WO2008130400A1

WO2008130400A1 - High efficiency compact water heater and method of operation

Info

Publication number: WO2008130400A1
Application number: PCT/US2007/066838
Authority: WO
Inventors: Marc W. Akkala; Kevin M. Field; Dennis R. Hughes; Brian Thomas Branecky; Charles W. Adams
Original assignee: Aos Holding Company
Priority date: 2007-04-18
Filing date: 2007-04-18
Publication date: 2008-10-30

Abstract

A water heater includes characteristics of both an instantaneous water heater and a storage-type water heater. The water heater comprises a tank with a cold water inlet and a hot water outlet. The tank has a capacity greater than two gallons and less than twenty gallons. The water heater also comprises a burner for causing products of combustion, and a heat exchange conduit which extends at least partially through the tank and through which the products of combustion flow.

Description

HIGH EFFICIENCY COMPACT WATER HEATER AND METHOD OF OPERATION

BACKGROUND

[0001] The present invention relates to water heaters.

SUMMARY

[0002] In one embodiment, the invention provides a water heater having characteristics of both an instantaneous water heater and a storage-type water heater. The water heater comprising a tank having a cold water inlet and a hot water outlet, the tank having a capacity greater than two gallons and less than twenty gallons. The water heater also comprising a burner for causing products of combustion, and a heat exchange conduit which extends at least partially through the tank and through which the products of combustion flow.

[0003] In another embodiment, the invention provides a water heater comprising a tank having opposite first and second end walls, a cold water inlet and a hot water outlet, the second end wall of the tank having therein a plurality of exhaust outlet openings. The water heater also comprising a combustion blower having an outlet, a combustion chamber tube inside the tank, the combustion chamber tube having an exterior surface and an inlet end communicating with the outlet of the blower, the combustion chamber tube also having an outlet end, and a burner at least partially inside the combustion chamber tube for combusting fuel mixed with air in the combustion chamber tube. The water heater also comprising a first plenum having an inlet opening communicating with the outlet end of the combustion chamber tube, and the first plenum having a plurality of outlet openings facing away from the first end wall of the tank. The water heater also comprising a plurality of flue tubes inside the tank, each of the flue tubes having an exterior surface and a first inlet end communicating with a respective one of the outlet openings in the first plenum, and each of the flue tubes having a lower outlet end communicating with a respective one of the exhaust outlet openings in the second end wall of the tank. The water heater also comprising an exhaust plenum communicating with the exhaust outlet openings in the lower end wall of the tank and thereby with the outlet ends of the flue tubes.

[0004] In another construction, the invention provides a water heater comprising a tank having a top and a bottom, a cold water inlet and a hot water outlet. The water heater also comprising a burner for causing products of combustion, the burner including at least one of an igniter and a flame sensor, a heat exchange conduit which extends at least partially through the tank and through which the products of combustion flow, and a mixing valve having a hot water inlet communicating with the tank. The mixing valve also having a cold water inlet and an outlet for mixed hot and cold water, the mixing valve mixing cold water with hot water from the tank as needed so that water flowing out of the mixing valve outlet is approximately at a set temperature. The water heater also comprising a circulation pump having an inlet communicating with the tank near one of the top and the bottom of the tank, and the pump having an outlet communicating with the tank between the top and the bottom of the tank.

[0005] In another construction, the invention provides a method of operating a water heater including a tank with a cold water inlet and a hot water outlet, a circulation pump with an inlet and outlet communicating respectively with the top and a middle section of the tank, a burner and a blower for causing products of combustion to flow through a heat exchange conduit within the tank, and a controller coupled to the circulation pump and the burner. The method comprising sensing a first temperature corresponding to the temperature of water near the top of the tank, sensing a second temperature corresponding to the temperature of water in the middle section of the tank, and comparing the first temperature to the second temperature. The method also comprising determining a temperature difference in response to comparing the first temperature and the second temperature, comparing the first temperature to at least one of a high threshold and a low threshold, and operating at least one of the burner and the blower in response to comparing the first temperature to at least one of the high threshold and low threshold.

[0006] In another construction, the invention provides a method of operating a water heater. The method comprising providing a water heater including a tank having a cold water inlet and a hot water outlet, a burner for causing products of combustion, and a heat exchange conduit which extends at least partially through the tank and through which the products of combustion flow. The method also comprising operating the water heater both as an instantaneous water heater and as a storage-type water heater.

