PATENT ABSTRACT
The temperature in a water tank (e.g., a residential water tank) is monitored, and a controller switches between a high temperature mode of operation and a low temperature mode of operation (e.g., based on a daily or weekly program). During the low temperature mode of operation, the controller generates signals to selectively activate a water heater to keep the water&#39;s temperature within a first range of values, (e.g., between 105 and 113° F). During the high temperature mode of operation, the controller generates signals that cause the water heater to heat the water to a temperature that is above the first range of values. This arrangement may be used for saving energy by operating in the low temperature mode at night and during those parts of the day when nobody is home, and by operating in the high temperature mode in the morning and evening when the demand for hot water is typically high. This arrangement is also well-suited for day-care centers and nursing homes, in which case the low temperature mode is used to reduce the risk of scalding. It is also useful in the homes of people who wish to avoid violating a religious injunction that prohibits heating liquids beyond a threshold temperature on the Sabbath.

PATENT DESCRIPTION
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of US Provisional Patent Application 60/698,024, filed Jul. 11, 2005 and also claims the benefit of US Provisional Patent Application 60/739,247, filed Nov. 23, 2005. 
     
    
     BACKGROUND  
       [0002]     Conventional water heaters for residential and mixed use buildings typically keep the water in the tank between about 140-150°, and a thermostat is usually used to control the water temperature. (Note that all temperatures mentioned herein are specified in degrees Fahrenheit.) However, since thermostats do not provide a high a degree of accuracy, the water temperature can often fluctuate by up to 10° before the heating system switches on. In many cases, the inaccurate nature of the temperature control in conventional water heaters is not a problem, because the user can compensate for temperature variations by mixing in more or less cold water at the tap. In other cases, e.g., when small children or infirm adults may be using the hot water, the 140-150° temperature posses a potential risk of scalding the user. This risk can be eliminated by reducing the temperature of the water in the tank at all times (e.g., to 110°). However, keeping the temperature that low makes it very likely that the users will run out of hot water during high demand periods (e.g., in the morning, when many members of the household may be showering). Moreover, for low temperature operation, the large fluctuations of conventional temperature controls becomes more of an issue, since a 10° increase would increase the risk of scalding, and a 10° decrease would cause the users to run out of hot water during high demand periods.  
       SUMMARY  
       [0003]     A controller monitors the temperature of water in a tank and switches between a high temperature mode of operation and a low temperature mode of operation based on time. During the low temperature mode of operation, the controller generates signals to selectively activate a water heater to keep the water&#39;s temperature within a first range of values (most preferably within a 4° or 6° subset of the 105°-113° F. range). During the high temperature mode of operation, the controller generates signals that cause the water heater to heat the water to a temperature that is above the first range of values. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a block diagram of a gas-fired water heater with a controller that is configured to provide a high temperature mode of operation and a low temperature mode of operation.  
         [0005]      FIG. 2  is a schematic diagram of an alternative temperature control circuit for the system shown in  FIG. 1 .  
         [0006]      FIG. 3  is a pictorial representation of the  FIG. 1  embodiment, with the controller mounted on the water tank.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0007]      FIG. 1  is a block diagram of a gas-fired water heater that has a high temperature mode of operation and a low temperature mode of operation. The water heater maintains accurate control over the water temperature in the low temperature mode of operation. The water heater uses a programmable controller/timer for switching between the high temperature and the low temperature modes at regular intervals. Note that the invention is described herein in the context of a gas-fired heater. However, the present invention is not limited to gas-fired heaters, and may be used with other types of water heating systems (e.g., electric and oil-fired water heaters) to provide similar results.  
         [0008]     A water tank  30  of any conventional construction may be used. One example of a suitable water tank is the Bradford White M-I series of upright residential gas water heaters. The water in the water tank  30  is heated by a gas burner  45  which is configured with a suitable valve  46  to control the flow of gas from the gas supply  48  into the burner  45 . One example of a suitable gas valve is the Robertshaw 722 series. The gas burner  45  and valve  46  are hooked up to the gas supply  48  using conventional techniques that are well known to persons skilled in the relevant art. A gas ignition controller  40  is hooked up to valve  46 , also using conventional techniques that are well known to persons skilled in the relevant art. One example of a suitable gas ignition controller is the Robertshaw 780 series. Taken together, the ignition controller  40 , the valve  46 , and the burner  45  are configured so that an electrical input signal to the ignition controller  40  provides control over the flow of gas through the valve  46  into the burner  45  in order to heat the water  32  in the tank  30 .  
