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A heat pump water heater has a tank portion, an electric heating structure for adding electrical heat to water stored in the tank, and a heat pump for adding refrigerant heat to the tank water. A control system associated with the water heater has three user-selectable heating modes for heating the

A heat pump water heater has a tank portion, an electric heating structure for adding electrical heat to water stored in the tank, and a heat pump for adding refrigerant heat to the tank water. A control system associated with the water heater has three user-selectable heating modes for heating the tank water during a given heating demand cycle—a first mode that initially heats the tank water with refrigerant heat while the electric heat is locked out for a first predetermined period before supplementing the refrigerant heat if necessary, a second mode similar to the first mode but with a longer electric heat lockout period, and a third mode in which only the electric heat is utilized to satisfy a tank water heating demand. Illustratively, the heat pump is disposed in a compact component arrangement on the top end of the water heater tank.

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1. Liquid heating apparatus comprising: first apparatus operative to transfer refrigerant heat to a liquid;second apparatus operative to transfer electrical heat to the liquid; anda control system having a heating mode operative, in a heating cycle initiated in response to a sensed demand for liquid

1. Liquid heating apparatus comprising: first apparatus operative to transfer refrigerant heat to a liquid;second apparatus operative to transfer electrical heat to the liquid; anda control system having a heating mode operative, in a heating cycle initiated in response to a sensed demand for liquid heating, to: (1) initially utilize said first apparatus, at the start of the heating cycle, to transfer refrigerant heat to the liquid while preventing operation of said second apparatus for a predetermined lockout time period,(2) thereafter utilize said second apparatus to supplement the refrigerant heating of the liquid with electrical heating thereof if the heating demand has not been satisfied by the previous refrigerant heating of the liquid, and then(3) terminate the operation of both the first heating apparatus and the second heating apparatus at the end of the heating cycle when the demand for liquid heating is satisfied, the lockout of the second apparatus during each heating cycle in said heating mode being initiated only at the start of such heating cycle. 2. The liquid heating apparatus of claim 1 wherein: said liquid heating apparatus includes an electric water heater having a tank for storing water to be heated,said first apparatus includes a refrigerant circuit structure operatively coupled to said electric water heater, andsaid second apparatus includes an electric resistance type heating structure extending through an interior portion of said tank. 3. The liquid heating apparatus of claim 2 wherein: said refrigerant circuit structure is a heat pump. 4. The liquid heating apparatus of claim 2 wherein: said refrigerant circuit structure has sequentially connected in series therein a compressor, a condenser coil operative to receive a throughflow of tank water to be heated by refrigerant passing through said condenser coil, an expansion valve, and an evaporator coil with an associated evaporator fan, said evaporator coil forming an outer wall portion of a plenum structure within the interior of which said compressor is disposed, said evaporator fan being operative, during operation of said refrigerant circuit structure, to flow air through the interior of said plenum structure and then outwardly through said evaporator coil, to thereby transfer heat from said fan and said compressor to said evaporator coil, andsaid first apparatus further includes a water circuit extending between said tank and said condenser coil and having connected therein a pump operative to sequentially flow water from said tank through said condenser coil and then back into said tank. 5. The liquid heating apparatus of claim 4 wherein: said tank has a top end, andsaid refrigerant circuit structure is mounted on said top end of said tank. 6. The liquid heating apparatus of claim 4 wherein: said pump is operative to draw water from a bottom portion of said tank and return the water to a top portion of said tank. 7. The liquid heating apparatus of claim 1 wherein: said first apparatus includes a compressor, andsaid control system is operative, if necessary, to utilize said second apparatus to heat the liquid to a predetermined minimum temperature prior to permitting operation of said compressor. 8. Liquid heating apparatus comprising: first apparatus operative to transfer refrigerant heat to a liquid;second apparatus operative to transfer electrical heat to the liquid; anda control system operative to control said first apparatus and said second apparatus in either one of user-selectable first and second heating modes, said first heating mode, when selected, being operative in response a sensed demand for liquid heating to initially utilize said first apparatus to transfer refrigerant heat to the liquid, while preventing operation of said second apparatus for a predetermined first lockout time period, and thereafter utilizing said second apparatus to supplement the refrigerant heating of the liquid with electrical heating thereof if the heating demand has not been satisfied by the previous refrigerant heating of the liquid, andsaid second heating mode, when selected, being operative in response a sensed demand for liquid heating to initially utilize said first apparatus to transfer refrigerant heat to the liquid, while preventing operation of said second apparatus for a predetermined second lockout time period of a different magnitude than said first lockout time period, and thereafter utilizing said second apparatus to supplement the refrigerant heating of the liquid with electrical heating thereof if the heating demand has not been satisfied by the previous refrigerant heating of the liquid. 