Abstract:
A water heater adapted to be positioned outside of a building and to interface with the plumbing system of the building. The water heater includes a water storage tank adapted to store water outside of the building, a means for heating the water in the tank, a base member supporting the tank outside of the building, and a plurality of water pipes communicating with the water tank. The water heater also includes a manifold that is mounted to the base member and includes a plurality of pipe unions adapted to interface between the plumbing system and the plurality of pipes to provide cold water to the tank and to remove heated water from the tank for use in the building.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. application Ser. No. 10/274,183, filed Oct. 18, 2002, U.S. Pat. No. 6,679,014 which is a continuation of U.S. application Ser. No. 09/732,472, filed Dec. 7, 2000 now U.S. Pat. No. 6,591,788. The entire contents of these related applications are incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The invention relates to water heaters, and more particularly to water heaters adapted to be mounted on the roof of a building. 
   BACKGROUND 
   It is known to provide a water heater within a building, and to mount other equipment (e.g., HVAC equipment) on the roof of a building. A primary concern with rooftop equipment is the load imposed on the roof by the weight of the equipment. Most rooftop equipment is supported on the roof by a roof curb. The roof curb provides a weather-tight seal between the equipment and the roof, and distributes the weight load of the equipment to the building&#39;s roof. The roof curb also prevents leakage of water into the building and equipment in the event of a roof flood. Roof curbs are typically built to match the pitch of the roof so that the equipment mounts on a level surface. Rooftop equipment is typically raised to the roof with a crane and set on the roof curb. A foam sealing tape is often used between the bottom of the equipment and the roof curb to provide a weather-tight seal. 
   Many commercial buildings include a wall around the rooftop to screen the rooftop equipment from view. Much of the HVAC equipment currently installed on rooftops is enclosed in a rectangular cabinet, and has a sufficiently low profile to be not visible from the ground level. 
   SUMMARY 
   The present invention identifies several advantages to providing a water heater on the roof of a building. In commercial applications, one factor affecting many aspects of some businesses, and particularly retail and restaurant businesses, is floor space. Moving a water heater out of a building and positioning it on the building&#39;s roof may permit more floor space to be used for conducting business. In addition to potentially freeing up floor space and the need for a separate boiler room in a building, the present invention provides other advantages over interior water heaters. A rooftop water heater embodying the present invention may make design and maintenance simpler and cheaper in some instances when compared to the design and maintenance of interior water heaters. 
   Several design parameters are imposed on the design of a rooftop water heater by weather conditions and the expectations of potential purchasers of such water heaters. The water heater must first account for weather conditions not normally encountered by interior water heaters. It would be desirable to mount the water heater on a weather-tight roof curb that would support the water heater above expected water levels in the event of a roof flood. The roof curb would also provide the required weight load distribution to the roof. Because roof curbs are already used to support other rooftop equipment, it is convenient to use a roof curb to support a rooftop water heater as well. 
   To maintain the aesthetics of their buildings, purchasers would likely want equipment that cannot be ordinarily seen from the ground level. The water heater therefore must have a sufficiently low profile and be of substantially the same height as most HVAC equipment to meet the expected demands of purchasers. 
   In light of the foregoing considerations, one embodiment of the present invention includes a water heater adapted to be positioned outside of a building and to interface with the plumbing system of the building. The water heater includes a water storage tank adapted to store water outside of the building, a means for heating the water in the tank, a base member supporting the tank outside of the building, and a plurality of water pipes communicating with the water tank. The water heater also includes a manifold that is mounted to the base member and includes a plurality of pipe unions adapted to interface between the plumbing system and the plurality of pipes to provide cold water to the tank and to remove heated water from the tank for use in the building. 
   Another embodiment of the invention includes a water heater having a water storage tank adapted to store water outside of the building, a gas burner, and a flue tube within the tank communicating with the gas burner for the flow of hot products of combustion from the burner through the flue tube to heat water in the tank. The water heater also includes a cabinet surrounding the tank and supporting the tank outside of the building. A vent communicates with the flue tube for the removal of the products of combustion from the flue tube and out of the cabinet. The vent includes first and second ducts within the cabinet and extending through first and second sides, respectively, of the cabinet. 
