Grant Vortex Pro boiler stops firing after half an hour

[turfymiler]

External Grant Vortex Pro boiler works when first switched on. The radiators get hot and all seems fine. It's only used for central heating, not hot water. The boiler switches off after about 30 minutes. After a few minutes it restarts but the problem is the Riello RDB1 Vortex 15-26 burner does not fire. The burner fan and water pump are running. I can hear water circulating in the radiators but they cool down. The thermostat in house is still calling for heat.

What might be causing the burner not to fire again after the first firing? All components are working for first half hour. Could it be the high limit thermostat tripping as the boiler's got too hot for some reason? If so, what might cause that?

Would really appreciate any suggestions for what might be wrong. Thanks

 

[Exedon]

If as you say burner fan is running but no ignition photocell is normal suspect.
Not high limit stat they have to be manually reset.
Depending on age of your boiler riello did have a problem with photocells we changed quite a few..