Abstract: A boiler includes a cylindrical shell sealed at both ends. A working liquid, typically water, circulates inside the shell. A tube bundle is seated inside the shell. Hot combustion gases circulate inside the tube bundle, heating the water circulating inside the shell. The tube bundle includes a relatively large inlet tube that runs straight to a distal end, where hot combustion gases are turned 180° by a concave inner surface on the tube bundle header, which is not a part of the shell, and are returned by a plurality of smaller diameter return tubes and then to an exhaust stack. The tube bundle includes a front tube sheet and a distal end tube sheet that the tubes are welded to, but these tube sheets are not part of the shell. The entire tube bundle floats inside the shell without being fastened to the shell except at the front of the tube bundle, where a baffle is secured to the front opening of the shell. This allows the tube bundle to be removed from the shell as a single unit.

Abstract: A gas fired tube and shell heat exchanger directs a flame directly into an inlet end of the tube bundle and the flame and exhaust are drawn through the tube bundle by an induced draft blower located in the exhaust stream outside of the shell. Liquid to be heated is pumped through the shell by an electric pump. In another preferred embodiment, a forced draft blower is placed on behind the burner to force air into the tube bundle. Control features prevent the burner from firing unless there is liquid flow through the shell.