Heat pump water heater and storage tank assembly

A water heater and storage tank assembly comprises a housing defining a chamber, an inlet for admitting cold water to the chamber, and an outlet for permitting flow of hot water from the chamber. A compressor is mounted on the housing and is removed from the chamber. A condenser comprises a tube adapted to receive refrigerant from the compressor, and winding around the chamber to impart heat to water in the chamber. An evaporator is mounted on the housing and removed from the chamber, the evaporator being adapted to receive refrigerant from the condenser and to discharge refrigerant to conduits in communication with the compressor. An electric resistance element extends into the chamber, and a thermostat is disposed in the chamber and is operative to sense water temperature and to actuate the resistance element upon the water temperature dropping to a selected level. The assembly includes a first connection at an external end of the inlet, a second connection at an external end of the outlet, and a third connection for connecting the resistance element, compressor and evaporator to an electrical power source.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to water heater and storage tank assemblies, and is 
directed more particularly to such an assembly of the heat pump type. 
2. Description of the Prior Art 
A widely accepted and used water heater for residential hot water 
production and storage is the electric resistance water heater and storage 
tank. Referring to FIG. 1, it will be seen that such water heaters 
typically include a tank 20 defining a chamber 22 for retention of water. 
A water inlet pipe 24 is provided with a first connection 25 for 
interconnection with a cold water supply line (not shown) and conveys 
fresh relatively cold water into the chamber 22. Electric resistance 
elements 26 heat the water in the tank. A hot water outlet pipe 28, 
provided with a second connection 29 for interconnection with a hot water 
discharge line (not shown), conveys relatively hot water from the chamber 
22. A pressure and temperature relief valve 34 is provided at the top or 
side of the tank 20 and typically is connected to a pressure and 
temperature relief valve drain tube 36 extending downwardly alongside the 
tank 20. An electrical power line 30 is provided with an electrical 
connector 32 for connection to a power source (not shown), such as a high 
voltage household outlet, and provides electrical power to the electric 
resistance elements 26. Such electrically powered assemblies typically 
have an Energy Factor (a measure of efficiency) of about 0.86 for a 50 
gallon tank. The assemblies are available in different sizes, usually 
40-80 gallon capacity, most typically 40 or 50 gallon capacity, and are 
dimensioned for acceptance in residential basements, kitchens, closets, 
and the like. Typically, the cold water inlet and hot water outlet are 
disposed at the top of the tank, and the electrical power line is disposed 
at the top (as shown) or side of the tank. 
Heat pump water heaters and storage tank assemblies are generally known in 
the art. In U.S. Pat. No. 2,516,094, issued Jul. 18, 1950, to A. W. Ruff, 
there is shown and described a heat pump water heater. In the Ruff 
assembly, a compressor and condenser are disposed within a water 
containing tank and an evaporator is disposed on top of the tank. These 
heat pump components occupy a large portion of the volume of the tank, 
leaving a limited volume for storage of water. The hot and cold water 
connections are on the side of the tank, the cold water inlet being near 
the bottom of the tank. 
In U.S. Pat. No. 2,696,085, issued Dec. 6, 1954, to A. W. Ruff, there is 
disclosed a similar heat pump water heater differing from the '094 
assembly in that the condenser portion of the refrigerant circuit 
comprises three sets of coils submerged high, low and midway inside the 
tank. 
In U.S. Pat. No. 2,575,325, issued Nov. 20, 1951, to E. R. Ambrose et al, 
there is shown a heat pump water heater and storage tank in which a 
compressor and condenser are located within the tank and two evaporators 
are located externally of the tank. 
While heat pump water heaters provide improved efficiencies over electric 
resistance water heaters, they have never attained wide acceptance. The 
reasons for the lack of acceptance appear to be (1) high initial costs; 
(2) the fact that water heater installers (a) are used to placement of hot 
water and cold water connections in about the same place relative to the 
tank, which typically is a 40 gallon or 50 gallon tank, and used to a 
single simple electrical connection, and (b) are not equipped to handle 
system components external to, and removed from, the tank; (3) the fact 
that often a water heater is located in a confined space which is 
constructed with enclosure of a 40 or 50 gallon residential electrical 
water heater of conventional shape in mind; and (4) the fact that heat 
pump water heaters can be slow to recover from a large draw-down of hot 
water. 
It has been recognized by the U.S. Department of Energy that wide-spread 
acceptance of heat pump water heaters would lead to substantial savings in 
energy consumption. 
