IL137546A - Programmable domestic water heating system - Google Patents

Abstract

A method for heating water in a domestic water heating system, comprising: providing a water tank containing mw liters of water; providing at least one temperature sensing unit (17) in said water tank for sensing the temperature of the water in the tank; providing a heating element (3) in said water tank; providing a control unit (21) for activating said heating element, said control unit continuously receiving indication for the water temperature from said temperature sensing unit; providing to said control unit a desired water temperature, and designating a time for using the water at said desired temperature; determining the current water temperature, the desired water temperature at said designated time, the power of the heating element, and the specific heat of the water, calculating by the control unit the heating period it needed for heating the water in the tank from the current temperature as measured by said temperature sensing unit, to the desired temperature; periodically repeating said calculation and updating said calculated period it according to the changes in the sensed water temperature; and when the designeated usage time is approaching, activating the heating element a it period before the said designated usage time. פירוט זה נבחן בהתאם לתקנה 35 לתקנות הפטנטים, תשכ" ח-1968 This specification was examined in accordance with regulation 35 of the Patent Regulations, 5728-1968. 3492 ט" ז בשבט התשס" ד - February 8, 2004

Description

mDJT? Jl_JT.il D1!. D1TTTV7 JlTT-l JIDIIJT] PROGRAMMABLE DOMESTIC WATER HEATING SYSTEM PROGRAMMABLE DOMESTIC WATER HEATING SYSTEM Field of the Invention The field of the invention generally relates to electrical home appliances. More particularly, the invention relates to an improved domestic water heating system.

Background of the Invention Hot water is an essential commodity in the modern world, and a water heating system is an appliance commonly used in households throughout the world.

In some countries where the price of the energy is negligible, it is common to activate the water heating system all the day, resulting in a significant waste of energy.

In other countries, where energy is relatively expensive, solar energy is used for heating the water. However, the solar energy cannot generally provide hot water 24 hours a day, 365 days a year, and therefore complementary heating involving energy consumption is required. The source of energy for this purpose is, in most cases, electricity or gas.

In order to save energy, activation of the water heating is only as needed. However, in most water heating systems of the prior art, the user is not provided with "any indication regarding temperature of the water in the tank, and moreover, he has no indication whatsoever regarding how long the heating system has to be ON in order to provide water in the desired amount and temperature. Generally, this causes the user to activate the heating element of the system a longer time than necessary resulting in a waste of energy, or a shorter time than necessary, resulting in a colder and insufficient amount of water than desired. Furthermore, even after the water heating is presumably completed, the user has no indication of the water temperature in the tank, and must open the tap and wait a relatively long time for regulating the temperature, resulting in a waste of water.

The lack of accurate temperature indication of the water in the tank, and the inability to plan in advance the necessary water amount and temperature causes inconvenience, waste of energy and water. In cases wherein the user constantly activates the water heating throughout the day and night, there is even more energy waste, particularly in times when there is no need for hot water. This energy waste is added to the energy loss resulting from the temperature difference between environment and the water in the tank, which in many cases is significant.

Of course there are times where a user requires a relatively hotter temperature than in other times. In the systems of the prior art, the pre-planning of the water temperature is either unavailable, or unsatisfactory. In conventional water heating systems of the prior art, and particularly for safety purposes, there is an adjustable thermostat mounted in a pocket in the water tank, which senses the Water temperature, and disconnects the electrical supply when a pre-assigned maximum temperature is reached. However, in this case, the regular user does not have access to the thermostat, or control over the pre-assigned temperature.

Some other prior art systems comprise a timer, either electrical or mechanical, for setting the duration of the. water heating. 1 Figure 1 shows a hot water tank commonly used in systems of the prior art. The water tank 1 comprises an electric heating unit 3 for supplying energy to the water. Heating unit 3 is essentially a resistor, heated by an electric current flowing through it, and transferring heat to the surrounding water. The water tank further comprises in its lower part an inlet water pipe 8, and in its upper part an outlet water pipe 9. A metal flange 2 at the bottom of the tank supports the heating unit 3. Also supported by the flange is a metal sleeve 4, serving as a pocket for a standard thermostat. Insulating layer 5 blocks heat transfer to the surroundings. Thin metal 10 encloses the tank the insulating layer 5. Remote ON/OFF switch 6, is usually located in an easily accessed place, and generally comprises a red indication which lights when the switch is ON. When the switch is ON and the water temperature rises to the preset temperature of the thermostat, the thermostat disconnects current to unit 3. When the water temperature falls below said preset temperature, the thermostat reconnects the current to the heating element.