[0007] In another construction, the invention provides a water heater operable both as a storage-type water heater and as an instantaneous water heater.

[0008] Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 is a schematic representation of a water heater according to the present invention.

[0010] Fig. 2 is a perspective view of an exemplary construction of a water heater.

[0011] Fig. 3 is a perspective view of the water heater shown in Fig. 2 with an outside cabinet in phantom.

[0012] Fig. 4 is a view similar to Fig. 3 with parts removed.

[0013] Fig. 5 is a perspective view of a water tank of the water heater.

[0014] Fig. 6 is another perspective view of the water tank shown in Fig. 5.

[0015] Fig. 7 is an elevation view of the water tank shown in Fig. 5.

[0016] Fig. 8 is a perspective view of a portion of the water tank

[0017] Fig. 9 is a perspective view of a mixing valve of the water heater.

[0018] Fig. 10 is a flow chart indicating the ignition sequence of an exemplary water heater. [0019] Fig. 11 is a schematic representation of a water heater according to another embodiment of the present invention.

DETAILED DESCRIPTION

[0020] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be 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," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings.

[0021] Figs. 1-9 illustrate one exemplary construction of a water heater 10 according to the present invention. As explained in more detail below, the water heater 10 incorporates aspects of both tank or storage type and instantaneous type water heaters. One advantage of the present invention is that the water heater 10 can substantially eliminate the effect generally known as the "cold water sandwich," in which a water heater delivers hot water with cold water pockets.

[0022] With reference to Figs. 5-7, the water heater 10 includes a substantially cylindrical tank 15 for the storage of water. In the construction shown, the water tank 15 has a capacity of about 9.5 gallons, although the preferred range is between about 2 gallons and about 20 gallons. Capacities outside that range are, however, within the scope of the invention. Though not shown, the water tank 15 can be enclosed in a layer of insulation to help prevent heat loss. The "capacity" of the tank 15 is the volume available for water, taking into consideration other elements (described below) of the water heater within the tank.

[0023] The water tank 15 is generally aligned with a vertical longitudinal axis Ac and includes a cylindrical outer wall or shell 20, an upper end wall or top head 25, and a lower end wall or bottom head 30. The outer wall 20 includes a cold water inlet or spud 35 located near the lower end wall 30, a hot water outlet or spud 40 near the upper end wall 25, a circulation inlet or spud 45, and a circulation outlet or spud 50. In the illustrated construction, the inlet spud 45 is relatively close to the lower wall 30 and the outlet spud 40 is relatively close to the upper wall 25. In other constructions, the vertical location of the inlet spud 45 can vary, as is discussed below. The outer wall 20 also includes a first probe spud 55 relatively near the lower wall 30, and a second probe spud 60 relatively near the upper wall 25. The upper wall 25 includes a centrally located burner opening 65 and the lower wall includes a number of exhaust outlet openings 70 (shown in Fig. 8).

[0024] With reference to Fig. 3, the water heater 10 also includes a combustion blower 75 outside of the water tank 15. The combustion blower 75 includes an upwardly-facing outlet 80 coupled to a U-shaped tube 85. In the construction shown, the U-shaped tube 85 includes an inlet 90 that fluidly communicates with the blower outlet 80, and an outlet 95 that fluidly communicates with the burner opening 65 of the upper wall 25. The U-shaped tube 85 is defined by a first vertical portion 100 and a second vertical portion 105 coupled to the combustion blower 75 and to the water tank 15, respectively, and a substantially horizontal portion 110 connecting the first and second vertical portions 100, 105. The tube 85 is coupled to the upper wall 25 of the water tank 15 with a coupling system including manifold flange 115, a burner flange 120, and a flue ring 125 (also shown in Fig. 7). It is to be understood that the tube 85 can include shapes other than a U-shape.

[0025] With reference to Fig. 7, the water heater 10 also includes a combustion chamber tube 130 inside the water tank 15 and fluidly connected to the U-shaped tube 85 and thereby to the combustion blower 75 (shown in Fig. 3). In the construction shown, the combustion chamber tube 130 is centered with respect to the axis Ac, similar to the tank 15. The combustion chamber tube 130 includes an outer surface 132, and an upper inlet end 135 communicating with the burner opening 65 of the upper wall 25, and therefore with the tube outlet 95. The combustion chamber tube 130 also includes a lower outlet end 140.