There is thus a need for a heat pump water heater having substantially 
improved efficiencies over a comparable electric resistance water heater, 
having substantially the same physical characteristics as a comparable 
electric resistance water heater, and having connections that can easily 
be made by anyone practiced in installing electric resistance water 
heaters. There is further a need for such a heat pump water heater wherein 
provision is made for quick recovery from large draw-downs. There is still 
further a need for such a heat pump water heater which is comparable in 
initial cost to that of an electric resistance water heater. 
SUMMARY OF THE INVENTION 
An object of the invention is therefore, to provide a heat pump water 
heater exhibiting efficiencies substantially improved over comparable 
electric resistance water heaters. 
A further object of the invention is to provide such a water heater which 
is of substantially the same size and configuration as a comparable 
electric water heater, is provided with water and electrical connections 
disposed similarly to such connections in electric water heaters, and 
wherein the heat pump components are fully integrated with the tank, such 
that the heat pump water heater may readily be substituted for an electric 
water heater without a requirement for an installer having specialized 
skills for heat pump water heater installation. 
A still further object of the invention is to provide such a heat pump 
water heater having facility for quickly reacting to large draw-downs, so 
as to promptly reestablish a heated condition for water in the tank. 
A still further object of the invention is to provide such a heat pump 
water heater as is competitively priced relative to a comparable electric 
resistance water heater. 
With the above and other objects in view, as will hereinafter appear, a 
feature of the present invention is the provision of a water heater and 
storage tank assembly comprising a housing defining a chamber for 
retaining water. An inlet pipe extends through the housing and into the 
chamber for admitting relatively cold water to the chamber and an outlet 
pipe extends through the housing for permitting flow of relatively hot 
water from the chamber. A compressor is mounted proximate and removed from 
the chamber, the compressor being adapted to receive refrigerant, to 
compress the refrigerant, and to discharge the refrigerant in a hot state. 
A condenser comprises one or more tubes adjacent the chamber and in 
thermal contact with the chamber, the tubes being adapted to receive the 
hot refrigerant from the compressor, and adapted to impart heat to water 
in the chamber adjacent the. An evaporator is mounted on the housing and 
removed from the chamber, the evaporator being adapted to receive cooled 
refrigerant from the condenser and comprising coils adjacent atmosphere 
external to the housing. The evaporator is adapted to discharge 
refrigerant to conduits in communication with the compressor for directing 
refrigerant from the evaporator to the compressor. An electric resistance 
element extends into the chamber, and a thermostat is disposed proximate 
the chamber and is operative to sense temperature corresponding to water 
temperature in the chamber and to actuate the electric resistance element 
upon the water temperature dropping to a selected level. The assembly 
includes a first connection at an external end of the cold water inlet 
pipe, a second connection at an external end of the hot water outlet pipe, 
and a third connection for interconnecting the electric resistance element 
and an electrical power source. 
The above and other features of the invention, including various novel 
details of construction and combinations of parts, will now be more 
particularly described with reference to the accompanying drawings and 
pointed out in the claims. It will be understood that the particular 
devices embodying the invention are shown by way of illustration only and 
not as limitations of the invention. The principles and features of this 
invention may be employed in various and numerous embodiments without 
departing from the scope of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 2-5, it will be seen that an illustrative heat pump 
water heater and storage tank assembly includes a housing 40 defining a 
chamber 42 for retaining water. An inlet pipe 44 extends through the 
housing 40 and into the chamber 42 for admitting relatively cold water to 
the chamber 42. An outlet pipe 46 extends through the housing 40 for 
permitting flow of relatively hot water from the chamber 42. The usual 
pressure and temperature relief valve 34 is mounted on the housing 40. 
A compressor 50 is mounted on a bottom wall 52 of the housing 40 and is 
separated from the chamber 42 by a partition 54 which extends widthwise in 
the housing 40, defining one end of the chamber 42 and defining, in 
cooperation with the bottom wall 52 and housing side walls 56, a 
compartment 58. The compartment 58 is thus removed from the chamber 42 and 
houses the compressor 50. The compressor 50 is adapted to receive 
refrigerant, compress the refrigerant, and to discharge the refrigerant in 
a hot state. 
Alternatively, the housing side walls 56 may end proximate the partition 54 
and the housing may be supported by legs (not shown) extending beneath the 
housing side walls. In this case, the bottom wall 52 may be omitted and 
the compressor 50 may be supported by the surface supporting the legs. 
A condenser 60 comprises one or more tubes 62 adjacent to, and preferably 
supported by, the side walls 56 of the housing 40. The condenser tube 62 
preferably is disposed within the side walls 56 and adjacent the chamber 
42. The tube 62 is disposed closer to an inside surface 66 of the housing 
side walls 56 than to an outside surface 68, as shown in FIG. 3, such that 
the heat supplied by the tube 62 radiates inwardly into the chamber 42. 
