A generally cylindrical housing containing a burner underlying a fluid conduit formed to provide a plurality of interconnected concentric helical coils, the outermost coil having a lower end portion projection below the lower ends of the inner coils. A reticulate baffle, in underlying engagement with the lower end of an inner coil, cooperates with the lower end portion of the outermost coil to define a combustion chamber, the baffle permitting flow of gases of combustion upwardly therethrough but retaining most of the burner flame within the combustion chamber. The fluid conduit and its mounting structure are arranged for easy removal from the housing and replacement therein.