Patent Publication Number: US-9845978-B2

Title: Residential heat pump water heater

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of prior-filed, U.S., non-provisional patent application Ser. No. 12/371,572, filed on Feb. 13, 2009 now U.S. Pat. No. 8,422,870. 
    
    
     FIELD OF INVENTION 
     Embodiments of the present invention relate to water heaters. More specifically, embodiments of the present invention relate to, heat pump water heaters. 
     BACKGROUND OF THE INVENTION 
     A widely accepted and used water heater for residential hot water production and storage is the electric resistance water heater and storage tank. Water heaters typically include a tank defining a chamber for retention of water. A water inlet pipe that is provided with a first connection for interconnection with a cold water supply line that conveys fresh relatively cold water into the chamber. Within the tank there are electric resistance elements that heat the water in the tank. In current embodiments, there are at least two electric resistance elements. A first electric resistance element positioned near the bottom of the tank and a second electric resistance element positioned near the top of the tank. There are also two sensors positioned on the exterior of the tank that measure the temperature of the tank near the top and bottom of the tank in proximity to the location of the electric resistance elements. When the temperature sensed by such sensors drops below a certain temperature level, these sensors close the contacts associated with the corresponding electric resistance elements causing the electric resistance elements to energize. 
     When water is supplied to the tank, it is supplied through a dip tube that pushes the cold water to the bottom of the tank and thereby pushes the hot water out of the top through the outlet pipe where water is the hottest. One of the problems with this configuration is that the sensor near the top of the tank can&#39;t detect that hot water is exiting and cold water is entering the tank near the bottom. The lower sensor detects that cold water is entering the tank when it detects a temperature drop at the thermostat, which is the primary purpose for having two sensors. When the lower sensor detects a temperature drop below a certain level, it closes the contact and energizes the lower electric resistance element until the temperature reaches a specified level. But, each time the lower electric resistance element heats the water; the heated water is buoyant and goes up to the top of the tank. For example, if the tank is holds 50 gallons of water, and three gallons of water flow into the tank, it may cause the lower electric resistance element to be energized for a few minutes in order to recover the temperature. If a few minutes later, there is a draw of another three gallons of water, the lower electric resistance element is energized again for another few minutes in order to recover the temperature. This causes the heated water to rise to the top creating a problem called stacking. Under sequential small draws of water, the lower electric resistance element is energized each time and runs until the lower sensor is satisfied that the lower part of the tank is sufficiently warm. When this is occurring, the top part of the tank continues to get a little bit hotter each time which causes over heating of water in the top of the tank, which can potentially lead to undesirably hot water being drawn from the tank. So there is a need for a configuration that solves the problem associated with stacking resulting from small sequential water draws made on current water heaters. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Consistent with embodiments of the present invention, systems for controlling a heat pump water heater (HPWH) are disclosed. The systems are configured to heat water within a water storage tank of a heat pump water heater wherein a controller within the system is operatively connected to a heat pump and a pair of electric resistance heaters. The heat pump condenser is positioned proximate the water storage tank to facilitate the transfer heat from the condenser to the water in the water storage tank. A first electric resistance heater is positioned within the water storage tank in proximity to the bottom of the water storage tank. A second electric resistance heater is positioned within the water storage tank in proximity to the top of the water storage tank. The system further includes a temperature sensor positioned to determine the temperature of water within and in proximity to the top of the water storage tank. The controller includes a module configured to process data representative of temperature readings of water within the water storage tank. Upon processing temperature readings, the controller automatically selects and controls energizing of a heat pump condenser and the pair of electric heating elements. Data representative of the rate at which water flows into and from the water storage tank, the temperature of refrigerant, typically in a gas or vapor state entering a condenser and the temperature of refrigerant, typically at least partially in liquid state exiting the condenser is also processed by the controller. The controller automatically energizes one or more of the heat pump, the first electric resistance heater, and the second electric resistance heater in response to data processed. 
     Still consistent with embodiments of the present invention, methods of heating water within a water storage tank of a heat pump water heater including a controller operatively connected to heating elements and sensors selects and utilizes the appropriate heating elements to heat the water within the water storage tank without overheating of the water are disclosed. The methods may include positioning condensers of a heat pump in proximate the water storage tank for a heat exchange relationship with the water contained in the tank to transfer heat to the water in the tank, positioning electric heating elements to heat water within the water storage tank, periodically processing the temperature readings measured by a plurality of sensors in order to automatically control the selection and energizing one or more of the heat pump and the electric heating elements. 
     Still consistent with embodiments of the present invention, a method and apparatus for heating water within a water storage tank of a heat pump water heater including a condenser configuration in which the inlet portion of the condenser is positioned proximate the bottom of the tank to deliver heat first to the water in the lowermost region of the tank are disclosed. 
     Still consistent with embodiments of the present invention, a method and apparatus for heating water within a water storage tank of a water heater including a controller operatively connected to heating elements, which avoids the aforementioned stacking problem while using a single sensor for monitoring the temperature of the water in the tank and an event flow module configured to receive and process data representative of temperature readings measured by the temperature sensor in order to determine if water is flowing through the water storage tank are disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Non-limiting and non-exhaustive embodiments are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG. 1  depicts a first embodiment of a heat pump water heater schematic consistent with embodiments of the invention; 
         FIG. 2  depicts a second embodiment of a heat pump water heater schematic consistent with embodiments of the invention; 
         FIG. 3  depicts a third embodiment of a heat pump water heater schematic consistent with embodiments of the invention; 
         FIG. 4  depicts a fourth embodiment of a heat pump water heater schematic consistent with embodiments of the invention; 
         FIG. 5A  depicts a fifth embodiment of a heat pump water heater schematic consistent with embodiments of the invention; 
         FIG. 5B  depicts a sixth embodiment of a heat pump water heater schematic consistent with embodiments of the invention; 
         FIG. 6  is a graph illustrating a comparison of sensor output near the top of a storage tank when the unit is in standby mode and when there is a rate of one gallon per minute of water flowing from the water storage tank; 
         FIGS. 7A and 7B  depict a control block diagram and a wiring diagram respectively, consistent with embodiments of the invention; and 
         FIGS. 8A-8E  illustrates a process flow of the temperature and flow module&#39;s automatic control of the heat pump condenser and electric heating elements. 
     
    
    
     GENERAL DESCRIPTION 
     Reference may be made throughout this specification to “one embodiment,” “an embodiment,” “embodiments,” “an aspect,” or “aspects” meaning that a particular described feature, structure, or characteristic may be included in at least one embodiment of the present invention. Thus, usage of such phrases may refer to more than just one embodiment or aspect. In addition, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments or aspects. Moreover, reference to a single item may mean a single item or a plurality of items, just as reference to a plurality of items may mean a single item. 
     Embodiments of the present invention utilize a controller programmed to control a heat pump water heater, though not all aspects of the invention are limited to heat pump water heaters, but may have other applications as well, such as for example, electric water heaters. The controller may be programmed to have preset modes of operation. In addition, the controller may be programmed to interpret various temperature and data inputs for use in controlling the heat sources of the water heater. Furthermore, the temperature and data inputs may be interpreted by the controller to automatically select and energize one or more of the electric heating elements and heat pump (via energization of the compressor) in an effort to efficiently heat the water in a manner that prevents over heating of the water caused by stacking. 
     DETAILED DESCRIPTION 
     Various embodiments are described more fully below with reference to the accompanying drawings, which form a part hereof, and which show specific embodiments of the invention. However, embodiments may be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Accordingly, the following detailed description is, therefore, not to be taken in a limiting sense. 
     The embodiments described set forth embodiments of a heat pump water heater system that utilizes one or more electric resistance elements, as well as a heat pump or refrigeration sealed system to impart heat to the water. The energy required to heat water is significantly reduced by utilizing a refrigeration sealed system to move the heat from the air of the surrounding warm environment into the water. In the embodiments described in  FIGS. 1-5 , each heat pump water heater system discloses various sensors, one of which is positioned to sense water temperature within the water storage tank. The data from this sensor is used not only to sense the temperature of the water in the tank but also, in embodiments lacking a flow meter, to indirectly detect the occurrence of a flow event, that is, the flow of hot water and the associated flow of cold water out of and into the water storage tank respectively. In some embodiments a flow meter is used to directly detect the occurrence of a flow event. The sensor positioned to sense water temperature within the water storage tank may be positioned within the water storage tank or alternatively on the exterior of the tank in contact with the tank sidewall. 
