Combi fault, Elec., Mech. or Both? Help, advice and problem solutions on all aspects of home appliances from washing machines to TV's

[ALDA]

What fault would cause a combi boiler to start operating without a demand?
Could it be electrical/electronic?
The boiler is a Worcester Bosch combi 240 rsf.
This fault only occasionally happens.
The combi does not have a frost thermostat fitted.
Once the boiler operates, it cannot be stopped by switching the combi advance switch or by turning the room thermostat off.
The boiler can only be shut down at the mains switch.
The boiler appears to be firing up in the DHW mode, as the burner pressure is greater.
Also, because there isn’t an HW tap open, the radiators are heating up fast until the system shuts down after about 3-5 minutes.
The fault will occur once and then boiler goes back to normal operation.
Any replies will be gratefully received.

[TheDoctor5]

Last year 64% of the questions asked in our forum were answered within our DIY project pages at www.diydoctor.org.uk/projects.htm The project pages are now separated alphabetically and your answers are accompanied by diagrams and the ability to see, and buy, the tools and/or required to complete your project. Use our search box to look for your answer and save a great deal of time and money!