A burner for burning a combustible gas comprising fuel gas and air that has been mixed before being supplied to the burner. The burner has a flame holder .[.concavely.]. .Iadd.axially and radially .Iaddend. recessed into a flame outlet. The .[.concave.]. configuration of the flame holder focuses the individual flames on the combustion surface toward a central location where the individual flames interact with and reinforce one another in a direction axial to the burner. Thus very little heat is transmitted directly from the burner in a direction normal to the burner axis. This characteristic of the burner allows it to be used to fire a flue type heat exchangers where the walls of the heat exchanger are very close to the burner without excessive temperatures being produced in the heat exchanger walls adjacent the burner.
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