     The data representative of water temperature within the water storage tank, and the occurrence of a flow event is transmitted to a controller for processing. The controller is operatively connected to electric resistance heaters and the heat pump and includes a module that facilitates the automatic selection and energizing of at least one of the heat pump and the electric resistance heaters in response to data received that is representative of water temperature within the water storage tank and the occurrence of a water flow event. 
     Referring now to the figures,  FIG. 1  depicts a heat pump water heater  100  schematic consistent with a first embodiment of the invention. The heat pump system comprises an evaporator  102 , a compressor  130 , a condenser  108 , a throttling device  106 , and at least one fan  104 . Condenser  108  is assembled in a heat exchange relationship with the water in the water storage tank  120 . During operation of the heat pump cycle a refrigerant exits the evaporator  102  as a fluid in the form of a superheated vapor and/or high quality vapor mixture. Upon exiting the evaporator  102  the refrigerant enters the compressor  130  where the pressure and temperature increase. The temperature and pressure are increased in the compressor  130  such that the refrigerant becomes a superheated vapor. The superheated vapor from the compressor  130  enters the condenser  108 . While in the condenser  108 , the superheated vapor transfers energy to the water within a storage tank  120 . Upon transferring energy to the water within the storage tank  120 , the refrigerant turns into a saturated liquid and/or high quality liquid vapor mixture. This high quality/saturated liquid vapor mixture exits the condenser  108  and travels through the throttling device  106 . Upon exiting the throttling device  106  the pressure and temperature of the refrigerant drop at which time the refrigerant enters evaporator  102  and the cycle repeats itself. 
     The heat pump water heater  100  includes a water inlet line  112  for allowing cold water to enter the heat pump water heater  100 , where it is directed to the bottom of the tank  120  via a dip tube  110 . The heat pump water heater  100  also has electric heating elements  122  and  124  placed near the top and bottom of the water storage tank  120  to heat the water. In the embodiments herein described the heating elements are shown projecting into the interior of the tank, however, other configurations providing for positioning the upper and lower elements to heat the water in the upper and lower regions of the tank respectively could be similarly employed. The heated water exits the heat pump water heater near top of tank  120  at exit  114  and flows to the residence or other place where heated water is desired. The heat pump water heater  100  has a temperature sensor  126  positioned to sense the temperature of the water in the upper region of the tank and may also have additional temperature sensors placed at various locations for sensing other temperatures, such as heat pump condenser inlet and outlet temperatures, ambient temperature, etc. 
     In the first embodiment illustrated in  FIG. 1 , a single water temperature sensor  126  is positioned toward the upper end of the tank  120 . The heat pump condenser  108  is positioned in a heat exchange arrangement with the water storage tank  120  to enable heat from the condenser to heat the water in the storage tank. The system includes a controller  152 , equipped with a microprocessor programmed to include a water temperature and flow module, which is operatively connected to the heat pump water heater and configured to receive data representative of temperature readings measured by the single sensor  126 . The temperature readings received by the controller  152  are processed by the water temperature and flow module to determine the temperature of the water in the tank  120 . The water temperature and flow module within the controller  152  is further configured to process data representative of temperature readings measured by the single sensor  126  to determine the rate at which the temperature of water in the water storage tank  120  changes. In response to the sensed water temperature and the rate at which the temperature of water in the water storage tank  120  changes, the controller  152  determines which of the compressor  130 , an upper electric resistance heater  122 , and a lower electric resistance heater  124  shall be energized, and for how long, in order to heat the water within the water storage tank  120 . The controller  152  and the water temperature and flow module stored therein, along with the single sensor  126 , are operatively configured to effectively respond to small amounts of water being withdrawn from the water storage tank which causes small temperature changes, thereby eliminating the need for a second sensor to be positioned in the lower portion of the water storage tank  120 . This rate of temperature change information could also be used in lieu of a flow meter to detect the approximate flow rate of water being withdrawn from the tank, for example, by comparing the detected rate of change of temperature with a look up table comprising a set of empirically determined temperature change rate/flow rate correlations and choosing the flow rate associated from the table that is closest to the detected temperature rate of change. 
     The system may also be programmed to determine how much water is used in a short period of time in order to use that information to determine the most efficient manner to heat the unheated water added to the water storage tank  120 . 
     As illustrated in  FIG. 2 , the second embodiment includes a flow meter  216  positioned in the water inlet line  212 . Flow meter  216  transmits data representative of the amount of water flowing into the water storage tank  220  to the water temperature and flow module within the controller  252 . Under circumstances in which large amounts of water are removed from the water storage tank  220  in short periods of time, single sensor  226  may not read changes in water temperature at a speed that facilitates immediate recognition by the controller  252  that large amounts of water have been removed from the water storage tank  220 . When large amounts of water are removed from water storage tank  220 , the energizing of alternate and/or additional heating elements may be necessary in order to heat the water in the most efficient and timely manner. The controller  252  processes data representative of the rate of flow of water into the water storage tank  220  received from the flow meter  216  along with the data received from single sensor  226  in order to determine which of the compressor  230 , the upper electric resistance heater  222 , and the lower electric resistance heater  224  shall be energized in order to heat the water within the water storage tank  220 . 
     The embodiment illustrated in  FIG. 2 , also includes a second temperature sensor  228  in the water inlet line  212  in order to sense the temperature of the water flowing into water storage tank  220  through the water inlet line  212 . The controller  252  processes data representative of the temperature of water flowing into water storage tank  220  through the water inlet line  212  in order to determine a projected temperature of the water within the water storage tank  220  when the water flowing into the water storage tank and the heated water already within the water storage tank are combined. The ability to project a resultant temperature from the combination of unheated water flowing into the water storage tank  220  and the heated water already within the water storage tank  220  allows the controller  252  to preemptively and automatically determine which of the compressor  230 , the upper electric resistance heater  222 , and the lower electric resistance heater  224  needs to be energized in order to heat the water within the water storage tank  220 . As illustrated, the heat pump water heater also includes an external temperature sensor  232  which is configured to transmit data representative of the temperature of the air surrounding the water storage tank  220  to the controller  252  for processing. The controller  252  processes data representative of the temperature of the air surrounding the water storage tank  220  in order to determine the efficiency of the heat pump system when compressor  230  is energized to heat the water within the water storage tank  220 . 
     In the first and second embodiments illustrated in  FIGS. 1 and 2  respectively, because thermistors are used to sense temperature as opposed to the bi-metal sensors which are commonly used in the industry, the system controller  152 / 252  can detect small changes in temperature. The water temperature and flow module within the controller  152 / 252  processes the temperature readings transmitted by temperature sensor  126 / 226  to detect the rate of change in temperatures measured by sensor  126 / 226  over time. For example, if water storage tank  120 / 220  is full of water heated to a previously defined temperature and then a user draws a small amount of water, such as three to five gallons, then temperature sensor  126 / 226  will detect some but not a significant change in water temperature. It is possible to distinguish between the change in temperature caused by a withdrawal of water from water storage tank  120 / 220  and a change in temperature resulting from the HPWH system being in standby mode and no water is being withdrawn because, the water temperature and flow module monitors the decline in water temperature measured by the temperature sensor  126 / 226  over time. In standby mode, the decline in temperature measured by temperature sensor  126 / 226  is a very slow decline. As soon as water is withdrawn, even at a low flow rate, the rate of decline measured by temperature sensor  126 / 226  is faster than the decline measured by sensor  126 / 226  when no water is being withdrawn. Accordingly, a fast decline in water temperature over time measured by sensor  126 / 226  is an indication to the controller  152  that a flow event has occurred. In embodiments such as  FIG. 1 , which does not include a flow meter to measure flow directly, flow events are determined by the controller  152  based on the speed with which the temperature of water within the water storage tank changes over time. 
