Patent Publication Number: US-10323858-B2

Title: Liquid heater with temperature control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/327,941, filed Jul. 10, 2014, which application is a divisional of U.S. patent application Ser. No. 12/889,581, filed on Sep. 24, 2010, which application is a continuation-in-part of U.S. patent application Ser. No. 11/352,184, filed on Feb. 10, 2006 and published as US Patent Application Publication No. US 2006/0291527 A1, now U.S. Pat. No. 7,817,906, which application claims benefit of the filing date of U.S. Provisional Patent Application Nos. 60/677,552, filed on May 4, 2005; 60/709,528, filed on Aug. 19, 2005; and 60/726,473, filed on Oct. 13, 2005. The disclosures of all of the aforementioned applications and publication are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to liquid heaters, and components thereof. 
     BACKGROUND OF THE INVENTION 
     As set forth in the aforementioned US Patent Application Publication No. US 2006/0291527 A1 (“&#39;527 Publication”), it is advantageous to heat fluids, particularly liquids such as water for use as domestic hot water using a “tankless” heating device. A tankless heating device is intended to heat the fluid as it flows from a source to a point of use. A tankless heater does not rely on a stored reservoir of preheated liquid, but instead is designed with sufficient capacity to heat the liquid to the desired temperature, even as the liquid flows through the heater at a rate equal to the maximum expected demand. For example, if a tankless heater is intended to provide hot water to shower in a home, the heater is designed with sufficient capacity to heat water at the lowest expected incoming temperature to the highest desired shower temperature at the maximum flow rate of the shower. 
     As disclosed in the &#39;527 Publication, one form of fluid heater particularly suitable for liquids such as domestic water heating is a direct electric resistance liquid heater. In a direct electric resistance liquid heater, electrical power is applied between electrodes immersed in the liquid to be heated so that current flows through the liquid itself and power is converted into heat due to the electrical resistance of the liquid itself. As also disclosed in the &#39;527 Publication, such a heater can be arranged with multiple electrodes defining numerous channels for liquid flow. The control system for such a heater may be arranged to connect and disconnect different ones of the electrodes to a power supply. The electrodes and associated elements of the heater can be arranged so that connection of different sets of the electrodes to the electrical power supply connection provides different levels of current passing through the liquid. These levels most preferably include a step-wise progression between zero current when none of the electrodes are connected and a maximum current when all of the electrodes are connected. As disclosed in the &#39;527 Publication, this progression desirably has substantially uniform ratios between the currents of adjacent steps of the progression having non-zero current levels. As explained in the &#39;527 Publication, heaters having such a set of possible current levels can provide progressive control of liquid temperature despite wide variations in incoming liquid temperature, desired outgoing liquid temperature, flow rate, and resistivity of the liquid. The desired step-wise progression desirably includes numerous steps as, for example, 60 or more steps or different current levels for fluid of a given resistivity. Most preferably, the steps are arranged so that the maximum ratio between the current levels in any two adjacent steps of the progression having non-zero currents is no more than about 1.22:1, and preferably no more than about 1.1:1, and so that the greatest difference between levels of current in any two adjacent steps of the progression is no greater than about 10% of the maximum current for the given level of fluid resistivity. 
     Because the heat is evolved within the liquid itself, such a heater can provide essentially instantaneous heating of the liquid flowing through it. Moreover, the heater can be controlled by simply connecting and disconnecting different ones of the electrodes to the power supply, allowing use of switching elements such as conventional relays or, more preferably, solid-state semiconductor switching elements such as triacs and field effect transistors. The preferred semiconductor switching elements can be brought to a conducting or “closed” state in which they have very low electrical resistance, or a substantially non-conducting state in which they have extremely high, almost infinite resistance and conduct essentially no current, and thus act as an open switch. Thus, the semiconductor elements themselves dissipate very little power, even though substantial electrical currents flow through them when they are in their closed states. 
