AU2013245437A1 - A heat exchanger element and a water heater using same - Google Patents

Abstract

Disclosed herein is a heat exchanger element 22 for carrying a refrigerant fluid, the heat exchanger element comprising an inner tube 22a and an outer tube 22b both formed from heat conductive material. Substantially the entire inner surface of the outer 5 tube 22b is in contact with the outer surface of the inner tube 22a. A transverse cross section of at least a portion of the inner and outer tubes has a non-circular shape and/or the inner tube may be formed from copper and the outer tube from stainless steel. Also disclosed is a heat pump water heater 10 comprising a bypass circuit having a first end 28 connected to the heat pump circuit between the compressor 20 and the condenser 22 10 and a second end 30 connected to the heat pump circuit between the expansion device 24 and the evaporator 18. The bypass circuit includes a valve 32 for selectively diverting a portion of relatively hot refrigerant output from the compressor 20 to the evaporator 18. 6g24 Fig. 1

Description

1 AUSTRALIA Patents Act 1990 INFINITY ENERGY TECHNOLOGIES PTY LTD COMPLETE SPECIFICATION STANDARD PATENT Title: A heat exchanger element and a water heater using same The following statement is a full description of this invention including the best method of performing it known to us:- 2 Cross-Reference to Related Applications [0001] The present application claims priority from Australian Provisional Patent Application No 2012904507 filed on 16 October 2012, the content of which is incorporated herein by reference. Technical Field [0002] The present disclosure relates to a heat exchanger and to a water heater using same. The presently disclosed heat exchanger and water heater have been developed primarily for use in generating domestic hot water and will described hereinafter with reference to this application. However, it will be appreciated that they may also be used in commercial or industrial applications for heating water or other fluids. Background [0003] Known water heaters include electrical water heaters, solar water heaters, gas fired water heaters and ambient air heat pump water heaters. However, all conventional water heaters have some disadvantages, as discussed below. [0004] Electrical water heaters convert high grade electrical energy, which has many uses, into relatively low grade thermal energy in the form of hot water, which is of limited use. In addition, as the insulation of the electrical wires in electrical water heaters deteriorates due to age, there is a risk of users being shocked. [0005] Gas fired water heaters can suffer gas leaks, which can be dangerous. Also, there have been numerous occurrences of combustion products from gas fired water heaters not being ventilated in time, thereby leading to dangerous explosions. [0006] Solar water heaters need a relatively large collector, and more importantly, rely on favourable weather in order to operate efficiently. Also, the collector is 2133973_2.doc 3 normally installed on the roof of a building, which makes installation difficult and can negatively affect the aesthetics of the building. [0007] Conventional ambient air heat pump water heaters cannot operate in cold ambient conditions. [0008] Australian Patent No. 603510 discloses a solar-boosted heat pump water heater. However, it requires a relatively large panel area. Moreover, the panel is normally installed on the roof of a building, which makes installation difficult and can negatively affect the aesthetics of the building. It also requires the connection of the refrigeration pipelines onsite, which requires skilled labour. [0009] Most conventional ambient air heat pump water heaters have high failure rate due to overheating of the compressor if forced to heat water to relatively high temperature, such as the 60'C temperature required by Australian and other industrial countries' standards for sanitary/potable water. Moreover, the heat pump of most conventional ambient air heat pump water heaters cannot operate below 7C ambient air temperature. As a result, conventional ambient air heat pump water heaters usually rely on a back-up electrical element to heat water when ambient air temperature drops below 7C. Some conventional heat pump water heaters do not have a double-wall heat exchange effect between refrigerant and water, nor the pressure of water higher than that of the refrigerant, both of which are required by standards in Australia and other industrial countries. Another problem with conventional heat pumps is that de-frosting of the evaporator is realised by reversing the refrigeration cycle. This reversing cycle de-frosting method is problematic firstly as it takes heat from the water being heated, which reduces efficiency, secondly because when the water is hot, the suction pressure in the de-frosting period will be too high for the compressor, and thirdly because in the short de-frosting time, the oil returning