I've an old Glow Worm 50B boiler which functions OK when heating the water i.e. pilot light runs, main burner comes on and off as required. However when switched to the heating the boiler fires up, runs for 6 secs and then there is a big 'solenoid' click and both the main burner and pilot light go out. The pilot light can then be re-lit manually and run quite happily so I'm assuming the thermo coupling is working OK. Any ideas?
This looks like an over-temperature trip, such as would be caused by no-flow thro' the radiators. This could be due to a malfunction of the diverter valve;-- they can stick or wear out, or burn-out;-- not an expensive job;-- you could manage to fit a new motorised actuator yourself, or manually exercise the valve stem with the motor head removed. Any water leakage from the valve gland means a new valve, and a plumber to do the work due to the draining/refilling/priming/fresh anti-corrosion protection chemicals.