A burner 5 is located within a flame tube 2 arranged coaxially in an outer tube 6 with a closed end. Air enters through inlet 15 and exhaust gases flow between the tubes to exhaust duct 10. To reduce heat transfer to the flame tube 2 adjacent to the burner 5, vanes 19 are associated with the burner which cause combustion air to swirl before it mixes with gas from the burner and produce a vortex which encourages the gas to flow, and the flame to extend, centrally of the flame tube adjacent to the burner, and also cause air to surround the flame adjacent to the burner. In one embodiment the air is divided by a shroud surrounding the burner. The shroud causes the air to converge towards the central axis of the flame tube as it approaches the vanes. The flame length can be varied by adjusting the distance between the shroud and the burner.