A burner for burning a combustible gas comprising fuel gas and air that has been mixedbefore being supplied to the burner. The burner has a flame holder concavely recessed into aflame outlet. The concave configuration of the flame holder focuses the individual flames onthe combustion surface toward a central location where the individual flames interact with andreinforce one another in a direction axial to the burner. Thus very little heat is transmitteddirectly from the burner in a direction normal to the burner axis. This characteristic of theburner allows it to be used to fire a flue type heat exchangers where the walls of the heatexchanger are very close to the burner without excessive temperatures being produced in theheat exchanger walls adjacent the burner.
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