         [0009]     The decision to turn on the burner  45  or turn off the burner  45  is made by a controller  20 , which sends appropriate electrical signals to the ignition controller  40  to control the flow of gas into the burner  45 . More specifically, when the controller  20  receives information indicating that the temperature of the water  32  in the tank  30  is too low, the controller  20  sends appropriate electrical signals to the ignition controller  40  which will, in turn, cause the burner  45  to turn on, thereby raising the temperature of the water  32 . If the controller  20  receives information indicating that the temperature of the water  32  in the tank  30  is at or above the desired temperature at any given moment, the controller  20  turns off the signal to the ignition controller  40 , which causes the valve  46  to turn off so that the burner  45  will stop heating the water  32  in the tank  30 .  
         [0010]     The controller  20  receives information about the temperature of the water  32  from a temperature probe  25 . The temperature-sensing portion of the probe  25  is in thermal contact with the water  32  in the tank  30 . For the application described below, the water temperature must be sensed with a higher degree of accuracy than in conventional water heaters. Accordingly, the temperature probe  25  must be designed to provide relatively high accuracy. One preferred approach for implementing a temperature probe with sufficiently high accuracy is to use a thermistor such as the Invensys-Robertshaw 54584-006. Alternative temperature sensors that can provide sufficient accuracy include RTDs and integrated circuit temperature sensors, such as the LM34 analog temperature sensor or the LM92 digital temperature sensor, both made by National Semiconductor. The latter can sense temperatures between 60 and 120° F. to an accuracy of less than 1° F.  
         [0011]     The interface between the temperature probe  25  and the controller  20  will depend on the particular sensor technology that is selected for use in the temperature probe  25 . However, for any given sensor technology, the interface between the controller  20  and the temperature probe  25  is preferable implemented using conventional techniques that are well known to persons skilled in the relevant art. Based on the signals arriving from the temperature probe  25 , the controller  20  obtains information about the temperature of the water  32  in the tank  30 . The controller  20  using this information to decide whether or not to turn on the burner  45  by sending appropriate control signals to the ignition controller  40 . Circumstances when the controller  20  turns the burner  45  on or off are described below.  
         [0012]     One useful application for the dual-mode water heater depicted in  FIG. 1  would be a day care center in mixed-use building. At night and in the morning, when the residents will likely be taking showers and using a lot of hot water, the controller  20  selects the high temperature mode and maintains the temperature of the water in the tank at about 140°. This is accomplished by reading the signals arriving from the temperature sensor  25  to sense the temperature of the water in the tank  30 , and sending appropriate signals to the ignition controller  40  to turn the burner  45  on or off The temperature monitoring via the sensor  25  may be done constantly or periodically, but in the latter case it should be done often enough so that the temperature cannot change too much between measurements, based on to the thermal inertia of the water in the tank  30 .  
         [0013]     When the controller  20  recognizes that it is time for the day care center to open (e.g., based on a time schedule that has been programmed into the controller  20 ), the controller  20  sets the system to operate in the low temperature mode. Optionally, the switch to the low temperature mode is programmed to occur an interval of time before the lower temperature water is actually required, since the tank will not cool instantly as soon as the mode is switched. For example, if the day care center opens at 8 AM, the controller may be programmed to switch to the low temperature mode a half hour in advance of that time, at 7:30 AM. The temperature will drop between 7:30 and 8 as hot water is drawn out of the tank (by ordinary use of hot water) and replaced by incoming cold water.  
         [0014]     Optionally, after the low temperature mode has been selected, hot water may be drained from the tank under control of the controller  20  in order to rapidly reduce the temperature of the water to the desired level. One way to accomplish this is by having the controller  20  send appropriate signals to an electrically operated valve (not shown) that draws hot water from any hot water pipe that is fed by the tank  30 . Of course, appropriate plumbing must be provided between the hot water supply, the valve, and an appropriate drain.  
