In message , Martin
writes
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.

It sounds like either the synchron motor has gone on the 3 port valve or
(and since you didn't say what the boiler is, it's impossible to say,
but lets assume that it's driven from the pcb) the PCB isn't driving the
valve
--
geoff