - water only works ?
I normally have the hot water & central heating on all the time and let the respective thermostats do the work. It was only recently though that i noticed the central heating was not going off but pumping round hot water around the rads even though the room thermostat and TRV's were set much lower.