Figure 2 shows a prior art system that further comprises a heat concentrator "7 in the water tank. The heat concentrator 7, which is used only in a vertically oriented tank, is a cup-like device made of any suitable material, and mechanically connected to the bottom of the water tank. The heat concentrator 7 has openings at its lower part 19 for enabling water passage, and at its upper part an additional outlet opening 20. The heat concentrator 7 encloses the heating unit 3 and the thermostat 4. When the heating unit 3 is activated, hot water in concentrator 7 flows to the upper opening 20, and cold water flows through the lower openings 19 to the concentrator, creating water circulation. Layers of hot water are therefore concentrated at the upper part of the water tank. After a long period of heating, all the water in the tank becomes hot, and the water temperature in different parts of the tank is relatively homogenic. Generally, it is common to use a heat concentrator 7 in water tanks of 80 liters or more.

Prior art US 6,002, 114, filed September 15, 1998, discloses a water heating system which comprises: 2. A water tank with four heating elements; 3. Temperature sensors for checking the temperature at the inlet and outlet of the water tank; 4. A sensor for checking the water flow rate at the inlet pipe of the tank; 5. CPU receiving sensor indications, for activating/deactivating said four heating elements, further comprising a circuitry for detecting failures; and 6. A display panel for showing the user the temperature of the water leaving' the tank. .

More particularly, US 6,002,114 deals with a commercial heating system having four electric heating elements, and a plurality of sensors. The heating elements are activated according to water temperature at the inlet and outlet of the tank, while further considering the inlet water flow rate. > DE 29719 267 discloses a microprocessor-based controller for an electric water heating system. The front panel of the housing of the controller has several push buttons for setting the desired temperature and various other parameters, for selecting from a function menu, and for activating a rapid heating mode. The controller further comprises a seven-segment display with a temperature bar indicating the thermal state of the heating system.

US 5,556,564 discloses a domestic water heating system having a unit for controlling the water temperature. The said system comprises: 1. Three temperature sensors, a first sensor at the top, next to the outlet of the water from the taiik, a second in the middle of the tank, and a third at the bottom of the tank next to the water inlet; 2. A display panel showing the temperature measured by the upper sensor, and enabling the user to set the required temperature of water leaving the tank; 3. Two light indicators which deactivate when the middle and the lower sensors measure temperatures above the set temperature. The light indicators indicate to the user when there is enough water in the tank for use. 4. The hot water tank and the control panel are distant one from the other, and are connected by only two electric wires. The same two electric wires provide the power to the heating element, and transfer the low voltage temperature indication from the upper sensor in the tank to the control panel.

FR 2 539 238 discloses a control method and device for an apparatus for heating a fluid to reach a predetermined temperature. The device comprises a central control unit receiving a signal from a temperature probe which identifies the temperature of the fluid, a storage unit for storing a characteristics data of the apparatus used, and a circuit for setting a predetermined temperature. The central control unit tries to activate the heating so that the fluid will reach the predetermined temperature at the predetermined time. The invention is particularly useful in electric water heating systems. The system of this patent particularly intends to activate the heating during low-rate electrical periods, for example, overnight, weekends, etc. This patent identifies the periods of low-cost electric energy in order to activate the heating particularly during these periods. The system follows the unclear and undefined expressions th=(TF -TD), and 1< K + ta. th is the temperature at the end of the low cost electric energy period. TF is the time at the end of the low cost electric energy, TD is the present time, K is a factor describing the intensity of the electric power at the heating element and the water volume in the tank. This formula cannot determine the time required to heat the water in the tank. All this is available for one cycle a day. The system also enables manual heat activation for times when the energy cost is higher.

TJS 4,568,821 discloses still another remote water heating system. The system comprises two water tanks, one tank solar heated, the other heated by electricity, oil or gas. The system comprises two temperature sensors located at the outlet pipes of each water tank. The . controller of said system uses a 24-hour clock, and is assembled with solid state electronic components.

All the above prior art systems are designed to provide better control over water heating systems, and to save energy. Some of the prior art systems allow the designating of a period for heating with a starting time. However, these systems do not consider the water temperature at the starting time for heating, in which the water is heated for the said designated period, resulting in hotter water than necessary (and waste of energy) or colder than necessary (resulting in inconvenience). In some other cases, the water reaches the desired temperature before the time planned for use, and the heating terminates. However, until the water is actually used, the temperature decreases, resulting in a waste of energy and inconvenience. The inventors have found that more energy can be saved in the system of the invention in comparison with the prior art water heating systems.

