heating comes on for 5 mins, then goes cold

[BRB]

wonder if I could have anyone\\\'s opinion

problem is - turn on the central heating and the boiler ignites, pump starts whirring, radiators become luke warm.
after 5 minutes they just turn cold even though the boiler is on and the pump is still going and the timer switch still shows ch should still be operating.

Wondering if valve is sticking and not letting hot water through. Pump seems to be working fine, as remains on even though heating has gone cold. Have bled the pumps and radiators.

Hot water is fine. Would appreciate any advice - thanks

 

[ChrisR]

If theres no corculation the boiler won't just stay on or it'll boil. COuld be a dud boiler stat. MIght have helped if you'd told us what the ****** thing IS!