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RELATED APPLICATIONS 
       [0001]    This application claims priority and benefit under 35 U.S.C. 119(e) from provisional Application No. 61/341,195 filed Mar. 29, 2010, the entire disclosure of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Many ways of recovering some of the heat in the grey waste water of a shower to preheat the incoming cold water have been attempted in the past. The prior art of Hunter&#39;s Shower Bath Economizer (U.S. Pat. No. 4,372,372) uses a chamber below the drain with a helical coil within forming a type of coil-in-shell heat exchanger. Nobile, in his heat recovery device of U.S. Pat. No. 5,791,401, uses a coil of tubing containing the incoming cold water wrapped around a formed section of the drain pipe to recover heat from the waste water. Vasile et al. in his U.S. Pat. No. 4,619,311 uses a contraflow heat exchanger formed around the straight drain pipe to the sewer to recover some heat. Sheffield, in his U.S. Pat. No. 4,821,793, uses an above the floor tub and shower floor heat exchanger for the same heat recovery function. 
         [0003]    The prior art of Cardone&#39;s U.S. Pat. No. 4,304,292 for a shower waste heat recovery system is described in detail in  FIGS. 1-6  provided below. While it is a workable system for recovery of heat from the grey waste water, it did have a shortcoming such that it was not compatible with some plumbing codes regarding removability of clogs in the drain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS OF THE PRIOR ART OF CARDONE &#39;292 
         [0004]      FIG. 1  is a simplified diagrammatic view of the water connections in Cardone &#39;292 to and from a household shower embodying a conservation technique according to the prior art; 
           [0005]      FIG. 2  is a front elevational view, on an enlarged scale, illustrating one embodiment of a heat exchanger in Cardone &#39;292 for practicing the method and for using the preferred structure of the prior art; 
           [0006]      FIGS. 3 and 4  are simplified instruction diagrams illustrating the boiler-heated warm water input to the household shower in Cardone &#39;292; 
           [0007]      FIG. 5  is a plan view of a removable floor panel of this shower embodiment in Cardone &#39;292; and 
           [0008]      FIG. 6 , like  FIG. 5 , is also a plan view, but on an enlarged scale and illustrating the heat exchanger in Cardone &#39;292 which is situated below the floor panel of  FIG. 5 . 
       
    
    
       [0009]    In the prior art of Cardone &#39;292, with the significant shortcoming subsequently noted above, it already is known that use is made of discharging water from a household shower, either of the tub-type  30  or stall-type  32 , to effectuate significant energy conservation, wherein as is typically the case, the household has a boiler  34  of the type which includes a storage tank  36  which tank, depending upon boiler-heated water available for use for such purposes as showering at locations  30 ,  32 . 
         [0010]    The prior art will be understood for the set-up for the showers  30  or  32  to include a tub drain conduit  40  or a stall shower drain  42 , wherein the drain channels the discharging water through a trap of U-shaped design designated  44 , on its way to a sewer connection  46 . 
         [0011]    It is known that the water discharging through the U-shaped conduit  44  is, in most cases, tepid, being a mixture of boiler-heated warm water and a cold water input to the showers,  30 ,  32 . 
         [0012]    In the above respect, it has been noted that the discharging water that normally drains is approximately 100° Fahrenheit at the time encountering the U-shaped conduit  44 . This otherwise wasted 100° Fahrenheit water is effectively passed in heat exchange relation to the cold water input, which in the community of Wantagh, N.Y., is typically supplied at 50° Fahrenheit. The heat exchange has been found in practice to provide a lukewarm water source at approximately 65° to 75° Fahrenheit for delivery to the faucet connection  58 ,  60  of the showers  30 ,  32 . This increase in approximately 15° to 25° Fahrenheit significantly diminishes the amount of boiler-heated water that is required to be delivered to the showers  30 ,  32 . While the beneficial results depend on different operating conditions and thus cannot be defined with precision, in practice use of the invention for 28 successive showers at a selected tepid temperature of 110° Fahrenheit for the premixed water and during a selected duration time for showering of 10 minutes which consumed approximately 15 gallons of the prior art set-up of  FIG. 1 . 
