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Water is further mixed by providing reduced-aperture fluid coupling 53 for discharging the water from chamber 40 into the outlet line 52 then upward along the length of the water outlet line 52 to the fluid outlet It can be seen that there are five thermistors in the hot water heater system. Thermistors 42 and 54 are located at the bottom of the heater chambers. Thermistors 30 and 49 are located at the top of the chambers 28 and 40, respectively, and thermistor 18 is positioned at the bottom of inlet line The number and position of these thermistors are the primary measurement means for controlling the heater system.

It should be obvious to one skilled in the art that the numbers and positions shown represent a preferred embodiment, but both the number and position of the thermistors or other temperature sensing devices, as well as their control temperatures, may be changed depending upon the applications requirements. High temperature limits 24 and 46 are located in the top of chambers 20 and Limits 24 and 46 are preset thermal switches, normally closed normally making electrical contact , that open to break the electrical connection only when a preselected high temperature condition is present.

Once a switch opens it remains open until it is manually or automatically reset to close the switch. Again, it would be understood that the number and position of the high limit switches may be changed depending upon the application. The limit switches 24 and 46 are wired in series with the control signal for eight relays.

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The relays break both sides of the power line to all four heater elements, regardless of any control logic command that may be present from the electronic control system. This is a mechanical backup for sensing a high temperature condition. In the event of a high temperature condition, the limit switch must be manually or automatically reset before the heater will operate again. The relays are principally used as safety and surge protection devices, and the use and the number of such devices will be determined by the objective of the heating system.

Water level detectors 32 and 48 sense the water level in modules 26 and 36, respectively. In the event the water level drops below the height of the water level detectors, all the control relays are de-energized by breaking both sides of the power line to each of the four heater elements. If the water level in module 36 only drops below its level, heating elements 22, 44 and 50 are preferably disconnected from the power line by the control relays.

If the water level in module 26 only drops below its level, heater 34 is preferably disconnected by its control relays.

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When heater 34 is disconnected, heaters 22, 44 and 50 cannot be energized from the control logic, as discussed subsequently. The use of water level sensing circuitry and sensors is primarily for safety purposes. The use of such circuitry and sensors, and the numbers and location of the sensors, will preferably be determined by the particular application. Still referring to FIG.


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This feature is accomplished while simultaneously allowing the water to reach its proper level in the chambers, thereby preventing damage to the heaters. A sediment bowl 9 is removably attached to the housing which defines each of the heating chambers 20, 28, 38 and Each sediment bowl 9 may be substantially transparent to allow the operator periodically to visably determine service requirements for the heater.

Each bowl 9 may be connected by conventional threads to its respective housing, thereby facilitating easy removal and replacement for draining and cleaning the heating chambers. Each of the five system thermistors 62 is preferably connected to its own amplifier circuit. The operational amplifier 63 converts the negative resistance thermistor to a linear positive voltage output 64 of approximately 30 millivolts per degree F within the working temperature range of the water heater. The voltage output 64 is thus proportional to the sensed temperature.

The circuit 60 includes appropriately sized scale resistor 66 and gain resistor The amplifier set point circuit 72 as shown in FIG.

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Temperature is set by means of a manually adjusted potentiometer The set point circuit establishes two separate reference voltages 78 and 80, i. In the preferred embodiment, the circuitry also provides a minimum standby set point temperature in order to prevent freezing. The circuit 72 includes an operational amplifier The voltage 84 is connected to a respective inlet temperature amplifier 63 disclosed above. The set point circuit 72 thus provide the voltage inputs 80 and 78 to the shutdown circuit and the pulse width modulator, as described subsequently.

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The high temperature shutdown voltage 78 is one input to the operational amplifier comparator 86 of the shutdown circuit , as shown in FIG. Temperature amplifiers 92 and 94 through ratioing resistors 88 and 90 provide the second input 96 to the comparator The output voltage 98 of the comparator is high when the fluid temperature in chamber 28 and chamber 40 are below the high temperature shutdown voltage. When either chamber or both chambers exceed the high temperature shutdown voltage, the comparator output voltage 98 drops to zero, disabling the AND gate as shown in FIG.

This inhibits the heater element 34 by control circuitry as shown in FIG. Heater 34 as controlled by the control circuit as shown in FIG. When the shutdown comparator 86 as shown in FIG. By inhibiting the command voltage pulses from the pulse width modulator to heater 34, through the AND gate as shown in FIG. Any of the heaters may be used to provide the desired control or sense heater, depending upon the application.