     The heater disclosed in the &#39;527 Publication includes a temperature sensor arranged to sense the temperature of the heated liquid near a controller responsive to the signal from the temperature sensor for controlling the switching elements, and thereby controlling the power applied by the heater to the flowing liquid. The preferred temperature sensor taught in the &#39;527 Publication includes a “thermally conductive temperature sensing plate” which is “placed as close as practicable to the end of the heating chamber and perpendicular to the flow of liquids such that the liquid leaving the heating chamber must pass through the perforations of the temperature sensing plate,” and also includes a “semiconductor junction based temperature sensor” mounted to the plate. As set forth in the &#39;527 Publication, however, such an arrangement suffers from “thermal lag or delay” between changes in temperature of the heated liquid and the signal output from the thermal sensor because of the thermal resistance of the thermal plate and packaging of the thermal sensor and the “thermal mass” of these components. To compensate for this, the control system includes a signal conditioner circuit which creates a signal which represents “the rate of change of the temperature as measured by the temperature sensor,” and this signal is summed with the signal representing the temperature itself. While this arrangement provides satisfactory operation, further improvement would be desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the invention provides a fluid heater including a channel structure defining a plurality of channels extending in a downstream direction so that fluid can flow in parallel downstream though the channels from the inlet to the outlet. The channel structure preferably includes one or more electrical energy application elements associated with each channel. For example, the energy application elements may be electrodes as discussed in the &#39;527 Publication. The heater desirably also includes a temperature-sensing wire extending across the plurality of channels adjacent the downstream ends thereof; and a control circuit connected to the energy application elements and the wire, the control circuit being arranged to monitor an electrical resistance of the wire and control application of power to the application elements responsive to the electrical resistance of the wire. The control circuit desirably is arranged so that in at least some control conditions, fluid flowing through different ones of the channels will be heated to different temperatures. As further discussed below, the electrical resistance of the wire represents an aggregate or average of the sections associated with the various channels, and thus represents the final temperature of the fluid which will result when the fluid passing from the channels mixes as it passes downstream from the channels. 
     A further aspect of the invention provides a fluid handling device which can be used, for example, in a heater as discussed above. The heater according to this aspect of the invention desirably includes a channel structure defining at least one channel extending in a downstream direction and an elongated wire extending across the channel in a widthwise direction adjacent a downstream end of the channel. The device further includes an exit structure bounding the channel at a downstream end of the channel. The exit structure most preferably defines a slot extending across the channel in the widthwise direction in alignment with the wire. The slot desirably has a cross-sectional area smaller than the cross-sectional area of the channel and desirably is open for flow of fluid exiting from the channel. The exit structure preferably also defines a pair of collection chambers disposed on opposite sides of the slot and offset from the slot in lateral directions transverse to the downstream direction and widthwise direction, and a pair of elongated lips extending in the widthwise direction and separating the chambers from the slot, the collection chambers being open in the upstream direction and extending downstream from the lips. The exit structure desirably further defining exit bores communicating with the collection chambers and open for flow of fluid exiting from the channel. Preferably, the exit bores collectively have cross-sectional area smaller than the cross-sectional area of the slot. The exit structure helps to prevent attachment of bubbles to the wire. Where the wire is a temperature-sensing wire as discussed above, this improves the sensing action. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exterior plan view of a heater according to one embodiment of the invention. 
         FIG. 2  is a perspective cut-away view of the heater according to  FIG. 1  with portions removed for clarity of illustration. 
         FIG. 3  is a sectional view along line  3 - 3  in  FIG. 1 . 
         FIG. 4  is a sectional view of the heater depicted in  FIG. 1 . 
         FIG. 5  is a fragmentary sectional view depicting the area indicated at  5  in  FIG. 4 . 
         FIG. 6  is a further sectional view along line  6 - 6  in  FIG. 5 . 
         FIG. 7  is a schematic view in block diagram form of an electrical circuit used in the heater of  FIGS. 1-6 . 
