boiler on off erratic

[chris-uk]

I have a Potterton Netaheat profile boiler which generally works v well.
The system is pumped, and the timer is a Drayton Lifestyle LP522.
Everything works fine when the system is cool, the timer switches the boiler on and off when required and all seems to function as you would expect.
The problem occurs when the system is hot. The timer is set to switch the boiler off however the boiler stays on. Even switching the timer manually to "off" still does not switch the boiler off. If the system is cold this works fine.
The boiler can be switched off using its own control or switching off mains supply to it.
If I set the timer to switch on heating etc for the morning this works fine overnight but only if this is set when cool. If set when hot the boiler ticks over on/off all night.
The timer was replaced as a precaution but this has made no difference.
Thank you in advance.

 

[Slugbabydotcom]

The timer was replaced as a precaution but this has made no difference.

Well I was about to say the timer until I saw that.

This post has been up for over 7 hours without a reply.

All I can say then is that it sounds like a wiring fault somewhere. The question you have to run through your mind here is: -

From where is the boiler picking up the live that keeps it on?