A radiant tube heater includes a housing and a burner tube extending outwardly therefrom for carrying products of combustion from the housing. When less heat is required, unless air from the blower is reduced, there is too much air for complete combustion of the fuel and excess air mixes with combustion gases causing cooling in the burner tube. This problem is solved by providing a short flame tube in the inlet end of the burner tube for receiving the flame. Excess air flow around the flame tube is heated. Thus, it is necessary to reduce only the supply of fuel to the burner.