It is therefore an object of the invention to increase energy and water savings in a domestic, water heating system.

It is another object of the invention to provide to the user better control and more reliable indications relating to the temperature of the water in the tank .

It is still another object of the invention to enable easy installation of the system of the invention, in existing water heating systems, on site.

It is still another object of the invention to provide electrical and electronic failure indications, by visual or audial means.

Summary of the invention The invention relates to a method for heating water in a domestic water heating system, which comprises: (a) providing a water tank containing mw liters of water; (b) providing at least one temperature sensing unit in said water tank for sensing the temperature of the water in the tank; (c) providing a heating element in said water tank; (d) providing a control unit for activating said heating element, said control unit continuously receiving indication for the water temperature from said temperature sensing unit; (e) providing to said control unit a desired water temperature, and designating a time for using the water at said desired temperature; (f) knowing the current water temperature, the desired water temperature at said designated time, the power of the heating element, and the specific heat of the water, calculating by the control unit the heating period At needed for heating the water in the tank from the current temperature as measured by the temperature sensing unit, to the desired temperature; (g) Periodically repeating said calculation and updatingsaid calculated period At according to the changes in the sensed water temperature; and (h) when the designated usage time is approaching, activating the heating element a At period before the said designated usage time.

Preferably, the method further includes in the calculation a heat loss factor.

Preferably, the formula used by the control unit is: At = Wherein: is the expected heating period by the heating element [seconds]; P H is the power of the heating element [Watts] ; m w is the volume of the water in the water tank measured in liters; Joules Cp is the specific heat capacity of the water ( = 4200 ); kg -°c -9a- AT is the difference between the designated temperature at a later desired time and the current temperature of the water in the tank, measured in degrees Celsius -10- Preferably, when the loss factor is considered, the calculation is made by the following formula: KmwCpAT wherein K is the loss factor; Preferably the loss factor is calculated'by the following formula: A B PH Wherein: A, B, and C are numerical values obtained by laboratory experiments. A is the volume of the tank used [liters], B is the difference between the required temperature in the tank and the air temperature surrounding the tank [°C], and C is the heat lost to the surroundings [Watts], as acquired by experimental results.

The invention further relates to a water heating system, which comprises: - a water tank; - a heating element in said water tank; - a temperature sensor for sensing the temperature of the water in the tank; -11- - a control unit located in a place accessible to the user, the control unit receives from said temperature sensor an indication to the current temperature. The control unit further comprises: a. a display for displaying the current water temperature as acquired by the said temperature sensor; 1 b. a display and push buttons allowing the user to designate time for having hot water at a desired water temperature; c. calculating means for calculating from the current water temperature, the desired water temperature, the power of the heating element and the specific heat of the water a heating period in which the heating element has to be activated in order to heat the water to the desired water temperature by the heating element; and d. switching means for providing voltage to the heating element during said heating period.

Preferably, the temperature sensor is positioned in a metal sleeve penetrating into the water tank, and the tip of the temperature sensor which is sensitive to temperature is forced to be in contact with- the top of the said sleeve. The element forcing the tip of the temperature sensor to be in contact with the top of the sleeve is a spring. Preferably, the temperature sensor is a PT 100 type sensor, having means for transforming a change in temperature into a proportional changes in voltage. -12- Brief description of the drawings - Fig.l is a schematic depiction of a domestic water heating system according to the prior art; - Fig. 2 is a schematic illustration of another domestic water heating system according to the prior art; . ' - Fig. 3 is a schematic illustration of the installation of a single temperature sensor in the standard metal sleeve of a water tank, of the type shown in Figs. 1 and 2; - Fig. 4 describes the front panel of the control unit, including the display and push buttons according to the first (most common) alternative; - Fig. 5 is a flow chart describing the logical operation of the first alternative; - Fig.6 describes the front panel of the control unit, including the display and push buttons according to the second (most sophisticated)alternative; - Fig. 7 describes the front panel of the control unit, including the display and push buttons according to the third and fourth alternatives; - Fig. 8 describes the control unit panel, including the display and push buttons of the fifth alternative; - Fig. 9 describes the front panel of the control unit, -including the display and push buttons according to the sixth alternative; - Fig. 10 describes the front panel of the control unit and push buttons according to either the sixth or the seventh alternative; and - Fig. 11 describes 3 alternatives (a,b,c) of a computer control over the heating system. -13- Detailed Description of Preferred Embodiments The invention provides improvements to domestic water heating systems. More particularly, the system of the invention provides an improved control over the water heating, enabling the user to plan and define in advance the exact temperature of the water in the water tank, and the time at which heated water will be needed at the defined temperature. As said, some of the domestic heating systems of the prior art enable the defining of a desired water temperature at a specific time. However, these systems are either not sufficiently accurate, particularly in determining the exact temperature of the mass amount of the water, or are not optimized in their energy consumption.