   An additional embodiment of the invention includes a water heater assembly having a water heater, a cabinet surrounding the water heater, and a manifold. The water heater includes a water storage tank adapted to store water outside of the building, a means for heating the water in the tank, and a plurality of water pipes communicating with the water tank. The manifold is mounted to the cabinet to interface between the plumbing system and the plurality of pipes to provide cold water to the tank within the cabinet and to remove heated water from the tank and the cabinet for use in the building. 
   Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a water heater embodying the present invention. 
       FIG. 2  is a view of the water heater of  FIG. 1  from a different perspective. 
       FIG. 3  is an exploded view of the cabinet of the water heater of FIG.  1 . 
       FIG. 4  is a perspective view of the tank assembly of the water heater of FIG.  1 . 
       FIG. 5  is a schematic illustration of the piping system associated with the tank assembly of FIG.  4 . 
       FIG. 6  is a cross-section view taken along line  6 — 6  in FIG.  4 . 
       FIG. 7  is a perspective view of the tank assembly of  FIG. 4  with selected elements removed for the purpose of illustration. 
       FIG. 8  is an exploded view of the tank assembly of  FIGS. 4 and 7 . 
       FIG. 9  is an end view of the tank assembly of  FIGS. 4 and 7 . 
       FIG. 10  is a side cross-section view of the tank assembly taken along line  10 — 10  in FIG.  9 . 
       FIG. 11  is a view of the outlet tube taken along line  11 — 11  in FIG.  10 . 
       FIG. 12  is an end view of a tank assembly of an alternative construction. 
       FIG. 13  is a side cross-section view taken along line  13 — 13  in FIG.  12 . 
       FIG. 14  is a perspective view of a water heater according to an additional embodiment of the present invention. 
       FIG. 15  is a view of the water heater of  FIG. 14  from a different perspective. 
       FIG. 16  is an exploded view of the cabinet of the water heater of FIG.  14 . 
       FIG. 17  is a perspective view of the tank assembly of the water heater of FIG.  14 . 
       FIG. 18  is a schematic illustration of the piping system associated with the tank assembly of FIG.  17 . 
       FIG. 19  is a perspective view of an electric water heater according to another embodiment of the present invention. 
   

   Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The use of “consisting of” and variations thereof herein is meant to encompass only the items listed thereafter. The use of letters to identify elements of a method or process is simply for identification and is not meant to indicate that the elements should be performed in a particular order. 
   DETAILED DESCRIPTION 
   The present invention is embodied in a water heater including a cabinet  10  illustrated in  FIGS. 1-3  and a tank assembly  14  illustrated in  FIGS. 4-13 . A first construction of the tank assembly  14  is illustrated in  FIGS. 4-11 . With particular reference to  FIGS. 9 and 10 , the tank assembly  14  includes a tank  18  having a substantially cylindrical shape and a horizontally-extending longitudinal axis  22 . The tank  18  includes a shell  24  that is closed at one end by a head  26 . A U-shaped flue tube  30  is positioned within the tank  18 . The U-shaped flue tube  30  includes two legs  31 ,  32  communicating through a curved or bent portion  33 . The legs  31 ,  32  terminate in a burner end  34  and an exhaust end  38 , respectively, both of which extend through the head  26 . 
   Turning to  FIGS. 1-3 , the cabinet  10  is mounted on a roof curb  42  on a rooftop  46  of a building, and encloses the tank assembly  14 . The cabinet  10  is generally rectangular in shape, having its major axis extending generally horizontally. In this regard, the cabinet  10  has a generally low profile and is similar to other rooftop units such as typical HVAC (e.g., air conditioning) units. As used herein, “low profile” means that the rooftop water heater cabinet  10  cannot typically be seen by a person of average height standing at ground level from a distance of about one city block. The low profile therefore permits the unit to be mounted on a rooftop without detracting from the aesthetics of the building. Roof rails and a pitch pocket or small roof curb may be used as an alternative mounting structure to the roof curb  42  illustrated. 
