Condensing water heater

A flue system is provided for a water heater having improved heat exchange efficiency. The flue system includes an upstream heat exchange portion providing a first pass for heat exchange with water in the water heater. The flue system further includes a downstream heat exchange portion providing a second pass for heat exchange with water in the water heater and a blower positioned between the upstream heat exchange portion and the downstream heat exchange portion. The blower is configured to urge combustion products from the upstream heat exchange portion to the downstream heat exchange portion.

FIELD OF THE INVENTION

The present invention relates to a high efficiency water heater and, more particularly, to a water heater having improved heat exchange performance.

BACKGROUND OF THE INVENTION

Commercial and residential water heaters typically heat water by generating tens of thousands, and even hundreds of thousands, of BTUs. For many years, manufacturers of water heaters have sought to increase the efficiency of the exchange of this heat energy from burned fuel to the water contained in the water heater. Accordingly, maximized heat exchange efficiency has long been an object of commercial and residential water heater manufacturers.

As heat exchange efficiency increases, however, such increased efficiency gives rise to the problems associated with condensation of water vapor from the products of combustion. More specifically, upon burning of a mixture of fuel and air, water is formed as a constituent of the products of combustion. It is recognized that as the temperatures of the combustion gases decrease as the result of successful exchange of heat from the combustion gases to water in the water heater, the water vapor within the combustion gases tends to be condensed in greater quantities. In other words, as the temperatures of the combustion gases decrease as a direct result of increasingly efficient exchange of heat energy to water, the amount of condensate forming on the heat exchange surfaces also increases.

Such condensate is typically acidic, with pH values often in the range of between about 2 to 5. The formation of increased amounts of such acidic condensate, even in relatively small quantities, can accelerate the corrosion of heat exchange tubing, increase oxidation and scale formation, reduce heat exchange efficiency and contribute to failure of the water heater.

Commercial and residential water heaters can be designed to operate below the efficiencies at which increased quantities of condensate are likely to form (i.e., below the condensing mode) so that acidic products of combustion are discharged in vapor form in higher temperature exhaust gas. To do so, however, compromises the efficiency of the water heater.

Accordingly, there continues to be a need for a water heater having improved heat exchange efficiency yet resisting the effects of water vapor condensation associated with such efficiency.

SUMMARY OF THE INVENTION

In one exemplary embodiment, this invention provides a water heater having improved heat exchange efficiency. The water heater includes a water tank and a flue system extending at least partially through an interior of the water tank and positioned to receive combustion products and to transfer heat from combustion products within the flue system to water in the water tank. The flue system includes an upstream heat exchange portion providing a first pass for heat exchange with water in the water tank. The flue system further includes a downstream heat exchange portion providing a second pass for heat exchange with water in the water tank, and a blower positioned between the upstream heat exchange portion and the downstream heat exchange portion. The blower is configured to urge the combustion products from the upstream heat exchange portion to the downstream heat exchange portion.

In another exemplary embodiment, a flue system is provided. The flue system includes an upstream heat exchange portion providing a first pass for heat exchange with water in the water heater. The flue system further includes a downstream heat exchange portion providing a second pass for heat exchange with water in the water heater and a blower positioned between the upstream heat exchange portion and the downstream heat exchange portion.

In yet another exemplary embodiment, a method of improving heat exchange efficiency of a water heater is provided. The method comprises the step of positioning a blower between an upstream heat exchange portion positioned at least partially within the water storage tank, and a downstream heat exchange portion positioned at least partially within the water storage tank. The combustion products are induced to flow from a combustion chamber of the water heater into the upstream heat exchange portion for exchanging heat between the combustion products and the water in the water storage tank. The combustion products are then delivered through a downstream heat exchange portion to exchange heat between the combustion products and the water in the water storage tank.

In still another exemplary embodiment, a water heater having improved heat exchange efficiency is provided. The water heater comprises a water tank and a flue system extending at least partially through an interior of the water tank and positioned to receive combustion products and to transfer heat from the combustion products within the flue system to water in the water tank. A blower is positioned outside of the water tank and downstream of the flue system. The blower is configured to urge the combustion products from the flue system. A thermal insulator is positioned over at least a portion of the blower for thermally insulating the blower.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary features of selected embodiments of this invention will now be described with reference to the figures. It will be appreciated that the spirit and scope of the invention is not limited to the embodiments selected for illustration. Also, it should be noted that the drawings are not rendered to any particular scale or proportion. It is contemplated that any of the exemplary configurations and materials and sizes described hereafter can be modified within the scope of this invention.

