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?
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.
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