The following equations are used in the control unit of the system for defining the required heating period, and the exact starting time in which the heating is initiated: (1) Watts - s∞ = mw - Cp - AT Where: Watt · sec is the heating energy provided to the water by the heating element; mw is the volume of the water in the water tank measured in liters (=Kg); Cp is the specific heat capacity of the water (= 4200 5— ); kg - C -14- ΔΓ is the difference between the desired temperature at a later desired time and the present temperature of the water in the tank, measured in degrees Celsius [° C].

A K - Watt - sec (2) At = P wherein: t is the expected heating duration of the heating element [seconds]; PH is the power of the heating element installed in the water tank [Watts]; K ' is an experimental factor which enables the algorithm to accurately calculate the time duration required to heat the water in the tank to the desired temperature. Formula (3) details how K is calculated. It is partially based on actual experiments that were performed by the inventors, taking into account the volume of the water tank (mW), the temperature difference (ΔΤ), and the power of the heating element (PH).

The control unit calculates the value of K according to the following formula: (3) K = 1 + - . ^ . C.

A B P„ A, B, and C are numerical values obtained by laboratory experiments. A=60 liters, is the volume of the tank used, B=20°C is the difference between the required -15- temperature in the tank and the air temperature surrounding the thermally insulated tank. C=70 Watts was the heat lost to the surroundings. These values may change by accumulation of experience, and with variations in materials and structure of the water tank.

Example 1: An 80 liter water tank having a heating element of 2500 Watts is provided. The present temperature of the water in the tank is 28° C . It is desired that at 19:00 this evening, the water temperature will be 50°C . ΔΤ = 50-28 = 22°C.

Therefore: Watts · sec = 80 · 4200 · (50 - 28) = 7.392 · 106 Joules 60 20 2500 . 1.041 · 7.392 - 106 „ . t = : = 3078.2 sec = 52 nun 2500 Therefore, the heating element will be activated at 18:08:00. If the user desires, the program may be set to continue water-heating for a specified duration of time. For example, if the user desires to keep the water in the tank at this temperature for an additional 40 minutes, the heating will resume each time the water temperature drops below 50° C , until * -16- According to the invention, the control unit operates continuously, checks the present date, time and temperature of the water in the tank, and calculates when to activate the heating element.

Example 1 shows that the system saves a significant amount of energy in comparison to systems of the prior art ; which include mechanical/electrical/electronic timers that do not consider the present temperature before activating the heating process. The systems of the prior art thus maintain water in the tank at higher temperatures than needed over long periods. The advantage of the algorithm of the invention is that the water in the tank is heated only towards the required time, in order to reach the exact desired temperature precisely at the set time, and thus heat loss to the environment is minimized. Therefore, the system of the invention provides appreciable energy savings.

The temperature of the water in the tank is measured by a sensor mounted in the tank, with data continuously provided to the control unit. The user defines the times, desired temperature, and the time duration to keep the said temperature. The control unit is located in a place convenient to the user and remote from the hot water tank.

The measured water temperature is displayed continuously on a front panel of the control unit. The user introduces to the control unit by means of push buttons the desired settings. For example, the user may set a required water temperature, a -17-date and time in which the required water temperature is desired, and the duration for which this temperature is desired.

The control unit retains the user settings in an internal memory. The user can also activate or deactivate the heating directly, or the timer operation of the control unit.

The preferred water temperature sensor according to the invention is, for example, PT100 type sensor (PT stands for "Platinum Temperature"), a thermocouple, or an equivalent temperature-sensing element. When a PT100 temperature-sensing element is used, it is connected to a matched transducer as commonly used in the art. Hereinafter, when the term "PT100" is used, it should be understood to include the transducer. The transducer receives from the control unit a _DC voltage, and provides in turn to the control unit a voltage proportional to the temperature sensed by the PT100. Installation of all parts of the system is simple, and any existing standard domestic water heating system can be upgraded to the system of the invention with relative ease.