9. The liquid heating apparatus of claim 8 wherein: said control system is additionally operative to control said first apparatus and said second apparatus in a third user-selectable heating mode which, when selected, is operative for only a predetermined time period to utilize only said second apparatus to transfer electric heat to the liquid in response to a sensed demand for liquid heating, said control system, after the expiration of said predetermined time period, automatically selecting one of said first and second heating modes for use in satisfying a sensed liquid heating demand. 10. The liquid heating apparatus of claim 8 wherein: said liquid heating apparatus includes an electric water heater having a tank for storing water to be heated,said first apparatus includes a refrigerant circuit structure operatively coupled to said electric water heater, andsaid second apparatus includes an electric resistance type heating structure extending through an interior portion of said tank. 11. The liquid heating apparatus of claim 10 wherein: said refrigerant circuit structure is a heat pump. 12. The liquid heating apparatus of claim 10 wherein: said refrigerant circuit structure has sequentially connected in series therein a compressor, a condenser coil operative to receive a throughflow of tank water to be heated by refrigerant passing through said condenser coil, an expansion valve, and an evaporator coil with an associated evaporator fan, said evaporator coil forming an outer wall portion of a plenum structure within the interior of which said compressor is disposed, said evaporator fan being operative, during operation of said refrigerant circuit structure, to flow air through the interior of said plenum structure and then outwardly through said evaporator coil, to thereby transfer heat from said fan and said compressor to said evaporator coil, andsaid first apparatus further includes a water circuit extending between said tank and said condenser coil and having connected therein a pump operative to sequentially flow water from said tank through said condenser coil and then back into said tank. 13. The liquid heating apparatus of claim 12 wherein: said tank has a top end, andsaid refrigerant circuit structure is mounted on said top end of said tank. 14. The liquid heating apparatus of claim 12 wherein: said pump is operative to draw water from a bottom portion of said tank and return the water to a top portion of said tank. 15. The liquid heating apparatus of claim 8 wherein: said first apparatus includes a compressor, andsaid control system is operative, if necessary, to utilize said second apparatus to heat the liquid to a predetermined minimum temperature prior to permitting operation of said compressor. 16. Liquid heating apparatus comprising: first apparatus operative to transfer refrigerant heat to a liquid;second apparatus operative to transfer electrical heat to the liquid; anda control system operative to control said first apparatus and said second apparatus in either one of user-selectable first and second heating modes, said first heating mode, when selected, being operative to utilize said first apparatus and, if needed, said second apparatus to transfer heat to the liquid in response to a sensed demand for liquid heating, andsaid second heating mode, when selected, being operative, for only a predetermined time period, to utilize only said second apparatus to transfer heat to the liquid in response to a sensed demand for liquid heating, said control system, after the expiration of said predetermined time period, automatically selecting said first heating mode for use in satisfying a sensed liquid heating demand. 17. The liquid heating apparatus of claim 16 wherein: said liquid heating apparatus includes an electric water heater having a tank for storing water to be heated,said first apparatus includes a refrigerant circuit structure operatively coupled to said electric water heater, andsaid second apparatus includes an electric resistance type heating structure extending through an interior portion of said tank. 18. The liquid heating apparatus of claim 17 wherein: said refrigerant circuit structure is a heat pump. 19. The liquid heating apparatus of claim 17 wherein: said refrigerant circuit structure has sequentially connected in series therein a compressor, a condenser coil operative to receive a throughflow of tank water to be heated by refrigerant passing through said condenser coil, an expansion valve, and an evaporator coil with an associated evaporator fan, said evaporator coil forming an outer wall portion of a plenum structure within the interior of which said compressor is disposed, said evaporator fan being operative, during operation of said refrigerant circuit structure, to flow air through the interior of said plenum structure and then outwardly through said evaporator coil, to thereby transfer heat from said fan and said compressor to said evaporator coil, andsaid first apparatus further includes a water circuit extending between said tank and said condenser coil and having connected therein a pump operative to sequentially flow water from said tank through said condenser coil and then back into said tank. 20. The liquid heating apparatus of claim 19 wherein: said tank has a top end, andsaid refrigerant circuit structure is mounted on said top end of said tank. 21. The liquid heating apparatus of claim 19 wherein: said pump is operative to draw water from a bottom portion of said tank and return the water to a top portion of said tank. 22. The liquid heating apparatus of claim 16 wherein: said first apparatus includes a compressor, andsaid control system is operative, if necessary, to utilize said second apparatus to heat the liquid to a predetermined minimum temperature prior to permitting operation of said compressor. 