         [0015]     A suitable program of operation for the controller  20  for use in a day care center that operates on weekdays only is set forth in Table 1 below.  
                           TABLE 1                       Day   5:00 AM-7:30 AM   7:30 AM-6:00 PM   6:00 PM-5:00 AM                   Sunday   140°   140°   140°       Monday   140°   110°   140°       Tuesday   140°   110°   140°       Wed-   140°   110°   140°       nesday       Thurs-   140°   110°   140°       day       Friday   140°   110°   140°       Satur-   140°   140°   140°       day                  
 
         [0016]     Another useful application for the dual-mode water heater depicted in  FIG. 1  would be in residential homes. In the evening and in the morning, when the residents will likely be taking showers and using a lot of hot water, the controller selects the high temperature mode and maintains the temperature of the water in the tank at a desired high temperature (e.g., 130°). At night and during portions of the day when nobody is home, the temperature may be set to a desired low temperature (e.g., 90°) in order to save energy. Temperature control is implemented in the same way as in the day care application discussed above, except that the timing of the transitions and the temperature set points are different. A suitable program of operation for the controller  20  for achieving energy savings in residential homes is set forth in Table 2 below.  
                               TABLE 2                       Day   6-8 AM   8 AM-5 PM   5-11 PM   11 PM-6 AM                   Sunday   130°   130°    130°   90°       Monday   130°   90°   130°   90°       Tuesday   130°   90°   130°   90°       Wednesday   130°   90°   130°   90°       Thursday   130°   90°   130°   90°       Friday   130°   90°   130°   90°       Saturday   130°   130°    130°   90°                  
 
         [0017]     A third example of a suitable application for the dual-mode water heater depicted in  FIG. 1  would be for use in the homes of people who observe the Jewish law that prohibits heating liquids beyond a certain threshold temperature on the Sabbath. Many Jewish legal authorities that were contacted by the inventor maintain that the threshold temperature is 113° F., and others maintain that the threshold temperature is as low as 106° F. However, all these authorities agree that heating liquids on Sabbath is not prohibited by Jewish law when the liquid is not heated beyond the threshold temperature.  
         [0018]     In a conventional water heater, when the water temperature is set at about 140°, when a person draws hot water from the tank, cold water flows into the tank via the cold water inlet pipe. When that cold water mixes with the hot water that is already present in the tank, its temperature will be raised above the threshold, which would violate the prohibition of heating liquids on the Sabbath. To avoid this, some observant Jews refrain from using hot water on the Sabbath, so that the incoming cold water is never heated beyond the threshold temperature.  
         [0019]     If, however, the hot water that is contained in the tank is always kept at or below 112°, which is below the threshold temperature according to the aforementioned Jewish legal authorities, when the cold water flows into the tank via the cold water inlet pipe, it will not be heated past the threshold temperature. Under these circumstances, Jewish law permits people to draw hot water out of the tank on Sabbath, even though cold water will flow into the hot water tank as the hot water leaves.  
         [0020]     Thus, for this application, the controller  20  is programmed to switch into the low temperature mode of operation about one hour before Sabbath (to provide a period of cool-down time), and to switch back to the normal high temperature mode when Sabbath is over. In the embodiment described above with rapid cooling, that time can be reduced. Since the Jewish Sabbath starts at sundown on Friday evening and lasts until the stars come out on Saturday night, and since the sun sets at different times during the year, a suitable program for the controller  20  for automatically entering the low temperature mode before Sabbath begins is shown in Table 3:  
                               TABLE 3                                       start low temp. mode   end low temp mode.           Month   Friday at   Saturday at                           January   3:30 PM   5:30 PM           February   4:00 PM   6:00 PM           March   4:30 PM   6:30 PM           April   5:00 PM   7:00 PM           May   5:30 PM   7:30 PM           June   6:00 PM   8:00 PM           July   6:00 PM   8:00 PM           August   5:30 PM   7:30 PM           September   4:30 PM   7:00 PM           October   4:00 PM   6:30 PM           November   3:30 PM   6:00 PM           December   3:00 PM   5:30 PM                      
 
         [0021]     Preferably, the controller is programmed to make suitable adjustments in regions that observe daylight savings time, to adjust for the changed time of sunset.  
