mouth 24 is vertical leaving substantial segmental spaces each side of head 22 for passage of said flow but in this example the lower horizontal edge of mouth 24 is in close proximity to the bottom of the tube in use a mix of gas fuel and air is projected from mouth 24 for combustion within tube 10 burning taking place as a flame directed downstream and approximately in the zone indicated by the wavy broken lines 26 in fig1 in known constructions for example as in our above co pending application problems arise due to reduced efficiency and over production of noxious emissions notably co due to chilling of the flame arising from its direct contact with the passing through flow of gases along tube 10 which will be relatively cool by the time they reach the respective burner head 22 the first burner head at the upstream end of a particular run or leg of tube 10 will be subjected to cold carrier air admitted at the end vent and burner heads downstream along the line are subjected to gases which have been heated by the upstream burner or burners but which have then been substantially cooled in their passage along tube 10 as the whole object is to emit maximum heat into the surrounding space the chilling of the flame by.