         [0013]    Cardone&#39;s U.S. Pat. No. 4,304,292 for heat conservation, unfortunately with the shortcoming noted above, is nevertheless commendably practiced as best illustrated in  FIGS. 5 and 6 , which correspond to FIGS. 8 and 9 of Cardone &#39;292, as well as what is shown in the crossectional detail view of FIG. 10 of Cardone &#39;292, in which a floor of shower  30 ′ includes a base  100  in the upper face or surface  102  of which there is embodied, in any appropriate manner, a spiral trough, generally designated  104  in  FIG. 6  and the individual helical turns of which are designated individually and collectively  106 . By slight increases in depth of the individual helical turns  106  the trough  104  is pitched to drain towards central opening  108  of base  100  of FIG. 10 of Cardone &#39;292, in which opening there is an appropriate drain fitting  110  which mounts a depending conduit  112  which will be understood to discharge into a sewer or the like. In accordance with the prior art, a pipe  114  suitable for flowing water to the shower  30 ′ and itself in a helical configuration as illustrated, is deposited in the correspondingly helically configurated trough  104 . That is, and as is perhaps best illustrated in  FIG. 6 , the individual helical turns of the spiral pipe  114 , designated individually and collectively  116  are each located in a cooperating one of the helical turns  106  of the spiral trough  104 . Connected at the center, as at  118  to the spiral pipe  114 , is the shower cold water inlet pipe or conduit  122 , as shown in Prior Art  FIG. 6  herein, and in FIG. 9 of Cardone &#39;292. The cold water outlet connection from the spiral pipe  114 , designated  120 , extends from the outermost helical turn and is connected to the shower faucet cold water conduit  56 ′. As a result, the cold water input to the shower  30 ′ is delivered through the helical heat exchanger  114  prior to delivery through the shower head  67 ′. More particularly, cold water from a suitable source initially flows through the inlet pipe  120 , then successively through each of the helical turns  116  to the helical pipe  114 , and then finally through the outlet pipe  120  into the faucet pipe  56 ′ where, upon opening of the valve  58 ′, the water is discharged through the shower nozzle  67 ′. 
         [0014]    However, the prior art of Cardone &#39;292 does not reveal a shower heat recovery system using a high efficiency flat plate heat exchanger with specific features for drain clog removal. 
       OBJECTS OF THE INVENTION 
       [0015]    It is therefore an object of the present invention to provide a shower heat exchanger which preheats incoming cold street water while reducing the need for a hot water heater/boiler to supply hot water for a shower. 
         [0016]    It is also an object of the present invention to provide a shower hot water heat exchanger which complies with applicable plumbing codes requiring access for removing clogs and which promotes the removal of standing stagnant water. 
         [0017]    Other objects which become apparent from the following description of the present invention. 
       SUMMARY OF THE INVENTION 
       [0018]    In keeping with these objects and others which will become apparent, the shower heat exchanger system of this invention, while complying with plumbing and sanitary standards for clog removal from drains, offers very high efficiency of heat recovery using a unique flat plate heat exchanger strategically placed below a tub or shower stall (or even below the bathroom floor). 
         [0019]    The concepts of this invention are designated by the acronym SHWERD™ which is derived from “Shower Hot Water Energy Recycling Device”. 
         [0020]    The standard drain inside the tub or shower stall accepts the grey waste water and directs it in a precise manner to impinge upon one end of a slightly tilted top plate (preferably copper) of the heat exchanger. This top plate is fastened robustly to a flat heat exchanger lower plate (preferably made of a sheet of rigid synthetic material such as PVC approximately ¼″-⅜″ thick or of a heat conductive material, such as copper). The lower plate is thermally insulated from below and on its sides. The lower plate has a serpentine pattern routed into its top side to a shallow depth such that the top plate, when fastened and sealed to it, forms a continuous chamber or serpentine conduit for containing the cold water flow. Thus it is understood that heat is transferred from the waste water falling on the top plate and spreading over it to the cold water flowing underneath the top plate in a tortuous serpentine pattern in contact with the bottom surface of the top plate. The cold water is thereby preheated and used as a substitute for the cold water that would be normally plumbed to the shower head. This therefore reduces the amount of hot water flow required to achieve the desired showering temperature. The geometric relationship between the drain and the placement of the heat exchanger underneath guarantees the counterflow of the waste water flowing atop the heat exchanger and the cold water flow going back and forth across the underside of the top plate and inching its way toward the vicinity of the tub drain, which is the hottest portion of the top heat exchanger plate. In this manner, the greatest rise in temperature of the preheated cold water is achieved, since the waste water enters the sewer outlet, at a temperature that can be significantly lower than that of the preheated cold water, while still transferring heat to the even colder inlet cold water, entering at the waste water discharge end of heat exchanger. At the top end of the heat exchanger near the drain outlet, the waste water is hotter than the preheated cold water beneath, so that it is still raising the temperature of the preheated cold water, just before discharge through the shower head. 