   As seen in  FIG. 3 , the cabinet  10  includes two side panels  50 ,  54  arranged generally parallel to each other and at right angles to a third side panel or end panel  58 . A second end panel or cabinet door  62  is releasably attached at right angles to the two side panels  50 ,  54  and generally parallel to the end panel  58 . The door  62  includes a cutout  66  and a recessed or raised handle  70  to facilitate removal of the door  62  from the rest of the cabinet  10  without the use of tools. An upper panel or top  74  is releasably affixed to the top edges of the other panels  50 ,  54 ,  58 ,  62  of the cabinet  10 . The top  74  is preferably removable from the rest of the cabinet  10  without the use of tools. Preferably the door  62  and top  74  are locked to the other panels  50 ,  54 ,  58  to resist tampering with the tank assembly  14 . 
   A base member  78  supports the cabinet panels  50 ,  54 ,  58 ,  62 ,  74  and provides the bottom of the cabinet  10 . The base member  78  is supported by the roof curb  42 , and a foam tape or other sealing member is preferably interposed between the roof curb  42  and the base member  78  to create a water-tight seal therebetween. A pair of rails  82  are mounted to the underside of the base member  78  and straddle the roof curb  42 . The rails  82  define channels along their lengths into which the prongs of a conventional fork lift may be inserted for lifting and lowering the water heater, and also include slots  86  to insert the forklift prongs transverse to the channels. The rails  82  also include apertures  90  for attaching the hooks of a lifting crane to facilitate lifting the water heater to the roof  46  of the building. 
   When the top and door  74 ,  62  are removed, the tank assembly  14  is sufficiently exposed to permit a service technician to perform service on the water heater. An electrical disconnect button  94  ( FIG. 1 ) is mounted to one of the side panels  50 . The disconnect button  94  permits the service technician to conveniently connect and disconnect power to the water heater while the technician is on the rooftop  46 . An electrical outlet  98  ( FIG. 1 ) is also provided to accommodate a service technician&#39;s power tools and electric lights. Another feature of the water heater is the provision of local and remote controllers  99 ,  100 , respectively, that include thermostats. The local controller  99  may be mounted on the side panel  50  as illustrated, or it may be mounted within the cabinet  10 . The remote controller  100  is mounted in a desirable place within the building. The local and remote controllers  99 ,  100  each include a digital display of the water temperature within the tank  18 , and permit adjustment of the temperature. Thus, the water heater may be monitored and controlled from both the rooftop  46  and from within the building. This arrangement creates convenience for both the building owner or occupant within the building and a service technician on the rooftop  46 . 
   The cabinet panels  50 ,  54 ,  58 ,  62 ,  74  are joined together around the tank assembly  14  in a weather-tight fashion. As used herein, “weather-tight” means that rain or other precipitation falling on the cabinet  10  from above are substantially prevented from entering the cabinet  10  and interfering with the operation of the water heater. For example, one of the side panels  54  includes an air inlet vent  102  that opens downwardly to cause precipitation to run off the cabinet  10 , and the other side panel  50  includes a downwardly-opening exhaust vent  106 . In addition to being weather-tight, the edges of the cabinet panels  50 ,  54 ,  58 ,  62 ,  74  are preferably substantially water-tightly joined to each other (e.g., by welding or with gasket material) to resist or prevent water from entering the cabinet  10  and interfering with the operation of the water heater. For example, the panels  50 ,  54 ,  58 ,  62  can be connected to each other with the use of mating tongue and groove connections such that one edge of a panel  50 ,  54 ,  58 ,  62  can be slid into a groove formed in the edge of an adjacent panel  50 ,  54 ,  58 ,  62 . 
   Turning now to  FIG. 4 , the tank assembly  14  will be discussed in more detail. A fan or combustion blower  110  communicates with the air inlet vent  102  through an inlet duct  112  to deliver air from outside the cabinet  10  to a gas fuel burner  114  positioned at the burner end  34  of the flue tube  30 . Gas fuel is also delivered to the burner  114  via a gas pipe  118  and regulator  122 . The burner  114  causes combustion of the air and gas fuel, and the products of combustion are forced through the flue tube  30  by the combustion blower  110 . The products of combustion heat the water in the tank  18  through the wall of the flue tube  30 , and then exit the exhaust end  38  of the flue tube  30 . The exhaust vent  106  communicates with the exhaust end  38  of the flue tube  30  through an exhaust duct  123  to permit the spent products of combustion to be exhausted to the surrounding atmosphere. 