     Upon detection of a flow event, the controller  152  may decide to energize the lower heating element or compressor and may transmit heat to the water for as long as is required to get the temperature back up to the set point temperature as detected by the sensor  126 . This allows the lower heating element or compressor to be energized upon detection of a flow event and heat the water in the lower portion of the tank without causing water in the top of the water storage tank to be overheated. By controlling the lower heating element  124  or compressor with the upper sensor  126 , in this manner, sequential small draws of water will not result in the water in the top of the water storage tank  120  being overheated as a result of stacking. 
     Referring still to the embodiment of  FIG. 2 , a temperature sensor is also placed at the outlet of the compressor  230  as indicated by reference numeral  234  to sense the temperature of the super heated vapor exiting the compressor, which is also essentially the temperature of the vapor entering the condenser  208 . A temperature sensor  236  is also placed at the outlet of condenser  208  in order to measure the temperature of the refrigerant exiting the condenser  208 . Temperature sensors  234  and  236  allow a system controller  252  to approximate energy transmitted to the water within the water storage tank by the condenser  208 . Data representative of the drop in temperature across the condenser as measured by temperature sensors  234  and  236  is transmitted to the controller  252  and processed along with the previously described temperature data to automatically determine whether additional heating elements  222  or  224  need to be activated in order to heat the water within the water storage tank  220 . Similarly, temperature sensors  242  and  244  are used to measure the inlet and outlet temperatures respectively of the evaporator, to monitor the evaporator “superheat”. When operating properly, the temperature difference between the outlet temperature and inlet temperature should be on the order of 10° F. For efficient operation of the sealed system and to avoid potential damage to the compressor resulting from refrigerant not fully evaporating, the controller  252  is configured to turn off the sealed system if the temperature difference between the outlet temperature and the inlet temperature is less than 5° F. Also, if the temperature difference is too high, a signal may be generated to inform the user of inefficient operation. 
     The system controller  252  is operatively connected to the heat pump water heater  200  and configured to receive data representative of temperature readings measured by the temperature sensors  226 ,  236 ,  232 ,  234 ,  242  and  244 . During operation of the heat pump water heater  200 , any one of the electric heating elements  222  and  224 , and compressor  230  may also operate at any given time. Generally, the compressor  230  and the electric heating elements  222  and  224  do not operate at the same time. However, it is contemplated that one of electric heating elements  222  or  224  and the compressor  230  may operate simultaneously. While it is contemplated that both electric heating elements  222  and  224  and the compressor  230  may operate at a given time, operation of both heating elements  222  and  224  at the same time may require special electrical considerations (e.g. a larger circuit breaker, a dedicated circuit, etc.) to accommodate an increased current draw. Therefore in the illustrative embodiments simultaneous energization of both heating elements is avoided. 
     Referring now to  FIG. 3 ,  FIG. 3  depicts a heat pump water heater  300  schematic consistent with embodiments of the invention. The heat pump system comprises an evaporator  302 , a compressor  330 , a condenser  308 , a throttling device  306 , and at least one fan  304 . During operation of the heat pump cycle a refrigerant exits the evaporator  302  as a superheated vapor and/or high quality vapor mixture. The heat pump water heater  300  may have sensors placed at various locations. In the embodiment of  FIG. 3 , a temperature sensor  328  is placed in tank  320  near upper heating element  322 . The positioning of a temperature sensor  328  inside of the tank, connected to a rod  340  allows for the water temperature to be sensed directly, rather than by measuring tank wall temperature and inferring water temperature. Positioning the temperature sensor  328  inside of the water storage tank  320  improves the response time and accuracy of water temperatures sensed. Temperature sensor  334  is placed at the outlet of the compressor  330  to measure the compressor discharge temperature to protect against overheating the compressor. A temperature sensor  332  is provided to measure ambient temperature. Additionally, temperature sensors  336  and  338  measure the evaporator  302  inlet and exit temperatures respectively. This embodiment of the heat pump water heater  300  also has electric heating elements  322  and  324  placed near the top and bottom of the water storage tank  320  respectively. 
     The HPWH system  300  includes a controller  352  that is operatively configured to receive data representative of temperature readings measured by the temperature sensors  328 ,  332 ,  334 ,  336  and  338 . Data from sensors  332 ,  336  and  338  are used by controller  352  in the same manner as was described with reference to their counterparts in the embodiment of  FIG. 2 . Controller  352  is configured to process the compressor discharge data and turn the compressor off if the sensed temperature exceeds a predetermined reference temperature selected to prevent overheating of the compressor. For example, in the illustrative embodiment, a reference temperature of 240° F. is used. The heat pump water heater  300  includes an inlet  312  for allowing cold water to enter the heat pump water heater  300 , where it is directed to the bottom of the tank  320  via a dip tube  310 . The heated water exits the heat pump water heater near top of tank  320  at exit  314  and flows to the residence or other place where heated water is desired. The heat pump water heater  300  also includes a flow meter  316  for measuring the amount and the flow rate of water into the water storage tank  320 . The flow meter  316  measures the total amount of water that has flowed into the water storage tank  320  during a given time interval. For example, the flow meter  316  may determine that in a given month a homeowner may have used 1,000 gallons of heated water. Temperature sensors  328 ,  332 ,  334 ,  336  and  338  are configured to transmit data representative of the temperatures measured to the controller  352  for processing. The controller  352  processes this temperature data and flow rate data to automatically determine which of the compressor  330 , the upper electric resistance heater  322 , and the lower electric resistance heater  324  shall be energized in order to heat the water within the water storage tank  320 . 
     Referring now to  FIG. 4 ,  FIG. 4  depicts a heat pump water heater  400  schematic consistent with embodiments of the invention. The heat pump system is of a configuration similar to that illustrated in  FIG. 3 , except that the condenser  408  substantially covers the water tank  420  side walls, facilitating the ability to provide more heat to the water within the water storage tank through the condenser wrapped around the exterior of the water storage tank  420 . 
     Referring now to  FIGS. 5A and 5B ,  FIG. 5A  depicts a heat pump water heater  500  schematic consistent with embodiments of the invention. The heat pump system is of a configuration similar to that illustrated in  FIG. 1 , except that the condenser  508 , includes a cylindrical portion which at least partially covers the side wall of water tank  520 , and a bottom portion which at least partially covers the bottom wall  518 , facilitating the ability to provide more heat from the condenser to the coldest water within the water storage tank through the bottom portion of condenser  508 . In the embodiment shown in  FIG. 5A , the bottom portion of condenser  508  is in contact with the bottom wall  518  in a spiral coil configuration. It is contemplated that the bottom portion of condenser  508  that is in contact with the bottom wall  518  may be in alternative configurations so long as the alternative configuration allows for increased surface area contact with bottom wall  518 . The heat pump system comprises an evaporator  502 , a compressor  530 , a condenser  508 , a throttling device  506 , at least one fan  504 , and electric heating elements  522  and  524  placed near the top and bottom of the water storage tank  520  respectively. A thermistor  526  is placed in tank  520  near upper heating element  522 . 
     The heat pump water heater  500  includes an inlet  512  for allowing cold water to enter the heat pump water heater  500 , where it is directed to the bottom of the tank  520  via a dip tube  510 . The heated water exits the heat pump water heater near top of tank  520  at exit  514  and flows to the residence or other place where heated water is desired. Temperature sensor  526  is configured to transmit data representative of the temperatures measured to the controller  552  for processing. The controller  552  processes data representative of the temperature of water within the water storage tank  520  in order to determine which of the compressor  530 , the upper electric resistance heater  522 , and the lower electric resistance heater  524  shall be energized in order to heat the water within the water storage tank  520 . 