         [0021]    In a typical example, cold street temperature water of about 50 degrees Fahrenheit is exposed to typical hot shower water of about 120 degrees Fahrenheit through the heat exchanger. This exposure quickly raises the cold water to a preheated temperature of about 90 degrees Fahrenheit, which mixes with a reduced amount of hot water, to a final temperature, also of about 102-108 degrees Fahrenheit, which also reduces the need for using a hot water heater/boiler to heat hot water for a shower. 
         [0022]    One way to insure the efficiency of the heat exchanger of this invention is to have many reversals of the cold water path close together and with small width. In another embodiment, a wide water path is used through the heat exchanger with fewer reversals in the serpentine. To increase efficiency, fins are optionally added to the underside of the copper plate in the region of the serpentine below that would be wet by the cold water flow; this increases the effective area of contact. It is also noted that the upper plate can be made integral with the serpentine sections and base supporting the serpentine sections by the base plate being made of copper or other heat conductive material, with the corrugated walls of the serpentine sections extending upward from the lower base plate to the upper heat conductive copper plate, so that the entire heat exchanger unit is heat conductive, but thermally insulated on its sides and from below. 
         [0023]    The serpentine sections can be either channels cut in the lower PVC plate or channels formed of copper walls. In addition, the serpentine sections can be formed from parallel tubes (of any geometric configuration, round or straight sided, with flat walls) with curved or angled manifolds, forming the curved or angled corners connecting the straight portions, formed from the parallel tubes. However, whether the bottom plate is PVC or copper, since the top plate is a good conductor for spreading heat from the waste grey water laterally in case the flow from the drain does not spread evenly; therefore the heat must be conducted through the plate to be deposited to the water flowing under it. It is beneficial for the heat to stay there and not leak out the bottom or sides of the base. This is why the heat exchanger is wrapped underneath in insulating foam or other insulating material. The base plate is thermally insulated at the sides and bottom, such as by a thick layer of solid or spray-on foam insulator. 
         [0024]    In the first embodiment of this invention, the heat exchanger is plumbed directly with the cold water supply at the bottom (cold end) with the cold water shower valve at the preheated end feeding the shower head mixing area. This means that the heat exchanger remains at supply pressure even when the shower is not in use. In an alternate embodiment, the plumbing is re-routed to feed the cold end of the heat exchanger through the shower cold water valve with the preheated cold water outlet of the heat exchanger directly plumbed to the shower head mixing area. While the operation to the user is identical, the heat exchanger is no longer pressurized when the shower is not in use. This is called a “Dry Base Exchanger” configuration, whereby if the heat exchanger were damaged or punctured, it would not leak profusely, since it is not pressurized. 
         [0025]    In the first embodiment, the drain is preferable at the end of the tub opposite the shower head, while the sewer connection is retained at its normal location at the shower head end. This can be easily accomplished by turning the tub around since it is reversed from normal practice. Note that an alternate embodiment retaining the drain at the shower head end of the tub is also possible, but the sewer connection will then be at the opposite end. Either will perform equally, but one may be easier to install than the other. 
         [0026]    In yet another embodiment, a service chamber is appended to the tub at the end near the sewer connection which discharges grey water after it has run down the heat exchanger top plate. This chamber has an easily removable top cover permitting direct access to the sewer opening so that a plumber&#39;s snake can be directly entered thereby bypassing the excursion down the length of the top plate. 