is often a problem, which can cause the compressor to lack oil and fail. [0010] A further disadvantage of conventional water heaters is that their water reservoir can be difficult to seal. 2133973_2.doc 4 [0011] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application. Summary [0012] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. [0013] In a first aspect, the present disclosure provides a heat exchanger element for carrying a refrigerant fluid, the heat exchanger element comprising: an inner tube and an outer tube, the inner tube and outer tube being formed from a heat conductive material, wherein substantially the entire inner surface of the outer tube being in contact with the outer surface of the inner tube, and wherein a transverse cross section of at least a portion of the inner and outer tubes has a non-circular shape; and/or the inner tube is formed from copper and the outer tube is formed from stainless steel. [0014] The transverse cross sectional shape may be a rounded polygonal shape. The rounded polygonal shape may be imparted by twisting said at least part of the tubes. Alternatively, the transverse cross sectional shape may be an oval shape. In some embodiments, said at least a portion of the inner and outer tubes may be the whole length of the tubes, and, in other embodiments, said at least a portion of the inner and outer tubes may be a downstream portion of the tubes. [0015] The element may be substantially helically shaped. 2133973_2.doc 5 [0016] An air vent may be provided between the inner and outer tubes. [0017] In a second aspect, the present disclosure provides a water heater comprising: a water reservoir; and a heat exchanger element as defined in the first aspect above extending into the water reservoir to heat water in the reservoir. [0018] The element may penetrate the water reservoir at a position below a minimum operational water level of the water heater. [0019] The water heater may be a heat pump water heater further comprising: a heat pump circuit including: an evaporator exposed to ambient air, a compressor downstream of the evaporator, a condenser downstream of the compressor, the condenser defining a heat exchanger element penetrating the reservoir for immersion in the volume of water, and an expansion device downstream of the condenser; a refrigerant in the heat pump circuit; a bypass circuit having a first end connected to the heat pump circuit between the compressor and the condenser and a second end connected to the heat pump circuit between the expansion device and the evaporator, the bypass circuit including a valve for selectively diverting a portion of relatively hot refrigerant output from the compressor to the evaporator; a controller for controlling actuation of the valve. [0020] The controller may periodically open the valve to defrost the evaporator. In one embodiment, the controller may be responsive to an evaporator temperature sensor for detecting the evaporator temperature and may open the valve to defrost the evaporator only when the evaporator temperature sensor detects that the evaporator temperature is less than a predetermined lower temperature. If the evaporator temperature sensor is connected to the evaporator, the predetermined lower temperature 2133973_2.doc 6 may be 0 0 C. If the evaporator temperature sensor is connected to the heat pump circuit between the evaporator and the compressor, the predetermined lower temperature may be between 0 0 C and 10 0 C. The controller may close the valve if the evaporator temperature sensor detects that the evaporator temperature is greater than a predetermined higher temperature. If the evaporator temperature sensor is connected to the evaporator, the predetermined higher temperature may be between 5'C and 10 0 C. If the evaporator temperature sensor is connected to the heat pump circuit between the evaporator and the compressor, the predetermined higher temperature may be between 10 0 C and 15'C. In another embodiment, the controller may be responsive to an ambient air temperature sensor for detecting the ambient air temperature and to an evaporator temperature sensor for detecting the evaporator temperature and may open the valve to defrost the evaporator only when the temperature detected by the ambient air temperature sensor is less than a predetermined ambient air temperature and the temperature difference between the temperature detected by the ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is greater than a predetermined higher temperature difference. The predetermined ambient air temperature may be between 10 0 C and 20'C. The predetermined higher temperature difference may be between 10 C and 20'C. The controller may close the valve if the temperature difference between the temperature detected by the ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is less than a predetermined lower temperature difference. The predetermined lower temperature difference may be between 0 0 C and 10 0 C. In either embodiment, opening of the valve may also be modulated by a timer. The controller may be responsive to the timer to close the valve after a predetermined time, such as 10 minutes. The controller may also be responsive to the timer to prevent the valve from being re-opened for 45 minutes. [0021] A fan may be provided to blow ambient air toward the evaporator. The fan may be responsive to the controller. The controller may deactivate the fan when the valve is opened. The controller may reactivate the fan before closing the valve to blow melted frost off the evaporator. 