Abstract:
A water heater has a water container and an element located to heat water in the water container. A sensor located to sense temperature proximate the element and a controller connected to the element and the sensor. The controller is programmed to compare a plurality of sensed temperatures and to disengage the element in the event that the difference between sensed temperatures over a predetermined time period is greater than a predetermined temperature difference and further to engage the element based on a comparison of sensed tempereatures.

Description:
FIELD OF THE INVENTION 
     This invention relates to an electric water heater, particularly to an electric water heater having an electronic control system that greatly improves manufacturing costs, reduces warranty expense and operating efficiencies. 
     BACKGROUND OF THE INVENTION 
     Typical electric water heaters are constructed with one or two electric-powered heating elements to heat water in the water tank, depending on the size and utilization of the water heater. Each element utilizes an electromechanical thermostat mounted onto the side of the tank at the point where the screw cap of the element connects to the side of the water tank. There are a number of disadvantages associated with such constructions. 
     Current electromechanical thermostats use bimetal technology for actuation of a set of contacts that either energize or deenergize the heating element. Such bimetal technology is comparatively imprecise and the response time to temperature changes in the water tank are relatively slow, thereby reducing water heater efficiency. 
     Another significant problem with present construction is the difficulty of protecting against “dry fired” elements. “Dry fire” occurs when power is applied to a heating element without water surrounding the element. Such dry firing rapidly causes damage to the heating element, thereby sharply reducing its useful life span. In most instances, dry firing can cause immediate failure of the element. 
     Current electromechanical thermostats also utilize a comparatively large and bulky thermostat bracket and occupy a comparatively large amount of surface area on the side of the water tank. This reduces energy efficiency since polyurethane foam insulation that surrounds the remainder of the tank is not used in this space. This occurs because the chemicals that form the polyurethane foam can interfere with the electromechanical thermostat controls during assembly and field service. Current methods for preventing such interference include foaming aprons, fiberglass batts or EPS foam dams, all of which have lower thermal efficiency (K-factors) than the polyurethane foam surrounding the remainder of the tank. 
     All of the above constructions result in a large number of manufacturing parts and steps, all of which add to the final cost of the product. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide a water heater that increases energy efficiency. 
     It is another object of the invention to provide a water heater that eliminates comparatively large electromechanical thermostats and reduces the number of component parts required to produce a water heater. 
     It is yet another object of the invention to provide a water heater that protects against dry firing of heating elements. 
     Other objects and advantages of the invention will become apparent to those skilled in the art from the drawings, the detailed description of the invention and the appended claims. 
     SUMMARY OF THE INVENTION 
     In one aspect, the water heater of the invention includes a water container; an element located to heat water in the water container; a sensor located to sense temperature proximate the element; and a controller connected to the element and the sensor, the controller being capable of disengaging the element in the event that a sensed temperature over a predetermined time interval is greater than a predetermined temperature difference, wherein there is substantially no degradation of the element within the predetermined time interval. 
     In another aspect of the invention, there is a water heater that includes a water container; an element located to heat water in the water container; a sensor located to sense temperature of water in the water container; and a controller connected to the element and the sensor, the controller being capable of comparing temperature information received from the sensor with a predetermined temperature and energizing the element based on the comparison. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic front elevational view of a water heater in accordance with aspects of the invention wherein dashed lines show interior portions of the water heater. 
     FIG. 2 shows a schematic side elevational view, taken partly in section, of the water heater of FIG.  1 . 
     FIG. 3 shows a schematic exploded top view of the water heater shown in FIG. 1 and a user interface. 
     FIGS. 4A and 4B show side and front elevational views, respectively, of a heating element utilized in accordance with aspects of the invention. 
     FIG. 5 discloses a circuit diagram of the control system of a water heater in accordance with aspects of the invention. 
     FIG. 6 shows a ladder diagram of the control system of a water heater in accordance with aspects of the invention. 
     FIG. 7 shows a simplified functional block diagram illustrating the function of prevention of dry fire in heating elements in accordance with aspects of the invention. 
     FIG. 8 is a simplified functional block diagram illustrating another embodiment of the function of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It will be appreciated that the following description is intended to refer to the specific embodiments of the invention selected for illustration in the drawings and is not intended to define or limit the invention, other than in the appended claims. 
     Turning now to the drawings in general and FIGS. 1-4B in particular, the number “ 10 ” designates an electric water heater of the invention. Water heater  10  includes an outer jacket  12  which surrounds foam insulation  14 . Foam insulation  14  surrounds water tank  16 . A top pan  18  caps jacket  12  on its upper end and bottom pan  20  caps jacket  12  on its lower end. An inlet  22  in the upper portion of tank  16  provides for cold water to enter the tank. Similarly, outlet  24  allows for hot water to exit through the upper portion of tank  16 . 
