Waste heat capture from a dual fuel gas and electric water heater

Abstract

A dual fuel—gas fuel and electric heat pump—water heater is provided that captures waste heat from the exhaust flue of the gas fuel heating system and utilizes it in the evaporator of the electric heat pump. Multiple mechanisms are disclosed for the transfer of heat from the exhaust flue gases to the evaporator. Heat from the exhaust gas that would otherwise be lost is used to help heat refrigerant in the evaporator to improve the efficiency of the system. This heat can also be used to prevent or remove frozen condensate on the evaporator.

What is claimed is: 
     
         1 . A water heater, comprising:
 a tank for holding water;   a gas fuel heating system configured for heating the water in said tank, said gas fuel heating system comprising an exhaust flue, said exhaust flue defining an exterior surface;   a heat pump heating system configured for heating the water in said tank, said heat pump heating system comprising an evaporator;   a pipe in thermal communication with said exhaust flue and said evaporator, wherein said pipe is configured for transferring heat from said exhaust flue to said evaporator; and   a fan configured for causing a flow of air past said exhaust flue and over said evaporator.   
     
     
         2 . A water heater as in  claim 1 , wherein said pipe is comprised of a solid material for conducting heat between said exhaust flue and said evaporator. 
     
     
         3 . A water heater as in  claim 1 , further comprising a single phase transfer fluid transported within said pipe for conducting heat between said exhaust flue and said evaporator. 
     
     
         4 . A water heater as in  claim 1 , further comprising a phase change fluid contained within said pipe for conducting heat between said exhaust flue and said evaporator. 
     
     
         5 . A water heater as in  claim 4 , wherein said phase change fluid is selected from the group consisting of dichlorodifluromethane, trichlorofluromethane, benzene, methanol, ammonia, water, mercury, and mixtures thereof 
     
     
         6 . A water heater as in  claim 1 , wherein said pipe wraps around the exterior surface of said exhaust flue one or more times. 
     
     
         7 . A water heater as in  claim 1 , wherein said flue defines an axial direction, and wherein said pipe extends adjacent to said flue and in contact with said flue along the axial direction. 
     
     
         8 . A water heater as in  claim 1 , wherein a portion of said pipe extends around a portion of said exhaust flue and forms a partial sleeve around said exhaust flue. 
     
     
         9 . A water heater as in  claim 1 , wherein a portion of said pipe runs adjacent to said evaporator. 
     
     
         10 . A water heater as in  claim 9 , wherein a portion of said pipe extends across the path of air flow to said evaporator at least one time. 
     
     
         11 . A water heater as in  claim 9 , wherein said evaporator defines a plurality of grooves, and wherein said pipe is positioned within the plurality of grooves defined by said evaporator. 
     
     
         12 . A water heater as in  claim 9 , wherein said pipe is attached to said evaporator using a plurality of hooks. 
     
     
         13 . A water heater as in  claim 9 , wherein said pipe is welded to said evaporator. 
     
     
         14 . A water heater as in  claim 1 , wherein a portion of said pipe is located adjacent to said evaporator in a path of air flow over the evaporator, and wherein a portion of said pipe comprises a plurality of fins. 
     
     
         15 . A water heater as in  claim 14 , wherein a portion of said pipe extends across the path of air flow to said evaporator at least one time. 
     
     
         16 . A water heater, comprising:
 a tank for holding water;   a gas fuel heating system configured for heating the water in said tank, said gas fuel heating system comprising an exhaust flue;   a heat pump heating system configured for heating the water in said tank, said heat pump heating system comprising an evaporator; and   a fan configured for causing an air flow over said evaporator and past said flue so as to provide for heat transfer from said exhaust flue to said evaporator.   
     
     
         17 . A water heater as in  claim 16 , wherein said exhaust flue defines an axial direction, and wherein said exhaust flue defines a plurality of branches extending along the axial direction with each branch configured for carrying exhaust gases, wherein the branches are positioned so as to allow the air flow from said fan to pass between the branches. 
     
     
         18 . A water heater as in  claim 16 , wherein a portion of said exhaust flue defines a plurality of fins. 
     
     
         19 . A water heater, comprising:
 a tank for holding water;   a gas fuel heating system configured for heating the water in said tank, said gas fuel heating system comprising an exhaust flue that facilitates the flow of exhaust comprising heated gases from the gas fuel heating system;   a heat pump heating system configured for heating the water in said tank, said heat pump heating system comprising an evaporator, wherein a portion of the exhaust from said exhaust flue flows over said evaporator.   
     
     
         20 . A water heater as in  claim 19 , wherein the amount of exhaust that flows through the evaporator is varied depending on operating conditions of the water heater.