Referring generally to the figures and according to one exemplary embodiment of the invention, this invention provides a water heater15having improved heat exchange efficiency. The water heater15includes a water tank22and a flue system50extending at least partially through an interior of the water tank22and positioned to receive combustion products and to transfer heat from the combustion products within the flue system50to water in the water tank22. The flue system50includes an upstream heat exchange portion32providing a first pass for heat exchange with water in the water tank22. The flue system50further includes a downstream heat exchange portion34providing a second pass for heat exchange with water in the water tank, and a blower54positioned between the upstream heat exchange portion32and the downstream heat exchange portion34. The blower54is configured to urge the combustion products from the upstream heat exchange portion32to the downstream heat exchange portion34.

Referring now toFIGS. 1 and 2, a residential water heating system embodying exemplary aspects of this invention is generally designated by the numeral “10.” In the residential water heating system, a gas-fired water heater15is attached to a gas supply line (not shown) and an exhaust conduit20. The gas supply line supplies natural gas to the water heater15for combustion, and the exhaust conduit20provides a conduit for exhausting the products of combustion from the water heater15.

The gas-fired water heater15comprises a water tank22for containing water, an outer shell24for encapsulating the water tank22, and an annular cavity formed between the water tank22and the outer shell24. Foam insulation26and an insulation member28are provided in the annular cavity to limit the escapement of thermal energy from the water storage tank22to the surrounding environment. A top cover30is fastened to the outer shell24, thereby enclosing the top surface of the water storage tank22. The top cover30includes apertures for accommodating a flue system50, a cold water inlet port11and a hot water outlet port13.

Although not shown, the cold water inlet port11is coupled to an unheated water supply line. In practice, unheated water is introduced into the water heater15through the cold water inlet port11. An inlet diptube25is coupled to the inlet port11and positioned within the water tank22for delivering unheated water into the bottom end of the water tank22.

The outlet port13of the water heater15is coupled to a heated water supply line (not shown) for distributing heated water from the tank22. An outlet diptube17is coupled to an opposing end of the outlet port13and positioned within the water tank22. The outlet dip tube17includes a circular inlet port21for drawing heated water from the top end of the water tank22. The heated water is ultimately distributed through the heated water supply line to one or more hot water distribution points. A sacrificial anode rod19is coupled to the end of the outlet diptube17. The anode rod19is configured for limiting corrosion of the metallic water tank22.

According to this exemplary embodiment, the water heater15is gas-fired. As will be appreciated by those skilled in the art, the invention disclosed herein is not limited to gas-fired water heaters. Many of the details of this invention may also apply to any other type of heat exchanger or insulated tank. Furthermore, although reference is made to “residential” water heaters, the descriptions herein also apply to industrial, commercial or domestic water heaters as well as other heat transfer systems.

The gas-fired water heater15includes a control unit36having a gas valve and thermostat. The control unit36includes an inlet (not shown) for receiving gas from a gas supply line (not shown). A thermocouple38extending from the control unit36measures the water temperature inside the water tank22. Apertures are provided in the outer shell24and the water tank22to accommodate the thermocouple38. In operation, the control unit36compares the temperature reported by the thermocouple38with the temperature setting of the thermostat (set by the user) and adjusts the amount of gas provided to a gas burner40accordingly.

The gas burner40receives gas via a conduit42. The gas burner40is positioned in a combustion chamber44that is disposed at an elevation beneath the water storage tank22. A pilot is positioned adjacent the gas burner40within the combustion chamber44for igniting the gas. The products of combustion are carried along a flue system50that is positioned at least partially within the interior of the tank22. The combustion products are ultimately exhausted through an exhaust conduit20. Although the gas burner40and the combustion chamber44are positioned at an elevation beneath the water tank22, they may also be positioned at an elevation above the water tank22, or at any other desired elevation.

Thermal energy is generated within the combustion chamber44for distribution to the contents of the water storage tank22. The flue system50is configured to transfer the thermal energy from the products of combustion emanating from the combustion chamber44to the water contained within the tank22. Arrows inFIG. 2indicate the flow of combustion products through the heat exchange system.