     A pair of heating elements  26  are mounted to the side of tank  16 . Elements  26  are electrically connected to an electronic controller  28  located in a recessed portion  30  of top pan  18 . Elements  26  are mounted to the side wall of tank  16  by means well known to those of ordinary skill in the art, such as threads  46 , and are covered by plastic caps  32  which snap into position through openings in jacket  12 . An upper foam dam  34  surrounds upper element  26  and extends between tank  16  and jacket  12 . Similarly, lower foam dam  36  surrounds element  26  and spigot  38 . Foam dam  36  also extends between jacket  12  and tank  16 . 
     Each heating element  26  includes a base  27 , a resistance heater  29 , a thermistor sensor  44  and a pair of thermistor connectors  45 . The thermistor  44  is embedded in base  27  between opposing legs of the resistance heater  29 . 
     Electronic controller  28  connects to elements  26  by way of wires  40 . Wires  40  extend between electronic controller  28  and elements  26  through the space between jacket  12  and tank  16 . That space is otherwise filled with insulation  14 . It is possible for wires  40  to be located such that foam-forming liquids form directly around wires  40  during the foaming process. Also, wires  40  can be located within a passageway created within the foam, if desired, such as with tubes, pipes and the like. Electronic controller  28  is a user interface and includes a water temperature adjustment dial  42  which can be rotated to select a variety of water temperatures at which the water within tank  16  will be maintained. 
     The specifics of the connections and operations of electronic controller  28  and heating elements  26  shown in FIGS. 5 and 6. Thermistor  44  is connected in a conventional manner through thermistor connectors  45  to electronic controller  28 . Resistance heater  29  is also connected to heater control board  47  via relays  50  on heater control board  47 . Electrical power is supplied to the system through power supply  48 , which include fuses  49  and  49 ′ for deenergizing the system in the event of an amperage surge. 
     Heater control board  47  preferably incorporates electronic control circuitry for controlling operation of the water heater, as described in more detail below. Such control circuitry may incorporate a number of electronic components, known to those of ordinary skill in the art, such as solid state transistors and accompanying biasing components, or one or more equivalent, programmable logic chips. The electronic control circuitry may also incorporate a programmable read only memory (PROM), random access memory (RAM) and a microprocessor. 
     The arrangement and/or programming of these components may take any number of forms well known to those of ordinary skill in the art to accomplish operation of the water heater as described below. For example, specific programming of the type described herein may be obtained from Therm-O-Disc, Inc. and United Technologies Electronic Controls. 
     When there is a call for hot water, hot water exits through outlet  24  and cold water is introduced through inlet  22 . Thermistor sensors  44  detect the temperature of water within tank  16  by way of their being embedded in bases  27  at positions interior of the water tank side wall. The temperatures of bases  27  reflect the temperature of water in tank  16 . Thermistors  44  then send temperature information, typically in the form of an electrical signal, to controller  28 . Controller  28  is programmed with predetermined set point temperatures to determine the temperature at which controller  28  energizes element  26 . The predetermined set point can be made to be variable if desired. When the temperature of the water within tank  16  decreases to that predetermined set point, controller  28  detects such temperature information received from thermistor sensor  44  and energizes element  26 . Element  26  continues in the energized state to heat the water until temperature information received from sensor  44  indicates that the water temperature has reached a second predetermined set point. 
     The second predetermined set point can be selected by adjustment dial  42  and is variable. When controller  28  detects that the second predetermined set point has been reached, controller  28  deenergizes element  26 . The second predetermined set point typically has five variable settings for deenergizing elements  26 . Such selectable settings are preferably between about 90° F.-180° F. The differential for energizing the elements can vary depending on the task to be performed. 
     Controller  28  also contains a lock-out set point which is preferably less than about 210° F. The control lock-out prevents elements  26  from energizing when the water temperature reaches an abnormal predetermined set point and the controller  28  will not permit energizing of elements  26  until controller  28  is reset by removing power and then subsequently reapplying power. This can be accomplished automatically by controller  28 , thereby reducing and possibly eliminating the need for a mechanical reset control. Such a reset could be performed by a reset user interface  31  on controller  28 . The sensing capabilities of sensors  44  are such that elements  26  can be energized and deenergized after only approximately 1.5 gallons of water have been drawn from tank  16 . This compares to about 3.0 gallons of water removal in prior art constructions. 
