Bocsh 280 Combi Boiler will not heat radiators?

I have a Bosch 280 Combi Boiler, when asking for hot water i have no problems.
When switching to heat the radiators the burners ignight and come on but for only 5-10 secs then go off.
The water circulation continues but with cold water.
I do not have a seperate room stat, the control panel is on the boiler and the pressure gauge is set and shows a steady 1.5 bar.Any advice would be welcomed.