The control unit includes the option of failure detection, which alerts the user to detected failures, such as in the heating element or water temperature sensor. Any of the above failures causes automatic termination of the voltage supply to the electric heating element. -18- In the system of the present invention, a single sensor is installed in the water tank. Figure 3 shows in schematic form the system according to one embodiment of the invention. A temperature sensor 17 is positioned in the metal sleeve 4 of the water tank 1. A metal sleeve 4 that penetrates into the water tank exists essentially in all standard water tanks of the prior art. However, in the prior art tanks, the sleeve contains electro-mechanical thermostats, which disconnect electricity to the heating element when the temperature of the thermostat (which often is not the actual temperature of the water) exceeds a predetermined setting. The metal flange 2 of the standard water heater supports the metal sleeve 4. Gasket 11 is tightened by nut 13 to prevent leaking of water. Cylindrical structure 14 connects the sensor to the metal sleeve, and is tightened by nuts 12 to the metal sleeve. As said, the temperature sensor 17 is preferably a PT100 sensor, a thermocouple, or equivalent device. Spring 15 ensures full contact between the sensor and the upper part 4A of the metal sleeve 4. This is an essential part of the invention, as the full contact of the sensor with the metal sleeve ensures an accurate estimate of the water temperature about the center of the tank. It has been found that a particular problem in prior art systems is inaccurate temperature measurement, as a result of the lack of contact between the sensor or thermocouple and the sleeve. Nut 16 adjusts the force the spring applies to the sensor, causing it to contact the top of the sleeve. Connecting cable 18 directs the temperature measurement of the sensor to the control unit. -19- According to actual measurements made by the inventors, it has been found that there is no deviation between the temperature read by the PT100, and the actual water temperature next to the sleeve's top (it should be noted that the PT100 accuracy is 0.1%).

Some observations on temperature sensor/sensors: a. According to the invention, more than one temperature sensor 17 can be installed in the well 4, (shown in Figures 1 and 2). b. One or more dipped-type sensors can be installed at different locations in the tank, to directly measure the water temperature. If more than one sensor is used, the algorithm is provided with information relating to which of the sensors to use, and at what time.

The operation of the control unit may vary. Hereinafter, several alternative modes of operation are described. Furthermore, the structure of the control unit and its display may also vary.

General: The control unit comprises a display, software for operating the unit, electronic components, and electrical and mechanical components. The algorithm according to which the unit operates is based on the formulas as given hereinbefore. As said, the algorithm uses at least three main parameters in order to calculate when and for how long to activate the heating: (a) the water temperature before the -20-heating; (b) the specific heat of the water; and (c) the desired water temperature at the time when the hot water is to be used. 1st alternative: a standard system, the control unit including software for enabling two modes of operation, manual or automatic.

Manual activation is provided by setting the ON/OFF switch 6 to the ON position, thereby enabling the control unit to activate the heating element. The heating terminates either by manually turning OFF switch 6, or by the control unit when the desired water temperature in the tank, as programmed by the user, has been reached. The programming of the control unit is performed by the user, using the display and the buttons of the unit.

Example 2: It is desired to have water in the tank in a temperature of 5tfC at 19:00. The present temperature in the tank is 28° C . The control unit uses the algorithm to calculate the time duration required for the heating element to heat the water up to 50°C The present calculation, using the algorithm of formula (2), results in 52 minutes of heating. The software continuously checks the calculation until 19:00 minus 52 minutes = 18:08. At the calculated time, 18:08, the heating element is activated automatically by the control unit. At 19:00, when the temperature reaches the desired temperature of 50°C, the control unit terminates the heating. If during the heating, i.e., between 18:08 and 19:00, hot water is consumed from the tank, and therefore the water temperature at 19:00 is found to be lower than the desired, the control unit continues to activate the heating -21-element, until the water temperature reaches the desired temperature. Furthermore, the user may be provided with the option of programming the unit to continue providing hot water at 50°C in a consuming duration of, for example, 40 minutes. In that case, the water will be heated to 5CPC at 19:00, and any time between 19:00 and 19:40 when the temperature drops below 50°C, the control unit activates the heating element 3.

