Abstract:
A compact tankless water heater of modular configuration to accommodate a range of potential demands is disclosed. A heat transfer chamber is divided into subchambers, each of which may may mount more than one heating element. Notched passageways at the top of each dividing wall allow water passage between chambers, and a plurality of temperature sensors act in concert with a flow sensor and microprocessor control of the heating elements to maintain a constant set point output water temperature.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of tankless fluid heaters. These are flow-through devices for the instantaneous heating of a fluid by passing the fluid through a chamber containing a heating element. Several versions of such an apparatus have been particularly adapted to heating water, with the objective of serving the function ordinarily performed by a standard tank-type water heater. 
     Tankless water heaters in general have an advantage over tank-style water heaters in that they apply energy to heat only water about to be used, rather than continually heat and reheat a stored reservoir of water. A principal challenge in tankless water heater design is that widely varying flow rate demands are present in a typical use, and ideally a constant set water temperature at the output will be available regardless of flow velocity or flow volume. Further, the bounds on flow demand in, for example, typical residences may vary widely by size of residence or size of family. 
     The latter problem has been addressed by modular tankless heater design, whereby one or more heating elements may be placed in contact with the moving water, according to the expected maximum flow to be serviced. The multiple elements may be in a single water chamber or in a set of chambers connected in series between cold water input and heated water output. For all such designs, it is advisable to maximize the transfer of heat to the flowing liquid by moving the liquid sequentially across the heating elements. Most inventions of this sort have disclosed a feedback mechanism, either analog, digital or microprocessor-based, to regulate the output temperature by turning off the elements when the water gets hot enough, and a flow sensor to assure that the elements operate when fluid is moving through the system, shutting off when the flow ceases. 
     For example, U.S. Pat. No. 5,408,578 to Bolivar discloses a tankless heater with a plurality of elongated chambers, each of which contains a heating element, water under pressure enters the first heating chamber at the bottom and fills it. A pair of ports of different sizes connect the first heating chamber with an adjacent heating chamber. The size difference allows better distribution of water to the heating elements. The design also includes an entrance chamber containing a flow control switch that activates the heating circuitry when water moves through the chamber. Hurko, U.S. Pat. No. 4,808,793 discloses a tankless electric water heater which includes an open ended folded tubular conduit having a separate metal-sheathed emersion heating element inserted into each end of the conduit. It also includes a self-regulatory heating cable, either in or wrapped around the tubular conduit, that is energized independently of the main heating elements and keeps the standing water in the chamber at a set temperature. Davidson, U.S. Pat. No. 4,604,515, discloses a chamber housing that is divided into a plurality of equal subchambers by barrier walls, with each subchamber having a heating element responsive to a separate temperature sensor. 
     White, U.S. Pat. No. 5,479,558, discloses a compact tankless water hater in which a single water chamber, filled from the bottom, contains four individually controlled heating elements. A pressure responsive flow switch activates circuitry which sequentially energizes the heating elements according to need. Posen, U.S. Pat. No. 5,438,642 discloses a serpentine chamber for water flow, carrying the water sequentially along in a plurality of heating elements, which can be either flat plate elements that constitute combination heating and chamber partition assemblies. Fernandez, U.S. Pat. No. 5,325,822, discloses modular units having two connected chambers, each with a heating element, that may be connected in series. Temperature sensors in the first and second chambers of each module provide signal inputs to energize each heating element of each chamber for a period of time proportional to the temperature difference between the first sensor and the desired set temperature. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a compact tankless water heater capable of configuration to accommodate a range of potential demands. A rectangular heat transfer chamber is divided by a central rib wall into two subchambers. An inlet opening at the bottom of the apparatus is centered on the rib wall so that water enters and fills both subchambers simultaneously. A plurality of heating elements is mounted in the heat transfer chamber, with the preferred design capable of fitting from one to four elements, depending upon the expected demand for hot water. Thus, the same configuration may be installed whether demand requires four elements (typically a house for a family of four), or just one (such as an individual sink). 
     A notch passageway at the top of the central rib permits water to flow between the subehambers if one fills faster than the other. An exit chamber adjacent to the heat transfer chamber is connected at the bottom to the plumbing in the facility being serviced. A notch passageway at the top of the heat chamber wall allows water to flow across and down into the exit chamber. 
     A flow sensor measures the rate of water movement, and temperature sensors are placed at the water inlet, the outlet, and near the tops of the two heating chambers. With flow rate, incoming temperature and outgoing temperature as inputs, a microprocessor based controller regulates the energy to the heating elements to maintain a set point water temperature. Safety of the unit is enhanced by a mechanical thermal cut-off switch as well as protective relays that open when an over-temperature condition in the chamber is sensed. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an exploded view of one embodiment of the invention. 
     FIG. 2 is an exploded view of the heat transfer chamber. 
     FIG. 3 is a cutaway view of the unit, showing the interior of the heat transfer chamber. 
    
    
     DETAILED DESCRIPTION 
     The overall configuration of one embodiment of the invention is shown in FIG.  1 . 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 FIGS. 1,  2  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 FIGS. 2 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.