[0026] Supported within the combustion chamber tube 130, the water heater 10 includes a burner 145 for combusting fuel mixed with air in the combustion chamber tube 130. Among other elements, the burner 145 can include an igniter and a flame sensor (not shown) to help control the burner 145. In some constructions, the burner 145 can be a pre-mix burner configured to accept a combustable mixture of fuel and air from the combustion blower 75 and directed to the combustion chamber tube 130 by the U-shaped tube 85. In the illustrated construction, the burner 145 has an output of about 199,000 BTUH (British Thermal Unit per Hour). The output is preferably lower than about 500,000 BTUH, and is more preferably between about 75,000 BTUH and about 200,000 BTUH.

[0027] With reference to Figs. 7-8, the water heater 10 also includes an upper plenum 150. In the construction shown, the plenum 150 is centered with respect to the axis Ac. The upper plenum 150 includes an exterior surface 155, an upwardly facing inlet opening 160 and a number of downwardly facing outlet openings 165. The inlet opening 160 is in fluid communication with the lower outlet end 140 of the combustion chamber tube 130 to receive gasses generated by the burner 145. The outlet openings 165 are arranged such that each of the outlet openings 165 is aligned with a respective one of the exhaust outlet openings 70 in the bottom of the water tank 15. In the illustrated construction, there are fourteen openings 70 and fourteen openings 165.

[0028] The water heater 10 also includes a number of flue tubes 170 inside the water tank 15. Each of the flue tubes 170 includes an outer surface 175, an upper inlet end 180, and a lower outlet end 185. The upper inlet end 180 of each flue tube 170 is coupled to one corresponding outlet opening 165 of the plenum 150, such that the flue tube 170 is in fluid communication with the plenum 150 and with the combustion chamber tube 130. The lower outlet end 185 of each flue tube 170 is coupled to one corresponding exhaust outlet opening 70 of the water tank 15. If desired, each of the flue tubes 170 can have therein at least one baffle to increase heat transfer. As shown in Fig. 7, the outer surface 132 of the combustion chamber tube 130, the exterior surface 155 of the upper plenum 150, and the outer surfaces 175 of the flue tubes 170 define in combination a heat exchanging surface that contacts the water within the water tank 15. The heat exchanging surface may vary based on the construction of the combustion chamber tube 130, the upper plenum 150, and each of the flue tubes 170. For example, in the illustrated construction, the heat exchanging surface can be about 1,367 square inches. However, other constructions can include a different surface area defining the heat exchanging surface. For example, the heat exchanging surface can be between about 1,200 square inches and about 1,500 square inches. Other areas are within the scope of the invention, and the area will vary with the volume of the tank. Also, the number of flue tubes can vary.

[0029] With reference to Figs. 5-7, the water heater 10 also includes an exhaust plenum 190 below the water tank 15. The exhaust plenum 190 includes substantially horizontal bottom and top walls 195 and 200 and a side wall 205. The top wall 200 has therein a generally circular opening (not shown) covered by the bottom end wall of the tank, so that the exhaust plenum 190 communicates with the lower outlet ends 185 of the flue tubes 170. A portion of the exhaust plenum 190 extends outwardly from beneath the tank 15 and includes an upwardly facing outlet 210 (also shown in Fig. 8). An exhaust conduit or vent 215 has a lower end 220 coupled to the outlet 210. The conduit 215 also has an upper end 225. The conduit 215 extends generally vertically.

[0030] With respect to Figs. 4 and 9, the water heater 10 can also include a mixing valve 230 that includes a hot water inlet 235, a cold water inlet 240, and a mixing valve outlet 245. The outlet 245 communicates with an outlet pipe 249. The hot water inlet 235 is in fluid communication with the hot water outlet 40 of the tank 15 through a hot water conduit 247. The cold water inlet 240 is in fluid communication with a main cold water conduit 250 through a cold water pipe 255. The cold water conduit 250 communicates with a cold water source (not shown). The cold water conduit 250 is also connected to the cold water inlet 35 of the tank 15 through a cold water pipe 260. The mixing valve 230, as in known in the art, mixes cold water and hot water to provide water at a desired temperature through the mixing valve outlet 245. In some constructions, the mixing valve 230 is configured to be mechanically adjusted to adjust the output temperature. In another construction shown in Fig. 9, the water heater 10 can include a motor 262 connected to the mixing valve 230, and a remote control 264 for operating the motor to allow a user to adjust the temperature of the mixed water.