Generally, the flue system50illustrated in the figures is a so-called “two pass” heat exchanger in which the combustion products make two passes through the water to be heated, thereby exchanging heat to the water in each of the two passes. In this particular embodiment, the first pass of combustion products through an upstream heat exchange portion32(also referred to as “upstream portion32”) provides for the primary heat exchange and the second pass of combustion products through a downstream heat exchange portion34(also referred to as “downstream portion34”) provides for the secondary heat exchange.

More particularly, the flue system50includes an upstream heat exchange portion32providing a first pass for heat exchange with water in the water tank22, a downstream heat exchange portion34providing a second pass for heat exchange with water in the water tank22, and an air blower54positioned between the upstream portion32and the downstream portion34. The air blower54is configured to urge the combustion products (emanating from the combustion chamber44) from the upstream portion32to the downstream portion34.

A series of baffles70are positioned along the length of the upstream and downstream portions32and34. The baffles70promote turbulence of the combustion products flowing therethrough. Increased turbulence of the combustion products produces greater heat transfer between the combustion products and the water within the water tank22. The number and arrangement of baffles70can be modified to optimize the efficiency of the water heater15.

The air blower54is configured to draw combustion products through the upstream portion32and deliver combustion products through the downstream portion34to facilitate both passes of the combustion products through the water tank22. In operation, the air blower54maintains a negative pressure (with respect to atmospheric pressure) within the upstream heat exchange portion32to urge the products of combustion from the combustion chamber44into the upstream portion32. The air blower54also maintains a positive pressure (with respect to atmospheric pressure or the pressure within the upstream heat exchange portion32) within the downstream portion34to urge the products of combustion through the downstream portion34.

The air blower54includes an inlet port52for coupling with the upstream portion32, an outlet port56for coupling with the downstream portion34, and an internal impeller (not shown) for urging the flow of combustion products from the inlet port52to the outlet port56of the air blower54. The air blower54is optionally positioned at an elevation above or coincident with the top end31of the water heater15. However, the air blower54may be positioned at any particular elevation, as shown inFIG. 6. A suitable air blower54is manufactured and distributed by the Fasco Corporation, a division of Regal Beloit of Beloit, Wis.

The flue system50is configured to limit condensation of the combustion products until the combustion products reach the downstream heat exchange portion34. Specifically, the blower54substantially reduces the formation of condensation on the surfaces of the burner40and the upstream portion32by urging the combustion products through the upstream portion32at a relatively high velocity. In the absence of a blower, condensation is more likely to collect on the surfaces of the burner40and the downstream portion32. As described in the Background section, the formation of acidic condensate, even in relatively small quantities, can accelerate the corrosion of heat exchange tubing, increase oxidation and scale formation, reduce heat exchange efficiency and contribute to failure of the water heater.

Delaying condensation of the combustion products until the combustion products reach the downstream heat exchange portion34provides for more consistent and reliable operation of the water heater15. As the combustion products travel downward through the downstream heat exchange portion34, the temperature of the combustion products continues to decrease until the temperature is equal to that of the water contained with in the storage tank22. Water vapor contained within the combustion products condenses once the temperature of the combustion products is equal to that of the dew point of the combustion products.

A number of variables may be controlled to limit the formation of condensation on the burner40and the downstream portion32, including, but not limited to: the hourly input (i.e., the rate at which fuel is combusted in units such as cubic feet per hour), the surface area of the heat exchange portions32and34, the pressure drop through the flue system50, and the speed of the air blower impeller.

In operation, condensation flows through the downstream heat exchange portion34under gravity. Accordingly, the entire length of the upstream portion34, or a significant portion thereof, is downwardly sloping to facilitate the flow of condensate under gravity. The condensation then travels into the collection device60of the exhaust conduit20. The collection device60is configured to separate condensation and combustion gases. The condensate collects in a container63, and drains through a tube64under gravity. The combustion gases are ultimately exhausted through an outlet port62of the exhaust conduit20.

According to one aspect of the invention, the upstream heat exchange portion32is a hollow tube of circular cross-section extending along the entire height of the water tank22between the inlet port52of the air blower54and the combustion chamber44. The upstream portion32provides a first pass for heat exchange of the combustion products with water in the water tank22. The upstream heat exchange portion32may be also commonly referred to in the art as a ‘flue tube.’

