Saunier Duvall F30E Hot water but no heating?

Hiya. Hot water operates fine however....demand for heat , boiler fires up ok, temp display increases to 77 degrees within about 10 seconds from cold, can see gas flow decreasing then flame off. Water pump continues to run and there is the sound of rushing water within the boiler. Both flow and return pipes within the boiler are red hot but no heat in the pipes below the boiler. Have replaced the thermistor but to no effect..Help!