Radiators warm when only hot water switched on. Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[JonW65]

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?

[JonW65]

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.

[bd3cc]

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.