Figure 4 shows a first possible structure for control panel 21 of the control unit. Control panel 21 comprises a numeric display 22, showing the time 22A (hour:minutes), and the current temperature in the tank 22B. Switch 23 activates the heating system. When switch 23 is ON, the control unit operates, to activate the heating element when needed. The digital display 22 functions always, whether switch 23 is ON or OFF. Red light 24 is activated when current flows through the heating element and it turns OFF when no current passes through it. The first push button 26 is used for setting the current time. The second push button 25 functions as follows: when activated, the threshold value of the desired water temperature appears on display 22B. The third push button 27 functions as follows: when activated, the desired time at which water at said threshold temperature is needed appears on the display 22A. The fourth push button 28 functions as follows: when activated, display 22A registers the additional duration at which the threshold temperature should exist in the water tank. The fifth and sixth push buttons 29A and 29B are adjustment buttons, button 29A upward and button 29B downward, respectively. -22- a. When pushing button 25 and one of buttons 29A or 29B the threshold temperature changes on the display 22B. Note that the threshold temperature cannot exceed a preset value, 70° C for example, particularly for the sake of safety. b. When pushing button 26 and one of buttons 29A or 29B, the current time can be adjusted. c. When pushing button 27 and one of buttons 29A or 29B, the desired time for using the water can be adjusted. d. When pushing button 28 and one of buttons 29A or 29B, the additional duration for which the threshold temperature should be maintained in the tank can be adjusted.

Figure 5 is a flow chart illustrating the functionality of the first alternative. The flow chart indicates principal operation steps of the control unit according to one embodiment of the invention. There are other alternatives, some simpler and some more complex. The control unit is always connected to the electric source 31 and in the operational state. Display unit 22 continuously displays the present time and the present water temperature in the tank (step 32). In step 33, if the main switch 23 is not ON, voltage supply to the heating element is disconnected (step 34). If the main switch 23 is ON in step 33, the system proceeds to step 35, and checks whether the water temperature sensor 17 is functioning properly (step 36). In the event of a malfunction, the voltage supply to the heating element is terminated, and an appropriate alarm initiated (step 37). When the water temperature sensor -23-functions properly, the system checks if a proper present time reading exists (step 38). If proper time reading does not exist, the voltage supply to the heating element is terminated and an appropriate alarm initiated (step 39). When it is found that the present time reading functions properly, the system is ready to accept, in step 40, new values for the desired water temperature (push buttons 25 and 29), the desired time for that temperature in the tank (push buttons 27 and 29), and the duration of the desired temperature (push buttons 28 and 29). The system advances to the calculation steps by the algorithm of equations 1, 2 and 3, resulting in a duration for which a supply voltage must be provided to the heating element 41. This duration ( td ) is provided to step 42, which checks if the value trf is greater than zero. If not, voltage to the heating element is terminated (step 43). If td is greater than zero, the program calculates the activation time at which to initiate the voltage supply (tA(i) to the heating element (44). Next, the system checks in step 45 whether the present time (tpres ) is greater than thtg . If not, the voltage to the heating element is terminated (step 43). If so, the system provides voltage, to the heating element 3 (step 46) and turns ON red light 24 of the main switch. Then, in step 47 the program checks whether the heating element functions properly. If the heating element is found to function improperly, then the voltage supply to the heating element is terminated, and an alarm is initiated in step 48. If the heating element is found to function properly, then the signal loop repeats again from step 35. -24- 2nd alternative: A standard system: the control unit includes software for enabling two modes of operation, manual or automatic, with extended display and functionality.

This alternative has, in addition to the first alternative, the option of programming the control unit to several heating objective times. In the event the user does not need this option, he can skip, and terminate it.

Figure 6 describes the control unit, which includes a control panel 51 on which digital display 52 includes three display lines. The first line, 52A, indicates the present data: day, time, date and temperature. The second line, 52B, indicates the desired water temperature, and the time (day, hour and date) at which the threshold water temperature is desired. The third line, 52C, indicates the end of the time (day, hour and date) at which the desired water temperature should be maintained. Main switch 53 activates the heating system. The heating element 3 is provided with electric voltage if the control unit enables it, and switch 53 is ON. Note that the digital display functions always, even if switch 53 is OFF. Red light 54 lights when current flows through the heating element and is turned OFF when no such current passes through. Normally, display 52 is lighted, and the upper right corner of it, i.e., the present water temperature in the tank, is lighted with a special emphasized lighting (54°C in Figure 6). When the user pushes any of the four push buttons 55, the light advances to a next field in display 52. When the user depresses one of the two buttons 56, the value of the emphasized field is altered up or down, -25-and only after the user pushes the set button 57 is the new value stored. Note that few values are unable to be set: (a) the present water temperature in the tank, as appearing in the field at the upper right corner; (b) the lower right corner field, i.e., the "end" temperature; (c) the leftmost field (days), if the date has been previously selected. The user may preprogram a plurality of scheduled future times, for example, nine, each scheduled time indicated by one or more digits of display 58. Pushbutton 59 adjusts this control unit from manual to automatic. When button 59 is not activated, the control unit operates in the manual mode, i.e., according to the position of the main switch 53, which is either ON or OFF. In this mode, only the present temperature is lighted. When button 59 is pushed, green light 60 turns on and the control unit operates in the automatic mode, i.e. as detailed above.