       FIGS. 5A and 5B  also illustrate a heat pump water heater system  500  wherein the condenser  508  is operatively connected to compressor  530  in a manner that facilitates delivery of the super heated refrigerant vapor from the compressor  530  to the condenser  508  in a manner whereby the super heated refrigerant vapor at its hottest state is channeled initially through the inlet portion of the condenser  508  which is proximate the bottom of the water storage tank  520 . Transitioning the super heated refrigerant vapor through the condenser  508  such that the refrigerant vapor transmits heat first to the lower portion of the water storage tank  520  allows for the super heated refrigerant vapor to transmit heat to water within the water storage tank  520  at its coldest point. The embodiments illustrated in  FIGS. 1-4  transmit super heated refrigerant vapor from the compressor  503  to the condenser  508  in a top to bottom manner whereby the super heated refrigerant vapor at its hottest state engages the upper and middle portion of the water storage tank  520  first. In these embodiments the water at the lowest portion of the water storage tank is heated by super heated refrigerant vapor within the condenser  508  that has been cycled through the condenser  508  and had heat removed from the upper portion of the water storage tank  520 . The embodiments illustrated in  FIGS. 5A and 5B  are configured to facilitate the transmission of heat from the super heated vapor traveling through the condenser  508  to the coldest water within the water storage tank  520  initially. More specifically, in the embodiment of  FIG. 5A , the super heated refrigerant vapor initially enters and flows through the bottom portion of the condenser  508  and then enters the cylindrical portion of the condenser at the lowermost point of the cylindrical portion, and from there flows gradually upward through the cylindrical spiral around the side wall of the water tank and exits the condenser  508  near the top of the water storage tank  520 . The result is a relatively low temperature gradient in the water storage tank  520  relative to that which is typical of the more conventional top to bottom condenser arrangements of the prior art. For example, in the configuration of illustrative embodiment of  FIG. 5A , temperature gradients in the tank on the order of 1° F.-3° F. have been achieved, as compared to gradients on the order of 15° F.-30° F. for conventional heat pump water heater configurations and 10° F. from the lower element to the top of the tank for conventional electric water heaters. If measured from the area beneath the lower element to the top of the tank in electric water heaters, the gradient can be closer to 50° F.-60° F. This means that in the illustrative embodiment, the entire tank  520  of water has been heated to the set-point temperature, not just the water in the top portion of the water storage tank  520 . This configuration is very effective in reducing the temperature gradient in the tank. However, it is vulnerable to a potential refrigerant migration problem under certain, relatively rare conditions. If the incoming water is particularly cold, e.g., 35° F.-40° F., and enough water is withdrawn rapidly so that at least half, but not substantially all, of the condenser coil is exposed to the incoming cold water, the refrigerant in the condenser will tend to migrate to the coldest portion of the condenser which is the portion extending beneath the bottom of the tank rather than circulating up through the portion of the condenser which circumscribes the tank side wall resulting in not enough refrigerant returning to the compressor for efficient operation of the sealed system. For example, in a 50 gallon storage tank, a withdrawal on the order of approximately 20 gallons of water could create such a condition. This vulnerability is avoided in the embodiment illustrated in  FIG. 5B  which includes a “Return Flow” condenser design component. In the design illustrated in  FIG. 5B , the vapor flow first enters the cylindrical portion of condenser  508 . The inlet to this portion of the condenser is located at the lower most point  509  of the cylindrical portion. The vapor flows up through the portion of the condenser coil  508  that wraps around the water tank and then flows down to and through the bottom portion  507  on the bottom of tank  518  exiting the condenser at the bottom of the tank. This arrangement still exposes the hot refrigerant vapor initially to the coldest portion of the tank proximate the bottom. However, by proceeding up the cylindrical portion before entering to the bottom portion, the coldest portion of the condenser is now near the exit of the condenser and the compressor is able to circulate the refrigerant through and out of the condenser thus avoiding the refrigerant migration problem. 
     Each of the embodiments of the heat pump water heater disclosed in  FIGS. 1-5  has four modes of operation. An electric mode, a heat pump mode a hybrid mode and a high demand mode. The electric mode—operates using only electric resistance heaters. Heat pump mode—uses only the sealed system driven by the compressor to heat water. Hybrid Mode—uses the sealed system driven by the compressor as the primary energy source for heating the water, but also uses the upper electric element to enable a more rapid recovery from events creating a relatively low temperature condition in the tank, such as when filling an empty or substantially empty tank, or following the withdrawal of a relatively large amount of hot water in a relatively short time. Like the hybrid mode, the high demand mode selectively uses the sealed system and the upper electric element, but it also selectively uses the lower heating element to enable rapid recovery when particularly large amounts of water are being withdrawn at a high rate over an extended time. 
     The controller is operative in all four modes to respond to standby cooling, that is, the gradual lowering of the temperature of the water in the tank due to heat loss through the insulated tank walls over time when no heat is being applied and no hot water is being withdrawn, and to flow event cooling, that is, the lowering of the temperature resulting from the withdrawal of hot water from the top of the tank which is replaced by cold water entering the bottom portion of the tank through the dip tube. When hot water is withdrawn from the water storage tank  120 , cold water is brought in by dip tube  110  to replace the water that has been removed. Dip tube  110  pushes the cold water brought in to the bottom of water storage tank  120 . The cold water begins to mix with the hot water already in tank  120 . However, when the cold water initially enters the tank  120 , the upper sensor  126  continues to read the temperature of water in the upper region of the tank which is normally at or close to the set point level. Over time, due to mixing, the temperature measured by upper sensor  126  begins to drop. Because the system uses only a single water temperature sensor located near the top of the tank, the controller needs to be able to respond to small temperature changes sensed by this sensor if occasioned by a flow event, by turning on the lower heat source to heat the cold water that has entered the bottom portion of the tank. Responding to such small changes in temperature if caused by the standby cooling rather than flow event cooling would result in unnecessarily short cycling. 
     Because the controller  152  is continually receiving data representative of the temperature of water within the water storage tank  120  as measured by upper sensor  126 , when the temperature of the water to drops the controller  152  is operative to generally distinguish between a drop in temperature due to standby cooling and a drop in temperature due to a flow event. The controller detects the first condition by detecting a temperature less than a threshold temperature which in the illustrative embodiments is a first predetermined off set from the set point temperature. The controller detects the second condition by detecting the occurrence of a flow event, either by input from the flow meter in those embodiments that employ such a device, or in embodiments not so equipped, by processing the water temperature data, as will be hereinafter described, and by detecting a temperature less than a threshold temperature which in the illustrative embodiment is a second predetermined offset from the set point temperature which is a smaller offset than the first offset so as to enable the controller to respond to a smaller temperature differential when a flow event is detected. 
     In the hybrid mode and in the electric mode, the controller is also operative to respond effectively to a condition in which the sensed temperature of the water in the tank is significantly lower than the set point temperature, such as might occur when initially filling the tank with cold water, or upon the withdrawal of an unusually large amount of hot water over a relatively short period of time. Such conditions are referred to herein as cold tank conditions. 
     For purposes of an illustrative example, assume that the water storage tank  120 , shown in  FIG. 1 , is full of cold water and the mode of operation is hybrid. When HPWH  100  is powered on, the upper sensor  126  senses the temperature of the water within water storage tank  120  and transmits data representative of the water temperature to the controller  152 . The controller  152  processes the data representative of the temperature of the water within the water storage tank  120  and determines that it is below a defined cold tank threshold selected to be representative of a water temperature low enough to require energization of the heat sources in a manner intended to rapidly heat the water at least at the top of the tank which would be withdrawn first, to a temperature at least near the set point temperature in a reasonable period of time. In this exemplary embodiment the cold tank threshold for the hybrid mode is set at 30° F. lower than the set point, e.g., T−30° F., where T is the set point temperature. In the illustrative embodiments, the set point is selectable by the user from a range on the order of 100° F.-140° F. A vacation mode set point of 50° F. is also available in the illustrative embodiments for a user anticipating a long period of non-use, for example. However, the set point temperature T could be selectable from a broader or narrower range, or predetermined factory set value or it could be a value selected automatically in accordance with temperature control algorithms implemented in the controller. Regardless of how selected, the set point temperature represents the desired or target temperature for the water stored in the tank. In hybrid mode, under these conditions, the controller  152  energizes the upper electric heating element  122  in order to heat the top portion of the water within water storage tank  120  until the water temperature reaches a second hybrid threshold temperature which is less than the set point temperature, but close enough to rely on the heat pump system to become the energy source to heat the water in the lower portion of the tank and to complete the recovery of the water temperature in the tank to the set point temperature within a reasonable time. The controller  152  is operative to continue to energize the upper heating element  122  until receiving data representative of temperatures at or above this second threshold temperature, at which time the controller  152  causes the upper electric heating element  122  to be de-energized. The second threshold is set below the set point temperature in order to compensate for any temperature overshoot, which may be caused as a result of the distance between the temperature sensor and the upper heating element  122 , since the temperature of the water proximate the upper heating element  122  when the element is energized is higher than that proximate the sensor  126  located on the wall of the tank. During the delay period while the warmer water travels from the upper heating element  122  to the sensor  126 , heat is still being added to the water by the electric heating element  122 . Accordingly, this excess heat may cause the temperature of the water to exceed the set point temperature. In the illustrative embodiment this second hybrid threshold temperature is selected to be 3° F. less than the set point temperature, that is, T−3° F. 