The shutdown comparator 86 will stay in a shutdown condition as long as the output voltage from the temperature amplifiers of chamber 28 and chamber 40 see FIG. The standby circuit as depicted in FIG.

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The voltage from each thermistor passes through an appropriate resistor and , respectively. The circuit also includes resistor , capacitor , quick discharge member , resistor , and amplifier The output from the circuit is fed to the shift register , as shown in FIG. The output voltage of the standby circuit goes to a high voltage level when thermistor 30 is just a few degrees warmer than thermistor 42 as indicated by the standby comparator circuitry This high level output voltage inhibits a shift register that sequences the activation of heaters 22, 44 and 50 by means of the shift register control circuitry as shown in FIG.

Operation of the shift register will be explained below. The high output voltage level from the standby comparator as shown in FIG. Under normal operation with water flowing through the heater 10, each chamber is successively hotter than the preceding chamber. Chamber 40 is thus hotter than chamber 38, which is hotter than chamber 28, which is hotter than chamber When the water flow is interrupted, latent heat in all the heaters will raise the temperature of the fluid within the chambers.

The top of each chamber is hotter than the bottom of the same chamber during no flow conditions thermocline. The standby circuit comparator circuitry as shown in FIG. The pulse width modulator as described below maintains chamber 28 at a reduced set point temperature while keeping the top of chamber 28 hotter than the bottom of chamber 28, thereby maintaining the high voltage level output from the standby circuitry as shown in FIG.

When flow is re-established through the water heater 10, the temperature sensed by termistor 30 approximates the temperature sensed by thermistor Output voltage from the standby circuit comparator changes to a low level, enabling the shift register which initiates the energizing of the control relays associated with heating elements 22, 44 and The control circuit logic is thus switched from the standby mode to the operational mode, and the set point is returned from the standby set point to the operational set point.

The heaters are activated in response to demand, incrementally seeking the operational set point temperature. If the water heater 10 is receiving preheated water above the operating set point, the control will remain in the standby mode. When the top of chamber 28 approximates the temperature sensed at the bottom of a downstream chamber, the output of the standby circuit goes to the zero state, taking the hot water heater 10 out of the standby mode.

The shutdown circuit as shown in FIG. When flow detection occurs, the modulation circuitry as shown in FIG. The pulse width modulator circuit as shown in FIG. Four amplifier outputs , , , and each monitor the temperatures in a respective chamber 20, 28, 38 and The voltage output is derived from the respective thermistor 30, 42, 49 and One voltage input is a control set point voltage manually or automatically set.

The final set point voltage input is an inlet thermistor 18 temperature compensation control reference set point.


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The modulator circuit has an oscillator typically running at Hz. The oscillator is specifically asynchronous to the power line frequency. The general function and operation of the circuit is provided by the above description and a review of FIG.

These conditions then cause the triacs as shown in FIG. The time constant of R11 and C1 is typically 30 seconds. The positive output voltage of OPAMP A2 will continue to increase within the limits of the heating means , until the proper power level is achieved to produce the operational set point temperature at the existing flow rate. As the water heats up, the output voltage of the thermistor amplifiers increases the thermistor amplifier circuit 60 is shown in FIG. As the differential input voltage to OPAMP A2 decreases because of the increase in output voltages of the thermistor amplifiers, the power to the heaters is reduced.

This control method will result in a smooth incremental transition of power until the proper heater power to each heater element is achieved, while also maintaining the output temperature at the set point with power sharing of the heater elements. This smooth incremented transition will occur for any flow rate within the power limitations of the water heater.

Now assume that the flow of water ceases and the fluid temperature rises. The water heater begins to incrementally reduce the power to each of the elements because of the decrease of differential input voltage to OPAMP A2. OPAMP A2 will eventually reduce the duty cycle of the pulse width modulator to the point where no more power is applied to the elements.

However, the latent heat of the heating elements carry the water temperature above the predetermined shutdown temperature, therefore the output of the shutdown circuit as shown in FIG. It should be understood that with element 34 disabled, all other heating elements are also disabled. Shutdown will also disable all of the control relays. Almost immediately following shutdown, the top of chamber 28 will become hotter than the bottom of chamber When this no flow condition occurs, the modulation detector output as shown in FIG.

The control system thus goes into the standby mode.