The upstream heat exchange portion32is positioned within the interior of the water tank22and may be substantially aligned with the longitudinal axis of the water tank22, as shown. Alternatively, depending upon the location of the air blower54, the upstream heat exchange portion32may be positioned in any other orientation within the water tank22, such as horizontal, for example. It should be understood that the position and orientation of the upstream heat exchange portion32is not limited to that shown and described herein, as the upstream heat exchange portion32may be positioned in any other orientation within the water tank22.

The upstream portion32may be a substantially straight tube, as shown. According to one aspect of the invention, the outer diameter of the upstream heat exchange portion32may be between 2 inches and 8 inches, more preferably between 4 inches and 6 inches and most preferably about 5 inches. The length of the upstream portion34may be between 20 inches and 80 inches, more preferably between 35 inches and 65 inches and most preferably between 45 inches and 50 inches.

The shape, size and number of upstream heat exchange portions may vary from that disclosed herein. Alternative upstream heat exchange portion routings could be vertically aligned with and offset from the water tank axis or diagonally aligned through the tank head and tank base of the water tank. In another embodiment, the upstream heat exchange portion32can take the form of a coil having any number of geometrical cross-sections. A helically shaped upstream portion may offer a relatively larger heat exchange area between the water in the water tank22and the combustion products. The baffles70may be positioned along the length of the upstream portion, regardless of its overall size, shape (e.g., straight or coiled) or cross-sectional shape (e.g., circular or square).

According to one aspect of the invention, the downstream heat exchange portion34is a hollow tube of circular cross-section extending between the outlet port56of the air blower54and the exhaust conduit20for providing a second pass for heat exchange of the combustion products with water in the water tank22.

The downstream heat exchange portion34includes a substantially straight segment that is oriented substantially parallel to the upstream heat exchange portion32, and a semi-helical segment69that is positioned to encircle or extend about the upstream heat exchange portion32. Because neither the substantially straight segment nor the semi-helical segment69of the downstream portion34are substantially horizontal, the condensate may drain along the entire length of the upstream portion34under gravity. It should be understood that the position and orientation of the downstream heat exchange portion34is not limited to that shown and described herein, as the downstream heat exchange portion34may be positioned in any other orientation within the water tank22.

The downstream heat exchanger provides sufficient surface area to transfer heat, and the interior diameter of the heat exchanger is preferably large enough to accommodate a baffle (such as baffle70ofFIG. 2) to promote heat exchange. The trajectory of the curved downstream heat exchange portion is tailored to provide sufficient clearance between the heat exchange portion and at least one sacrificial anode rod and the inlet diptube to prevent erosion of a protective enamel coating covering the heat exchange portion. Furthermore, the trajectory of this heat exchange portion is also tailored to clear the gas valve thermocouple that is used to sense the temperature of the water contained within the tank, and the temperature sensing probe of a temperature and pressure relief valve. The foregoing positional relationships are beneficially maintained within the generally cylindrical structure of a tank having an external diameter between 10 and 30 inches, or more preferably between 14 and 22 inches, and most preferably about 18 inches.

The semi-helical segment69extends outside of the water heater15through an aperture provided in the water tank22and the outer shell24for connection with the collection device60of the exhaust conduit20. The exit point of the semi-helical segment69is in close proximity to the bottom of the tank22.

The shape, size, orientation and number of downstream heat exchange portions may vary from that disclosed herein. More particularly, both the upstream and downstream heat exchange portions32and34could consist of multiple tubes. The number of upstream and downstream heat exchange portions32and34need not be equal. Nevertheless, it is preferred to distribute the heat exchange surface area along the heat exchange portions32and34such that the temperature of the combustion products is reduced to a point below the dew point of the combustion products. The baffles70may be positioned along the length of the downstream portion34, regardless of its overall size, shape (e.g., straight or coiled) or cross-sectional shape (e.g., circular or square).

According to one aspect of the invention, the outer diameter of the downstream heat exchange portion34may be between ½ inch and 5 inches, more preferably between 2 inches and 4 inches, or most preferably about 3 inches. The length of the downstream portion34may be between 20 inches and 200 inches, more preferably between 40 inches and 120 inches and most preferably 70 inches. Although only one downstream heat exchange portion34is shown, the flue system50may contain any number of downstream heat exchange portions.