     When the upper electric heating element  122  is de-energized, in response to the temperature in the upper portion of the tank exceeding the second hybrid threshold, the water in the lower portion of the water storage tank  120  continues to be cold, because, the water within the lower portion of the water storage tank  120  cannot be effectively heated with upper electric heating element  122 . To address this condition, upon de-energizing upper electric heating element  122 , the controller  152  energizes the compressor  130 , driving hot refrigerant through the condenser  108  around water storage tank  120  to heat the water in the lower portion of the tank, and continues to do so until the sensor  126  reads and transmits data to the controller  152  representative of temperature greater than or equal to the set point, T. Upon the controller  152  receiving and processing data representative of a sensed water temperature greater than or equal to the set point temperature, T, the controller  152  transmits a signal to de-energize the compressor  130  and thereby discontinue transmission of heat to the water in water storage tank  120 . In this hybrid mode example, following recovery from the cold tank condition, that is having brought the temperature sensed by the sensor to the set point temperature, and in the absence of a flow event, the heat pump will remain de-energized as long a the temperature in the tank remains above a third hybrid threshold temperature selected to be sufficiently close to the set point temperature to maintain an acceptable temperature in the tank without excessive cycling to recover from the slow heat loss through the tank walls over time, which in the illustrative embodiment is set at T−5° F. If the sensed water temperature drops below T−5° F., the sealed system is energized until the sensed temperature is restored to the set point temperature, T. By this arrangement, heat loss due to standby cooling is addressed. 
     As briefly mentioned above, a flow event is characterized by the withdrawal of hot water from the tank. More particularly, a flow event for temperature control purposes is characterized by the withdrawal of hot water in such a manner that the rate of change of temperature sensed in the tank exceeds a flow event detection threshold rate. If the sensed temperature drops below the set point temperature, the controller checks for a flow event. Upon detection of a flow event, the sensed temperature is compared to a flow event threshold temperature which is less than the set point temperature, and if the sensed temperature becomes less than this threshold temperature, the sealed system is energized to restore the temperature to the set point temperature. The flow event threshold temperature is selected to be sufficiently close to the set point to enable the system to promptly respond to the flow event so as to minimize the time required for the water temperature in the tank to be restored to the set point temperature. In the illustrative embodiment, the flow event threshold is set at one degree F. less than the set point temperature, that is, T−1° F. 
     In this example, operation in the electric mode is similar to the hybrid mode, the primary difference being that the lower electric heating element is energized instead of the sealed system when heat to the lower portion of the tank is needed. Another associated difference is that the cold tank threshold temperature for the electric mode is selected to be T−25° F., which is slightly higher than the hybrid cold tank threshold temperature. The same threshold temperature offset could be used in both modes, however, in keeping with the intent of the hybrid mode to provide improved energy efficiency by relying primarily on the use of the sealed system with only limited use of the electric element, use of the lower threshold temperature in that mode results in less use of the electric element without significantly compromising recovery time. 
     Considering again a cold tank example to demonstrate the electric mode, upon detection of a water temperature less than the electric mode cold tank threshold temperature of T−25° F., the controller  152  energizes the upper electric heating element  122  in order to heat the top portion of the water within water storage tank  120  until the water temperature reaches a second hybrid threshold temperature which is less than the user set point temperature, but close enough to rely on the lower electric heating element to become the energy source to heat the water in the lower portion of the tank and to complete the recovery of the water temperature in the tank to the set point temperature within a reasonable time. In the illustrative embodiment, this is T−3° F., just as in the hybrid mode. The controller  152  is operative to continue to energize the upper heating element until receiving data representative of temperatures at or above this second threshold temperature, at which time the controller  152  causes the upper electric heating element  122  to be de-energized. As in the hybrid mode, the water in the lower portion of the water storage tank  120  continues to be cold, so, the controller  152  energizes the lower heating element  124 , driving heat into the water in the lower portion of the water storage tank  120  until the sensor  126  reads and transmits data representative of an overshoot threshold temperature which in the electric mode is greater than the set point temperature. Upon the controller  152  receiving and processing data representative of a temperature equal to or greater than the overshoot threshold temperature, the controller  152  transmits a signal to de-energize the lower heating element  124  and thereby discontinue transmission of heat to the water in lower portion of the water storage tank  120 . In the electric mode, the overshoot threshold temperature is used rather than the set point temperature to account for the relative locations of the lower heating element and the temperature sensor. It has been empirically determined that when heating the water in the tank using the lower element, the heat tends to flow outwardly toward the tank side wall and then upward along the wall. Since the temperature sensor is located on an upper portion of the tank wall, it responds to the temperature of the water near the wall which heats up faster than the water in the center of the tank, so the threshold temperature is set at a temperature higher than the set point temperature to allow the water in the center of the tank to reach the set point temperature. In the illustrative example, the overshoot threshold temperature is set at T+5° F. 
     Continuing with the electric mode example, following recovery from the cold tank condition, that is having brought the temperature sensed by the sensor to the electric mode recovery threshold temperature of T+5° F., in the absence of a flow event, the lower heating element will remain de-energized as long as the temperature in the tank remains above a third hybrid threshold temperature selected to be sufficiently close to the set point temperature to maintain an acceptable temperature in the tank without excessive cycling due to heat loss through the tank walls, which in the illustrative embodiment is set at T−5° F. If the sensed water temperature drops below T−5° F., the lower heating element is energized until the sensed temperature is restored to the overshoot threshold temperature, T+5° F. As in the hybrid mode, upon detection of a flow event, the sensed temperature is compared to a flow event threshold temperature which is less than the set point temperature, and if the sensed temperature becomes less than this threshold temperature, the lower heating element is energized to restore the sensed temperature to the overshoot threshold temperature. The flow event threshold temperature is selected to be sufficiently close to the set point to enable the system to promptly respond to the flow event so as to minimize the time required to recover from the flow event. In the illustrative embodiment, the flow event threshold is set at 1° F. less than the consumer selected set point temperature, that is, T−1° F. 
     If the mode of operation is heat pump, the upper sensor  126  senses the temperature of the water within water storage tank  120  and transmits data representative of the water temperature to the controller  152 . The controller  152  processes the data representative of the temperature of the water within the water storage tank  120  and determine that it is below the set point. The controller  152  energizes the compressor  130 , driving hot refrigerant through the condenser  108  around water storage tank  120  until the sensor  126  reads and transmits data representative of the set point temperature having been reached or exceeded to the controller  152 . Upon the controller  152  receiving and processing data representative of set point temperature having been reached or exceeded, the controller  152  transmits a signal to de-energize the compressor  130  and thereby discontinue transmission of heat to the water in water storage tank  120 . 
     Each of the operative modes relies upon the detection of a flow event to control heat sources in response to routine withdrawal of hot water from the tank. In embodiments employing a flow meter, the actual flow rate of the water exiting, or entering the water heater is directly measured and if it exceeds a predetermined threshold rate, a flow event is signified and the controller responds accordingly. A threshold rate on the order of 2-3 gallons per minute should provide satisfactory results in a 50 gallon tank. In embodiments not equipped with a flow meter, use is made of water temperature data to indirectly detect the occurrence of a flow event. 
       FIG. 6  illustrates an example of data representative of temperature readings as a function of time for a 50 gallon water heater in which the water has been heated to approximately 120° F. as measured by upper sensor  126 . Line  610  illustrates temperature readings measured by an upper sensor  126  when upper heating element  122 , lower heating element  124  and compressor  130  are de-energized and no water is being withdrawn. As line  610  illustrates the temperature of water being measured drops at a rate over time that is characteristic of standby heat loss that is heat loss through the insulation of the water storage tank side walls. Line  614  of the temperature readings  600  illustrates data representative of temperature readings measured by upper sensor  126  as hot water is being withdrawn from the tank at a rate of 1 gallon per minute, and line  616  represents the temperature data as hot water is being withdrawn from the tank at a rate of 3 gallons per minute. 