2133973_2.doc 7 [0022] The controller may be responsive to a compressor temperature sensor for detecting the compressor temperature and may open the valve when the compressor temperature sensor detects that the compressor temperature is greater than a predetermined temperature, such as 90'C. The controller may close the valve when the compressor temperature sensor detects that the compressor temperature has fallen to a predetermined temperature, such as 80'C. The compressor temperature sensor may be connected to the heat pump circuit between the compressor and the reservoir. [0023] The controller may be responsive to an ambient air temperature sensor for detecting the ambient air temperature and may periodically open the valve when the ambient air temperature sensor detects that the ambient air temperature is less than a predetermined temperature, such as 10 0 C. [0024] In a third aspect, the present disclosure provides a heat pump water heater comprising: a water reservoir for containing a volume of water to be heated; a heat pump circuit including: an evaporator exposed to ambient air, a compressor downstream of the evaporator coil, a condenser downstream of the compressor, the condenser defining a heat exchanger element penetrating the reservoir for immersion in the volume of water, and an expansion device downstream of the condenser; a refrigerant in the heat pump circuit; a bypass circuit having a first end connected to the heat pump circuit between the compressor and the condenser and a second end connected to the heat pump circuit between the expansion device and the evaporator, the bypass circuit including a valve for selectively diverting a portion of relatively hot refrigerant output from the compressor to the evaporator; a controller for controlling actuation of the valve. 2133973_2.doc 8 [0025] The controller may periodically open the valve to defrost the evaporator. In one embodiment, the controller may be responsive to an evaporator temperature sensor for detecting the evaporator temperature and may open the valve to defrost the evaporator only when the evaporator temperature sensor detects that the evaporator temperature is less than a predetermined lower temperature. If the evaporator temperature sensor is connected to the evaporator, the predetermined lower temperature may be 0 0 C. If the evaporator temperature sensor is connected to the heat pump circuit between the evaporator and the compressor, the predetermined lower temperature may be between 0 0 C and 10 0 C. The controller may close the valve if the evaporator temperature sensor detects that the evaporator temperature is greater than a predetermined higher temperature. If the evaporator temperature sensor is connected to the evaporator, the predetermined higher temperature may be between 5'C and 10 0 C. If the evaporator temperature sensor is connected to the heat pump circuit between the evaporator and the compressor, the predetermined higher temperature may be between 10 0 C and 15'C. In another embodiment, the controller may be responsive to an ambient air temperature sensor for detecting the ambient air temperature and to an evaporator temperature sensor for detecting the evaporator temperature and may open the valve to defrost the evaporator only when the temperature detected by the ambient air temperature sensor is less than a predetermined ambient air temperature and the temperature difference between the temperature detected by the ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is greater than a predetermined higher temperature difference. The predetermined ambient air temperature may be between 10 0 C and 20'C. The predetermined higher temperature difference may be between 10 C and 20'C. The controller may close the valve if the temperature difference between the temperature detected by the ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is less than a predetermined lower temperature difference. The predetermined lower temperature difference may be between 0 0 C and 10 0 C. In either embodiment, opening of the valve may also be modulated by a timer. The controller may be responsive to the timer to close the valve after a predetermined time, such as 10 minutes. The controller 2133973_2.doc 9 may also be responsive to the timer to prevent the valve from being re-opened for 45 minutes. [0026] A fan may be provided to blow ambient air toward the