central heating not shutting down

[chelsea]

got called to a job yesterday as the central heatiing would not shut down on the controls and the hot water cylnder started leaking so they drained the tank before i got there and all the power was off so i could not test

what would be the most likley reason that the heating will not shut down?

its a conventional system on a 3 port mid position valve

 

[quality]

Probably the mid position valve has stuck

 

[PlumbBobBob]

i agree

 

[grahamgas]

Ditto

 

[dontknowitall]

I agree, cos Quality, Steveb and GrahamGas have said the same thing - and I couldn't think what the answer might have been (without being in the house and having a rootle around) - and they know more than me!

 

[SimonJohns]

And mee

 

[Roger Welton]

I supose it could also have been the leaky cylinder causing a short circuit, either 3 port v/v head or cylinder stat.