     
    
    
     DETAILED DESCRIPTION 
     A heater according to one embodiment of the invention includes a housing  10  ( FIG. 1 ). The housing  10  includes a first end cap  12 , second end cap  14 , and a generally tubular enclosure  16  extending between these end caps. The first and second end caps are provided with mounting feet  18 . The first and second end caps desirably are formed from a metallic material as, for example, a die cast or machined metal. Enclosure  16  desirably has substantially constant cross-section along its length between the end caps and desirably is formed from a metallic material. For example, enclosure  16  may be formed from an extruded metal such as extruded aluminum. Enclosure  16  is removed in  FIG. 2  for clarity. Enclosure  16  and caps  12  and  14  cooperatively define a pressure-tight vessel. The first end cap  12  is provided with a fluid inlet port  20 , whereas the second end cap  14  has a fluid outlet port  22 . A shroud  24  covers the first end cap  20 , whereas a further shroud  26  covers the second end cap  14 . As explained below, the second shroud  26  encloses certain electrical components. Shroud  26  and the associated electrical components are removed in  FIG. 2  for clarity of illustration. 
     A dielectric structure  30  is mounted within enclosure  16 . The dielectric structure  30  desirably includes numerous intermediate sections  32  identical to one another, the intermediate sections  32  being stacked one upon the other along the lengthwise direction of enclosure  16 . The stacked intermediate sections define slots  49 . The dielectric structure also includes a first interior end piece  34  mounted within first end cap  12  and a second interior end piece  36  mounted within second end cap  14 . Portions of these pieces are removed in  FIG. 2  for clarity of illustration. Dielectric structure  30  defines a fluid intake channel  38  extending lengthwise within enclosure  16 , a fluid outlet channel  40  extending lengthwise within housing  10 , a fluid outlet channel  40  also extending lengthwise within the housing and within enclosure  16 , and a pair of heating chambers  42  and  44  ( FIG. 3 ) also extending lengthwise within housing  10  and enclosure  16 . Chamber  42  is referred to herein as the “upper” heating chamber, whereas chamber  44  is referred to herein as the “lower” heating chamber, but such designation does not imply any particular orientation relative to the gravitational frame of reference. 
     As best seen in  FIGS. 3 and 5 , numerous flat, plate-like electrodes  46  are mounted to the polymeric structure  30  and subdivide upper heating chamber  42  into 10 individual, generally rectangular channels  48 . Two of the electrodes  46  are mounted at the edges of the chamber, and bound the channels nearest the edges. As further discussed below, the spacing between electrodes  46  are not uniform, so that different channels  48  have different widths. Lower heating chamber  44  contains further flat, plate-like electrodes  50  subdividing chamber  44  into numerous generally rectangular individual channels  52  ( FIG. 3 ) which also have differing widths. 
     As best seen in  FIGS. 4, 5, and 6 , an exit structure  54  bounds chambers  42  and  44  and hence channels  48  and  52  at downstream ends of the channels  48  and  52  near the first end plate  12  and first interior end piece  34 . The exit structure  54  thus separates the channels and heating chambers from an exit chamber  56  ( FIGS. 4 and 5 ) within first interior end piece  34 . 
     As best seen in  FIG. 5 , exit wall structure  54  has an upstream side (toward the top of the drawing in  FIG. 5 ) facing toward the channels  48  and a downstream side (toward the bottom of the drawing in  FIG. 5 ) facing towards exit space  56 . The electrodes  46  are received in grooves (not shown) extending into the upstream side of the exit structure  54 . The exit structure  54  also has dividing walls  58  which are substantially coplanar with the individual electrodes so that the dividing walls  58  effectively maintain each channel  46  separate from the adjacent channel  46 . There is a small gap  60  between each electrode and the coplanar dividing wall  58 , but such gaps are substantially inconsequential with respect to fluid flow. The end of each channel  48  at exit structure  54  is effectively closed by the exit structure apart from the openings in the exit structure discussed below. 
