Heating system for liquids

A heater for heating liquids comprising:

• a housing having a base and an upper exhaust end for forming a plenum which conducts a flow of hot flue gas from a burner positioned adjacent the housing's base;
• a heat exchanger residing within the plenum and forming an annular space therebetween through which hot flue gases flow upwardly to the exhaust end;
• an inlet adjacent the top of the heat exchanger;
• an outlet adjacent the bottom of the heat exchanger so that the liquid flows downwardly and countercurrent to the hot flue gas and is heated before being discharged from the heat exchanger;
• an annular jacket having inner and outer walls which are closed at a lower and an upper end and forming an annular cross-sectional space therebetween, the inner wall forming the housing and which is in heat conductive communication with the hot flue gases in the plenum;
• an inlet at the jacket's lower end for admitting feed liquid to the jacket;
• and
• an outlet at the jacket's upper end so that liquid can flow form the inlet to the outlet and be preheated through the inner wall before the preheated liquid is conducted from the jacket to the inlet to the heat exchanger.