central heating problem.. radiators stay cold

not sure why central heating systems always breakdown in winter and at the start of the week...but hey..
My problem is that the boiler comes on, there is hot water in the tank but the radiators are cold. Pump is working fine. If i twiddle the living-room thermostat the burner fires up for about 10 secs and then turns off.
If i manually lock open the 3-way valve then then the radiators get hot, but otherwise the 3-way valve stays closed.
I'm thinking that the living room thermostat has gone?? If so, it is simply a case of swapping it out? Or could it be the PCB that has a problem??
cheers martin
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Tell us wther the Hot Water is on the "A" or "B" side of the 3 way valve. It could be that yout tank thermostat has welded itself clsoed, thus demanding Hot Water all the time, and over riding the central heating demand.
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but
The most likely culprit is the 3 port valve, although Y-plan wiring can have some odd failure modes with failed contacts or damaged wiring.
Christian.
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start with the simple things. Is there power to the motor in the valve? Valve motors are the commonest failure
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