The ratio of the surface area of the downstream portion34to that of the upstream portion32may also be tailored to optimize the efficiency of the water heater. For example, the ratio can be adjusted by modifying the size and/or number of tubes in each of the heat exchange portions32and34. In one exemplary embodiment, the ratio of the surface area of the downstream heat exchange portion34to that of the upstream heat exchange portion32is maintained between about 1.1:1 and about 4:1, more preferably between about 1.3:1 and 2:1 and most preferably about 1.5:1. Other ratios may be acceptable as well. As discussed in greater detail later, the surface area of the downstream heat exchange portion34necessary to promote condensation of water vapor contained in the combustion gases is nearly equal to, or perhaps greater than the surface area of the upstream heat exchange portion32.

According to one aspect of the invention, the upstream portion32removes significantly more heat from the combustion gases than the downstream portion34. For example, the upstream portion32might receive combustion gases at about 2500° F. and the combustion gases might exit the upstream portion32at about 300° F. The downstream portion34might receive the combustion gases at about 300° F. and the combustion gases might exit the downstream portion34at about 110° F. The preferred temperature of combustion gases exiting the downstream portion is less then the average temperature of the water contained in the tank. For example, the average temperature of the water contained within the tank might be 135° F. and the combustion gases exiting the downstream portion34might be 125° F. This is achievable by delivering the incoming water from the diptube to the lowest portion the tank, thereby surrounding the semi-helical portion of the downstream portion, the tank base and at least a portion of the upstream portion in the coldest water within the tank.

FIGS. 3-5depict another exemplary embodiment of a water heating system110including a water heater115. The water heater115illustrated inFIGS. 3-5is substantially similar to the water heater15shown inFIGS. 1 and 2, with the exception of the position of the downstream portion134within the water tank122. Additionally, unlike the water heating system ofFIGS. 1 and 2, an exhaust conduit is omitted and a gas supply line118is included inFIGS. 3-5.

The water heater115includes a water tank122for containing water, an outer shell124for encapsulating the water tank122, and a flue system150for distributing combustion products for heat exchange with water in the water tank122. A top cover130is fastened to the outer shell124, thereby enclosing the top surface of the water storage tank122. The top cover130includes apertures for accommodating the flue system150, a cold water inlet port111and a hot water outlet port113.

The gas-fired water heater115includes a control unit136having a gas valve and thermostat. The control unit136includes an inlet for receiving gas from a gas supply line118, and a thermocouple138extending into the water that measures the water temperature inside the water tank122. The gas burner140receives gas via a conduit142. The gas burner140is positioned in a combustion chamber144that is disposed at an elevation beneath the water storage tank122.

Similar to the flue system50depicted inFIG. 2, the flue system150includes an upstream heat exchange portion132providing a first pass for heat exchange with water in the water tank122, a downstream heat exchange portion134providing a second pass for heat exchange with water in the water tank122, and a blower154positioned between the upstream portion132and the downstream portion134.

As shown inFIG. 5, the air blower154includes an inlet port152for connection to the outlet end180of the upstream heat exchange portion132, an outlet port156for connection to the inlet end182of the downstream heat exchange portion134, and an internal impeller for urging combustion products from the upstream portion132to the downstream portion134.

FIG. 6depicts another exemplary embodiment of a water heating system210including a water heater215. The water heater215illustrated inFIG. 6is substantially similar to the water heater15ofFIG. 1, and operates under the same principles. Unlike the water heater15depicted inFIGS. 1 and 2, however, the air blower254of the water heater215is positioned at an elevation beneath the top surface231of the water heater215. Positioning the air blower254beneath the top surface231of the water heater215reduces the overall height of the water heater, and improves manufacturability of the tank.

The water heater215includes a “two-pass” flue system250at least partially positioned within the water tank222. The flue system250includes an upstream heat exchange portion232providing a first pass for heat exchange with water in the water tank222, a downstream heat exchange portion234providing a second pass for heat exchange with water in the water tank222, and a blower254positioned between the upstream portion232and the downstream portion234.

The downstream portion234includes a semi-helical segment286extending from the air blower254, a second semi-helical segment288extending from the exhaust conduit220, and a substantially straight segment284extending between the semi-helical sections286and288. The substantially straight segment284is entirely positioned within the water tank222, whereas a portion of the semi-helical segments286and288are positioned within the water tank222. The remaining portions of each of the semi-helical segments286and288are positioned outside of the water heater215for connection to the air blower254and the collection device260of the exhaust conduit220, respectively. The water tank222and the outer shell224both include apertures to accommodate the semi-helical segments286and288.