23. Liquid heating apparatus comprising: first apparatus operative to transfer refrigerant heat to a liquid;second apparatus operative to transfer electrical heat to the liquid; anda control system operative to receive a desired liquid heating temperature set point input by a user of said liquid heating apparatus, and having a user-selectable first heating mode which, in response to a sensed demand for liquid heating, initially utilizes said first apparatus to transfer refrigerant heat to the liquid, while preventing operation of said second apparatus for a predetermined first lockout time period, and then utilizes said second apparatus to supplement the refrigerant heating of the liquid with electrical heating thereof if the heating demand has not been satisfied by the previous refrigerant heating of the liquid, said control system being automatically operative, if the user-input temperature set point is equal to or greater than a predetermined magnitude, to implement a second heating mode similar to said first heating mode but having a predetermined second lockout time period greater than said first lockout time period. 24. The liquid heating apparatus of claim 23 wherein: said liquid heating apparatus includes an electric water heater having a tank for storing water to be heated,said first apparatus includes a refrigerant circuit structure operatively coupled to said electric water heater, andsaid second apparatus includes an electric resistance type heating structure extending through an interior portion of said tank. 25. The liquid heating apparatus of claim 24 wherein: said refrigerant circuit structure is a heat pump. 26. The liquid heating apparatus of claim 24 wherein: said refrigerant circuit structure has sequentially connected in series therein a compressor, a condenser coil operative to receive a throughflow of tank water to be heated by refrigerant passing through said condenser coil, an expansion valve, and an evaporator coil with an associated evaporator fan, said evaporator coil forming an outer wall portion of a plenum structure within the interior of which said compressor is disposed, said evaporator fan being operative, during operation of said refrigerant circuit structure, to flow air through the interior of said plenum structure and then outwardly through said evaporator coil, to thereby transfer heat from said fan and said compressor to said evaporator coil, andsaid first apparatus further includes a water circuit extending between said tank and said condenser coil and having connected therein a pump operative to sequentially flow water from said tank through said condenser coil and then back into said tank. 27. The liquid heating apparatus of claim 26 wherein: said tank has a top end, andsaid refrigerant circuit structure is mounted on said top end of said tank. 28. The liquid heating apparatus of claim 26 wherein: said pump is operative to draw water from a bottom portion of said tank and return the water to a top portion of said tank. 29. The liquid heating apparatus of claim 23 wherein: said first apparatus includes a compressor, andsaid control system is operative, if necessary, to utilize said second apparatus to heat the liquid to a predetermined minimum temperature prior to permitting operation of said compressor. 30. Water heating apparatus comprising: an electric water heater having a tank for storing water to be heated, and an electric heating element extending through an interior portion of said tank and operative to add electric heat to water therein;a refrigerant circuit structure having sequentially connected in series therein a compressor, a condenser coil operative to receive a throughflow of tank water to be heated by refrigerant passing through said condenser coil, an expansion valve, and an evaporator coil with an associated evaporator fan, said evaporator coil forming an outer wall portion of a plenum structure within the interior of which said compressor is disposed, said evaporator fan being operative, during operation of said refrigerant circuit structure, to flow air through the interior of said plenum structure and then outwardly through said evaporator coil, to thereby transfer heat from said fan and said compressor to said evaporator coil;a water circuit extending between said tank and said condenser coil and having connected therein a pump operative to sequentially flow water from said tank, through said condenser coil and then back into said tank; anda control system operative to utilize said refrigerant circuit structure and said electric heating element to maintain a predetermined water temperature in said tank. 31. The water heating apparatus of claim 30 wherein: said control system has a user-selectable heating mode operative in a given heating cycle to sequentially operate said refrigerant circuit structure and then operate said electric heating element, if necessary, to supplement the water heating of said refrigerant circuit structure. 32. The water heating apparatus of claim 30 wherein: said tank has an upper end, andsaid refrigerant circuit structure is a heat pump disposed on the upper end of said tank.