DIY Doctor UK DIY Forums

I had a new Worcester 30CDI conventional boiler installed recently to replace an old Potterton that was about to expire. At the same time I had the pump replaced.
After installation there was a problem that the system wouldn't switch off, so my Drayton timer was replaced with a Honeywell one, and a capacitor was added in the junction box by the boiler.
Since then I have a problem with some radiators getting warm about 30 mins after the hot water has been on. The 3-way valve has been replaced as has all wiring. A Worcester expert has checked the boiler. My installer tells me there is excess voltage being returned to the boiler which is forcing the 3-way valve to open for the radiators.
He is now running out of ideas on how to resolve this problem. Any suggestions?

Still no idea what's causing this.
Having discussed it over the weekend we're now thinking of changing from a Y plan to an S plan system configuration. I can see the logic in why this would resolve it, but I'm still nervous that the cause hasn't been identified.
The work will be done later this week.

C/H systems are notoiously difficult to wire. You have so many different diagrams, and dont know which one to follow.
I don't know either of your components, but would go with the Drayton diagram.
Pump over run can often cause a problem. and it can be as simple as keeping/removing a pre-installed link on the boiler, or even a faulty sender, even on a new boiler.