Unlike the upstream heat exchange portion32ofFIG. 1, the upstream heat exchange portion232ofFIG. 6extends outside of the water heater215and includes a u-shaped segment290extending between the top surface231of the water heater215and the inlet port of the air blower254.

FIGS. 7A and 7Bdepict perspective views of a residential water heating system410. The system410is tailored to address a problem of a unique water heater structure including a blower which receives and impels hot gas. The system410is substantially similar to system10ofFIG. 1(i.e., it includes a “two-pass” flue system), with the exception that system410includes a thermal insulator497positioned over at least a portion of the air blower454for thermally insulating the blower. InFIGS. 7A and 7B, the thermal insulator497is partially cut-away to reveal the details of the air blower454. Accordingly, although not shown, the thermal insulator497may encapsulate the entire portion of the air blower454residing above the top cover430of the water heating system410.

The thermal insulator497is positioned to thermally insulate the components of the air blower454positioned above the top cover430of the water heater. Additionally, the thermal insulator497is also positioned to thermally insulate the transition components (not shown, but may be a clamp, for example) coupled between the inlet port452of the blower454and the upstream heat exchange portion, as well as the transition components (not shown, but may be a clamp, for example) coupled between the outlet port456of the blower454and the downstream heat exchange portion.

Positioning a thermal insulator497over the air blower454greatly improves the thermal efficiency of the residential water heating system410. More particularly, the components of the air blower454and the aforementioned transition components are optionally composed of materials having a high thermal conductivity, such as steel, for example, suitable for the transfer of hot flue gases from the upstream heat exchange portion to the downstream heat exchange portion. It is contemplated that the temperature of the hot flue gases may exceed the safe operating limits of many plastic materials (a common material of air blower components).

The thermally conductive components of the air blower454and the aforementioned transition components dissipate heat both during burner operation and during burner standby periods. Dissipation of heat through the air blower reduces the thermal efficiency of a water heating system. To counteract thermal efficiency losses, a thermal insulator497is positioned over at least a portion of the air blower454. The thermal insulator497is configured to reduce the dissipation of heat from the air blower454and the air blower transition components. The thermal insulator497is composed of insulative materials, such as fiberglass, high-density rigid polyurethane, or both, for example, or any other thermally insulative material known to those skilled in the art.

Surrounding the exposed, thermally conductive, components of the air blower454with the thermal insulator497increases the heat contained within the residential water heating system410, and reduces the heat dissipated by the residential water heating system410to the atmosphere. Insulating the air blower454enhances the natural heat trapping effect of the air blower454. The natural heat trapping effect of the air blower454combined with the insulation benefits conferred by the thermal insulator497greatly improves transfer of heat to the water within the water tank during burner operation, and significantly reduces heat loss during periods when the air blower454is not actively operating.

The thermal insulator497is optionally composed of two half sections (only one section is illustrated inFIGS. 7A and 7B). Each section of the thermal insulator497is fixedly connected to the top cover430by one or more “L”-shaped brackets499. Although not shown, fasteners may be employed to couple the respective ends of the brackets499to the top cover430and the thermal insulator497. The brackets499may also be adhered to both the top cover430and the thermal insulator497by an adhesive, for example. Those skilled in the art will recognize that numerous ways of attaching the thermal insulator497to the system410exist.

The thermal insulator497includes an opening496, a portion of which is illustrated inFIG. 7A, for accommodating the inducer motor498of the air blower454and exposing the inducer motor498to atmospheric, ambient air. The opening496may also be referred to herein as an air vent. By providing an opening496in the thermal insulator497, the inducer motor498is neither covered nor insulated by the thermal insulator497. Covering the inducer motor498with insulation could potentially result in overheating and/or failure of the inducer motor498. The opening496of the thermal insulator497promotes cooling of the inducer motor498by isolating the inducer motor from surrounding insulation and providing direct access to ambient air. Moreover, the opening496of the thermal insulator497maintains a lower temperature of the inducer motor498through unrestricted access to ambient air, thereby enhancing the performance and reliability of the air blower454, as well as extending the useful life of the air blower454.

