High efficiency water heating system

A storage water heater of the type having an insulated water storage tank, a cold water inlet and a hot water outlet to and from the water heater, and a gas fuel combustion assembly in which gas initially at line pressure or less mixes with combustion air without power assist and is ignited in a combustion chamber to heat water in the storage tank, the water heater characterized in that:

• the combustion chamber is positioned below and external to the storage tank, the combustion chamber walls comprising a plurality of generally vertical riser tubes surrounding a burner and through which all of the products of combustion flow radially, the upper ends of the riser tubes being in fluid communication with the storage tank and the lower ends of the riser tubes being in fluid communication with the bottom of the storage tank through at least one downcomer tube, the riser and downcomer tubes defining a heat exchange water passage with high and low points in the passage in liquid communication with the interior of the storage tank for flow of water through the heat exchange passage to the storage tank by natural convection;
• in an aspirator/mixer assembly comprising a venturi mixer tube, an air inlet to the mixer tube and a fuel gas nozzle at the inlet which receives fuel gas at line pressure of 6.5 mm Hg or less, a gas jet is formed to draw air through the inlet to mix with the gas in the mixer tube, such that gas initially at line pressure of 6.5 mm Hg or less aspirates and mixes with an amount of air sufficient for complete combustion of the gas;
• the premixed gas/air mixture is introduced from the preceeding aspirator/mixer assembly to a burner surrounded by the riser tubes of the combustion chamber and is substantially completely combusted within the water-walled combustion chamber;
• and
• the products of combustion exit the combustion chamber through an exhaust flue which is thermally isolated from the water in the storage tank.