     In the illustrative embodiments utilizing this technique, a rate of 0.3° F. per minute has been selected as the threshold or reference rate for detecting a flow event. Line  618  in  FIG. 6  represents this threshold flow event rate. If the controller  152  detects a drop in temperature sensed by sensor  126  at a rate greater than this flow event reference rate of 0.3° F./minute, then the controller  152  knows that flow event has occurred. In this example, receipt and processing of data representative readings illustrated by line  616  by the controller  152  allows the controller  152  to determine that the sensed rate of change is greater than the threshold or control limit rate of change signifying that a flow event occurred. The use of this threshold rate in the illustrative embodiment enables the controller to reliably respond to flow rates on the order of 3 gallons per minute or higher as flow events. It is to be understood however, that this threshold rate may be set at any level that facilitates detection of water being withdrawn from the water tank and is not limited to detection of a drop in temperature at a rate greater than illustrative threshold rate of 0.3° F./minute. In the illustrative embodiments, the controller looks for a drop of 0.3° F. over one minute moving windows checking every five seconds to detect a flow event. If a drop greater than 0.3° F. is detected within a one minute window the system recognizes a flow event and responds accordingly. In connection with this description of this technique for detecting flow events, it should be noted that dip tubes in commercially available water heaters include an anti-siphon aperture located near the upper end of the tube, which may have a diameter on the order of 0.120 inches. When cold water enters the tank via the dip tube, a small portion of the entering water bleeds through this anti-siphon aperture into the upper region of the tank. In addition, dip tubes in commercially available residential water heaters such as those commercially available under the GE brand have structure proximate the exit end of the dip tube to introduce turbulence into the entering water which produces a flow restriction. This flow restriction increases back pressure in the tube which increases the rate of bleeding through the ant-siphon aperture. As illustrated in  FIGS. 1-5 , in the illustrative embodiments, the anti-siphon aperture ( 110   a  in  FIG. 1 ) is at roughly the same height in the tank as the water temperature sensor  126  (in  FIG. 1 ). The bleeding of cold water into the tank through the anti-siphon aperture is important to the effectiveness of the upper temperature sensor in detecting a flow event. Satisfactory results were achieved in the illustrative embodiment with the commercially available dip tube anti-siphon aperture and outlet turbulence structure employed in electric residential water heaters commercially available under the GE brand. However, the sensitivity of detection may be adjusted by adjustments to the aperture size and/or the amount of flow restriction introduced at the exit of the dip tube for optimization for particular water storage tank configurations. 
     In prior art water heater systems that include an upper sensor toward the top of the water tank and a lower sensor toward the bottom of the water tank, the upper sensor detects changes in the temperature of the water in the upper portion of the water storage tank and causes an upper heating element to be turned on until the upper sensor senses that the temperature in the water in the top portion of the tank is heated to a defined set point temperature. The lower sensor in these systems detects changes in the temperature of the water in the lower part of the tank and causes the lower heating element to be turned on until the lower sensor senses that the temperature in the water recovers to the defined set point temperature. One of the problems with such a configuration is that sequential small flow events will cause the water in the top of the tank to overheat as a result of stacking. In such prior art systems, each time cold water is added to the bottom of a water storage tank, energy is added to the water because the lower heating element is turned on each time as a result of the detected temperature change. As a result, heat rises to the top of the water storage tank, causing the water in the top of the water storage tank which is already at the set temperature level to over heat when the additional energy is added. When a number of the small flow events occur sequentially, the additional energy added to the water in the top of the water in the water storage tank begins to stack up and causes overheating of the water. The use of a single sensor in the manner hereinbefore described solves that problem. While the exemplary embodiments of this aspect of the present invention are heat pump water heaters, that include an electric mode, it will be appreciated that this aspect of the invention is not limited to such embodiments and could be similarly employed for example in water heaters heated only by electric heating elements. 
     Referring now to  FIG. 7A ,  FIG. 7A  depicts a control block diagram consistent with embodiments of the invention. The control block diagram indicates some of the inputs, processing, and outputs that may be required during operation of the heat pump water heater. For example, the inputs may include inputs from one or more temperature sensors, depending on the particular embodiment, collectively represented here as the temperature sensors  702 . In the illustrative embodiments, the temperature sensors are thermistors, however, other types of temperature sensors could be similarly employed. Other inputs may include feedback  703  from the fans  704  indicative of fan speed. Also, inputs may be received from a flow sensor  716 , a float switch  714 , and a conductivity sensor  706 . Flow sensor  716  could be used to monitor hot water usage. Float switch  714  may be used to monitor the accumulation of condensation from the evaporator and to cause a pump or other device to be activated to remove the condensation or to provide a signal to the user that condensate needs to be removed. Conductivity sensor  706  may be used for monitoring condensate accumulation in lieu of a float switch, or may be used to detect water near the base of the water heater indicating a potential leak in the water storage tank. The inputs may further comprise inputs from a user interface  708 , a clock and/or a calendar  750 . In one embodiment, the clock is powered by non-volatile memory/battery/capacitor in order to maintain time-of-day clock such that if power is lost, a user does not have to re-set the date/time (as is required on many household appliances with clocks). This may also be accomplished by more elegant methods of reading the atomic clock satellite output, etc. Inputs may also be received from an energy monitoring billing device  772 . Energy monitor billing devices comprise devices installed by a utility company used to limit the power draw during peak demand times. For example, during summer months it is common for power companies to provide consumers with rebates for the privilege of allowing the power company to shut down devices which draw large amounts of power such as water heaters, heat pumps, and air conditioning systems. 
     The processing may be done by a main PCB, which may be a microcontroller or PLC controller  760 , etc. The main PCB may also regulate a power supply  770 . For example, the main PCB  760  includes a water temperature and flow module that processes data representative of the temperatures measured by a plurality of thermistors. The outputs for the control system may include power supply to fans  704 , power to the compressor  730 , upper heating element  720 , and lower heating element  118 . The outputs may also include indicating information on user interface  708  (not shown). The indications may be in the form of an LCD display and or LED lights as indicated by reference numeral  710  respectively. 
       FIG. 7B  is a representative wiring diagram for the illustrative embodiment of  FIG. 1 . The power input for the heat pump water heater  100  may be standard residential power. For example, the power supply may be a 240 volt alternating current (VAC) circuit operating at 60 Hz. This generally consists of three wires; two 120 VAC inputs and one ground, (i.e. no neutral wire). A Switch Mode Power Supply  224  is provided in the form of a conventional rectification circuit to provide a 12 volt dc power supply for the fans  104  and for the relay drivers and other electronic controller needs. System operation is controlled by a main controller  152 . The main controller  152  receives inputs such as the input from sensor  126 . In addition, the main controller  152  receives feedback inputs from and controls operation of the fans  104  as indicated by reference numerals  154  and  156 . In the illustrative embodiment, fans  104  are variable speed dc fans. However, ac fans could be similarly employed. Operation of the fans  104  includes monitoring and controlling fan speed, and providing power to the fans  104  for operation by way of pulse width modulated pulses from signal generator  158 . In one embodiment, fan speed is monitored via tachometer feedback built into the fan. The fans utilized in the present embodiment may be of a magnet/hall-effect sensor design. When a fan rotates, the magnet passes near the hall-effect sensor resulting in a pulse signal output. The frequency of the pulses generated is analyzed and used to calculate the rotational speed of the fan. Notwithstanding the specific method of monitoring fan speed in the above described embodiment, it is contemplated that fan speed may be monitored in plurality many different ways. The main controller  152  may also be configured to recognize a fan malfunction such as burnt out motors, excess winding temperatures, vibration, inadequate fan speed, etc. Using the above described tachometer feedback; the signal sent to the fan may be compared with the speed feedback. For example, if a 50% input is given, it would be expected that the tachometer feedback should indicate an approximate 50% of the max RPM. Also, if a signal is transmitted to the fan to facilitate operation at any speed, and there is no feedback indicating fan rotation, this can be interpreted as a fan failure. 