   With additional reference to  FIG. 8 , a baffle or turbulator  124  is positioned within one of the legs  31 ,  32  of the U-shaped flue tube  30 , and preferably in the upper leg  32 . The legs  31 ,  32  each include a longitudinal axis  125  (FIG.  10 ), and the longitudinal axes  125  together define a flue plane FP ( FIG. 9 ) that may be disposed substantially vertically, or may be angled with respect to vertical an angle labeled α. Preferably, the angle α is between 30-60° with respect to vertical. 
   Referring to  FIGS. 8-11 , an inlet tube  126  and outlet tube  130  are also mounted in the head  26 , and extend substantially parallel to the longitudinal axis  22  of the tank  18 . The inlet tube  126  is positioned in the bottom portion of the tank  18  below the longitudinal axis  22 , and the outlet tube  130  is positioned in the top portion of the tank  18  above the longitudinal axis  22 . Preferably, the inlet tube  126 , outlet tube  130 , and longitudinal axis  22  of the tank  18  are aligned parallel to each other in a vertical plane. This arrangement is made possible in part because the flue plane angle α is greater than zero (i.e., the flue plane FP is non-vertical), which moves the legs  31 ,  32  of the flue tube  30  from a position in which they would interfere with the extension of the inlet and outlet tubes  126 ,  130 . The tubes  126 ,  130  are substantially identical to each other and preferably have a length  132  ( FIG. 11 ) slightly shorter than or substantially the same as the interior length  134  of the tank  18  so that cold water is provided and hot water is removed along the entire interior length  134  of the tank  18 . The preferred length  132  is between about 37 and 38 inches, but the length  132  will vary depending on the dimensions of the water tank  18 . A threaded end  138  is attached at one end of the tubes  126 ,  130 . The threaded end  138  is threaded into a threaded spud  142  in the tank head  26 , and includes additional threads to receive a nipple, water pipe, or other plumbing fixture. In a less preferred embodiment, one or both of the tubes  126 ,  130  may be significantly shorter than illustrated, or the outlet tube  130  may be removed, leaving only the conventional spud  142  to which the building&#39;s hot water pipe communicates. 
   The end  146  opposite the threaded end  138  is closed. The tubes  126 ,  130  include a plurality of holes, apertures, or openings  150  along their lengths. The tubes  126 ,  130  preferably have an outer diameter of about one inch, with the holes  150  having a diameter of about 0.25 inches. The hole spacing  154  is preferably about five inches, with the last hole being spaced from the closed end  146  a distance  158  of about 0.5 inches. 
   In the illustrated construction, all of the openings  150  in the inlet tube  126  face down and all of the openings  150  in the outlet tube  130  face up. Thus, the inlet tube  126  directs cold water toward the bottom of the tank  18  and the outlet tube  130  draws hot water from the top of the tank  18 . This is advantageous because the hottest possible water is drawn from the top by the outlet tube  130  while the inlet tube  126  introduces cold water directly at the bottom of the tank  18 . The inlet tube  126  therefore evenly distributes water at the bottom of the tank to minimize mixing and thereby maximize heated water drawn from the tank  18 . In alternative less preferred constructions, the openings  150  may be arranged around the periphery of the tubes  126 ,  130  instead of opening in only one direction. 
   As can be seen in  FIGS. 3 ,  4 , and  6 , the base member  78  includes a water pipe aperture  162  surrounded by a vertical wall  166 . Four water pipes  168   a ,  168   b ,  168   c ,  168   d  (collectively referred to as  168 ) extend up from the building, through the roof curb  42 , and through the aperture  162 , and communicate with the water tank  18 . A grommet  170  includes apertures  174  water-tightly slip fit around the water pipes  168 , and has a depending wall  178  ( FIG. 6 ) water-tightly slip fit over the vertical wall  166 . The grommet  170  therefore provides a water-tight seal between the pipes  168  and the base member  78 . Suitable clamps  182  can be employed to further tighten the grommet  170  around the vertical wall  166  and pipes  168 . 
     FIGS. 4-6  illustrate the piping system associated with the tank assembly  14 . One of the water pipes  168   a  provides cold water to the water inlet tube  126 . Another water pipe  168   b  removes hot water from the tank  18  through the outlet tube  130 , and this hot water is used for dishwashers and other applications requiring very hot water (e.g., between about 120° F. and about 150° F., or a higher temperature if necessary). Another water pipe  168   c  communicates with a mixing valve  186 , and delivers a mixture of hot water and cold water to the warm water faucet of the building&#39;s bathrooms and kitchen sink. 