evaporator. The fan may be responsive to the controller. The controller may deactivate the fan when the valve is opened. The controller may reactivate the fan before closing the valve to blow melted frost off the evaporator. [0027] The controller may be responsive to a compressor temperature sensor for detecting the compressor temperature and may open the valve when the compressor temperature sensor detects that the compressor temperature is greater than a predetermined temperature, such as 90'C. The controller may close the valve when the compressor temperature sensor detects that the compressor temperature has fallen to a predetermined temperature, such as 80'C. The compressor temperature sensor may be connected to the heat pump circuit between the compressor and the reservoir. [0028] The controller may be responsive to an ambient air temperature sensor for detecting the ambient air temperature and may periodically open the valve when the ambient air temperature sensor detects that the ambient air temperature is less than a predetermined temperature, such as 10 0 C. [0029] In a fourth aspect, the present disclosure provides a water heater comprising: a water reservoir; and a heat exchanger element for carrying a hot refrigerant fluid, the heat exchanger element penetrating the water reservoir at a position below a minimum operational water level of the water heater and extending into the water reservoir to heat water in the reservoir. [0030] The heat exchanger element may be as defined in the first aspect above. [0031] The water heater may be a heat pump water heater further comprising: a heat pump circuit including: 2133973_2.doc 10 an evaporator exposed to ambient air, a compressor downstream of the evaporator, a condenser downstream of the compressor, the condenser defining a heat exchanger element penetrating the reservoir for immersion in the volume of water, and an expansion device downstream of the condenser; a refrigerant in the heat pump circuit; a bypass circuit having a first end connected to the heat pump circuit between the compressor and the condenser and a second end connected to the heat pump circuit between the expansion device and the evaporator, the bypass circuit including a valve for selectively diverting a portion of relatively hot refrigerant output from the compressor to the evaporator; and a controller for controlling actuation of the valve. [0032] The controller may periodically open the valve to defrost the evaporator. In one embodiment, the controller may be responsive to an evaporator temperature sensor for detecting the evaporator temperature and may open the valve to defrost the evaporator only when the evaporator temperature sensor detects that the evaporator temperature is less than a predetermined lower temperature. If the evaporator temperature sensor is connected to the evaporator, the predetermined lower temperature may be 0 0 C. If the evaporator temperature sensor is connected to the heat pump circuit between the evaporator and the compressor, the predetermined lower temperature may be between 0 0 C and 10 0 C. The controller may close the valve if the evaporator temperature sensor detects that the evaporator temperature is greater than a predetermined higher temperature. If the evaporator temperature sensor is connected to the evaporator, the predetermined higher temperature may be between 5'C and 10 0 C. If the evaporator temperature sensor is connected to the heat pump circuit between the evaporator and the compressor, the predetermined higher temperature may be between 10 0 C and 15'C. In another embodiment, the controller may be responsive to an ambient air temperature sensor for detecting the ambient air temperature and to an evaporator temperature sensor for detecting the evaporator temperature and may open the valve to defrost the evaporator only when the temperature detected by the ambient air 2133973_2.doc 11 temperature sensor is less than a predetermined ambient air temperature and the temperature difference between the temperature detected by the ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is greater than a predetermined higher temperature difference. The predetermined ambient air temperature may be between 10 0 C and 20'C. The predetermined higher temperature difference may be between 10 C and 20'C. The controller may close the valve if the temperature difference between the temperature detected by the ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is less than a predetermined lower temperature difference. The predetermined lower temperature difference may be between 0 0 C and 10 0 C. In either embodiment, opening of the valve may also be modulated by a timer. The controller may be responsive to the timer to close the valve after a predetermined time, such as 10 minutes. The controller may also be responsive to the timer to prevent the valve from being re-opened for 45 minutes. [0033] A fan may be provided to blow ambient air toward the evaporator. The