     The main controller  152  also includes a relay  212  for controlling the upper heating element  122 , a relay  214  for controlling the lower heating element  124 , and a relay  216  for controlling the compressor  130  relays  212 - 216  are cascaded such that only one of the heat sources is energized at any one time. The cascaded relays are coupled to power supply line L 1  through contacts  1  and  2  of thermal cutout switch  218 . Similarly, the power circuit is coupled to power supply line L 2  through contacts  3  and  4  of switch  218 . Switch  218  is a convention thermal cut out switch which is mounted to the wall of tank  120  to be responsive to the temperature of the tank wall. If the tank wall overheats to a temperature in excess of the cut out threshold temperature, which in the illustrative embodiment is 170° F., the switch element coupling contact  1  to contact  2  opens breaking the connection to L 1  and the switch element coupling contacts  3  and  4  opens breaking the connection to L 2 , thereby limiting the temperature of the tank. Relay  220  couples contact  3  of cut out switch  218  to L 2 , to provide a double break between the AC power supply and the power control circuitry when the system is in the off state. Controller  152  switches relay  220  to couple L 2  to contact  4  of switch  218 , when the system is on and relay  220  is in its normally open state otherwise. Referring again to the cascaded arrangement of relays  212 - 216 , terminal c of relay  212  is/connected to contact  2  of switch  218 . Its normally open contact is connected to upper heating element  122 , and its normally closed contact is connected to terminal c of relay  214 . The normally open contact of relay  214  is connected to lower heating element  124  and its normally closed contact is connected to terminal c of relay  216 . The normally open contact of relay  216  is connected to compressor  130  through discharge pressure cutoff switch  222 . Cutoff switch  222  is a conventional pressure switch employed in a conventional manner to protect the sealed system from excessive pressure. By this arrangement, to energize upper element  122 , controller  152  switches relay  212  to its normally open state thereby connecting heating element  122  across L 1  and L 2 . When relay  212  is in this state, L 1  can only be connected to heating element  122 . To energize lower heating element  124 , controller  152  switches relay  212  to its normally closed state and relay  214  to its normally open state. This connects heating element  124  across L 1  and L 2 . When relay  212  is in its normally closed state and relay  214  is in its normally open state L 1  can only be connected to lower element  124 . To energize compressor  130 , controller  152  switches relays  212  and  214  to their normally closed states and switches relay  216  to its normally open state. This connects pressure switch  222  and compressor  130  in series across L 1  and L 2 . The main controller  152  also accepts inputs from a user interface  202  as indicated by reference numeral  230 . The main controller  152  also may include an integral timer that is configured as part of the heat pump water heater electronic control, providing a user with the ability to control and program the heating activity of the heat pump water heater, such that energy may be conserved when there is no need for water to be heated. 
     In the circuit configuration for the embodiment illustrated in  FIG. 7B , during operation of the heat pump water heater  100  only one of heat sources, that is, heating elements  122  and  124  and compressor  130  may operate at any given time. This limits the electrical load. However, it is contemplated that in alternative configurations, that one of heating elements  122  or  124  and the compressor  130  may operate simultaneously. Furthermore, it is contemplated that in alternative configurations both heating elements  122  and  124  and the compressor  130  may operate simultaneously. However, operation of both heating elements  122  and  124  at the same time may require special electrical considerations (e.g. a larger circuit breaker, a dedicated circuit, etc.) to accommodate an increased current draw. Notwithstanding, it is contemplated that operation of both heating elements  122  and  124  may occur at the same time. Similar circuitry with additional sensor and other inputs can be employed for the embodiments of  FIGS. 2-5 . 
       FIG. 8  is an illustration of the process flow within the water temperature and flow module within the controller during operation of the HPWH. As illustrated, following the system being energized  792 ; a determination is made as to whether the water storage tank is full  794 . The method of determining whether the water storage tank is full is performed by the controller initiating a plurality of steps. First, the condenser, which is in contact with the exterior of the water storage tank is initiated for a defined period of time and heats the exterior walls of the water storage tank. If the tank is empty, the water storage tank will begin to warm up at a rate faster than when there is water in the tank. The controller facilitates monitoring of the temperature of the exterior walls by way of a sensor positioned in sufficient proximity to the water storage tank wall. In the illustrative embodiments sensor  126  is used for this purpose, however, a separate temperature sensor could be similarly employed. If the tank is full or at least has water at an acceptable level then the rise in temperature will not exceed a defined limit. If the temperature measured by the sensor rises above this limit it is an indication that the water storage tank is empty or the water level within the tank is below a desired level. If the water storage tank is not full, the water temperature and flow module within the controller facilitates the initiation of a display illustrating that the tank is dry or not full  796 . The system suspends further operation until the tank is filled sufficiently to satisfy the tank full test. Upon the water storage tank being filled with water, the query as to whether the water storage tank is full  794  will result in an affirmative answer. Next, the system determines an appropriate mode of operation. The illustrative embodiments have four modes of operation, a standard electric mode, a heat pump mode, a hybrid mode and a high demand mode comprised of a combination of the use of electrical elements and the heat pump. The system allows for the use of the mode of operation previously in use  798  or an operator may select a mode of operation  802 . During operation, the water temperature and flow module must first verify the mode of operation selected by an operator. As part of the verification process, the controller first queries whether the mode of operation is the standard electric mode at  804 . If the mode of operation is not standard electric mode, the controller next queries whether the mode of operation is the heat pump mode at  806 . If the mode of operation is not heat pump mode, the controller next queries whether the mode of operation is the high demand mode at  807 . If not the high demand mode, by default the water temperature and flow module switches the system into the hybrid mode at  808 . 
     When the selected mode of operation is standard electric mode, the controller implements the standard electric mode  810  ( FIG. 8B ). In this mode the temperature and water flow module obtains water temperature data T 2  from sensor  126  ( 812 ). The temperature and water flow module is configured to check first for a Cold tank condition signified by a value of T 2  indicating a water temperature which is less than the electric mode cold tank threshold temperature T−25° F. ( 814 ) where T is the set point. If T 2  is less than T−25° F., then the upper heating element  122  is energized and the lower element is de-energized in the event it happens to be already energized when a cold tank condition is detected ( 816 ). Since, the module is configured to give priority to the cold tank condition, and in the illustrative embodiments, both electric elements are not to be energized at the same time, if the lower element is already energized to satisfy another condition when a cold tank condition is detected, it is necessary to de-energize the lower element  124 . Heating element  122  will continue to be energized until T 2  rises to within 3° F. of the set point ( 818 ). When T 2  exceeds T−3° F., the upper element  122  is de-energized ( 820 ) and the lower element  124  is energized ( 822 ). This operating condition will continue until T 2  exceeds the overshoot threshold of 5° F. above the set point, that is, T 2  is greater than T+5° F. or unless interrupted by detection of another cold tank condition at which time the lower heating element is denergized and the module continues to monitor T 2  ( 810 ). 
     Returning again to  814 , if T 2  is not less than T−25° F., the temperature and water flow module checks next to determine heat is required due to standby cooling, by determining if the sensed water temperature is less than T−5° F. ( 824 ). If yes, then the lower heating element  124  is energized and remains energized until the overshoot threshold temperature of T+5° F. is reached ( 822 ) or a cold tank condition is detected ( 814 ). 
     Returning to  824 , if the sensed temperature is not less than T−5° F., the temperature and water flow module checks next to determine if heat is needed due to a flow event by first comparing the sensed temperature T 2  to the set point temperature T ( 826 ). If T 2  is not less than T, no energization of heat sources is needed and the system continues to monitor T 2  ( 812 ). If T 2  is less than T, the module next looks for the occurrence of a flow event ( 828 ). As hereinbefore described, this is determined in some embodiments from the output of a flow meter and in others from temperature rate of change data. If no flow event is detected, the module continues to monitor T 2  ( 812 ). If a flow event is detected the module determines if T 2  is less than the set point minus 1 degree F. ( 830 ). If not, the module continues to monitor T 2  ( 810 ). If T 2  is less than T−1° F., then the lower element is energized and remains energized until T 2  equals or exceeds the overshoot threshold T+5° F. ( 822 ) unless interrupted by detection of a cold tank condition ( 814 ). 