     The second interior element  36  at second end gap  14  defines a fluid inlet space, schematically shown at  62  ( FIGS. 2 and 4 ), open to the ends of the channels adjacent the second end gap  14 . Fluid intake passage  38  communicates with the fluid inlet port  20  in the first end cap  12 , and with the fluid inlet space  62  ( FIGS. 2 and 3 ) adjacent the second end cap  14 . Fluid outlet channel  40  ( FIGS. 2 and 3 ) communicates with the exit space  56  ( FIGS. 4 and 5 ) adjacent the first end cap  12 , and also communicates with the fluid outlet port  22  of second end cap  14  ( FIG. 1 ). Thus, as indicated by the curved flow path  63  shown in  FIG. 2 , fluid passing through the device enters first end cap  12  and passes through fluid inlet channel  38  to inlet chamber  62  adjacent the second end cap  14 . The fluid then passes through channels  48  and  52  of the flow chambers  42  and  44  toward the first end cap  12 , and passes from the channels through the openings in exit structure  54  into exit chamber  56 . The fluid then passes from exit chamber  56  through fluid outlet channel  40  ( FIGS. 2 and 3 ) and out of the device through outlet port  22  in the second end cap  14 . Thus, the fluid flowing within channels  48  and  52  passes in the direction from second end cap  14  toward first end cap  12 . In referring to the structures of the channels and the exit structure, that direction is referred to herein as the “downstream direction” and is indicated by arrow D in each of  FIGS. 2, 4, and 5 , whereas the opposite direction is referred to herein as the “upstream” direction. 
     As best seen in  FIGS. 5 and 6 , exit structure  54  includes a pair of lips  64  extending across each channel  48  in directions referred to herein as the “wire” or “widthwise” directions of the channel W ( FIG. 6 ). The widthwise direction is into and out of the plane of the drawing in  FIG. 5 . The lips  64  define a slot  66  between them. The slot is elongated and extends across the channel  48  in the widthwise direction W. As best seen in  FIG. 5 , slot  66  is open to the exit space  56 , so that the slot is open for flow of fluid exiting from the channel  48 . 
     The exit structure also defines a pair of collection chambers  70  which are offset from the slot  66  in opposite lateral directions symbolized by arrows L in  FIGS. 5 and 6 . The lateral directions are transverse to the widthwise direction W and also transverse to the downstream direction D. The collection chambers  70  associated with each channel  48  are separated from the slot  66  by the lips  64  and extend downstream from the lips. The collection chambers are open in the upstream direction. The exit structure also defines exit bores  72  connecting the downstream ends of the collection chambers  70  with the exit space  56 . Thus, the exit bores are also open for flow of fluid exiting from the channel  48 . The slot  66  associated with each channel has a smaller cross-sectional area than the channel. The exit bores  72  associated with each channel also have a smaller cross-sectional area than the channel and, preferably, an aggregate cross-sectional area less than the cross-sectional area of the slot. 
     As best seen in  FIG. 5 , each of the collection chambers  70  has a bounding wall which is generally in the form of a semicircle having its axis extending in the widthwise direction W (the direction into and out of the plane of the drawing in  FIG. 5 ). The bounding wall of each collection chamber  70  includes an inner bounding wall extending along the side of one of the lips. Such bounding wall slopes away from the slot in the lateral direction toward the downstream end of the collection chamber. Each collection chamber  70  also has an outer bounding wall remote from the slot and sloping generally inwardly toward the slot toward the downstream end of the collection chamber. The bounding walls slope towards each other and meet at the point of the collection chamber furthest downstream, at the intersection of the chamber and the exit bore  72  associated with the chamber. 
     The exit structure  54  defines a similar arrangement of a slot collection chambers and exit bores for each channel  48  in the upper flow chamber  42  and for each channel  52  in the lower flow chamber  44 . 
     As best seen in  FIG. 6 , the slots  66  of all of the flow channels  48  in the upper flow chamber  42  are aligned with one another, as are the exit chambers of all of the channels  48 . The slot, lips, and exit chambers occupy substantially the entire cross-sectional area of each channel. The slot associated with each channel is the same width in the lateral direction L, but extends across the entire extent of the channel in the wire direction W. As best appreciated with reference to  FIG. 6 , and also with reference to  FIG. 3 , the various channels  46  in the upper flow chamber differ from one another in their dimensions in the wire direction W, and hence in cross-sectional area. Likewise, the various channels  52  in the lower flow chamber  48  differ in wire-direction dimensions, and hence in cross-sectional area from one another. This is a consequence of the unequal spacings between the electrodes  46  and between the electrodes  50  associated with the various flow channels. However, each slot has a cross-sectional area substantially smaller than the associated channel. Merely by way of example, the width of each slot  66  in the lateral direction L may be on the order of 0.115 inches, whereas the dimension of each channel  46  and  52  in the lateral direction may be about 0.929 inches, so that the ratio of slot cross-sectional area to channel cross-sectional area is about 0.12. 