[0031] As shown in Figs. 3-4, the water heater 10 also includes a circulation pump 265 with an inlet 270 and an outlet 275. A lower circulation conduit 280 couples the outlet 275 to the circulation inlet 45 of the tank 15, and an upper circulation conduit 285 couples the inlet 270 to the circulation outlet 50 of the tank 15.

[0032] With reference to Figs. 2-4, the water heater 10 also includes a cabinet or jacket 290 for containing and supporting the tank 15, the blower 75, the mixing valve 235, the pump 265, the exhaust conduit 215, and other elements of the water heater 10 such as a controller 400 (shown schematically in Fig. 1). The cabinet 290 includes a generally horizontal top wall 295. The top wall 295 includes a first opening 315 through which the upper end 225 of the exhaust conduit 215 extends. The top wall also includes openings for the upper portions of the cold water conduit 250 and the mixed water outlet pipe 249. The cabinet 290 may also include other elements such as a user interface or a control panel.

[0033] Fig. 1 illustrates a schematic representation of the water heater 10. As shown in Fig. 1, the controller 400 is coupled to a first or lower temperature sensor 405, and to a second or upper temperature sensor 415. The lower temperature sensor 405 is configured to generate a signal indicative of the water temperature relatively near the bottom of the tank 15. The upper temperature sensor 415 is configured to generate a signal indicative of the water temperature relatively near the top of the tank 15. Though not shown, other sensors or switches may be coupled to the controller to control the operation of the water heater 10. The controller 400 is also connected to the pump 265, to the combustion blower 75, and to the igniter and/or flame sensor of the burner 145. [0034] Fig. 11 illustrates a schematic representation of a water heater 1000 similar to the water heater 10. In general, the water heater 1000 includes the same elements as the water heater 10, thus elements are numbered the same for ease of description. The water heater 1000, however, has a different configuration of the heat exchange conduit defined by the burner 145, the combustion chamber tube 130, the plenum 150, and flue tubes 170. In the construction shown in Fig. 11, the burner 145 is centrally located near the bottom of the water tank 15. The combustion chamber tube 130 supports the burner 145 and can receive a fuel and air mixture from a tube (similar to tube 85) extending through a center section of the exhaust plenum 190 and through the lower wall 30.

[0035] The combustion chamber tube 130 extends upwardly to the upper plenum 150, which is connected to a number of flue tubes 170. Like in the water heater 10, the flue tubes 170 connect to the lower wall 30 and to the exhaust plenum 190. In the particular construction shown in Fig. 11, the heat exchange conduit defined by the combustion chamber tube 130, the upper plenum 150, and the flue tubes 170 partially extends through the water tank 15.

[0036] In Figs. 1 and 11, arrow 447 indicates the flow of cold water from a cold water source (not shown). Arrows 440 and 445 respectively indicate the flow of hot water and the flow of cold water to the mixing valve 230. Arrow 450 indicates the flow of mixed water exiting the mixing valve 230. Arrows 455 indicate the flow of water from the top of the tank and through the circulation pump 265 to a lower point in the water tank 15. The location of this point is further discussed below. Arrows 470 indicate the flow of flue gas generated within the combustion chamber tube 130, circulating through the upper plenum 150, the flue tubes 170, the exhaust plenum 190, and exiting the water heater 10 through the exhaust conduit 215.

[0037] Still with reference to Figs. 1 and 11, the upper temperature sensor 415 can be positioned about 1.5 inches below the upper wall 25, and the lower temperature sensor 405 can be positioned about 6.5 inches above the lower wall 30 of the tank 15. It is possible to have an additional high limit temperature safety sensor near the top of the tank 15 and electrically coupled to the controller 400. [0038] Temperature control of the water heater 10, 1000 is accomplished by setting two temperature trigger points such as a high temperature threshold and a low temperature threshold. For example, if the water temperature detected by the upper temperature sensor 415 is less than the low temperature threshold, the controller 400 commands the burner 145 to turn on. Additionally, if the water temperature detected by the upper temperature sensor 415 is greater than the high temperature threshold, the controller 400 can command the burner 145 to turn off.

