M
md68
Pound to a peenny it just the thermostat which has failed. These things happen so it wouldn't be the original installers fault after everything has been ok for 2 years so IMO your plumber is just playing on your fears. Does he know your a sparky?
If it's a vented direct cylinder / combi cylinder it's doing exactly what it should do as thats what overflows are for.
If it's a vented direct cylinder / combi cylinder it's doing exactly what it should do as thats what overflows are for.