         [0027]    Because of the continuous preheating of cold inlet water, there are less BTU&#39;s consumed, and subsequent showers remain warm even after multiple showers. 
       BRIEF DESCRIPTION OF THE DRAWINGS OF THE INVENTION 
       [0028]    The description of the invention which follows, together with the accompanying drawing, should not be construed as limiting the invention to the example shown and described, because those skilled in the art to which this invention appertains will be able to devise other forms thereof. The drawings are diagrammatic, and not drawn to scale of actual use. 
         [0029]      FIG. 7  is an exploded perspective view of the bathtub, partially in section, and of the heat exchanger, both in accordance with the present invention; wherein the flow of street water being preheated is indicated by single line arrows and wherein the flow of exiting grey hot water is indicated by double line arrows; 
         [0030]      FIG. 7A  is an exploded perspective view in partial cutaway, similar to  FIG. 7 , but optionally showing the addition of fins to the underside of the top plate of the heat exchanger to enhance heat transfer; 
         [0031]      FIG. 7B  is a side view crossection of the assembled heat exchanger of  FIG. 7A  showing the added fins immersed in cold water flow within the serpentine channels; 
         [0032]      FIG. 7C  is a close-up detail perspective view of a fluid path schematic diagram of an alternate embodiment for serpentine sections where parallel round tubings are connected by corner sections; 
         [0033]      FIG. 7D  is a close up detail perspective view of a portion of parallel tubings of angled corrugated fluid path sections in serpentine configuration, with straight rectangular walls connected by angled corner sections; 
         [0034]      FIG. 8  is a side elevational view, in assembled condition, of the bathtub and heat exchanger of  FIG. 7 ; 
         [0035]      FIG. 9  is a partial elevational view, in section, of the structure noted by the reference arrow  9  in  FIG. 8 ; 
         [0036]      FIG. 10  is also a partial elevational view, in section, but of the structure noted by the reference  10  in  FIG. 8 ; 
         [0037]      FIG. 11  is a side view schematic representation illustrating the cold water piping flow of a Dry Base Exchanger embodiment whereby the input to the heat exchanger is controlled by the cold water valve; 
         [0038]      FIG. 12  is a side view schematic showing the heat exchanger of this invention located below the bathroom floor and servicing a tub above; 
         [0039]      FIG. 13  is a side view schematic showing the heat exchanger located below the bathroom floor and servicing a shower stall; 
         [0040]      FIG. 14  is a side elevation of an alternate embodiment for a tub with a side attached service chamber providing unobstructed access to the sewer pipe connection, where the side attached service chamber comprises a wide pipe with a breather cap; 
         [0041]      FIG. 14A  is a side elevational view in crossection of the tub and heat exchanger of  FIG. 14 ; 
         [0042]      FIG. 15  is an exploded perspective view of an alternate embodiment, showing an access plate over the chamber having a pipe wherein the porcelain tub is shown separated from the heat exchanger plate, which includes a concave end portion in the vicinity of the sewer connection drain, and wherein a series of elevated water-sealed peripheral walls are provided with a minimum 2″ clearance under the bottom of the tub/shower stall, for removing clogs pursuant to the Uniform Plumbing Code; 
         [0043]      FIG. 16  is an exploded perspective view of the shower heat exchanger showing an auxiliary peripheral jet flush-out system, as well as an optional central divider brace; 
         [0044]      FIG. 17  is a perspective view of a factory produced tub/shower stall modular unit with liner walls having the show heat exchanger built integral therein; and, 
         [0045]      FIG. 17A  is a perspective view of a factory produced shower stall modular unit with liner walls having the show heat exchanger built integral therein. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0046]    In the preceding discussion of the prior art U.S. Pat. No. 4,304,292 of Cardone, reference was made to a shortcoming. This shortcoming is, as required by the sanitary code of most intended sites of use, that the system used have the ability of clog-removable functioning in the drain to the sewer. In the system now to be described, it should be readily understood that hypothesizing the “clog” as a two-inch-diameter ball stuck in the drain to the sewer, that the removal with a snaking cable or the like is readily achieved. Thus this invention complies with the Uniform Plumbing Code. 
