reference numerals common with those used in the description and drawings of australian patent no . 531449 again have been used , and the function of those components is as described in the noted patent . the boiling water unit shown in fig1 has an electronic temperature sensing device in the form of a thermister 50 located within the level control tank 3 at a position remote from the interconnecting conduit 21 . the thermister 50 is sensitive to detect temperature and is connected with an electronic controller 52 by an interconnecting wire 56 . the thermister 50 detects that the water in the heating tank 1 is boiling by sensing the temperature of the surrounding air . the air temperature is elevated due to the presence of steam coming - off the water and passing through the conduit 21 into the space above the water level in the level control tank 3 . by way of example , when the water in the heating tank 1 reaches 100 ° c . and boils ( i . e . at an atmospheric pressure corresponding to sea level ), some of the steam given off travels into the level control tank 3 and raises the ambient air temperature to about 85 ° c . a thermister 54 , forming a part of the over - pressure safety device situated atop the level control tank 3 , replaces the capillary 29 shown in patent no . 531449 . operation of the boiling water unit will be described with reference to fig2 to 6 . the boiling water unit has four main ` routines ` controlling its operation . fig3 shows a flow diagram of the primary routine ( or main loop ). within this main loop a subordinate temperature control routine , shown in fig4 takes place , as does an after hours operation routine , shown in fig6 . the overload protection routine , shown in fig5 functions independently of the main loop . implementation of these routines is achieved by means of software executed by the microprocessor 66 of the electronic controller 52 . the relevant routines are stored in the memory 70 associated with the processor 66 . it would be a matter of routine for a computer programmer to write the necessary software to implement the routines as presently will be described . the controller 52 receives the active power wire 14 , passing through two inline relay controlled contacts 60 , 82 . the output signal of the thermister 50 , passed along wire 56 , is supplied to an input buffer device 64 . similarly , the output signal from the thermister 54 , passed by the wire 58 , is supplied to the input buffer device 64 . these two signals are converted into an appropriate data format suitable to be read by the processor 66 . that data is passed from the input buffer device 64 to the processor 66 via a data bus 68 . the processor 66 outputs signals to control each of the relay control devices 76 , 80 . the relay control device 76 receives its controlling signal on output line 74 , whilst the relay control device 80 receives its controlling signal by output line 78 . each relay control device 76 , 80 cause opening and closing of a respective contact 60 , 82 . the contacts 60 , 82 control the supply or the cut - off of electricity to the heating element 2 . fig2 also shows a user control panel 90 , for convenience indicated as a part of the electronic controller 52 . it is equally possible that the user controls 90 are remote from the control 52 , but connected thereto by an extended form of the bus arrangement 91 . the user controls include a time of day clock that is user adjustable with respect to the time of day , and also with respect to the hours between which the after hours operation is to take place . a pushbutton 92 is provided that allows selection of either continuous operation or after hours operation . a ready led 94 is provided as is an after hours led 96 . as shown in fig3 the main loop commences at step 100 , and firstly requires setting of a time - of - day clock ( steps 101 and 102 ). this is followed by setting of the time - of - day pertaining to the energy saving function ( steps 103 and 104 ), by which a time is set , typically outside of normal business hours , in which the boiling water unit is not required to continuously maintain boiling water for use . the energy saving mode therefore allocated a time of day where the boiling water unit automatically switches off , thus providing a significant energy saving . step 105 determines whether a 24 hour mode is selected . this is a determination of whether continuous boiling water is required for 24 hours of the day . if so , then the routine continues to step 106 , but if not , the energy saver operation is commenced ( steps 108 - 112 ). the after hours ( energy saving ) operation ( step 112 ) is shown in more detail in fig6 . in the case of 24 hour operation , the temperature control routine ( step 107 ) is continuously performed . referring to fig4 it can be seen that the temperature control firstly determines whether the water temperature sensed by the thermister 50 is greater than an adjustable &# 34 ; set temperature &# 34 ;, corresponding to boiling of the water in the water heating tank 1 . the &# 34 ; set temperature &# 34 ; value is variable , since the temperature of boiling will reduce with increasing altitude . the set level is adjusted by the supplier before delivery by means of the potentiometer 75 shown in fig2 . if the water temperature is below the &# 34 ; set temperature &# 34 ;, then the element 2 is caused to turn on ( step 124 ). if the water temperature is greater than or equal to the &# 34 ; set temperature &# 34 ;, it is not necessary to heat the water , but at the same time a three minute timer commences ( step 121 ). if that time period has not elapsed , then the heating element 2 remains turned off ( step 122 ). therefore , in this embodiment , the thermister 50 is used only to determine when the temperature of water in the heating tank 1 is boiling . the determination of whether the water temperature has decreased