Leaky Motorised Valve

[Affick]

We have oil-fired CH with 3 motorized valves (for HW, CH and UFCH, downstairs). Recently, I have noticed that the boiler comes on at times when it should not, according to timers and thermostats. WHen this happens, the valves (ACL Lifestyle) are all in the closed position. THe only way to turn off the boiler is to turn the power off. The problem will last for a day or two and then appear to improve, only to return some days later. The pipe to the underfloor system after the valve to this area appears to get quite warm and the manifold will appear warm (even though the valve is apparently closed).
1. Does this suggest that this particular valve is the problem?
2. Can a valve be a little open and trigger the boiler to fire?
3. WHy does the problem go away from time to time....dodgy switch/dodgy valve?
4. If I replace this old style valve (not a separate actuator and valve), should I bite the bullet and do all 3 valves as the system will have to be drained anyeay?
Many thanks

[Roger Welton]

Maybe a micro switch in one of the actuators sticking, need to check which has 240v on the orange wire when everything is off.

[diamondgas]

one of the three is most likely suspect