         [0047]    The present system of heat recovery of this invention has no such shortcomings. Shown in  FIGS. 7 and 8 , in the former in exploded relation, and in the latter in assembled relation, is a bathtub, generally designated  11  in its use mode as illustrated in  FIG. 8  as a shower, as noted by arrow  13 , received from a shower head  15 , the volume of the shower  13  being a function of the valving of two on/off faucets  17 , one for cold water and the other for boiler-heated water, wherein the result of this mixing of inputs results in a tepid temperature of the shower  13 , as previously explained in the discussion of the prior art. 
         [0048]    As shown in  FIGS. 7-10 , the tepid water  13  drains at the remote bathtub end  19 , i.e. an end usually not the conventional bathtub location, and flows, as noted by single line arrows  21  in the clearance  23 , which serves as a passageway above the heat exchanger  25 , emptying as best shown in  FIG. 10  in an outlet  27  which is in fluid communication with the exiting boiler-heated hot water source used for the shower  13 , exiting as grey water, and identified as double line arrows cascading across heat exchanger plate  29  and exiting through outlet  27 . 
         [0049]    The heat exchange function results from the use of a closure plate  29  of copper construction material, appropriately attached over a base  31  having edges bounding a passageway outlet  33  of tepid water  13  exiting from the bathtub remote end  19 . As shown in  FIGS. 9 and 10 , base  31  includes a layer of insulation  31   a , such as STYROFOAM® foam or other insulating material, which may be an integral layer, or may be attached by spraying or attaching from below. The insulating material can be any kind of natural or synthetic resinous cellular material. The insulating layer applies to all embodiments herein. 
         [0050]    Passing in heat exchange with the tepid water  13  is the cold water input into the heat exchanger  25 , as explained in the previous discussion of the prior art, the input being at the heat exchanger end and flowing in a sinusoidal flow pattern, as noted by arrow  37 , said sinusoidal pattern lengthening the heat exchange duration of the tepid water and cold water inputs constituting the showering water  13 . 
         [0051]    Note in  FIG. 7  base  31  has few direction reversals. In actuality, much narrower water channels  37  formed between sinusoidal heat conductive walls  37 ′ in a separate pathway and reversals every 2″ or so are used to enhance heat transfer. Moreover, the depth is preferably about ⅛″ in depth as used, although depth may vary, as long as effective heat transfer occurs. 
         [0052]    An alternative method of increasing heat transfer from top plate  29  to the water in channels  37  is to maintain the broad serpentine pathway of  FIG. 7 , but to add heat transfer fins  29 A (as shown in  FIGS. 7A and 7B ) attached to the underside of plate  29  and dipping into the water channels below. Heat transfer fins  29 A can extend all the way down from heat conductive plate  29  to base  31 , or they can extend partially down (not shown). 
         [0053]    Although drawing  FIG. 7  shows a meandering serpentine pathway which is in contact with the heat exchanger plate  29  above, in  FIG. 7C  there is shown a fluid path schematic diagram of an alternate embodiment for serpentine sections where parallel round tubings  37   a  are connected by corner sections  37   b , and wherein the tubings contact the heat exchanger plate  29  at the tangent where the tops of each tubing portion  37   a  contact heat exchanger plate  29  above. Optionally, in another embodiment, where the tubes are heat conductive (such as copper), use of heat exchanger plate  29  can be dispensed with. In that use, the hot shower grey water can flow over the tops of round tubings  37   a  without heat exchanger plate  29 , so that heat is exchanged directly through the tubings  37   a  to the cold street water being preheated. 
         [0054]    Likewise, in  FIG. 7D  there is shown a portion of parallel tubings of angled corrugated fluid path sections  38  in serpentine configuration, with straight rectangular walls  38   a  connected by corner sections, where the tops of the straight walls  38   a  contact the flat heat exchanger plate  29  above. In  FIG. 7D  the walls  38   a  are foreshortened in the hollow corner areas  38   b , to provide the serpentine pathway for the incoming cold street water to flow therethrough, forming the preheated water as the cold water is in thermal contact with the heat generated by hot shower water passing over flat heat exchanger plate  29 , located above the serpentine pathway portions  38  formed by straight walls  38   a . Some of the walls  38   c  extend at one side all the way to an outer wall, to form a closed area, to direct fluid flow around corners and through the serpentine configuration. 