     The diameters of the exit bores, such as exit bores ( FIGS. 5 and 6 ) desirably are selected so that the exit bores associated with the smallest channel have the minimum diameter which will reliably allow bubbles to pass through the bores. Although the present invention is not limited by any theory of operation, it is believed that this minimum diameter is related to the surface tension of the liquid. For domestic hot water at about 100-120° F., the minimum diameter is about 0.070 inches. This minimum diameter yields a ratio of about 0.35 between the aggregate area of the exit bores and the open area of the slot  66  associated with the smallest channel (after deducting area blocked by the wire  76  discussed below). The exit bores associated with larger channels are of larger diameter so as to maintain a reasonably uniform ratio between the cross-sectional areas of the exit bores associated with each channel and the cross-sectional area of the slot associated with each channel. For example, this ratio can be about 0.3 to about 0.45 for all of the channels. 
     A unitary elongated wire  76  is mounted to the exit structure and extends in the widthwise direction W in alignment with the slots  66  associated with all of the channels  48  in the upper chamber  42 . Wire  76  is supported in small notches in the dividing walls  58  of exit structure  54 . Wire  76  extends along the slots of all of the chambers. A portion of the wire (not shown) extends between the slots of the upper flow chamber and the slots associated with the lower flow chamber. This portion is positioned within exit space  56 . Wire  76  is a fine diameter wire having resistance which varies with temperature. For example, wire  76  may be a wire formed from a nickel-iron alloy such as a 70% nickel, 30% iron alloy of the type sold under the commercial designation Balco 120 ohm alloy, and may be about 40 gauge (0.079 mm diameter) with a thin dielectric covering. The dielectric covering preferably is formed from a polymer as, for example, a fluoropolymer such as a PTFE polymer sold under the trademark Teflon®. The dielectric covering insulates the wire from the fluid flowing in the heater. The dielectric covering should be as thin as practicable without pinholes or other gaps. 
     The upstream ends of electrodes  50  and  48  project through the second interior end structure  36  and second end cap  14  as best appreciated with reference to  FIG. 2 , where the upstream ends of electrodes  50  associated with the lower flow chamber are visible. The electrodes  46  associated with the upper flow chamber  42  are removed in  FIG. 2  for clarity of illustration. The electrodes are sealed to the second interior end structure  36 . The upstream ends of the electrodes are connected to switching elements mounted within shroud  26  ( FIG. 4 ). A few of the switching elements are schematically indicated by arrows  82  in  FIG. 7 . The switching elements may be relay-actuated mechanical switches, but most preferably are semiconductor switching elements such as triacs, field effect transistors or the like. The switching elements associated with each electrode desirable are operable to connect each electrode to either pole  84  or  86  of an AC power supply connection. The AC power supply connection in this embodiment is a single-phase AC connection arranged for connection to the ordinary household electrical power supply. When the poles of the power supply are connected to the household current supply, there is an alternating voltage, typically 220 volts in the US, between poles  84  and  86 . Although only a few electrodes  46  and  50  are depicted in  FIG. 6  for clarity of illustration, each electrode has switching elements  82 , and each electrode can be independently connected to either pole of the power supply. 
     Wire  76  is connected in a control circuit schematically shown in  FIG. 7 . The control circuit includes a resistance monitor  78  arranged to detect the electrical resistance of wire  76  and to supply a signal representing the resistance of wire  76  as a temperature signal representing the temperature of fluid within or passing through the heater. The control circuit further includes a control logic unit  80  which is linked to the resistance monitor so that the control logic receives the temperature signal. The control logic unit is also connected to a source  81  of a set point value. This set point value may be a permanent setting or may be a user selectable setting, in which case the source  81  of the set point may be a user-operable control such as a dial, keypad, or the like. 