[0039] The controller 400 can include instructions to operate the water heater 10, 1000 to heat water (turn on the burner 145) and circulate water (turn on the circulation pump 265) as water is being extracted from the tank 15 through the mixing valve 230. In one example, if the temperature detected by the lower temperature sensor 405 is less than the low temperature threshold, the controller 400 can command the circulation pump 265 to move water from the top of the tank 15 to a lower point in the tank, the lower point being determined by the location of the spud 45. As mentioned above, in the illustrated construction the spud 45 is near the bottom of the tank. It may be preferable, however, to have the spud 45 at a higher point, such as near the middle of the tank or even higher. It has been found that the flow of water into the tank at a higher location may reduce extreme temperatures near the plenum 150. The optimal location will depend on the characteristics of a particular water heater. In any event, water at a higher temperature is moved from near the top of the water tank 15 to a lower point in the tank by action of the circulation pump 265. As a result, the temperature detected by the upper temperature sensor 415 drops and the controller 400 can subsequently turn on the burner 145 to heat incoming cold water. Turning on the burner 415 causes water to be heated prior to depleting the water tank 15 of hot water.

[0040] The temperatures detected by the temperature sensors 405 and 415 are used by the controller 400 to control the thermal stratification in the tank 15 via operation of the recirculation pump 265 and the burner output rate. The burner output rate can be controlled by operating of the blower 75. In some constructions, the blower 75 has an ignition speed mode, a low speed mode and a high speed mode, which can respectively result in at least three different output rates of the burner 145. For example, the low speed mode can be set to yield about 107,000 BTUH output while the high speed mode can be set to yield about 199,000 BTUH output. Generally, the ignition speed mode can be used specifically during an ignition sequence, and the low speed mode can be used to control heating of the water in the tank 15 while water temperature reaches a point near the high temperature threshold.

[0041] For example, Fig. 10 illustrates the ignition and shut-down sequence of the water heater 10, 1000 according to one embodiment of the present invention. The water heater 10, 1000 can be initiated manually by a user or automatically in response to the upper temperature sensor 415 detecting a temperature lower than the high temperature threshold (step 500). The controller 400 turns on the blower 75 to the high speed mode for about three seconds (step 505) to clear the water heater 10, 1000 of vapors or fuel mixture in the air-flow passages (e.g., U-shaped tube 85, combustion chamber tube 130, and flue tubes 170). In some constructions, the controller 400 can monitor the operation of the blower 75 with a pressure sensor (not shown) placed near the blower outlet 80. To ignite the burner 145, the blower 75 is set to the ignition speed mode (step 510) and power is supplied to an igniter (not shown) for a period of about two seconds (step 515). The controller 400 opens a gas valve (not shown) and continues supplying power to the igniter for about three seconds (step 520). Subsequently, the controller 400 turns off the igniter (step 525) and determines whether a flame is generated by the burner 145 (step 535) based on a reading from the flame sensor, for example.

[0042] If the controller 400 determines that the burner 145 is not producing a flame, the controller 400 closes the gas valve (step 540) and proceeds to clear the air-flow passages as described in step 505. If the controller 400 determines the burner 145 is producing a flame, the controller 400 switches the blower 75 to the high speed mode (step 545) and proceeds to sense the temperature from temperature sensors 405 and 415 (step 550). As a result of detecting temperature, the controller 400 determines whether the temperature detected by the upper temperature sensor 415 (T_TANK) is between about the low temperature threshold (T_LOw) and the high temperature threshold (T_HIGH) (step 555). If the temperature detected is not in the range defined by

hence the temperature is lower than the low temperature threshold, the controller 400 continues to operate the blower 415 in the high speed mode as described in step 545.

