Patent Publication Number: US-8971694-B2

Title: Control method for a hybrid tankless water heater

Description:
PRIORITY CLAIM AND RELATED APPLICATIONS 
     This divisional patent application claims the benefit of priority from provisional application U.S. Ser. No. 61/149,418 filed on Feb. 3, 2009 and U.S. Ser. No. 12/692,628 filed on Jan. 24, 2010. Each of said applications is incorporated by reference in its entirety. 
    
    
     FIELD OF INVENTION 
     This invention relates to apparatus and control methods for water heaters, and more specifically, to hybrid tankless water heaters. 
     BACKGROUND ART 
     Most water heaters are capable of delivering water at a desired temperature in steady state conditions where the water demand or flow rate is substantially constant. However, water heating systems presently available or prior art water heating systems fall short when attempting to maintain a water output at a constant predetermined temperature level during rapid shifts or fluctuations in water demand. It is understood that the demand for water is directly related to the output flow rate requested from the water heating system. Prior art water heating systems will provide the water output flow rate corresponding to the water demand placing the predetermined temperature setting as a secondary consideration. Placing predetermined water output temperature as a secondary consideration creates two major thermal related problems. The first issue is encountered during a rapid increase in water demand, wherein the user or device experiences a sudden drop in water temperature or a cold water splash. The remaining issue occurs during a rapid decrease in water demand, wherein the user or device experiences a sudden spike in water temperature, thereby creating a possible burn or scald type hazard. Furthermore, none of the prior art water heaters are capable of delivering water at the predetermined or desired temperature range without substantial delays. The rapid shifts in water demand creates a transient condition within the water heating system wherein such existing systems are ill equipped to handle. 
     On-demand water heaters are gaining popularity because of their reduced space requirement in addition to improved energy advantages. The current on-demand water heaters have well known drawbacks, most notably, the uncontrollable and undesirable fluctuation of temperature of the output water during water usage. When output water flow increases, the temperature of the output water decreases. Conversely, when output water flow decreases, the temperature of the output water increases. This creates undesirable temperature fluctuations for users, appliances, and the like. Disadvantages of these tankless water heaters are well known in the art and general population, such a discussion is described in Wikipedia, and reads as follows:
         Installing a tankless system comes at an increased cost, particularly in retro-fit applications. They tend to be particularly expensive in areas such as the US where they are not dominant, compared to the established tank design. If a storage water heater is being replaced with a tankless one, the size of the electrical wiring or gas pipeline may have to be increased to handle the load and the existing vent pipe may have to be replaced, possibly adding expense to the retrofit installation. Many tankless units have fully modulating gas valves that can range from as low as 10,000 to over 1,000,000 BTUs. For electrical installations (non-gas), AWG 10 or 8 wire, corresponding to 10 or 6 mm 2 , is required for most POU (point of use) heaters at North American voltages. Larger whole house electric units may require up to AWG 2 wire. In gas appliances, both pressure and volume requirements must be met for optimum operation.   There is a longer wait to obtain hot water. A tankless water heater only heats water upon demand, so all idle water in the piping starts at room temperature. Thus there is a more apparent “flow delay” for hot water to reach a distant faucet.   There is a short delay between the time when the water begins flowing and when the heater&#39;s flow detector activates the heating elements or gas burner. In the case of continuous use applications (showers, baths, washing machine) this is not an issue. However, for intermittent use applications (for example when a hot water faucet is turned on and off repeatedly) this can result in periods of hot water, then some small amount of cold water as the heater activates, followed quickly by hot water again. The period between hot/cold/hot is the amount of water which has flowed though the heater before becoming active. This cold section of water takes some amount of time to reach the faucet and is dependent on the length of piping.   Since a tankless water heater is inactive when hot water is not being used, they are incompatible with passive (convection-based) hot water recirculation systems. They may be incompatible with active hot water recirculation systems and will certainly use more energy to constantly heat water within the piping, defeating one of a tankless water heater&#39;s primary advantages.   Tankless water heaters often have minimum flow requirements before the heater is activated, and this can result in a gap between the cold water temperature, and the coolest warm water temperature that can be achieved with a hot and cold water mix.   Similarly, unlike with a tank heater, the hot water temperature from a tankless heater is inversely proportional to the rate of the water flow—the faster the flow, the less time the water spends in the heating element being heated. Mixing hot and cold water to the “right” temperature from a single-lever faucet (say, when taking a shower) takes some practice. Also, when adjusting the mixture in mid-shower, the change in temperature will initially react as a tanked heater does, but this also will change the flow rate of hot water. Therefore some finite time later the temperature will change again very slightly and require readjustment. This is typically not noticeable in non-shower applications. A temperature compensating valve tends to eliminate this issue. Tankless systems are reliant on the water pressure that is delivered to the property. In other words, if a tankless system is used to deliver water to a shower or water faucet, the pressure is the same as the pressure delivered to the property and cannot be increased, whereas in tanked systems the tanks can be positioned above the water outlets (in the loft/attic space for example) so the force of gravity can assist in delivering the water, and pumps can be added into the system to increase pressure. Power showers, for example, cannot be used with tankless systems because it cannot deliver the hot water at a fast enough flow-rate required by the pump.       