Preferably, the number indicated by display 58 is the number of the future, next scheduled time. When the user finishes programming, indicator 58 returns after a short time to the normal state as well as the other lights of the panel of the control unit 52.

There are many other options that may be applied to the control unit of the 2nd alternative, and the following only a few examples: 1. The user can change the one of 10 possible scheduled times already programmed in the control unit. As said, the set indicator 58 shows the next scheduled time at which the unit heats the water, for example, n=3. In order to change any value in this scheduled line, the user must move the light by button 55 to the -26- field he wants to change. After changing all required fields, the user pushes the setting button 57, the program accepts the changes and advances the indicator 58 to the next scheduled time, n=4. If the user does not want to change the scheduled time n=4, but wants to change n=5, he must advance the scheduled time shown on the indicator by pushing the set button 57. > 2. If the user wants to have hot water every day at the same time, he has to go to the required scheduled time line, adjust the temperature field 52B, and the end time field 52C, and then to push the set button 57. Then, on the panel of the control unit there will be no values shown at the day field 52D and date field 52F of the two lines 52B and 52C, indicating that this line indicates a periodically repeated heating. 3rd alternative: Manual activation is performed by setting switch 23 to ON, and this enables the program to activate the heating element. Terminating the heating is done by setting the switch 23 to OFF, or by the control unit. The control unit terminates the heating when the water temperature reaches the threshold value, and restarts heating when the temperature falls below the threshold value.

Generally, it is preferable to set the maximum water temperature to 60° C , but it is possible also for the manufacturer to set it differently.

Figure 7 shows the panel of the control unit according to this alternative. The control unit comprises control panel 21, with a digital display 22B showing the -27-water temperature. At rest, display 22B continuously shows the current temperature of the water. Switch 23 serves to activate the heating system. The heating element is provided with a voltage supply only if switch 23 is ON and the control unit enables heating. The digital display shows the temperature always, even when switch 23 is off. Red light 24 lights when a voltage supply is provided to the heating element and turns OFF when no such voltage supply is provided to the heating element. 4th alternative: This alternative is similar to the third, with the following difference: heating is terminated when the temperature of the water reaches the threshold value. To restart heating, the user switch 23 is turned to OFF and then to ON position when the temperature falls below the threshold value.

Figure 7 shows also the control unit for this alternative. The control unit comprise a control panel 21 with a digital display 22B showing the water temperature. At rest, the display shows the current temperature of the water. Switch 23 serves to activate the heating system. The digital display 22B functions continuously, even when switch 23 is OFF. Red light 24 lights when voltage is provided to the heating element 3 and turns OFF when the voltage supply is terminated. 5th alternative: Figure 8 shows the control unit for this alternative. The user is provided with the option of selecting by push button 71 the operation according to alternative 3 or according to alternative 4. When push button 71 is in the upper -28-position, the control unit operates according to alternative 4, i.e., in the manual mode. When push button 71 is in the lower position, the green lamp 72 turns ON and the control unit operates according to alternative 3, i.e., in the manual continuous mode. 6th alternative: Figure 9 shows the control unit for this alternative. The operation is similar to alternative 3, with the addition of adjustment to the temperature threshold by the user. Ordinarily, display 22B shows the present water temperature in the tank. When the user wants to adjust the threshold temperature he has to hold push button 25. Then, display 22B shows the currently adjusted threshold temperature. The threshold temperature can be adjusted up or down by push button 29. It is to be noted that when ordering the control unit, the manufacturer limits the standard maximal threshold value, usually to 70°. If as mentioned above the user tries to adjust the threshold temperature above 70° C , the control unit will not allow it. It should be noted that all the functions of alternative 3 work also in this alternative. The program terminates heating when the water temperature reaches the threshold value, and restarts when the water temperature falls below the threshold value. 7th alternative: This alternative is similar to the sixth alternative with the following difference: The control unit terminates the heating when the water temperature reaches the threshold value, and when it falls below the threshold value, nothing happens. To restart the heating, the user must turn OFF switch 23, -29-and then ON again after the temperature falls below the threshold value. Figure 9 illustrates this alternative, as well as the sixth alternative. 8th alternative: Figure 10 describes the control unit for this alternative, which operates either according to 6 or 7, and the user selecting by push button 71 which of the two options to operate. When push button 71 is in the upper position, the control unit operates according to alternative 7, i.e., in the manual mode. When push button 71 is in the lower position, green lamp 72 turns ON and the control unit operates according to alternative 6, i.e., in the manual continuous mode.

