Abstract: In a heat exchanger employing double wall tubing comprising concentric inner and outer tubes, the inner and outer tubes are disposed within apertures of spaced tube sheets and connected thereto by roller expansion of the tubes. To maintain venting of the air space between the tubes, a split wall ferrule is located over the inner tube and within the outer tube in the region of the tube sheet to which the outer tube is connected. The ferrule, which has a length sufficient to traverse the thickness of the tube sheet, transmits expansion forces exerted on the interior of the inner tube to the outer tube so as to expand the outer tube to connect it to the tube sheet. The split in the ferrule wall forms a passageway that connects the air space between the tubes to the space between the tube sheets.

Abstract: A water heater comprises an elongated vessel having a heat exchanger that occupies a substantial portion at one end and leaves a minimum volume storage and blending zone at the other end of the vessel. Water is recirculated through the heat exchanger, preferably continuously, through a recirculation conduit that takes water through its inlet essentially solely from the outlet end of the heat exchange zone and returns it to the inlet end, the intake of the recirculation conduit being located at the lower end of the volume storage and immediately above an apertured partition extending across the interior of the vessel immediately above the heat exchanger. The temperature sensor of the control thermostat is received within a tunnel secured to the partition. The tunnel has two side outlet openings away from the aperture so that the thermostat is always subject to the hottest water just above the heat exchanger for fast and accurate control of the heating fluid on failure of the circulating pump.

Abstract: A water heater comprises an elongated vessel having a heat exchanger that occupies a substantial portion at one end and leaves a minimum volume storage and blending zone at the other end of the vessel. Water is recirculated through the heat exchanger, preferably continuously, through a recirculation conduit that takes water through its inlet essentially solely from the outlet end of the heat exchange zone and returns it to the inlet end, the intake of the recirculation conduit being located very close to the end of the heat exchanger. The temperature sensor of the control thermostat is received entirely within the recirculation conduit so that it is responsive to a high velocity flow of water drawn from the heat exchange zone.