Abstract: An apparatus (10) for transferring heat to a fluid from combustion gases including a housing (12) extending between a first end (14) and a second end (16). The apparatus further includes a burner (24) in which a combustible gas and oxygen react to form hot combustion gases which flow from the burner to a gas outlet (28) in the second end. The fluid flows in a length of helically coiled tubing (30) extending the length of the housing. The tubing includes a fin portion (36) which is spirally wound about the length of tubing. The apparatus further includes a core member (40) which extends within the coils of the tubing, and which engages the tubing. The core member includes a spirally-wound threaded area (62) about which the tubing is wrapped. The core member further includes a recessed portion (66) which creates stagnant air pocket adjacent the end of the core member.