The system described so far includes a control unit operated by push-buttons. Figures 11a lib and 11c describe three alternatives in which a computer is used in conjunction with the control unit, or with a reduced control unit.

Figure 11(a) describes a computer (81) which is capable of exchanging information with the control unit (21) via wires, and in turn, the control unit (21) controls the heating element (3) and the PT-100 (17).

Figure 11(b) represents one possible way of introducing the software of the control unit inside a computer (81) indicated as control unit program (82). The computer (81) controls a reduced control unit (21A), which does not contain the said software, which as said is contained within computer (81). -30- Figure 11(c) is similar to figures 11(a) and 11(b), but contains a wireless communication between the computer 81 containing a control unit program 82, and the reduced control unit 21 by means of antennas 83 and 84. It is also possible to replace the wire connection between the control unit (21 or 21A) and the sensor 17 by a wireless connection, by providing a transmitter at the sensor 17, and a receiver at the control unit (21 or 21A).

While some embodiments have been illustrated by means of the above examples, it should be understood that the invention may be carried out with many variations, modifications and adaptations, without departing from its spirit or exceeding the scope of the claims. 10685/00 137546/2 -31-

Claims (1)

1. CLAIMS 1. A method for heating water in a domestic water heating system, comprising: Providing a water tank containing mw liters of water; Providing at least one temperature sensing unit in said water tank for sensing the temperature of the water in the tank; Providing a heating element in said water tank; Providing a control unit for activating said heating element, said control unit continuously receiving indication for the water temperature from said temperature sensing unit; / Providing to said control unit a desired water temperature, and designating a time for using the water at said desired temperature; Knowing the current water temperature, the desired water temperature at said designated time, the power of the heating element, and the specific heat of the water, calculating by the control unit the heating period At needed for heating the water in the tank from the current temperature as measured by said temperature sensing unit, to the desired temperature; Periodically repeating said calculation and updating said calculated period At according to the changes in the sensed water temperature; and When the designated usage time is approaching, activating the heating element a At period before the said designated usage time. 10685/00 -32- 2. A method according to claim 1, further including in the calculation a heat loss factor. 3. A method according to claim 1, wherein the formula used by the control unit is: Wherein: At is the expected heating period by the heating element [seconds]; PH is the power of the heating element [Watts]; mw is the volume of the water in the water tank measured in liters; Joul&s Cp is the specific heat capacity of the water (= 4200 - — ); kg - C AT is the difference between the designated temperature at a later desired time and the current temperature of the water in the tank, measured in degrees Celsius 4. A method according to claim 2, wherein the calculation is made by the following formula: 10685/00 -33- At wherein K is the loss factor; 5. A method according to claim 4, wherein the loss factor is calculated by the following formula: AT C B PH Wherein: A, B, and C are numerical values obtained by laboratory experiments. A is the volume of the tank used [liters], B is the difference between the required temperature in the tank and the air temperature surrounding the tank [°C], S and C is the heat lost to the surroundings [Watts], as acquired by experimental results. 6. A domestic water heating system comprising: J - a water tank; - a heating element in said water tank; - a temperature sensor for sensing the temperature of the water in the tank; 10685/00 137546/2 -34- - a control unit located in a place accessible to the user, the control unit receives from said temperature sensor an indication to the current temperature, the control unit further comprises: a. a display for displaying the current water temperature as acquired by the said temperature sensor; b. a display and push buttons allowing the user to designate time for having hot water at a desired water temperature; c. calculating means for calculating from the current water temperature, the desired water temperature, the power of the heating element and the specific heat of the water a heating period in which the heating element has to be activated in order to heat the water to the desired water temperature by the heating element; and d. switching means for providing voltage to the heating element during said heating period. 7. A domestic water heating system according to claim 6, wherein the temperature sensor is positioned in a metal sleeve penetrating into the water tank, and the tip of the temperature sensor which is sensitive to temperature is forced to be in contact with the top of the said sleeve. 10685/00 wherein the element h the top of the sleeve is a spring. 9. A domestic water heating system according to claim 6 wherein the temperature sensor is a PT 100 type sensor, having means for transforming a change in temperature into a proportional changes in voltage. 10. A method according to claim 1, essentially as described and illustrated. 11. A system according to claim 8 essentially as described and illustrated.