the overall configuration of one embodiment of the invention is shown in fig1 . an external housing is configured to contain the apparatus and adapted to mount on a wall . rear plate 102 attaches directly to the wall and mounts the other components . front plate 103 serves as a cover and incorporates a window 104 for viewing the controls and settings . a microprocessor - based control module 105 allows setting of the desired temperature and functions to regulate the energization of the heating elements . heat transfer chamber 110 is seen in fig1 and 3 . it comprises a main housing 111 divided into subchambers by a rib support wall 112 . a notch or u - shaped opening 113 at the top of the rib wall permits water to flow easily between the two sub - chambers . a similar notch 117 in the bottom of the rib wall facilitates water level balance between chambers . water enters the heat transfer chamber through an inlet pipe 114 , attached to a central aperture 115 in the bottom 116 of the chamber housing . in one embodiment , a metal plate 125 reinforces the bottom cap 116 . the rib support wall 112 is centered over aperture 115 . this causes the inflowing water under pressure to fill both subchambers at approximately the same time , reducing the chance of burnout of a heating element due to energization while dry . the main housing 111 is preferably made from an aluminum alloy such as al6036 - t6 . the aluminum enhances system safety by providing an electrical ground and also readily conducts heat across the chambers . the interior walls of the chamber should be coated with a material such as a fusion bonded epoxy , in order to retard corrosion and to maintain the water &# 39 ; s potability . in one embodiment , the aluminum housing is powdercoated inside and out to achieve this result . the unit may seat from one to four standard heating elements 120 in the heat chamber 110 . one embodiment of the invention uses incoloy sheathed elements , but other heat sources known in the art may be employed . as illustrated in fig2 and 3 , elements 120 fit snugly into the heat chamber 110 . a flexible silicone gasket 109 , preferably ethylene propylene dilene methylene , rests between the main housing and the chamber cap 118 . a similar gasket 108 is placed between the main housing and the chamber bottom 116 . another similar gasket 119 fits atop the cap and receives the heating elements with a tight seal to withstand the expected pressures . an exit chamber 121 located at one side of the main housing 111 , is approximately the same size and shape as the plumbing that carries the hot water to its intended destination . an exit pipe 122 connects the exit chamber 121 to the plumbing . the wall 123 between the heat transfer chamber and the exit chamber is similar to the central rib wall 112 and has a u - shaped opening 124 at the top like that in the rib wall 113 this causes water under pressure in the heat transfer chamber to flow from the top to the bottom of the exit chamber and then out the exit pipe 122 . by constructing the exit chamber integrally with the main housing and causing hot water to flow along the length of the heat transfer chamber , some radiant heat loss is mitigated . the number of heating elements to be installed will depend upon the desired flow capacity . for low demand installations , a single heating element 120 might be satisfactory . this element would preferably be positioned nearest the exit chamber . an installation requiring more flow would require two elements next to each other in the subchamber adjacent to the exit chamber . a third element would preferably be placed in the other subchamber , nearest the dividing wall , and if four heating elements are called for , two will be set into each subchamber . the heater unit includes a flow sensor to detect the volume of hot water being demanded by the user at any given time . in the preferred embodiment a standard turbine - type flow sensor 130 is placed in the inlet pipe 114 and connected to the control circuitry . both the existence of demand and the actual volume per unit of time of water moving through the system are detected and transmitted to the control unit . a temperature sensor 131 , which may be a standard thermistor or another type of sensor , is placed in the water inlet line to measure and report the temperature of incoming water . a second temperature sensor 132 , preferably of like type , is placed in the water outlet pipe 122 . the preferred embodiment additionally includes a pair of temperature sensors 133 , one located near the top of each heating subchamber . all of the temperature sensors provide input to the control circuitry . a safety temperature sensor integrated with a thermal cut - off switch 134 having a manual reset button is placed at the top of the unit where heated water enters the exit chamber , 121 . if water temperature exceeds a predetermined maximum indicative of system failure , the cut - off switch disables electricity flow to the entire unit and can be reset only after the temperature drops below the predetermined danger level . a set of circuit relays 140 under software control provides an additional level of safety . relay contacts are present in the electrical circuit for the installed heating elements 120 . if temperature in one of the heating subchambers , as measured by the chamber temperature sensors 133 rises above a predetermined level , a signal will open the relay contacts in the circuits controlling the elements in the overheating chamber . the software - controlled cutoff will thus reduce the hazard before a mechanical shutoff becomes necessary . the system controller is run by microprocessor 150 . based on the water flow , the measured temperature of incoming water , the desired set point and the measured temperature of outgoing water , the microprocessor uses standard methods to calculate the amount of energy necessary to elevate incoming water to the correct temperature . the microprocessor sends a signal to the triacs , 141 , each of which is connected to an element 120 , causing the elements to energize . operation of the heater is under microprocessor control in a manner generally known in the art . temperature sensor 131 at the water inlet and temperature sensor 132 at the water outlet feed data to the microprocessor , while flow sensor 130 provides flow rate to the microprocessor . a control program operates in a predetermined manner using rapidly repeated polling of sensors providing input temperature , output temperature , flow rate and a set point temperature which has been entered externally by the user via input buttons 106 or another device , to generate signals to triacs 141 associated with each heater element , thereby energizing the heater elements . when the unit starts from a zero flow state , the microprocessor also reads the chamber temperature sensors 133 to determine the temperature of the residual water inside the unit and adjusts the energy levels accordingly . constant monitoring of the critical parameters , i . e ., flow rate , incoming water temperature and outgoing water temperature allows the microprocessor to control energy to the heating elements such that outgoing water temperature remains near the set point without substantial fluctuations . it will be understood that various details of the invention may be changed without departing from the scope of the invention . the forgoing description is for the purpose of illustration only , and not for the purpose of limitation .