     One particular sequence of operational steps to achieve operation of the water heater in this matter is shown in more detail in FIGS. 7 and 8. When the water heater control system is first started, the control electronic circuitry of heater control board  47  records the initial temperature at bottom element  26  and then turns on the bottom element  26  for ten seconds and then off for two minutes. Heater control board  47  then records the file temperature of the bottom element  26  as measured through thermistor  44  and calculates the difference between the final temperature and initial temperature. 
     If the difference between these temperatures is greater than five degrees, then heater control board  47  turns off both elements  26  through relays  50 . Heater control board  47  then checks to see if system power has been turned off or reset through incoming power supply  48 . Once the system has been reset, heater control board  47  then begins this process from start. 
     If, however, the temperature differential is less than five degrees, then heater control board  47  energizes bottom element  26  to heat the water in tank  16  until it reaches the temperature set on temperature adjust dial  42 . 
     If the temperature of temperature adjust dial  42  is less than 110° F., then the top element  26  remains off. Otherwise, heater control board  47  checks the temperature at thermistor  44  in upper element  26 . If the temperature of thermistor  44  in upper element  26  is equal to the temperature of dial  42  minus 5° F., then heater control board  47  does not energize upper element  26  until the temperature at thermistor  44  in upper element  26  is less than the turn on temperature (which is typically the temperature set on temperature adjust dial  42  minus some increment such as 5°) minus 5° F. Heater control board  47  then energizes top element  26 . 
     Heating of the water in tank  16  then continues in a conventional manner until the turn off temperature of temperature adjust dial  42  is achieved. 
     By energizing upper and lower elements  26  in the manner described above, the significant advantages of the invention can be achieved. For example, energizing the element briefly (e.g., about 5-10 seconds) and detecting temperature with a thermistor allows heater control board  47  to prevent elements  26  from being energized for long periods of time in a “dry fire” condition, thereby avoiding substantial degradation of the elements and significantly extending their life. Thus, the terms “substantially no degradation” refers to little or no element degradation that occurs for an element energization period of about 5 seconds and up to about 10 seconds. Energizing the element for longer than about 10 seconds can result in substantial degradation under dry fire conditions. 
     Use of thermistor  44  allows for a much more accurate and responsive detection of temperature than the use of more conventional temperature-sensing technology, such as bimetallic strip. This allows the significant temperature changes which occur in a short period of time under a dry fire condition to be detected with only a short (e.g., about 5-10 seconds) energizing of the heating element  26 . In this way, a dry fire condition can be detected virtually immediately to prevent overheating of the element, which significantly reduces its useful life. 
     Also, use of thermistors  44  eliminates the electromechanical thermostats and their associated foaming aprons, fiberglass batts and the like. Small doughnut-shaped foam dams surround the bases  27  and permit foam insulation to cover more surface area of the tank. 
     An alternative set of operational steps in accordance with the invention is shown in FIG.  8 . In this embodiment of the invention, during control power up of the water heater, heater control board  47  checks to see if there is a need for heating of the water at lower element  26  by measuring the temperature at thermistor  44  and comparing the measured temperature with that of temperature adjust dial  42 . 
     If such a demand exists, heater control board  47  energizes lower element  26  and continuously checks to see if the water heating demand is satisfied. 
     Once this heating demand is satisfied, heater control board  47  then repeats this process for the upper element  26 . 
     The improvements described above result in a highly energy efficient water heater. The result is that the thickness of the foam insulation positioned between tank  16  and jacket  12  can be reduced by up to 50%. In other words, a 2″ foaming cavity can be reduced to a 1″ cavity, and still retain the same energy input. 
     Although this invention has been described in connection with specific forms thereof, it will be appreciated that a wide variety of equivalents may be substituted for the specific elements described herein without departing from the spirit of the scope of this invention as described in the appended claims. For example, water tank  16  may be made of a number of sizes and shapes and may be made from a wide variety of materials such as metals and/or plastics. Foam insulation  14  may similarly be made from any number of high energy efficient foam insulations well known in the art. 
     The bottom of the water tank  16  may have various shapes, either with lower flanges as shown or as a flat construction. Other modifications may be made, including use of foam insulation between the bottom of tank  16  and bottom pan  20 . Also, outer jacket  12  may be made from any number of materials such as rolled metals, preferably steel, or extruded vinyl materials and the like. Also, top pan  18  and bottom pan  20  may be deep-drawn, stamped or the like, or be made from metal, plastic or other suitable materials. Various types of heating elements may be utilized so long as they are used in conjunction with thermistor sensors  44 .