[0043] If the temperature detected by the upper temperature sensor 415 is determined to be in the range defined by T_IOW^TANK^HI_GH, the controller 400 switches the blower 75 to the low speed mode (step 560). The controller 400 continues to monitor temperature to determine whether the temperature detected by the upper temperature sensor 415 is higher than the high temperature threshold, lower than the low temperature threshold, or between the high and low temperature thresholds (step 565). If the temperature is between the high and low temperature thresholds, the controller 400 continues to operate the blower 75 in the low speed mode as described in step 560. If the controller 400 determines that the temperature detected by the upper temperature sensor 415 is lower than the low temperature threshold, the controller 400 switches the blower 75 to the high speed mode as described in step 545. If the controller 400 determines that the temperature detected by the upper temperature sensor 415 is higher than the high temperature threshold, the controller 400 closes the gas valve (step 570) and proceeds to clear the air-flow passages of residual gasses produced by the burner 145 (step 575). The controller 400 finally turns off the blower (step 580) and continues to monitor the temperature of the water in the tank 15 (step 585).

[0044] Subsequent to starting operation of the water heater 10, 1000, either automatically or by actuation of a switch, the controller 400 can include instructions to operate the water heater 10, 1000 and maintain water in the tank 15 under predetermined conditions. For example, one instruction can include turning on the circulation pump 265 when the burner 145 is on. The controller 400 can also turn on the circulation pump 265 if the temperature detected by the lower temperature sensor 405 is less than the low temperature threshold. Generally, the controller 400 operates the water heater 10, 1000 to maintain a substantially constant water temperature in the tank 15. To reduce stratification (temperature difference between to top and bottom portions of the tank 15), the controller 400 can be programmed with an upper difference threshold temperature and a lower difference threshold temperature. [0045] For example, the controller 400 can turn on the circulation pump 265 when the temperature difference between the top and bottom portions of the tank 15 is greater than the upper difference threshold. Another command includes turning off the circulation pump 265 when the temperature difference between the top and bottom portions of the water tank 15 is lower than the lower difference threshold. In one construction, the upper difference threshold and the lower difference threshold can be 20° F and 10° F, respectively. The controller 400 can also include safety commands. For example, the controller 400 can turn off the burner 145 and turn on the circulation pump 265 when the temperature detected by the upper temperature sensor 415 is greater than a safety temperature threshold (e.g., 185° F). The controller 400 can also turn off the circulation pump 265 when the temperature detected by the upper temperature sensor 415 drops below another safety threshold temperature (e.g., 185° F). It is envisioned that the water tank 10 can also include elements, such as a high limit sensor and a temperature/pressure relief valve, associated with the controller 400 to operate the water heater 10, 1000.

[0046] Various features and advantages of the invention are set forth in the following claims.

Claims

CLAIMSWhat is claimed is:

1. A water heater having characteristics of both an instantaneous water heater and a storage-type water heater, the water heater comprising a tank having a cold water inlet and a hot water outlet, the tank having a capacity greater than two gallons and less than twenty gallons, a burner for causing products of combustion, and a heat exchange conduit which extends at least partially through the tank and through which the products of combustion flow.

2. A water heater as set forth in claim 1 and further comprising a mixing valve having a hot water inlet communicating with the tank, the mixing valve also having a cold water inlet and an outlet for mixed hot and cold water, the mixing valve mixing cold water with hot water from the tank as needed so that water flowing out of the mixing valve outlet is approximately at a set temperature.

3. A water heater as set forth in claim 2 wherein the set temperature is at or below 180 degrees Fahrenheit.

4. A water heater as set forth in claim 2 wherein the mixing valve is coupled to a motor operable to adjust the mixing valve to provide water at the set temperature.

5. A water heater as set forth in claim 4 further comprising a remote control operable to control the motor allowing a user to adjust the set temperature.

6. A water heater as set forth in claim 2 wherein the tank has a top and a bottom, and further comprising a circulation pump having an inlet communicating with the tank near one of the top and the bottom of the tank, and the pump having an outlet communicating with the tank between the top and the bottom of the tank.

7. A water heater as set forth in claim 1 wherein the tank has a top and a bottom, and further comprising a circulation pump having an inlet communicating with the tank near one of the top and the bottom of the tank, and the pump having an outlet communicating with the tank between the top and the bottom of the tank.

8. A water heater as set forth in claim 1 wherein the burner has an output of greater than 75,000 BTUH and less than 500,000 BTUH.

9. A water heater as set forth in claim 8 wherein the burner has an output of greater than 75,000 BTUH and less than 200,000 BTUH.