fan may be responsive to the controller. The controller may deactivate the fan when the valve is opened. The controller may reactivate the fan before closing the valve to blow melted frost off the evaporator. [0034] The controller may be responsive to a compressor temperature sensor for detecting the compressor temperature and may open the valve when the compressor temperature sensor detects that the compressor temperature is greater than a predetermined temperature, such as 90'C. The controller may close the valve when the compressor temperature sensor detects that the compressor temperature has fallen to a predetermined temperature, such as 80'C. The compressor temperature sensor may be connected to the heat pump circuit between the compressor and the reservoir. [0035] The controller may be responsive to an ambient air temperature sensor for detecting the ambient air temperature and may periodically open the valve when the ambient air temperature sensor detects that the ambient air temperature is less than a predetermined temperature, such as 10 0 C. 2133973_2.doc 12 Brief Description of Drawings [0036] Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic front view of an embodiment of a water heater according to the present disclosure; Fig. 2 is a schematic front view of the water heater of Fig. 1 is a defrost cycle; Fig. 3 is an enlarged side elevational view through a portion of the heat exchanger element of the water heater of Fig. 1; and Fig. 4 is an enlarged transverse cross sectional view through a portion of the heat exchanger element of the water heater of Fig. 1. Description of Embodiments [0037] Referring to the drawings, there shown a heat pump water heater 10 comprising a water reservoir 12 for containing a volume of water to be heated. The reservoir 12 includes a cold water inlet 14 near its base and a hot water outlet 16 near its top. The water heater 10 includes a heat pump circuit including an evaporator 18 exposed to ambient air, a compressor 20 downstream of the evaporator, a condenser 22 downstream of the compressor, and an expansion device 24 downstream of the condenser. [0038] The condenser 22 defines a helically shaped heat exchanger element penetrating the reservoir 12 at a position below a minimum operational water level of the water heater for immersion in the volume of water to be heated. Since the condenser 22 penetrates the reservoir 12 below water level, it is relatively easy to seal between the condenser 22 and the reservoir 12, since the seal needs only be water tight and not vapour tight. [0039] The expansion device comprises a throttling restriction or an expansion valve or other means to drop the pressure and temperature of the refrigerant. The evaporator 2133973_2.doc 13 18 is in the form of a finned coil. A fan 26 is associated with the evaporator 18 to blow ambient air toward the evaporator. [0040] As best seen in Fig. 2, a bypass circuit is provided and has a first end 28 connected to the heat pump circuit between the compressor 20 and the condenser 22 and a second end 30 connected to the heat pump circuit between the expansion device 24 and the evaporator 18. The bypass circuit includes a solenoid valve 32 for selectively diverting a portion of relatively hot refrigerant output from the compressor 20 to the evaporator 18. [0041] A controller 34 and associated timer are provided for controlling actuation of the valve 32 and the fan 26. [0042] A filter/drier 36 is provided in the heat pump circuit between the reservoir 12 and the expansion device 24 to clean and dry the refrigerant prior to its passing to the expansion device. [0043] As best seen in Figs. 3 and 4, the condenser (heat exchanger element) 22 comprises an inner tube 22a and an outer tube 22b both formed from a heat conductive material. In one embodiment, both tubes are formed from copper. In another embodiment, the inner tube 22a is formed from copper and the outer tube 22b from stainless steel. Substantially the entire inner surface of the outer tube 22b is in contact with the outer surface of the inner tube 22a. The inner and outer tubes 22a, 22b have a twisted configuration such that a transverse cross section through the tubes has a rounded hexagonal shape to increase the heat-exchange area of the condenser 22 compared to a circular section tube of similar diameter. [0044] In use, the compressor 20 circulates refrigerant through the heat pump circuit. The compressor 20 draws refrigerant vapour from the evaporator 18, and compresses the low pressure, low temperature refrigerant vapour to a high pressure and high temperature superheated state, before exhausting the refrigerant vapour to the condenser 22. Heat from the high temperature refrigerant vapour is transferred via the 2133973_2.doc 14 condenser 22 to the water in the reservoir 12. Accordingly, the refrigerant is de superheated, condensed, and sub-cooled to sub-cooled liquid state. The sub-cooled refrigerant from the condenser 22 then passes through the