     The switching elements  82  are actuated by the control logic  80 . As explained in greater detail in the &#39;527 Publication, control logic  80  can connect the electrodes to the poles of the current supply and can leave some or all of the electrodes unconnected. By connecting and disconnecting the different electrodes to the power supply, the control logic can create current paths of differing lengths and hence differing electrical resistance. Merely by way of example, connecting electrodes  46   a  and  46   b  at the extreme ends of chamber  42  to opposite poles of the current supply while leaving all of the other electrodes  46  disconnected from the power supply creates a relatively long, high resistance current path through the fluid in all of the flow channels  48  of upper chamber  42 . By contrast, connecting any two immediately adjacent electrodes to one another creates a very short, low-resistance and hence high-current flow path. The unequal spacings between electrodes allow for creation of a wide variety of flow paths of different lengths. A plurality of current flow paths can be created by connecting more than two electrodes to the poles of the power supply, and each current flow path may include a single flow channel or multiple flow channels. The flow channels of lower chamber  44  provide a similar action. As explained in greater detail in the &#39;527 Publication, the spacings of the electrodes provide current flow paths having differing electrical resistance, and hence differing electrical conductance when filled with fluid of a given conductivity. The conductances and hence the current which will flow along each path desirably include numerous different conductances and currents. The different conductances and currents desirably include conductances and currents defining a step-wise progression of conductances and currents forming a substantially logarithmic progression between a minimum non-zero conductance (and minimum non-zero current flow) and a maximum conductance and maximum current flow. For each step in the progression, the conductance and is the sum of the conductances between all of the pairs of electrodes which are connected to the power supply, and the current flow is the sum of all of the current flows between the connected electrodes. Desirably, the ratios of current flow, and hence conductance, of the steps in the progression are substantially uniform. Most preferably, the progression includes at least 60 steps, and desirably more, and is selected so that the difference in current flow between any two steps of the progression is no greater than about 25% of the maximum current flow and desirably less, more preferably about 10% of the maximum current flow or less. The available conductances and current flow values may also include redundant values not necessary to form the progression as, for example, a current flow value which is exactly the same as or almost exactly the same as another current flow value incorporated in the progression. 
     As described in greater detail in the &#39;527 Publication, control logic  80  responds to a signal indicating the temperature of the fluid flowing through the heater, or present in the heater, which in this case is the signal from resistance monitor  78 , by picking a step having a greater or lesser aggregate current value. Most preferably control logic  80  is arranged to evaluate the signal and change the current value accordingly at numerous times per second, most preferably once on each cycle of the AC voltage applied to the power supply  84 ,  86 . In a particularly preferred arrangement, the control logic is arranged to switch any of the switching elements as required to change the combination of inactive electrodes at about the time the voltage on the power supply crosses zero during the normal AC cycle. This helps to assure that the switching action does not generate electrical “noise” on the power line or radio frequency interference. Moreover, the control logic desirably is arranged to change the set of connected electrodes one step on each cycle. That is, if the temperature signal indicates that a greater current flow is required, the control logic will select the connection which gives the next higher step of the step-wise progression and energize the electrodes in that pattern, and repeat as required until the temperature signal indicates that the temperature of the liquid is at the desired value. Stated another way, the control logic desirably does not “jump” immediately to a much higher step. This helps to assure that the switching action does not cause voltage fluctuations on the supply line, and hence does not cause, for example, dimming of lights in a building where the heater is installed. 
     Leakage electrodes  90  are mounted in intake passage  38  and outlet passage  40 . The leakage electrodes also extend through the second interior end structure  36  and second end cap  14 . The leakage electrodes are permanently connected to the ground connection of the power supply. The leakage electrodes assure that current cannot pass from any of the electrodes  46  or  50  through the flowing liquid to the plumbing system or to the fluid flowing through the system. The leakage electrodes also assure that current cannot pass to either of the end caps or to the enclosure  16 . The enclosure and end caps also may be electrically connected to the ground connection of the power supply for even further assurance. 