The tube 62 is adapted to receive the hot refrigerant from the compressor 
50. The tube 62 winds around the chamber 42 and imparts heat to the water 
in the chamber. While the tube 62 is illustrated in FIG. 3 as being 
disposed in a generally spiral configuration, it will be appreciated that 
the tube 62 alternatively may be wound around the chamber 42 in a 
serpentine manner, or in any selected configuration for imparting heat to 
the water in the chamber 42 as desired. The tube 62 is in communication 
with a conduit 64 disposed along the housing side wall 56 and having a 
flow restriction device 69 therein. The flow restriction device 69 may be 
a thermostatic expansion valve, an orifice plate, or, preferably, a 
capillary tube, or the like. The conduit 64 may be provided with a filter 
65 and/or a sightglass 67, as is known in the art. 
An evaporator 70 is mounted on the housing 40, preferably at a top portion 
of the housing, and is removed from the chamber 42. The evaporator 70 is 
adapted to receive cooled refrigerant from the flow restriction device 69 
in the conduit 64 extending from the condenser 60. The evaporator 70 
includes coils 72 adjacent the atmosphere A external to the housing 40. 
The evaporator 70 may include a fan 74 for blowing air past the coils 72 
and to the atmosphere A, cooling the atmosphere and warming the 
refrigerant in the coils 72. The coils 72 may be disposed above an upper 
end 78 of the housing 40, as shown in FIG. 3. The evaporator 70 is adapted 
to discharge refrigerant to a conduit 76 mounted on the housing side wall 
56 and in communication with the compressor 50. In the conduit 76 is 
disposed an accumulator 79 which serves as a reservoir for liquid 
refrigerant. 
The assembly further includes at least one electric resistance element 80 
extending into the chamber 42 and preferably mounted on the housing side 
wall 56 in an upper portion of the chamber 42, between the condenser 60 
and the evaporator 70. A thermostat 82 is disposed proximate the chamber 
42, and preferably is mounted in the side wall 56 near the resistance 
element 80. The thermostat 82 is operative to sense the temperature of the 
housing wall 56, which substantially corresponds to the water temperature 
in the chamber 42, and to actuate the resistance element 80 upon dropping 
of the water temperature to a selected level. Thus, in the case of a rapid 
draw-down of hot water, the water temperature in the chamber drops 
precipitously, which causes cooling of the wall temperature, which is 
detected by the thermostat 82, which operates to turn on the resistance 
element 80 to boost heating of the water in the chamber 42. The thermostat 
82 is a standard readily available thermostat. 
A second thermostat 83 is similarly disposed in side wall 56, preferably in 
a lower region of the chamber 42, and is similarly operative to sense the 
temperature of the housing wall 56 and, in response thereto, to terminate 
electrical power to the compressor 50 and the evaporator fan 74 upon the 
sensed temperature rising to a selected level, and to activate electrical 
power to the compressor and evaporator fan upon the sensed temperature 
dropping to a selected level. The thermostat 83 is a standard readily 
available thermostat. Thus, the thermostats 82, 83 are both simple and 
inexpensive devices which operate to control the temperature of the water 
in the chamber 42. 
In a preferred embodiment of the invention, the evaporator 70 is adapted to 
operate without causing formation of condensation. Alternatively, there 
may be provided a condensate drain tube 92 (FIGS. 2 and 3) for delivering 
condensate to a household drain line or other receptacle, such as a 
condensate pan (not shown). There may be provided in connection with the 
condensate pan, a re-evaporation assembly (not shown) for evaporating the 
condensate collected in the condensate pan. Alternatively, there may be 
provided in the pan a trip switch (not shown) actuated by the presence of 
condensate in the pan to shut off the evaporator 70. In still another 
embodiment there is provided a sensor and microprocessor (not shown) for 
detection of the formation of condensate by the evaporator 70 and 
operative to shut off the evaporator. 
The upper end of the inlet pipe 44 is provided with a first connection 100 
for interconnection with a cold water supply line (not shown). Similarly, 
the upper end of the outlet pipe 46 is provided with a second connection 
102 for interconnection with a hot water discharge line (not shown). The 
first connection 100 is disposed at about the same place as the first 
connection 25 of the electric resistance heater of FIG. 1, and the second 
connection 102 is disposed at about the same place as the second 
connection 29 of the electric resistance heater of FIG. 1. 