   The last water pipe  168   d  communicates between the building&#39;s hot water pipes and a circulation pump  190 . The circulation pump  190  performs two functions. First, the circulation pump  190  is turned on by the controller  99  ( FIG. 1 ) each time the combustion blower  110  is turned on, and remains active for a set period of time (e.g., about nine minutes) after the combustion blower  110  is turned off. A bypass valve  191  is actuated to route water from the outlet tube  130 , through the circulation pump  190 , and back into the tank  18  through the inlet tube  126 . In this manner, the circulation pump  190  causes hot water to be drawn off the top of the water tank  18  through the outlet tube  130  and recirculated through the cold inlet tube  126  to even out the temperature of the water in the tank  18  and reduce the effects of stacking. 
   The second function of the circulation pump  190  is to maintain a supply of hot water in the pipes of the building. A thermostat  192  ( FIG. 5 ) may be employed to determine when the temperature in the building&#39;s hot water pipes has dropped below an desired temperature (e.g., when a hot water draw has not occurred for an extended period of time). In this case, the bypass valve  191  is turned to permit the circulating pump  190  to circulate the water in the building&#39;s hot water pipes into the inlet pipe  126 , which forces hot water out the outlet pipe  130  and into pipe  168   b . The hot water replaces the water in the building&#39;s hot water pipes. The piping system also includes check valves CV and an adjustable gate valve GV (which may be replaced with a fixed valve). The gate valve GV may be used to control the flow rate of recirculated water into and out of the water tank  18 . 
   Referring again to  FIG. 3 , the base member  78  also includes a drain opening  194  that receives a drain member  198 . The drain member  198  communicates with a drain pipe  202  (shown in phantom in  FIGS. 1 ,  2 , and  4 ) in the building, and the drain pipe  202  communicates with the building&#39;s sewage system. The drain opening  194  is the only opening in the base member  78  through which water is permitted to flow in the event of a water leak within the cabinet  10 . Thus, any water flowing freely within the cabinet  10  drains through the drain opening  194  and is routed to the building&#39;s sewage. The tank assembly  14  also includes a drain valve  204  ( FIG. 5 ) that permits the tank  18  to be drained. The valve  204  is preferably positioned over the drain opening  194 . 
   As seen in  FIGS. 4 and 8 , the water heater also includes a temperature and pressure valve  206 , which opens in the event the temperature of the water in the tank  18  becomes too high, or if unacceptable pressure levels are present within the tank  18 . A hose or pipe  210  extends down from the temperature and pressure valve  206 , and terminates above the drain opening  194 . Water is drained from the top of the tank  18  and is fed into the building&#39;s sewage system in the event of an overtemperature or overpressure condition in the tank  18 . 
   Referring again to  FIGS. 7 and 8 , the water tank assembly  14  also includes a pair of support rails  214  extending transverse to the longitudinal axis  22  of the tank  18 , and a pair of wedge-shaped supports  218  welded or otherwise affixed to each support rail  214 . Tank mounting brackets  222  are attached (e.g., welded) to the tank  18 , and a thermally insulated spacer  226  is interposed between the tank mounting brackets  222  and the wedge-shaped supports  218 . Suitable fasteners couple the supports  218  to the brackets  222 . The tank  18  is surrounded with insulation  230  to reduce heat loss from the tank  18  to the ambient air. The head  26  includes an access opening  234  for cleaning the tank  18  and for applying a glass coating to the inside of the tank  18  during manufacture. An access cover  238  is mounted over the opening  234  and a gasket  242  is employed to prevent leakage of water through the access opening  234 . A thermostat and/or an anode tube are mounted in spuds  246  in the access cover  238 . 
   An alternative construction of the tank assembly  14  is illustrated in  FIGS. 12 and 13 . Here the flue plane FP is substantially vertical, and alternative inlet and outlet tubes  254 ,  258 , respectively, are positioned below and above, respectively, the flue tube  30 . A 90° elbow  262  is provided on both the inlet and outlet tube  254 ,  258  so that the tubes communicate with the building&#39;s pipes through the cylindrical shell  24  of the tank  18  and through the insulation  230 , rather than through the head  26 . 
