CH will not switch off

[Royboy]

Timer calls for heat, thermostat calls for heat, boiler comes on, when temperature reached heat does not switch off.
Checked the stat with a meter between L & N is 240v, between yellow switch wire and N is 240v which goes to zero when temperature reached, therefore the problem does not appear to be the thermostat, could it be the 3 way valve?
Only way to shut down heating is to set the timer to off.

Thanks for any advice.

 

[Chalked]

Probably the end switch in the motorised valve

 

[Riley]

Yep valve

 

[Royboy]

Don't know if this helps but been playing around with different settings of timer and thermostat.
Timer calls for heat and hot water plus thermostat on - result radiators and water warm up.
Thermostat reaches desired temperature and clicks off but the radiators get hotter.
Switch heating off from timer radiators continue to get hotter.
Switch hot water off from timer, radiators cooldown.
Therefore tried switching the hot water off completly from timer then radiator system works fine switching on and off via the thermostat.

 

[Riley]

Still valve

 

[fixitflav]

Don't know if this helps but been playing around with different settings of timer and thermostat.
Timer calls for heat and hot water plus thermostat on - result radiators and water warm up.
Thermostat reaches desired temperature and clicks off but the radiators get hotter.
Switch heating off from timer radiators continue to get hotter.
Switch hot water off from timer, radiators cooldown.
Therefore tried switching the hot water off completely from timer then radiator system works fine switching on and off via the thermostat.

From those symptoms it seems likely the 3-port valve is faulty, stuck in the CH position. In that case the cylinder will get no heat, so the cylinder stat remains unsatisfied. You could run in CH only mode by having no HW on the programmer, or by turning the cylinder stat right down (it should have an Off position).
Assuming you have an immersion, you could use that for HW while you get round to checking and fixing the 3-port valve.

 

[Royboy]

From those symptoms it seems likely the 3-port valve is faulty, stuck in the CH position. In that case the cylinder will get no heat, so the cylinder stat remains unsatisfied. You could run in CH only mode by having no HW on the programmer, or by turning the cylinder stat right down (it should have an Off position).
Assuming you have an immersion, you could use that for HW while you get round to checking and fixing the 3-port valve.

Thanks for your help will get in plumber.