FIG. 8depicts another exemplary embodiment of a water heating system510including a water heater515. A cross-sectional view of a portion of the water heater515is illustrated inFIG. 9. Arrows inFIG. 8indicate the flow of combustion products through the heat exchange system510. The water heater515illustrated inFIGS. 8 and 9is substantially similar to the water heater15ofFIG. 2, and operates under the same principles. Unlike the water heater15depicted inFIG. 2, however, the downstream heat exchange portion534includes a bent segment569in lieu of a helical segment. The bent segment569may comprise, for example, a 90 degree bend, as shown. By way of non-limiting example, the outer diameter of the downstream heat exchange portion534may be about 3 inches.

The bent segment569extends outside of the water heater515through an aperture provided in the water tank522and the outer shell for connection with the collection device of the exhaust conduit. The exit point of the bent segment569is in close proximity to the bottom of the tank522.

Example

A water heater corresponding to the exemplary embodiment illustrated inFIG. 7Awas built and tested to determine its thermal performance. The results of the five tests, labeled Examples 1-5, are summarized in Table #1.

TABLE #1Thermal Performance MeasurementsTime ofAverageAverageTankBurnerStartingUpstreamDownstreamAverageTempExampleCapacityOperationTankFlue1OutletFlue2OutletTankIncreaseCO2LevelCO LevelCOafBurner InputNo.(gal)(min)Temp (° F.)Temp (° F.)Temp (° F.)Temp (° F.)(° F.)(%)(ppm)(ppm)3(btu/hr)1461570269108.1101.831.810.52032.250,0262461569.5262108.510232.510.5252951,0053461570.1259109.2101.631.510.22023.949,9874461569.9266108.4101.93210.32023.749,5595461569.8263108.6102.132.310.22023.950,5451The ‘upstream flue’ refers to the upstream heat exchange portion 32 of FIG. 2. The outlet of the upstream heat exchange portion 32 is coupled to the inlet port (item 152 of FIG. 5) of the air blower (item 154 of FIG. 5). The temperature reading was taken at the outlet of the upstream heat exchange portion 32.2The ‘downstream flue’ refers to the downstream heat exchange portion 34 of FIG. 2 The outlet of the downstream heat exchange portion 34 is coupled to the exhaust conduit (item 20 of FIG. 1). The temperature reading was taken at the outlet of the downstream heat exchange portion 34.3The term ‘COaf’ denotes the amount of Carbon Monoxide (i.e., CO) in an air free sample of combustion gases.

The results of the test indicate a significant transfer of heat from the combustion gases through the heat exchanger material and into the water contained within the tank at a low Carbon Monoxide emission level.

The thermal efficiency of the water heater illustrated inFIG. 7Ais well above the typical thermal efficiency of conventional gas-fired, tank-style water heaters. The thermal efficiency of the water heater ofFIG. 7Awas determined by measuring several variables, as shown in Table #2 below, and inputting those measurements into a thermal efficiency formula, as described hereinafter.

After taking the measurements reported in Table #2, a “Correction Factor” accounting for gas pressure, barometric pressure and gas temperature was calculated using Equation #1 below.

After determining the “Correction Factor”, the thermal efficiency of the water heater ofFIG. 7Awas calculated using Equation #2 below. For reference, the “Temp. Change” listed in Equation #2 is the difference between the “Average Outlet Water Temp” and the “Average Inlet Water Temp” values reported in Table #2.

Substituting the values listed in Table #2 into Equation #2 yields a thermal efficiency of 92.5%. The calculated thermal efficiency of 92.5% is well above the typical thermal efficiency of conventional gas-fired, tank-style water heaters, which is reportedly 77%. The improved thermal efficiency of the water heater ofFIG. 7Ais believed to result from features including the unique two-pass flue system (items50,150and250) depicted in the figures and the thermal insulator (item497ofFIGS. 7A and 7B).