         [0022]     Alternatively, instead of roughly estimating the time when Sabbath begins based on the month, a more precise start time for switching to the low temperature mode can be determined based on the date. The controller  20  can obtain knowledge of the date by keeping track of time after being set once by the user in any conventional manner. In alternative embodiments, an appropriate receiver (not shown) that receives the atomic clock signals broadcast by the National Institute of Standards and Technology in Boulder, Colorado, may be added so the system to determine the date and time. The controller  20  would then determine the correct time to switch modes based on the expected time of sunset on the day in question (e.g., by using an appropriate look-up table indexed by the date).  
         [0023]     Optionally, a Jewish calendar may be programmed into the controller  20 , and the controller may be programmed to select the low temperature mode during those Jewish holidays when similar prohibitions on heating water are applicable.  
         [0024]     Controlling the temperature with a high degree of accuracy is particularly important in the first and third applications described above, especially in the low-temperature mode of operation. For example, in the context of a daycare center, if the water is 10° too hot while the daycare center is opened, it would increase the risk of accidentally scalding, and temperatures above 113° are problematic for the Jewish Sabbath. (Note that for those users who choose to comply with a lower threshold temperature, such as 106°, all the relevant temperature values set forth herein must be adjusted accordingly). Conversely, if the water temperature drops too far (e.g., to 100°), it may not be hot enough for the user&#39;s desired use (e.g. washing hands or doing dishes), especially during periods of high demand. Accordingly, the controller  20  should make appropriate and timely adjustments to minimize the temperature fluctuations, preferably to within a 6° F. range, and more preferably to within a 4° range (e.g., to manage the temperature fluctuations within the tank  30  so that it always stays between 105° and 111°, or more preferably between 107° and 111°).  
         [0025]     In the embodiment illustrated in  FIG. 1 , the controller  20  turns the burner  45  on and off as required in both the low temperature mode and the high temperature mode by sending appropriate signals to the ignition controller  40 . Thus, the controller  20  has direct control over the burner  45  in both the low temperature and the high temperature modes. In an alternative embodiment, the controller  20  retains direct control over the burner  45  in low temperature mode, but passes responsibility for controlling the temperature to another device (e.g., a conventional thermostat) in the high temperature mode where accuracy is less important. This may be accomplished as shown in  FIG. 2 , for example, by having the controller  20 ′ selectively actuate a relay  90  to connect a thermostat  92  to the control input of the ignition controller  40  in high temperature mode, and to connect the control output of the controller  20  to the ignition controller  40  in low temperature mode. In that case, the controller  20 ′ would have direct control over the burner  45  in low temperature mode, but would have indirect control over the burner in the high temperature mode (by letting the thermostat bring the temperature up to a range that is higher than the temperatures associated with the low temperature mode).  
         [0026]      FIG. 3  shows how the system can be physically connected to a water heater. A control panel  80  is shown mounted to the body of the water heater  30  at a convenient height. The control panel  80  preferably houses the controller  20  (shown in  FIG. 1 ), as well as a display and buttons for implementing a user interface, which may be implemented in any of a variety of ways that will be apparent to persons skilled in the relevant arts. For example, a user interface similar to those used for programmable air conditioning thermostats may be used, preferably including a digital display that displays the current temperature of the water in the tank and/or the temperature setting. The control panel  80  may be mounted to the tank&#39;s wall using any suitable approach including but not limited to screws, glues, magnets, straps that surround the tank, etc. A magnetic mount may make it easier to install the above-described embodiments in retrofits of existing tanks. Preferably, the control panel  80  is relatively small and as lightweight to facilitate easy mounting.  
         [0027]     The tank  30  has a utility compartment  82 , which may be used to house the gas ignition controller. In electric heat embodiments, the utility compartment  82  may be used to house components like a controller power module, a transformer, a heating element control switch, and the connection to the temperature probe. Optionally, a cover for the utility compartment  82  may be shaped to accommodate the control cable connection, and the power module may be fixed to the cover itself to simplify the installation and retrofitting of existing tanks. A cable  84  connects the control panel  80  to the components housed in the utility compartment  82 . Optionally, the cable  84  may be coiled to simplify installation onto different sized tanks or at other locations that may be preferred by the user.