         [0055]    As a result of the foregoing, the “cold” water input into the showering water  13  is raised to an elevated temperature than it would have had otherwise, with a first important consequence that less boiler-heated water  17  is required for the showering water  13 . 
         [0056]    A second important consequence is that the just noted savings is achieved by the described operating mode which complies with the sanitary code of most intended sites of use, i.e. that that system used, as more particularly described and illustrated in  FIGS. 7 ,  8  and  9 , have the ability of clog-removable functioning in the drain to the sewer. For example, the region below tub/shower stall  19  and above heat conductive plate  29 , through which shower waste grey water passes, is at least two inches in height, to permit a snake to clear out that area of any clogs, pursuant to the Uniform Plumbing Code. To enhance aiming and movement of the snake, the region near sewer outlet  27  at the opposite end of the heat exchanger can be optionally provided with a concave region  25   a  surrounding the sewer outlet  27 , as shown in  FIG. 15 , so that when the distal end of the snake approaches the sewer outlet  27 , it is directed to the sewer outlet  27 . This option is applicable to any of the embodiments for the heat exchanger of the present invention. 
         [0057]    The embodiment shown in the side schematic of  FIG. 11  has two notable changes from that of  FIG. 8 . The first change is that drain  40  is located at the same end as the shower head and valves which is the conventional arrangement. The second is the routing of the cold water relative to cold water valve  17  and heat exchanger  25 . This latter routing is called a Dry Base Exchanger. 
         [0058]      FIG. 11  illustrates a tub  11  with floor sloping downward toward left toward drain  40  with water  42  discharging through it. Heat exchanger  25  slopes downward toward the right with grey water  47  discharging from the top plate to the sewer connection and pipe  27  on the end opposite from that of  FIG. 8  (now placed away from the shower head). Water heater  45  receives cold water at 50 degrees Fahrenheit and feeds 120 degree water through hot water valve  46 . As opposed to  FIG. 8  practice, cold water is fed directly to cold water valve  17  and then to the cold water input of heat exchanger  25 . It is boosted in temperature to 90 degrees Fahrenheit and connects directly through pipe  48  to the mixing region to then be discharged through shower nozzle  15  at 103 degrees Fahrenheit. This Dry Base Exchanger cold water routing through valve  17  prior to heat exchanger  25  means that exchanger  25  is not pressurized when not in use. If the top plate or base portion were to be damaged during the standby period (most of the time), it will not leak profusely, damaging the surroundings, since the system only contains the water in the serpentine and that in the piping to the shower head to leak out. The Dry Base Exchanger piping hook-up is the preferred embodiment and may be used as a substitute for the pressurized heat exchanger hook-up in any of the illustrations and embodiments shown. 
         [0059]    While heat exchanger  25  can be installed below an elevated tub as shown in  FIG. 8 , the heat exchanger can be conveniently placed below the bathroom floor leaving the tub at it&#39;s normal height. This is especially convenient if the bathroom is over an unfinished basement.  FIGS. 12 and 13  are schematic views showing such an installation for a tub and for a shower stall respectively.  FIG. 12  shows tub  11  with sloping internal floor  41  at its normal height resting on bathroom floor  52 . Heat exchanger  25  is installed below floor  52  level and possibly attached to conveniently located beam  53 . In  FIG. 13 , shower stall  54  rests normally on bathroom floor  52  while heat exchanger  25  is below floor  52  level and can be attached to beam  53 . Note that concave inner shower stall floor  55  drains atop the heat exchanger top plate in a similar fashion to a tub installation. 
         [0060]    The embodiment of  FIGS. 14 and 14A  show the use of an optional service chamber  62   a  attached to the end of tub  11  adjacent to the sewer pipe connection  64 . It has an easily removed decorative access cover  62 . If one looks straight down chamber  62   a  with cover  62  removed, the sewer pipe connection  64  can be see in unobstructed view at the bottom of pipe  63   a , so that a plumber&#39;s snake and/or chemical clog removers can be administered along the straight short path  63  (shown in arrows). This direct access to sewer connection  64  bypasses the bend at drain  40  and the length of the top plate  29  of heat exchanger  25  which would have to be traversed through drain  40  otherwise. This area within chamber  61  can also be used to flush out the top plate surface with a water hose or pressure cleaner since the entire width of plate  29  is accessible through it. 