The electric resistance element 80 is provided with an electrically 
conductive line 84, shown schematically in FIG. 5, and having an 
electrical connector 86 (FIG. 3) for connection to a power source, such as 
a 220 volt household outlet, similar to electrical connector 32 of the 
water heater of FIG. 1. Thus, a water heater installer familiar with the 
hot and cold water connections and the single electrical connection, can 
readily install the heat pump water heater in place of an electric 
resistance water heater. While the electrical connector 32 shown in FIG. 1 
and the electrical connector 86 shown in FIG. 3 are each illustrated as a 
common male connector, it will be appreciated that water heaters often are 
hard-wired to a power source and that the connectors 32, 86 are merely 
representative of electric line terminal portions adapted for connection 
to the power source. 
In installation of the above-described heat pump water heater assembly in 
place of the above-described commonly used prior art electric resistance 
water heater, the latter is disconnected at the first and second 
connections 25, 29 and at the electrical connector 32 and taken away. The 
heat pump water heater is placed in the location formerly occupied by the 
electric resistance water heater. The first and second connections 100, 
102 are connected to the cold water and hot water pipes (not shown) to 
which the electric resistance water heater connections 25, 29 were 
formerly connected. The electric connector 86 is connected to the power 
source (not shown) to which the electrical connector 32 was formerly 
connected. In new construction situations, the heat pump water heater 
described herein is installed virtually identically to the installation of 
an electric resistance water heater. 
In start-up operation, cold water is admitted to the chamber 42 through the 
inlet pipe 44. The heat pump circuit is energized through electrical lines 
disposed in the housing walls (shown schematically in FIG. 5) and in 
communication with the electrical line 84. The thermostat 82 detects the 
cold temperature of the water in the chamber 42 and activates the 
electrical resistance element 80. The second thermostat 83 similarly 
detects the cold temperature of the water in the chamber 42 and activates 
the compressor 50 and evaporator 70, thus starting operation of the heat 
pump system. The water in the chamber 42 is heated by the resistance 
element 80 and by the compressor 50 which heats the partition 54 which, in 
turn, imparts heat to the water. The water in the chamber 42 is further 
heated by the condenser tube 62 carrying hot refrigerant from the 
compressor 50. Upon the water in the chamber 42 reaching a selected 
temperature, the thermostat 82 shuts down the electric resistance element 
80 and the thermostat 83 shuts down the heat pump system. The water 
temperature is then maintained by periodic operation of the heat pump 
system which requires only sufficient electrical power to run the 
compressor 50 and evaporator fan 74. 
As water is drawn from the assembly, the heat pump system operates, in 
known fashion, to raise the temperature of the water in the chamber 42, 
cooled by incoming cold water replacing the outgoing hot water, to a 
selected temperature. In the event of a large draw-down of hot water, the 
resistance element 80 activates and assists the heat pump system in 
reestablishing the selected temperature. 
The heat pump components of the system above described are relatively 
commonly used, relatively inexpensive, components. For example, the 
compressor 50 preferably is a small hermetic compressor of the type 
commonly used in domestic refrigerators, and is of about 800-1300 BTU/hr. 
nominal capacity for a 40 or 50 gallon tank. The small capacity compressor 
50 draws little electrical power, allowing the electric resistance element 
80 and the heat pump system to operate simultaneously. The evaporator 70 
is a forced convection finned-tube evaporator or a natural gill convection 
evaporator. The condenser tube 62 preferably is of 1/4 inch outside 
diameter and is of copper or aluminum. The capillary tube 69 is sized to 
limit the refrigerant flow rate to the relatively low compressor capacity 
at the maximum rated evaporator temperature, allowing operation at higher 
ambient temperature without overloading the compressor. The system, by 
heat pump standards, is of low heating capacity, which is required to 
enable use of low cost components, but which also affords reliability 
through system simplicity. The condenser tube 62, the inside surface 66, 
and the partition 54, in combination, impart heat to the water in the 
chamber 42 at a rate of no more than about 5000 BTU/hr., permitting use of 
a compressor of the type described above and having the aforesaid capacity 
of no more than about 1300 BTU/hr. at refrigerator compressor rating 
conditions of -10.degree. F. evaporating temperature and 130.degree. F. 
condensing temperature. The condenser 60 eliminates the usual requirement 
of a double-walled condenser, is not submerged in the tank, and requires 
no costly tank modifications. 
It will be noted that the heat pump system herein described does not 
include a water circulating pump. Having no requirement for a water 
circulating pump contributes to low costs in manufacture and consumer 
purchase, but also represents a substantial contribution to subsequent low 
maintenance and repair costs. 