filter/drier 36 and into the expansion device 24. After the expansion device 24, the temperature of the refrigerant is below the temperature of the surrounding ambient air, such that heat is transferred from the ambient air to the refrigerant in the evaporator 18 to vaporise the refrigerant. Actuation of the fan 26 facilitates heat transfer between the ambient air and the refrigerant in the evaporator. The cycle is continued until the water temperature in the reservoir 12 reaches a desired temperature. [0045] In some embodiments, the controller 34 periodically opens the valve 32 to divert a small portion of hot refrigerant gas through the bypass circuit to defrost the evaporator 18. In the illustrated embodiment, however, the controller 34 is responsive to an evaporator temperature sensor 38 on the evaporator for detecting the temperature of the evaporator 18 and opens the valve 32 to defrost the evaporator 18 only when the evaporator temperature sensor 38 detects that the evaporator temperature is less than 0 0 C. The evaporator temperature sensor 38 may alternatively be connected to the heat pump circuit between the evaporator 18 and the compressor 20, in which case the controller 34 opens the valve if the sensor 38 detects that the evaporator temperature is less than 5'C. The controller 34 also deactivates the fan 26 when the valve 32 is opened and reactivates the fan 26 before closing the valve 32 to blow melted frost off the evaporator 18. The controller 34 closes the valve 32 if the sensor 38 detects that the evaporator temperature has increased to 8'C (or to 13 0 C if the sensor 38 is connected to the heat pump circuit between the evaporator and the compressor) or after 10 minutes. The controller 34 prevents the valve 32 being reopened for 45 minutes. [0046] In another embodiment, the controller 34 is responsive to the evaporator temperature sensor 38 and to an ambient air temperature sensor 42 for detecting the ambient air temperature. In such embodiment, the controller 34 opens the valve 32 to defrost the evaporator 18 only when the temperature detected by the ambient air temperature sensor 42 is less than 15 C and the temperature difference between the temperature detected by the ambient air temperature sensor 42 and the temperature 2133973_2.doc 15 detected by the evaporator temperature sensor 38 is greater than 15 C. The controller 34 closes the valve 32 if the temperature difference between the temperature detected by the ambient air temperature sensor 42 and the temperature detected by the evaporator temperature sensor 38 is less than 5 0 C or after 10 minutes. Again, the controller 34 prevents the valve 32 being reopened for 45 minutes. [0047] In cold weather conditions, low ambient temperatures can cause the density of suction gas in the heat pump circuit to become so low that it can be difficult for the refrigerant vapour to carry oil from the evaporator 18 or the compressor suction line returning to the compressor 20. Accordingly, in a yet further embodiment, the controller 34 is responsive to an ambient air temperature sensor for detecting the ambient air temperature and periodically opens the valve 32 when the ambient air temperature sensor detects that the ambient air temperature is less than around 7 0 C. Such opening of the valve 32 diverts some of the hot refrigerant vapour discharged from the compressor 20 to the evaporator 18 to facilitate the oil returning to the compressor 20. [0048] Returning to the illustrated embodiment, the controller 34 is also responsive to a compressor temperature sensor 40 for detecting the temperature of the compressor 20 and opens the valve 32 when the compressor temperature sensor 40 detects that the compressor temperature is greater than 90'C. The controller 34 closes the valve 32 when the compressor temperature sensor detects 40 that the compressor temperature has fallen to 80'C. The compressor temperature sensor 40 is connected to the heat pump circuit between the compressor 20 and the reservoir 12. [0049] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. Examples of possible variations and/or modifications include, but are not limited to: 2133973_2.doc 16 e the heat exchanger element 18 may be used in a water heater without a heat pump circuit, wherein hot refrigerant fluid is pumped through the heat exchanger element 18 to heat water in the reservoir of the water heater; e the condenser tubes 22a, 22b having a different cross-sectional shape, such as an oval shape or a rounded square, pentagonal, octagonal or other rounded polygonal shape; e the non-circular rounded polygonal or oval cross-sectional shape of the tubes 22a, 22b may be limited to a downstream portion of the tubes, with an upstream portion of the tubes having a circular cross-sectional shape; e forming both the tubes 22a, 22b from copper or both the tubes 22a, 22b from stainless steel; and/or e providing an air vent between the inner and outer heat exchanger tubes 22a, 22b. 2133973_2.doc

