Beside the nozzle outlet 6 air mixing ports 7 are located radially around the nozzle and are arranged and positioned to draw a part of the combustion air into the nozzle for mixing with the gas to facilitate the flame. Between the mixing nozzle and the burner housing is defined an annular air chamber 8 which acts as an airflow passageway to deliver and convey combustion air from the air inlet port to the nozzle mixing ports and also to the inlet side of the primary combustion passage 9. This is coaxially aligned with and receives the end of the mixing nozzle. To reduce the diameter of the combustion passage by half as it extends from the combustion passage entrance to the combustion outlet 10, the combustion passage includes a necked down region 11.