         [0061]      FIGS. 14 and 14A  also show a side elevation in crossection of an alternate embodiment for a tub  60  with side attached service chamber  62   a  providing unobstructed access to the sewer pipe connection  64 , where the side attached service chamber comprises a wide pipe  63   a  with a breather cap  61  underneath access cover  62 . Typically, pipe  63   a  is a wide pipe of approximately 5″ in diameter leading to sewer connection  64 . As can also be shown in  FIG. 14 , tub  60  having a depth “D 1 ” of approximately 7″ indicates that the bottom of pipe  63   a  can be reached with a plumbing snake or even manually by a person cleaning pipe  63   a  with their hand and forearm. Depth D 1  can vary from 2″ up to high tube depths of 14 inches or more. 
         [0062]      FIG. 15  is an exploded perspective view of an alternate embodiment for the system  60  of  FIGS. 14 and 14A  showing an access plate  62  over chamber  63  having pipe  63   a  wherein in the porcelain tub  11  is shown separated from the heat exchanger plate  29 , which includes an optional concave end portion  25   a  in the vicinity of the sewer connection drain  64 , and wherein a series of elevated peripheral water sealed walls  25   b - 25   e  are provided with a minimum 2″ clearance under the bottom of the tub  11  for removing clogs pursuant to the Uniform Plumbing Code 710.3.3, which provides “In other than single-dwelling units, the ejector or pump shall be capable of passing a two (2) inch (51 mm) diameter solid ball, and the discharge piping of each ejector or pump shall have a backwater valve and gate valve, and be not less than three (3) inches (80 mm) in diameter.” 
         [0063]      FIG. 16  shows a shower heat exchanger showing an auxiliary peripheral jet flush-out system  70  with jets  71  and control valve  72 , as well as an optional central divider brace  80  for stability. In this embodiment, two separate sets of serpentine water channels  37  are provided on each side of the divider brace  80  and flowing jointly into drain  40 . The auxiliary flush out set of jets  71  can be provided within the space above the pair of separate heat conductive upper plates  29  and  29 ′ of the heat exchanger  25  and a floor of the tub  11  above the heat exchanger  25 . 
         [0064]    As shown in  FIG. 17 , although the shower heat exchanger  25  can be retrofit and sealed to existing tub/shower stalls  11 , it is further noted that the heat exchanger  25  can be built integral into a factory produced tub/shower stall modular unit  90  with liner walls  91 - 93  and tub  11  and/or optimal access plate  62 . 
         [0065]    Additionally, as shown in  FIG. 17A , although the shower heat exchanger  25  can be retrofit and sealed to existing tub/shower stalls  11 , it is further noted that the heat exchanger  25  can be built integral into a factory produced shower stall modular unit  90   a  with liner walls  91   a - 93   a , but with a shower stall  11   a , instead of a tub/shower stall  11  shown in  FIG. 17 , and/or with access plate  62   a.    
         [0066]    In the foregoing description, certain terms and visual depictions are used to illustrate the preferred embodiment. However, no unnecessary limitations are to be construed by the terms used or illustrations depicted, beyond what is shown in the prior art, since the terms and illustrations are exemplary only, and are not meant to limit the scope of the present invention. 
         [0067]    It is further known that other modifications may be made to the present invention, without departing the scope of the invention, as noted in the appended Claims.

Summary:
A heat exchanger placed underneath an elevated tub or shower stall or under the bathroom floor includes a flat top heat conductive plate. The top plate is fastened to a flat lower plate, having a serpentine pattern with a shallow depth embedded into the top surface. When the top plate is fastened and sealed to the lower plate, the flow of the incoming cold water is contained within the confines of the serpentine lower plate of the heat exchanger. Heat is transferred from the grey waste water falling onto the top plate to preheat the incoming cold water flowing underneath the top plate in the serpentine conduit chamber of the lower plate in contact with a bottom surface of the top plate. The cold water is thereby preheated and used as a substitute for the incoming cold water that would be normally plumbed to the discharge shower head.