10. A water heater as set forth in claim 9 wherein the burner has an output of approximately 199,000 BTUH.

11. A water heater as set forth in claim 1 wherein the burner is a pre-mix burner that combusts a fuel and air mixture.

12. A water heater as set forth in claim 11 wherein the burner has an output of approximately 199,000 BTUH.

13. A water heater as set forth in claim 1 wherein the tank has a capacity of less than ten gallons of water.

14. A water heater as set forth in claim 13 wherein the tank has a capacity of approximately 9.5 gallons of water.

15. A water heater as set forth in claim 1 wherein the heat exchange conduit has heat exchanging surfaces exposed to water in the tank, the surfaces having an area of approximately 1,367 square inches.

16. A water heater as set forth in claim 1 wherein the heat exchange conduit includes a plurality of flue tubes communicating with the burner and extending at least partially through the tank.

17. A water heater as set forth in claim 16 wherein each of the flue tubes has therein a baffle for maximizing heat transfer.

18. A water heater as set forth in claim 16 wherein the burner is placed above the flue tubes and within the water tank.

19. A water heater as set forth in claim 16 wherein the burner is placed adjacent to the flue tubes and within the water tank.

20. A water heater as set forth in claim 1 further comprising a controller, a first sensor coupled near the top of the tank, and a second sensor coupled near the bottom of the tank, the controller operable to control the burner based on an input from at least one of the first sensor and the second sensor.

21. A water heater as set forth in claim 20 further comprising a blower for supplying air and fuel to the burner, and a circulation pump having an inlet communicating with the tank near one of the top and the bottom of the tank, the pump also having an outlet communicating with the tank between the top and the bottom of the tank, the controller operable to control the blower and the circulation pump based on the input from at least one of the first sensor and the second sensor.

22. A water heater comprising a tank having opposite first and second end walls, a cold water inlet and a hot water outlet, the second end wall of the tank having therein a plurality of exhaust outlet openings, a combustion blower having an outlet, a combustion chamber tube inside the tank, the combustion chamber tube having an exterior surface and an inlet end communicating with the outlet of the blower, the combustion chamber tube also having an outlet end, a burner at least partially inside the combustion chamber tube for combusting fuel mixed with air in the combustion chamber tube, a first plenum having an inlet opening communicating with the outlet end of the combustion chamber tube, and the first plenum having a plurality of outlet openings facing away from the first end wall of the tank, a plurality of flue tubes inside the tank, each of the flue tubes having an exterior surface and a first inlet end communicating with a respective one of the outlet openings in the first plenum, and each of the flue tubes having a lower outlet end communicating with a respective one of the exhaust outlet openings in the second end wall of the tank, and an exhaust plenum communicating with the exhaust outlet openings in the lower end wall of the tank and thereby with the outlet ends of the flue tubes.

23. A water heater as set forth in claim 22 wherein the combustion chamber tube inside the tank is adjacent to the first end wall of the tank.

24. A water heater as set forth in claim 22 wherein the first plenum is inside the tank.

25. A water heater as set forth in claim 22 wherein the outlet openings in the first plenum are arranged such that each of the outlet openings in the first plenum is aligned with a respective one of the exhaust outlet openings in the second end wall of the tank.

26. A water heater as set forth in claim 22 wherein the exhaust plenum is outside the second end wall of the tank.

27. A water heater as set forth in claim 22 wherein at least one of the plurality of flue tubes includes a baffle for maximizing heat transfer.

28. A water heater as set forth in claim 22 wherein the first plenum further comprises an outer surface, such that the exterior surface of the combustion chamber tube, the outer surface of the first plenum, and the exterior surfaces of the plurality of flue tubes form a heat exchanging surface between about 1,200 square inches and about 1,500 square inches.

29. A water heater comprising a tank having a top and a bottom, a cold water inlet and a hot water outlet, a burner for causing products of combustion, the burner including at least one of an igniter and a flame sensor, a heat exchange conduit which extends at least partially through the tank and through which the products of combustion flow, a mixing valve having a hot water inlet communicating with the tank, the mixing valve also having a cold water inlet and an outlet for mixed hot and cold water, the mixing valve mixing cold water with hot water from the tank as needed so that water flowing out of the mixing valve outlet is approximately at a set temperature, and a circulation pump having an inlet communicating with the tank near one of the top and the bottom of the tank, and the pump having an outlet communicating with the tank between the top and the bottom of the tank.