   A water heater according to another embodiment of the present invention is illustrated in  FIGS. 14-18 . Reference numbers used with respect to the embodiments illustrated in  FIGS. 1-13  are also used in  FIGS. 14-18  to indicate like components. 
   As shown in  FIG. 16 , the water heater includes local and remote controllers  99 ,  100  respectively. The local controller  99  may be mounted adjacent to the tank  18  within the cabinet  10 , and the remote controller  100  may be mounted in a convenient location within the building. The local controller  99  and the remote controller  100  are electrically connected through a cable  101  that extends through an aperture  103  in the base member  78 . The cable  101  is water-tightly sealed within the aperture  103  to deter any water within the cabinet  10  from escaping through the aperture  103 . The local and remote controllers  99 ,  100  each include a digital display of the water temperature within the tank  18 , and permit adjustment of the temperature. Thus, the water heater may be monitored and controlled from both the rooftop and from within the building. This arrangement creates convenience for both the building owner or occupant within the building and a service technician on the rooftop. 
   The cabinet panels  50 ,  54 ,  58 ,  62 ,  74  are joined together around the tank assembly  14  in a weather-tight fashion. As discussed above, “weather-tight” means that rain or other precipitation falling on the cabinet  10  from above are substantially prevented from entering the cabinet  10  and interfering with the operation of the water heater. For example, one of the side panels  50  includes an air inlet vent  102  that opens downwardly to cause precipitation to run off the cabinet  10 , and one of the side panels  50  includes a sidewardly-opening exhaust vent  278 . In addition to being weather-tight, the edges of the cabinet panels  50 ,  54 ,  58 ,  62 ,  74  are preferably substantially water-tightly joined to each other (e.g., by welding or with gasket material) to resist or prevent water from entering the cabinet and interfering with the operation of the water heater. 
   Turning now to  FIG. 17 , in this embodiment a T-shaped exhaust duct  280  is connected to the exhaust end  38  of the flue tube. The T-shaped exhaust duct  280  includes a first end  282  to deliver spent products of combustion in one direction through one of the side panels  50  and a second end  284  that delivers spent products of combustion in the opposite direction through the other side panel  54 . The ends  282 ,  284  of the T-shaped duct  280  are connected to each other with a slip-fit connection that includes a sealing member to resist the escape of products of combustion through the slip-fit connection. The ends  282 ,  284  of the T-shaped duct  280  can be connected to exhaust vents  278  to deliver products of combustion out both side panels  50 ,  54  of the cabinet  10  to the surrounding atmosphere. Otherwise, one of the ends  282 ,  284  of the T-shaped exhaust duct  280  can be connected to a blocking plate  286  to prevent the products of combustion from exiting that side of the cabinet  10  and to thereby force all of the products of combustion out of the opposite side of the cabinet  10  through the opposite end  284 ,  282  of the T-shaped duct  280 . The blocking plate  286  can be used when it is preferable to direct exhaust gases to one side of the cabinet  10 , for example when one side of the cabinet  10  is open to the atmosphere and the other side of the cabinet  10  is obstructed. Also, a riser  288  (shown in phantom lines in  FIG. 16 ) can be connected to the ends  282 ,  284  of the T-shaped exhaust duct  280  to elevate the exhaust over top of the cabinet  10 . 
   As can be seen in  FIGS. 16 and 17 , the base member  78  includes a rectangular aperture  290 . A manifold  292  is sealingly connected to the base member  78  to cover the aperture  290 . The manifold  292  includes an adapter plate  294  and a gasket  296  interposed between the adapter plate  294  and the base member  78  to create a substantially water tight seal. The manifold  292  also includes five pipe unions  298 , each of which includes a first attachment end  300  within the cabinet  10  on one side of the adapter plate  294  and a second attachment end  302  outside of the cabinet  10  on the opposite side of the adapter plate  294 . In one embodiment, each attachment end  300 ,  302  includes a threaded coupling. Five water pipes  304   a ,  304   b ,  304   c ,  304   d ,  304   e  (collectively referred to as the piping system  304 ) connect between the tank  18  and the first ends  300   a ,  300   b ,  300   c ,  300   d ,  300   e  of the pipe unions  298 . Five water pipes  168   a ,  168   b ,  168   c ,  168   d ,  168   e  (collectively referred to as the building&#39;s plumbing system  168 ) extend up from the building and through the roof curb  42  to connect to the second attachment ends  302   a ,  302   b ,  302   c ,  302   d ,  302   e  of the pipe unions  298 . 