     In operation, the inlet port  20  is connected to a source of the liquid to be heated, such as the plumbing system of a home, and the outlet port  22  is connected to a point of use. A liquid such as water flows through the heater, as discussed above, through intake channel  38 , passing generally in the upstream direction U from the first end cap  12  toward the end cap  14  in the inlet channel and contacting the leakage electrode in such channel. The liquid then passes downstream through the various channels  48  and  50  while being heated by passage of current through the liquid between the electrodes. As the liquid reaches the downstream end of each channel, the major portion of the liquid flowing in each channel passes out of the channel into the exit space  56  ( FIGS. 5 and 6 ) through the slots associated with each channel, and thus passes over the wire  76 . 
     The wire  76  extends along the slots associated with all of the channels, and thus is exposed to the liquid flowing in all of the channels. The liquid flowing in different ones of the channels will be heated by different amounts. For example, if the particular combination of electrodes which are connected to the power supply is such that no current is flowing across a particular channel, the liquid flowing in such channel will not be heated directly at all, although it may be heated slightly heat transfer from adjacent channels. The liquid flowing in the various channels mixes in exit space  56  and passes out of the heater through outlet channel  40 , where it again contacts the current leakage electrode  90  and passes out of the system through outlet port  22 . The actual temperature of the liquid passing out of the outlet will reflect the temperature of the liquid passing out of the various channels in combination; the hotter and colder liquids will mix to form a liquid having a final average temperature. 
     Because wire  76  is exposed to the liquid passing out of all of the channels, the resistance of the wire will reflect the final average temperature of the liquid passing out of the heater. However, by measuring the temperature as close as practicable to the downstream end of the individual channels, prior to mixing, the resistance of the wire will measure the final average without the time delay required for the mixing process to occur. Moreover, because the wire  76  has very low thermal mass, its resistance will follow the temperatures of the liquids flowing from the channels almost instantaneously. These factors minimize “loop delay” in the control system. This can best be understood with reference to a hypothetical system in which the average temperature is measured downstream from the heating channels as, for example, at the fluid outlet port  22  of the heater. In such a system, if the temperature of the liquid is less than the desired set point temperature, the control logic will bring the electrodes to a higher current setting and thus apply more heat. However, until the heated liquid passes downstream to the outlet port, the liquid passing over the sensor remains below the set point temperature, and hence the control logic will continually increase the amount of current applied. This may cause the control logic to apply much greater current than is actually required to produce the desired set point, leading to an “overshoot” condition. By minimizing loop delay, the heater according to this embodiment provides a more effective control system. The resistance signal from resistance monitor  78  so closely tracks the temperature that it is normally not necessary to provide a signal representing the change in the resistance signal to the control logic. However, such a signal can be applied if desired. 
     Wire  76  is disposed very close to the downstream ends of the electrodes and channels. Thus, wire  76  is in effective thermal communication with the fluid contained within the channels themselves, even when no liquid is flowing. Thus, the control system can maintain the temperature of the liquid within the channels at the desired set point, even while no liquid flows through the system. It is not necessary to provide a separate sensor for use during such no-flow conditions. Moreover, it is not necessary to provide a flow sensor or other device to detect the occurrence of a no-flow condition. 
     All of these benefits are provided with an extremely simple temperature-sensing arrangement. The single wire used in the embodiments discussed above provides the ultimate in simplicity, and requires only one or two connections to the exterior of the pressurized, fluid-filled space. 
     In a further arrangement, unitary wire  76  may have multiple passes or turns, with each pass or turn extending across all of the slots associated with all of the flow channels. This provides increased sensitivity or change in resistance per unit change in temperature. In yet a further variant, the wire may be provided in sections, with each section extending across only a few of the channels and with the resistance of each section being monitored separately by the control system. In such an arrangement, however, the control system preferably would include a circuit which mathematically combines the resistance values as, for example, by taking an average. In a still further variant, an individual wire or other sensor could be provided for each channel. However, such an arrangement would require a more complex circuit, more complex logic programming in the circuit, or both. Moreover, an arrangement using multiple sensors associated with multiple channels would require multiple electrical connections passing out of the fluid flow space, thus increasing the possibility for leakage or other failure of the connections and increasing the cost of the system. 