For reference, in Table #2, the “Heating Value” was determined by a calorimeter, which measures how much heat is contained in 1 ft^3 of gas. The term “Heating Value” may also be referred to as a calorific value. The Barometric Pressure was measured by a barometer positioned adjacent the water heater. The “Gas Pressure @ Exit of Gas Valve” was measured by a pressure gauge positioned at the exit of the gas valve. The gas valve was positioned within the interior of the control unit36shown inFIG. 2. The “Gas Pressure @ Location Between Pressure Regulator and Gas Valve” was measured by a pressure gauge positioned at a location between the gas valve the pressure regulator. The pressure regulator was positioned upstream of the gas valve, but is not depicted in the Figures. The “Gas Consumed by Water Heater over 30 minute Period” was measured by a conventional gas meter over a period of 30 minutes. The weight of the “Water Expelled by Water Heater over 30 minute Period” was measured by a weight scale. More specifically, hot water was delivered from the hot water outlet port (item13ofFIG. 1) into an empty barrel over a 30 minute period. The empty barrel was first weighed before the 30 minute test period and was weighed again after being filled with hot water over a 30 minute period. The difference between those weight measurements was reported in Table 2.

The “Average Water Inlet Temp.” was periodically measured using a thermometer positioned at the cold water inlet port (item11ofFIG. 1) of the water heater, and the average of those measurements over a 30-minute period was reported in Table 2. The “Average Water Outlet Temp.” was periodically measured using a thermometer positioned at the hot water outlet port (item13ofFIG. 1) of the water heater, and the average of those measurements over a 30 minute period was reported in Table 2.

The combustion efficiency of the water heater illustrated inFIG. 7Ais also well above the typical combustion efficiency of conventional gas-fired, tank-style water heaters. The term ‘combustion efficiency’ is a measure of the percentage of total energy that escapes from the water heater. One method of calculating the combustion efficiency is to compare the theoretical amount of condensation produced by a water heater with the measured amount of condensate produced by a water heater. Several steps and measurements were generally used to determine the combustion efficiency of a water heater, as described hereinafter.

The stoichiometric combustion equation for burning a natural gas in the presence of air is shown below in Equation #3.
CH4+2O2+2(3.76)N2→CO2+2H2O+2(3.76)N2(Eq. 3)
To promote complete combustion of the gas, combustion chambers are typically supplied with excess air. Excess air increases the amount of oxygen thereby increasing the probability of combustion of all of the gas supplied to the burner. The water heater ofFIG. 7Awas operated at 15% excess air (a measured quantity) to promote complete combustion of the gas fuel. The stoichiometric combustion equation (i.e., Equation #3) does not account for excess air. A balanced combustion equation accounting for 15% excess air is shown below (i.e., Eq. 4).
CH4+2.1699O2+8.158N2→CO2+2H2O+0.1699O2+8.159N2(Eq. 4)
According to Table #4 shown below, the total molecular mass of the product side of the equation is 314 grams and the total mass of water is 36 grams. Thus, the percentage of water by mass is 11.47%.

Over the course of the testing period, the consumption rate of natural gas (composed primarily of methane) was 2.228 lb/hour. The consumption rate may be defined as the quotient of the average burner input (see Table #1) and the heating value of natural gas (see Table #1). Over the course of the testing period, the consumption rate of air was 39.761 lb/hour. The sum of the consumption rate of both natural gas (i.e., CH4) and air was 41.898 lb/hour. The product of the percentage of water by mass (11.47%) and the total consumption rate of both methane and air (41.898 lb/hour) yields a theoretical rate of condensate over the test period of 4.816 lb/hour. In comparison, the measured rate of condensate over the test period was 2.238 lb/hour.

The formula for determining the combustion efficiency is shown below in Equation #5. Substituting the above-reported values of the measured rate of condensate and the theoretical rate of condensate into Equation #5 yields a combustion efficiency of 93.041%. A combustion efficiency of 93.041% is well above the typical combustion efficiency of conventional gas-fired, tank-style, water heaters, which is approximately 76% according to the Energy and Environmental Building Association. The improved combustion efficiency of the water heater ofFIG. 7Ais believed to result from features including the unique two-pass flue system (items50,150and250) depicted in the figures and the thermal insulator (item497ofFIGS. 7A and 7B).
Combustion Efficiency=87+(13*Measured Condensate)/(Theoretical Condensate)  (Eq. 5)

Although this invention has been described with reference to exemplary embodiments and variations thereof, it will be appreciated that additional variations and modifications can be made within the spirit and scope of this invention. Although this invention may be of particular benefit in the field of residential water heaters, it will be appreciated that this invention can be beneficially applied in connection with commercial or domestic water heaters and other heating systems as well.