   The adapter plate  294  is rigidly mounted to base member  78  and lends structural stability to the manifold  292 . The rigid connection allows each of the water pipes  168 ,  304  to be connected with and disconnected from its respective end  300 ,  302  of the pipe union  298  regardless of whether or not a pipe  168 ,  304  is connected or disconnected to the opposite end  300 ,  302  of that specific pipe union  298 . The freedom to independently connect and disconnect the water pipe  168 ,  304  from their respective pipe union  298  simplifies the service of the water heater by allowing easier removal and replacement of the water pipes  168 ,  304 . 
     FIG. 18  illustrates the piping system  304  associated with the tank assembly  14 . The piping system  304  includes a bleed vent  305  at the high point of the piping system  304 . The bleed vent  305  allows air to bleed from the piping system  304  during startup. A cold water supply pipe  168   a  is connected to one end  302   a  of a first pipe union  298   a , and a cold water inlet pipe  304   a  is connected between the other end  300   a  of the first pipe union  298   a  and the inlet tube  126 . In this manner, the cold water inlet pipe  304   a  provides cold water to the water inlet tube  126 . 
   A hot water pipe  168   b  of the plumbing system is connected is connected to one end  302   b  of a second pipe union  298   b , and a hot water outlet pipe  304   b  is connected between the other end  300   b  of the second pipe union  298   b  and the outlet tube  130  such that the hot water outlet pipe  304   b  removes hot water from the tank  18  through the outlet tube  130 . This hot water is used for dishwashers and other applications requiring very hot water (e.g., between about 120° F. and about 150° F., or a higher temperature if necessary). 
   A mixed water pipe  168   c  of the plumbing system is connected to one end  302   c  of a third pipe union  298   c , and a mixed water outlet pipe  304   c  is connected between the other end  300   c  of the third pipe union  298   c  and a mixing valve  186  such that the mixed water outlet pipe  304   c  delivers a mixture of hot water and cold water through the mixed water pipe  168   c  to the warm water faucets of the building&#39;s bathroom and kitchen sinks. 
   A first circulation pump  306  causes hot water to be drawn off the top of the water tank  18  through the outlet tube  130  and hot water outlet pipe  304   b  and recirculated through the cold inlet tube  126  to even out the temperature of the water in the tank  18  and reduce the effects of stacking. The first circulation pump  306  is turned on by the controller  99 ,  100  each time the combustion blower  110  is turned on, and remains active for a set period of time (e.g., about nine minutes) after the combustion blower  110  is turned off. The first circulation pump  306  routes water from the outlet tube  130 , through the first circulation pump  306 , and back into the tank  18  through the inlet tube  126 . The piping system  304  also includes a flow control valve FC that may be used to control the flow rate of the water recirculated by the first circulation pump  306  into and out of the water tank  18 . The flow control valve FC preserves a minimal amount of stacking effects within the tank  18  to maintain the most heated water near the top of the tank  18  for a hot water draw. 
   Having a dedicated pump  306  within the cabinet  10  allows recirculation of the tank  18  without recirculating the water throughout the entire plumbing system  168  of the building. Because the first circulation pump  306  does not use the building&#39;s plumbing system  168  to recirculate the water within the tank  18 , operation of the first circulation pump  306  is not affected by the opening and closing of a recirculation valve in the building&#39;s plumbing system  168  during service of the building&#39;s plumbing system  168 . 
   A hot water return pipe  168   d  communicates between the building&#39;s hot water pipes  168   b  and one end  302   d  of a fourth pipe union  298   d . A hot water return pipe  304   d  of the piping system is connected between the other end  300   d  of the fourth pipe union  298   d  and a second circulation pump  308 . The second circulation pump  308  maintains a supply of hot water in the hot water pipes  168   b  of the building. A thermostat or temperature sensor  192   d  may be employed to determine when the temperature in the building&#39;s hot water pipes  168   b  has dropped below a desired temperature (e.g., when a hot water draw has not occurred for an extended period of time). In this case, the second circulation pump  308  is activated to circulate the water in the building&#39;s hot water pipes  168   b , into the hot water return pipe  304   d  of the piping system  304  and into the inlet tube  126 , which forces hot water out the outlet tube  130 , into the hot water outlet pipe  304   b  and into the building&#39;s hot water pipes  168   b . The hot water replaces the water in the building&#39;s hot water pipes  168   b . The flow control valve FC may be used to control the flow rate of the water recirculated by the second circulation pump  308  into and out of the water tank  18 . In one embodiment, the flow control valve FC allows the water to flow at approximately four gallons per minute. 