It is desirable that the compressor 50 be a domestic refrigerator type 
compressor inasmuch as this type of compressor is produced in the largest 
volumes of any commercially produced refrigerant compressor and is 
consequently the least expensive and most readily available refrigerant 
compressor. However, over the range of heat pump water heater operating 
conditions, especially at higher water temperatures, the motor (not shown) 
in this type of compressor could operate at an overload and may tend to 
overheat. A novel approach to capillary tube refrigerant flow control is 
used herein to provide extra cooling to the compressor motor by returning 
liquid refrigerant to the compressor 50, along with the vapor returning 
from the evaporator 70. The capillary tube restriction 69, the refrigerant 
charge quantity, and the accumulator 79 interact such that at low water 
temperatures sufficient refrigerant flows to the evaporator 70 such that 
by boiling and superheating, the heat extracted from the air is absorbed 
by the refrigerant. When the water temperature, and the corresponding 
condenser temperature and pressure, increase to levels that could result 
in compressor motor overheating, the increased pressure results in 
increased flow through the capillary tube 69 and a transfer of refrigerant 
charge from the condenser 60 to the low pressure (evaporator) side of the 
system. Consequently, more refrigerant flows through the evaporator 70 
than can be evaporated by the heat extracted from the air by the 
evaporator 70. The excess refrigerant flows to the compressor 50 and 
directly cools the motor windings. 
Inasmuch as the heat pump system herein described is of low capacity, which 
normally requires a long time to recover from a draw that substantially or 
fully exhausts the supply of stored hot water, the resistance element 80, 
having a conventional capacity of 4,500 watts, is needed to provide a 
shortened recovery time. If desired, a second resistance element (not 
shown) can be added to the assembly for even faster recovery time. 
Aside from attaining an Energy Factor of about 1.5 to 2.0, the 
above-described system tolerates a wide range of evaporating and 
condensing temperatures, allowing operation in low ambient temperatures. 
Accordingly, operation is feasible in unheated spaces such as attics, 
garages, carports, crawl spaces, and the like, even in cold weather. 
It has been determined by analysis that the cost of the above-described 
assembly with a 50 gallon capacity exceeds the cost of a conventional 
standard electric resistance water heater with a 40 gallon capacity by 
about $260. The potential energy cost savings have been determined to 
range from about $75/year for an average household to about $400/year for 
a large household. Thus, payback periods are expected to be about 31/2 
years for average households, and about 8 months for large households. 
In FIG. 6, there are shown typical yearly operating cost savings realized 
by the substitution of more efficient water heaters for standard water 
heaters. As shown in the FIG. 6 graph, by substituting a "high efficiency" 
electric resistance water heater for a standard electric resistance water 
heater, one may reduce operating costs per year by about 4%, and by 
substituting a "high efficiency" gas water heater for a standard gas 
heater, one may realize an 11% operating cost savings. However, by 
substituting the herein-described heat pump water heater for a standard 
electric resistance water heater, one may realize a savings of about 60% 
in annual operating costs. The heat pump water heater savings is exceeded 
only by the substitution of a solar water heater, which requires more 
expensive components and extensive installation skills usually not 
possessed by typical water heater installers. The savings noted in FIG. 6 
are based on typical yearly operating costs for a family of four. 
Referring to FIGS. 7 and 8, it will be seen that in an alternative 
embodiment components are arranged similarly to the arrangement shown in 
FIGS. 2 and 3, except that the compressor 50 is disposed on the housing 
upper end 78. This alternative arrangement reduces the cost of the 
assembly and increases the volume of the chamber 42 without substantially 
increasing the space required by the assembly. Further, the alternative 
arrangement simplifies maintenance and repair. However, in this embodiment 
the water in the chamber 42 does not receive the benefit of heat radiated 
from the compressor 50 or the compressor-heated partition 54. Accordingly, 
it is expected that a slight reduction in efficiency may be realized. If 
heat radiation and convection from the compressor 50 is deemed likely to 
present a problem in a specific application, a shield 104 (FIG. 8) may be 
mounted around the compressor. The installation and operation of the 
embodiment of FIGS. 7 and 8 is substantially the same as described above 
with respect to the embodiment of FIGS. 2-5. 
There is thus provided a heat pump water heater and storage tank assembly 
of reasonable initial cost compared to an electric resistance water 
heater, requiring installation very similar to the installation of the 
electric resistance water heater, and which is adapted for placement in 
virtually any place occupied by an electric resistance water heater. The 
assembly further is adapted to provide highly efficient operation, having 
an Energy Factor of about 1.5 to 2.0, and to provide an acceptable 
recovery time after a large hot water draw-down. 
It is to be understood that the present invention is by no means limited to 
the particular constructions herein disclosed and/or shown in the 
drawings, but also comprises any modifications or equivalents within the 
scope of the claims.