     As the liquid passes downstream through the channels and is heated by the current passing through it, gas bubbles tend to evolve within the liquid. For example, gases dissolved in the liquid tend to come out of solution as the liquid is heated. If such gas bubbles cling to the sensing wire  76 , they can impede heat transfer to the sensing wire and thus cause delayed or erroneous temperature signals. The exit structure and related components minimize the possibility that gas bubbles will cling to the exit wire. The relatively small cross-sectional area of slot  66  tends to create a high-velocity liquid flow through the slot, which aids in stripping bubbles from the wire. Moreover, the collection chambers  70  will tend to catch bubbles present in the liquid so that the bubbles pass out of the channel through the exit ports  72 , and thus do not cross the wire at all. Surprisingly, the arrangement of exit ports, collection chambers, and slot tends to provide this action regardless of the orientation of the heater relative to gravity. The precise shape of the collection chambers and associated elements may be varied somewhat. For example, the collection chambers need not be of semicircular shape as shown, but may have a generally polygonal cross-section. 
     The relatively small cross-sectional areas of the slots and exit bores provide flow resistance which is appreciable in comparison to the flow resistance of the channels  46  and  52 . This helps to equalize the velocity of liquid flowing in the various channels. 
     The modular design of the heater as described herein allows for simple production of heaters having numerous different capacity ranges. A heater with a greater capacity can be provided by simply using longer electrodes, a longer casing  16 , and more intermediate elements  32 . 
     In the embodiments discussed above, the different conductances of the different flow paths  46  and  52  are provided by the different spaces between the various electrodes in the wire direction W ( FIG. 6 ). This is desirable, because essentially the entire area of each electrode is exposed to the flowing fluid for transfer of current, and the current densities are substantially uniform over the entire surface area of each electrode. Other, more complicated arrangements could be used to provide the same difference in conductance between the various channels. For example, the channels could be of uniform width in the wire direction, but some channels could have a dielectric barrier extending within the channel in the lateral direction L ( FIG. 6 ) so as to narrow a portion of the conductive path. Alternatively, some of the electrodes could be coated over portions of their surface with a dielectric material so as to reduce the area of the current path and thus increase the electrical resistance of the channel. Such arrangements are less preferred, as they imply non-uniform current densities across the surfaces of the electrodes. 
     The physical arrangement of the flow channels in two sets—flow channels  46  in the upper flow chamber  42  and flow channels  52  in the lower flow chamber  44 —helps to provide a more compact arrangement having a small dimension in the widthwise or wire direction, i.e., in a direction transverse to the upstream and downstream directions. This, in turn, facilitates the construction of the pressurized enclosure, including casing  16 . To comply with regulatory and safety requirements, casing  16  typically must be arranged to withstand an internal pressure far above that normally encountered in service. 
     Heaters as discussed above can be utilized in a variety of applications, but are particularly useful in domestic hot water heating. A single heater may be provided for an entire home or, even more preferably, individual heaters may be associated with individual water-consuming devices or with a subset of the devices in the home as, for example, an individual heater for each bathroom or kitchen. In a system where an individual heater is associated with an individual water-using device such as a faucet or shower, the set point may be set by a knob on the using device. 
     Although the control system elements, such as the temperature sensing wire, and the bubble-eliminating elements, such as the slot and collection chambers, have been described herein in conjunction with a direct electric resistance heater where the electrical energy application elements of the heater are electrodes, the wire and bubble-eliminating elements can be used in other applications as well. For example, a liquid heater can include multiple channels with individual heating elements exposed to the fluid flowing in each channel, the heating elements being arranged to dissipate electrical power in the heating elements themselves and transfer the heat to the fluid flowing in the individual channels. Such a heater could be equipped with a sensing wire and bubble-eliminating elements as discussed herein. 
     As these and other variations and combinations of the features discussed above can be utilized without departing from the present invention as defined by the claims, the foregoing description should be taken by way of illustration rather than by limitation of the present invention.