   A mixed water return pipe  168   e  communicates between the building&#39;s mixed water pipes  168   c  and one end  302   e  of a fifth pipe union  298   e . A mixed water return pipe  304   e  of the piping system  304  is connected between the other end  300   e  of the fifth pipe union  298   e  and a third circulation pump  310 . The third circulation pump  310  maintains a supply of mixed water in the mixed water pipes  168   c  of the building. A thermostat or temperature sensor  192   e  may be employed to determine when the temperature in the building&#39;s mixed water pipes  168   c  has dropped below a desired temperature (e.g., when a mixed water draw has not occurred for an extended period of time). In this case, the third circulation pump  310  is activated to circulate the water in the building&#39;s mixed water pipes  168   c  partially into the mixing valve  186  and partially into the inlet tube  126 , which forces an equal amount of hot water out the outlet tube  130  and into the mixing valve  186 . From the mixing valve  186 , mixed hot and returned warm water are distributed to the mixed water pipes  168   c  to replace the water in the building&#39;s mixed water pipes  168   c.    
   Each of the circulation pumps  306 ,  308 ,  310  includes a one-way check valve  312  that ensures unidirectional flow when each of the circulation pumps  306 ,  308 ,  310  are activated. Because they are in the pumps  306 ,  308 ,  310 , there is no need for external check valves. 
   The cabinet  10  and piping system  304  within the cabinet  10  are preferably mounted outside of a building, and, as such, are exposed to the temperature of the outside atmosphere. In environments where the temperature reaches below freezing, the water within the piping system  304  could potentially freeze, cracking the pipes of the piping system  304 . In order to minimize the risk of water freezing within the piping system  304 , each of the circulation pumps  306 ,  308 ,  310  can be activated to replace the near-freezing water with warmer water. Additional thermostats  192 , such as thermostat  192   a  can be used to measure when the temperature of the water in the piping system  304  has dropped below a safe temperature. In this case, one or all of the pumps  306 ,  308 ,  310  can be activated by the controller  99 ,  100  to increase the temperature of the water within the water within the piping system  304 . 
   The invention provides a self-contained water heating system conveniently assembled within a cabinet  10  allowing the water heater to be easily assembled to the plumbing system  168  of a building. Specifically, there is no need for any pumps, sensors, or valves external to the cabinet  10  because the water heater includes all of these components already assembled for operation within the cabinet  10 . Additionally, the controls  99 ,  100  of the water heater are pre-wired allowing the water heater to be completely ready for operation as soon as the building&#39;s plumbing system  168  has been connected to the manifold  292  and the gas supply is connected to the burner. Another advantage is that the water heater includes modular parts (e.g., tank, burner, manifold, pumps, piping system) that allow for removal and replacement without disconnecting the building&#39;s plumbing system  168  from the manifold  292 . 
   An electric water heater according to another embodiment of the present invention is illustrated in FIG.  19 . Reference numbers used with respect to the embodiments illustrated in  FIGS. 1-18  are also used in  FIG. 19  to indicate like components. The electric water heater is substantially similar to the water heater illustrated in  FIGS. 14-18  except that the electric water heater is heated electrically. Because the electric water heater is heated electrically, the electric water heater does not include a fuel gas burner  114  and other components associated with the burner  114  such as a blower  110 , an air inlet vent  102 , an air inlet duct  112 , a flue tube  30 , an exhaust duct  280 , and exhaust vents  278 . Rather, the electric water heater includes an electrical heating control  314  electrically connected to four electrical heaters  316 . Alternatively, less than four heaters  316  may be used, and the unused sockets may be plugged. Each electrical heater  316  includes multiple U-shaped heating elements  318 . The electrical heaters are located near the bottom of the tank  18  and are substantially equally spaced from each other.