     When the selected mode of operation is the heat pump mode, the module implements the heat pump mode ( 832 ) ( FIG. 8C ). In the heat pump mode, only the compressor driven sealed system is used to heat the water. The module is not configured to detect and respond to a cold tank condition in this mode, so the module monitors T 2  ( 834 ) checking first to determine if; heat is required due to standby cooling, by determining if the sensed water temperature is less than T−5° F. ( 836 ). If yes, then the sealed system is energized and remains energized until the user selected set point temperature is reached ( 838 ). 
     Returning to  836 , if the sensed temperature is not less than T−5° F., the temperature and water flow module checks next to determine if heat is needed due to a flow event by first comparing the sensed temperature T 2  to the set point temperature T ( 840 ). If T 2  is not less than T, no energization of heat sources is needed and the system continues to monitor T 2  ( 834 ). If T 2  is less than T, the module next looks for the occurrence of a flow event ( 842 ). If no flow event is detected, the module continues to monitor T 2  ( 834 ) If a flow event is detected the module determines if T 2  is less than the set point minus 1 degree F. ( 844 ). If not, the module continues to monitor T 2  ( 834 ). If T 2  is less than T−1° F., then the sealed system is energized and remains energized until T 2  equals or exceeds the user selected set point temperature ( 838 ). 
     When the selected mode of operation is the hybrid mode, the module implements the hybrid mode ( 846 ) ( FIG. 8D ). In this mode the temperature and water flow module obtains water temperature data T 2  from sensor  126  ( 848 ). As in the standard electric mode, the temperature and water flow module is configured to check first for a cold tank condition signified by a value of T 2  indicating a water temperature which is less than a cold tank threshold temperature ( 850 ). However, in the illustrative embodiment, as hereinbefore described, the hybrid cold tank threshold temperature is T−30° F., which is less than the electric cold tank threshold temperature. If T 2  is less than T−30° F., then the upper heating element  122  is energized. Heating element  122  will continue to be energized until T 2  rises to within 3° F. of the set point ( 854 ). When T 2  exceeds T−3° F., the upper element  122  is de-energized ( 856 ) and the operation of the sealed system is initiated ( 858 ). The sealed system will continue to run until T 2  equals or exceeds the set point temperature, (unless interrupted by detection of another cold tank condition) at which time the sealed system is denergized and the module continues to monitor T 2  ( 848 ). 
     Returning again to  850 , if T 2  is not less than T−30° F., the temperature and water flow module checks, next to determine if heat is required due to standby cooling, by determining if the sensed water temperature is less than T−5° F. ( 860 ). If yes, then the operation of the sealed system is initiated and the sealed system continues to run until the set point temperature, T, is reached or exceeded ( 858 ) (unless interrupted by detection of another cold tank condition) at which time the sealed system is denergized and the module continues to monitor T 2  ( 848 ). 
     Returning to  860 , if the sensed temperature is not less than T−5° F., the temperature and water flow module checks next to determine if heat is needed due to a flow event by first comparing the sensed temperature T 2  to the set point temperature T ( 862 ). If T 2  is not less than T, no energization of heat sources is needed and the system continues to monitor T 2  ( 848 ). If T 2  is less than T, the module next looks for the occurrence of a flow event ( 864 ). If no flow event is detected, the module continues to monitor T 2  ( 848 ). If a flow event is detected the module determines if T 2  is less than the set point minus 1 degree F. ( 866 ). If not, the module continues to monitor T 2  ( 848 ). If T 2  is less than T−1° F., then the operation of the sealed system is initiated and continues to run until T 2  equals or exceeds the set point temperature T ( 858 ) (unless interrupted by detection of another cold tank condition) at which time the sealed system is denergized and the module continues to monitor T 2  ( 848 ). 
     The high demand mode is a variation of the hybrid mode provided to respond to higher than typical hot water usage conditions, such as can occur in homes with high flow shower heads, e.g., flow rates on the order of 5 gallons per minute as compared to more typical shower heads with flow rates of 2 gallons per minute. In the high demand mode the system uses the heat pump to recover standby losses and small draws as in the hybrid mode. However, if a large flow event is detected, for example a water temperature drop of 3° F. in ten minutes, then the system uses the lower electric element to recover. In addition in a manner similar to hybrid mode, but with a higher threshold, the upper heating element is used to recover the water temperature in the top part of the tank and then the lower element is used to recover the water temperature in the lower part of the tank. As previously described herein, the system is configured to detect “flow events” by detecting a rate of change of temperature on the order of 0.3° F. over a period of one minute, using a one minute moving window, checked every five seconds. To detect a “large flow event” the system looks for a change in temperature of 3° F. over a period of ten minutes using a ten minute moving window also checked every five seconds, however, every thirty seconds may be sufficient. 
     When the selected mode of operation is the high demand mode, the module implements the high demand mode ( 870 ) ( FIG. 8E ). In this mode the temperature and water flow module obtains water temperature data T 2  from sensor  126  ( 872 ). As in the hybrid and standard electric modes, the temperature and water flow module is configured to check first for a cold tank condition signified by a value of T 2  indicating a water temperature which is less than a cold tank threshold temperature ( 874 ). However, in the illustrative embodiment, as hereinbefore described, the hybrid cold tank threshold temperature is T−20° F., which is greater than the electric or hybrid cold tank threshold temperatures. This is to enable a quicker response to a cold tank condition, since high demand mode is intended for situations where cold tank conditions are likely to be more frequent. If T 2  is less than T−20° F., then the upper heating element  122  is energized and the lower element is de-energized in the event it happens to be energized when the cold tank condition is detected ( 876 ). Heating element  122  will continue to be energized until T 2  rises to within 3° F. of the set point ( 878 ). When T 2  exceeds T−3° F., the upper element  122  is de-energized and the lower element is energized ( 880 ). As in the electric mode, this operating condition will continue until T 2  exceeds the overshoot threshold of 5° F. above the set point, that is, T 2  is greater than T+5° F. or unless interrupted by detection of another cold tank condition, at which time the lower heating element  124  is de-energized and the module continues to monitor T 2  ( 872 ) 
     Returning again to  874 , if T 2  is not less than T−20° F., the temperature and water flow module checks next to determine if heat is required due to standby cooling, by determining if the sensed water temperature is less than T−5° F. ( 884 ). If yes, then the operation of the sealed system is initiated and the sealed system continues to run until the set point temperature, T, is reached or exceeded ( 886 ) (unless interrupted by detection of another cold tank condition or a large flow event) at which time the sealed system is de-energized and the module continues to monitor T 2  ( 872 ). 
     Returning to  884 , if the sensed temperature is not less than T−5° F., the temperature and water flow module checks next to determine if heat is needed due to a flow event by first comparing the sensed temperature T 2  to the set point temperature T ( 888 ). If T 2  is not less than T, no energization of heat sources is needed and the system continues to monitor T 2  ( 848 ). If T 2  is less than T, the module next looks for the occurrence of a flow event ( 890 ). If no flow event is detected, the module continues to monitor T 2  ( 872 ). If a flow event is detected the module determines if T 2  is less than the set point minus 1 degree F. ( 892 ). If not, the module continues to monitor T 2  ( 848 ). If T 2  is less than T−1° F., then the operation of the sealed system is initiated and continues to run until T 2  equals or exceeds the set point temperature T ( 858 ) (unless interrupted by detection of another cold tank condition or large flow event) at which time the sealed system is de-energized and the module continues to monitor T 2  ( 848 ). 
     Returning to  872 , if at any time during operation in the high demand mode, unless the system is in the process of responding to a cold tank condition, detection of a large flow event ( 894 ) takes priority. If a large flow event is detected, that is if the controller detects a drop in water tank temperature sensed by sensor  126 , of 3° F. or more in a running ten minute window, the lower heating element is energized ( 896 ) and remains energized until T 2  exceeds the overshoot threshold of 5° F. above the set point, that is, T 2  is greater than T+5° F. or unless interrupted by detection of another cold tank condition, at which time the lower heating element  124  is de-energized and the module continues to monitor T 2  ( 872 ). 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.