30. A water heater as set forth in claim 29 further comprising a controller coupled to at least one of the burner and the circulation pump to maintain water at a predetermined temperature.

31. A water heater as set forth in claim 30 wherein the controller is coupled to at least one of the igniter and the flame sensor.

32. A water heater as set forth in claim 30 further comprising a first temperature sensor coupled to the tank near one of the top and bottom of the tank, and a second temperature sensor coupled to the tank near the other of the top and bottom of the tank.

33. A water heater as set forth in claim 32 wherein the controller is operable to control at least one of the burner and the circulation pump in response to detecting a temperature sensed by at least one of the first temperature sensor and second temperature sensor.

34. A water heater as set forth in claim 29 further comprising a blower for supplying a fuel and air mixture to the burner.

35. A water heater as set forth in claim 34 further comprising a controller coupled to at least one of the burner, the circulation pump, and the combustion blower to maintain water at a predetermined temperature.

36. A water heater as set forth in claim 35 further comprising a first temperature sensor coupled to the tank near one of the top and bottom of the tank, and a second temperature sensor coupled to the tank near the other of the top and bottom of the tank.

37. A water heater as set forth in claim 36 wherein the controller is coupled to at least one of the burner, the circulation pump, and the combustion blower in response to detecting a temperature sensed by at least one of the first temperature sensor and second temperature sensor.

38. A water heater as set forth in claim 37 wherein the controller is coupled to at least one of the igniter and the flame sensor.

39. A method of operating a water heater including a tank with a cold water inlet and a hot water outlet, a circulation pump with an inlet and outlet communicating respectively with the top and a middle section of the tank, a burner and a blower for causing products of combustion to flow through a heat exchange conduit within the tank, and a controller coupled to the circulation pump and the burner, the method comprising: sensing a first temperature corresponding to the temperature of water near the top of the tank, sensing a second temperature corresponding to the temperature of water in the middle section of the tank, comparing the first temperature to the second temperature, determining a temperature difference in response to comparing the first temperature and the second temperature, comparing the first temperature to at least one of a high threshold and a low threshold, and operating at least one of the burner and the blower in response to comparing the first temperature to at least one of the high threshold and low threshold.

40. A method as set forth in claim 39 wherein the operating step includes generating an air flow to clear the heat exchange conduit, opening a gas valve and starting an igniter to create a flame in response to the first temperature being less than low threshold.

41. A method as set forth in claim 40 wherein generating the air flow includes operating the blower in a high speed mode.

42. A method as set forth in claim 39 further comprising comparing the second temperature to at least one of the first threshold and the second threshold.

43. A method as set forth in claim 42 further comprising operating the circulation pump to move water from the top of the tank to the middle section of the tank in response to the second temperature being less than the low threshold.

44. A method as set forth in claim 39 further comprising operating the circulation pump to move water from the top of the tank to the middle section of the tank in response to the temperature difference being greater than a high difference threshold.

45. A method as set forth in claim 44 wherein the high difference threshold is about 20 degrees Fahrenheit.

46. A method as set forth in claim 39 further comprising stopping the circulation pump from moving water in response to the temperature difference being lower than a low difference threshold.

47. A method as set forth in claim 46 wherein the low difference threshold is about 10 degrees Fahrenheit.

48. A method as set forth in claim 39 further comprising comparing the first temperature to at least one of a high safety threshold and a low safety threshold.

49. A method as set forth in claim 48 further comprising turning off the burner in response to the first temperature being greater than the high safety threshold.

50. A method as set forth in claim 49 wherein the high safety threshold is about 185 degrees Fahrenheit.

51. A method as set forth in claim 48 further comprising stopping the circulation pump from moving water in response to the first temperature being lower than the low safety threshold.

52. A method as set forth in claim 51 wherein the low safety threshold is about 180 degrees Fahrenheit.

53. A method of operating a water heater, the method comprising providing a water heater including a tank having a cold water inlet and a hot water outlet, a burner for causing products of combustion, and a heat exchange conduit which extends at least partially through the tank and through which the products of combustion flow, and operating the water heater both as an instantaneous water heater and as a storage- type water heater.

54. The method of claim 53 wherein the tank has a capacity greater than two gallons and less than twenty gallons.

55. A water heater operable both as a storage-type water heater and as an instantaneous water heater.