to a point where it requires to be reheated is governed entirely by the timer . when the three minute interval times out the heating element 2 is caused to turn on . it is possible , in other embodiments , to utilise the output signal of the thermister 50 in determining when the element is to be turned on . the time period of three minutes is variable , and dependents upon the normal convective heat losses from the boiling water unit . therefore , the &# 34 ; set time &# 34 ; interval also is factory adjustable . the steps 124 , 122 that turn the heating element 2 on and off therefore cause an output to be placed on the line 74 , respectively to cause the relay control device 76 to close or open the contact 60 , thereby either supplying or cutting - off electricity to the heating element 2 . the after hours operation , as shown in fig6 operates such that the boiling water unit is turned off unless required for use . upon selection of after hours service ( step 152 ), by virtue of the after hours pushbutton 92 , the water is heated to boiling and maintained in this condition by temperature control routine ( step 159 and fig3 ) for a period of fifteen minutes , after which time the boiling water unit is switched off . during heating , the ready led 94 flashes , and when the water is boiling , the led is maintained as a steady light ( steps 158 , 157 ). steps 151 , 153 and 155 also control operation of the ready led 94 and the after hours led 96 . fig5 shows the overload protection routine . this routine operates independently of the main loop shown in fig3 . an electrical input signal from the safety or overload thermister 54 is separately processed by the processor 66 to gain an indication of temperature , and thereby obtain a back - up system should the primary temperature sensing thermister 50 fail . another failure mode will occur if the contact 60 welds in the closed position . even if the relay control device 76 is attempting to open the contact 60 to cut - off the supply of electricity , such will not occur . by virtue of the contact 82 being in series with the contact 60 , opening of that normally - closed contact will remove the supply of electricity to the heating element 2 . therefore , the step 141 represents a determination of the sensing of excess temperature . if this is not the case , then step 144 enables the heating element to be on being effected by maintaining the contact 82 as closed . on the other hand , if there is an overload condition , then step 142 disables the supply of electricity to the element by opening the contact 82 . the step 143 contemplates the situation where the overload sensing device is resettable . the thermister 54 can be mounted at any convenient location on the boiling water unit . one embodiment has the thermister 54 as a part of the over - pressure safety device 24 . the boiling water unit is the subject of convective heat loss , in which case regular reheating of the water to boiling is required , and takes place in accordance with the methodology discussed above . the methodology equally applies to the instance where water is drawn - off from the outlet connection 23 . replenishing water is supplied to the level control tank through activation of the inlet valve 6 , allowing water to discharge from the inlet pipe 7 . the replenishing water is directed towards , and washes over , the thermister 50 . the temperature detected by the thermister 50 then correctly determines that the water within the heating tank 1 is no longer boiling . this arrangement also is disclosed in commonly owned australian patent no . 637412 . it is preferred , but not essential , that the thermister 50 is located in the position shown to come under the influence of the replenishing inlet water , since the ambient temperature in the level control tank 3 will , in any event , reduce . a number of advantages occur by virtue of the embodiment described , in that the replacement of the control and protection devices described in australian patent no . 531449 with electronic devices and controls results in greater linearity , accuracy and reliability . furthermore , the cost of these components is significantly less than the previously used componentry , thus reducing the overall cost of the water heating unit . assembly too is simplified , which further leads to a reduction in unit cost . the after hours function has the great benefit of saving on electrical consumption , and hence costs by , for example , shutting off the boiling water unit out of normal business hours . in another embodiment , the thermister 50 can be located on the external wall surface of the level control tank 3 , and the methodology for protection and control still is achievable by obtaining a knowledge of the heat loss ( thermodynamic ) relationship between the presence of steam within the level control tank and the surface temperature of the level control tank 3 . an advantage in locating the thermister 50 at the external surface is that construction of the water heating unit becomes even simpler , and therefore less costly . numerous alterations and modifications , as would be apparent to one skilled in the art , can be made without departing from the broad inventive scope of the present invention .
a continuous boiling water unit is disclosed and comprises a water heating tank containing an electric heating element near the bottom of the tank and a level control tank mounted alongside the heating tank . an interconnecting steam communication port is located between the level control tank and the heating tank , together with a draw - off connection and a flow connection . there further comprises an electronic temperature sensor in the form of a thermister operable to sense the temperature of water in the heating tank , and upon sensing boiling of the water in the heating tank , operable to cause cut - off of the supply to the heating element .