Boiler does not shut down Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[furryferret]

I have a fully pumped, open vented system using a Gloworm Economy Plus boiler and a Drayton Digistat +3. The problem is that sometimes the boiler continues to run even though the thermstat is not calling for heat. The result is a very hot house and big bills. The thermostat is fitted with two wires, though there is an extra wire loose at the back od the box. I assume the third wire serves no purpose on this thermosat. Anyone got any ideas?

[plumbbob]

I guess at some point an old dial mechanical thermostat has been replaced with a new digital programmer. This would explain why you have a spare wire. Old stats required a permanent neutral connection as well as the two lives.

As for your problem, there are various possibilities.

Your digistat could be faulty.
A frost stat may be set too high
A motorized zone valve may be partially seized.
A boiler overrun fault.

Unless you have some electrical test equipment, and the expertise to use it, you may have a problem in narrowing down the fault possibilities.