Claims (20)

1. A heat exchanger element for carrying a refrigerant fluid, the heat exchanger element comprising: an inner tube and an outer tube, the inner tube and outer tube being formed from a heat 5 conductive material, wherein substantially the entire inner surface of the outer tube being in contact with the outer surface of the inner tube, and wherein: a transverse cross section of at least a portion of the inner and outer tubes has a non circular shape, and/or the inner tube is formed from copper and the outer tube is formed from stainless 10 steel.

2. A heat exchanger element according to claim 1, wherein the transverse cross sectional shape is a rounded polygonal shape.

3. A heat exchanger element according to claim 2, wherein the rounded polygonal shape is imparted by twisting said at least part of the tubes. 15

4. A heat exchanger element according to claim 1, wherein the transverse cross sectional shape is an oval shape.

5. A heat exchanger element according to any one of the preceding claims, being substantially helically shaped.

6. A heat exchanger element according to any one of the preceding claims, comprising an air 20 vent between the inner and outer tubes.

7. A water heater comprising: a water reservoir; and a heat exchanger element as defined in any one of the preceding claims extending into the water reservoir to heat water in the reservoir. 25

8. A water heater according to claim 7, wherein the element penetrates the water reservoir at a position below a minimum operational water level of the water heater.

9. A heat pump water heater comprising: a water reservoir for containing a volume of water to be heated; a heat pump circuit including: 30 an evaporator exposed to ambient air, a compressor downstream of the evaporator coil, a condenser downstream of the compressor, the condenser defining a heat exchanger element penetrating the reservoir for immersion in the volume of water, and an expansion device downstream of the condenser; 35 a refrigerant in the heat pump circuit; a bypass circuit having a first end connected to the heat pump circuit between the compressor and the condenser and a second end connected to the heat pump circuit between the expansion device and the evaporator, the bypass circuit including a valve for selectively diverting a portion of relatively hot refrigerant output from the compressor to the evaporator; 18 a controller for controlling actuation of the valve.

10. A water heater according to claim 9, wherein the controller periodically opens the valve to defrost the evaporator.

11. A water heater according to claim 10, wherein: 5 the controller is responsive to an evaporator temperature sensor for detecting the evaporator temperature and opens the valve to defrost the evaporator only when the evaporator temperature sensor detects that the evaporator temperature is less than a predetermined lower temperature, and the controller closes the valve if the evaporator temperature sensor detects that the 10 evaporator temperature is greater than a predetermined higher temperature.

12. A water heater according to claim 10, wherein the controller is responsive to an ambient air temperature sensor for detecting the ambient air temperature and to an evaporator temperature sensor for detecting the evaporator temperature and wherein the controller: opens the valve to defrost the evaporator only when the temperature detected by the 15 ambient air temperature sensor is less than a predetermined ambient air temperature and the temperature difference between the temperature detected by the ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is greater than a predetermined higher temperature difference, and closes the valve if the temperature difference between the temperature detected by the 20 ambient air temperature sensor and the temperature detected by the evaporator temperature sensor is less than a predetermined lower temperature difference.

13. A water heater according to claim 11 or claim 12, wherein actuation of the valve is modulated by a timer.

14. A water heater according to claim 13, wherein the timer modulates actuation of the valve 25 to cause the valve to close if it is still open after a predetermined time and/or to prevent the valve from being re-opened for a predetermined period.

15. A water heater according to any one of claims 9 to 14, comprising a fan to blow ambient air toward the evaporator.

16. A water heater according to claim 15, wherein the fan is responsive to the controller, and 30 wherein the controller deactivates the fan when the valve is opened and/or activates the fan before closing the valve to blow melted frost off the evaporator.

17. A water heater according to any one of claims 9 to 16, wherein the controller is responsive to a compressor temperature sensor for detecting the compressor temperature and wherein the controller: 35 opens the valve when the compressor temperature sensor detects that the compressor temperature is greater than a first predetermined temperature, and closes the valve when the compressor temperature sensor detects that the compressor temperature has fallen to a second predetermined temperature. 19

18. A water heater according to any one of claims 9 to 17, wherein the controller is responsive to an ambient air temperature sensor for detecting the ambient air temperature and periodically opens the valve when the ambient air temperature sensor detects that the ambient air temperature is less than a predetermined temperature. 5

19. A water heater according to any one of claims 9 to 18, wherein the heat exchanger element is a heat exchanger element as defined in any one of claims 1 to 6.

20. A heat exchanger element or water heater substantially according to any one embodiment as hereinbefore described with reference to the accompanying drawings.