     A typical water demand scenario is provided in the following example. A first user draws water at a desired temperature at a bathroom faucet while simultaneously a second user opens a kitchen faucet. The output water temperature experienced by both users dramatically decreases since the total flow rate through the water heater increases, and thus, the volume of water to be heated per unit of time has increased while the burner output remains constant (or the system is not capable of keeping pace with the increased water demand). At the other end of the spectrum, in a situation where two users are using water at desired temperature at two separate faucets, where one user closes a faucet, the remaining open faucet will experience a spike (dramatic increase) in temperature. This is due to a decrease in the volume of water to be heated per unit time resulting in a reduction of water flow through the water heater resulting in an increase in output water temperature. 
     Other well known drawbacks associated current on demand water heaters include the cold sandwich effect, freeze hazards, and dead zones. Controls for water heaters are plagued with limitations and lack the sophistication to maximize system efficiency. 
     The purpose of the present invention is to overcome several shortcomings in the aforementioned prior art as well as the introduction of additional novel features. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the shortcomings of the prior art by providing several new and novel structures and methods for an improved hybrid tankless water heater. Briefly described, in a preferred embodiment, the improved hybrid tankless water heater overcomes the aforementioned prior art disadvantages, and better aligns with the needs and desires of a typical user by the addition of several subsystems assemblies, one such subsystem includes a thermally insulated mixing buffer tank disposed downstream from the heat exchanger located within the primary heating subsystem. The mixing buffer tank has turbulence inducing baffles to facilitate mixing. There is further provided a secondary heating element to supply supplemental heat, preferably disposed in the mixing buffer tank. Further provided is a differential pressure switch that functions to rapidly detect trickle or low flow conditions and activate or pre-activate primary and/or secondary heating elements depending on the detected real time water demand. Pre-activation shall be understood as a sensitive detection scheme that predicts usage given relatively small input signals. Also included is a recirculating system, and an inverted burner system in the primary heating subsystem comprising an array of coiled tubing surrounding an axially disposed radial burner, and a buffer tank which forms the core plug at exit end of the coil tubing forcing heated air drawn in from the entry end of the coil tubing to surround the exit end of the coil tubing effectively transferring heat to the water flowing inside the coil tubing. 
     One aspect of the novel controls method of the present invention includes receiving inputs, such as pressure differential to detect low flow levels and has outputs that eliminate hunting, allow fine adjustment of temperature, and prevent overshoot. Another aspect of the novel control method of the present invention maintains water at a predetermined target temperature, e.g. above 33 degrees Fahrenheit, by re-circulating, initiating a secondary electric heating element, pulse firing the burner of a primary heating element, or any combination thereof. There is provided a controller comprising a processor, memory and software for managing the outlet temperature to a desired temperature corresponding to given portions within a given day, week, or the like. 
     It is an object of the present invention to minimize variation of the water heater&#39;s predetermined outlet temperature regardless of rapid shifts or fluctuations in water demand. 
     It is another object of the present invention to minimize delays associated with achieving predetermined target temperatures at the water heater output. 
     It is yet another object of the present invention to minimize or eliminate the cold sandwich effects in transient system situations. 
     It is yet another object of the present invention to minimize or eliminate dead zones (conditions where water flow is below detection using typical sensing means) in a transient system. 
     It is yet another object of the present invention to minimize or eliminate the problem of bacteria growth in stored water supplies and water system components. 
     It is yet another object of the present invention to provide a method of controlling temperature and water flow in a water heater to maximize efficiency and response time. 
     It is yet another object of the present invention to provide fine heating modulation. 
     It is yet another object of the present invention to minimize or avoid both firing and transient delays. 
     It is yet another object of the present invention to minimize or eliminate freeze hazards. 
     It is yet another object of the present invention to actively monitor and detect system leaks. 
     It is yet another object of this invention to provide a hybrid tankless water heater that is relatively economical from the viewpoint of the manufacturer and consumer, is susceptible to low manufacturing costs with regard to labor and materials, and which accordingly is then susceptible of low prices for the consuming public, thereby making it economically available to the buying public. 
     Whereas there may be many embodiments of the present invention, each embodiment may meet one or more of the foregoing recited objects in any combination. It is not intended that each embodiment will necessarily meet each objective. 
     In this respect, before explaining at least one embodiment of the invention 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 present invention is capable of other embodiments and of being practiced and carried out in various ways. Thus, having broadly outlined the more important features of the present invention in order that the detailed description thereof may be better understood, and that the present contribution to the art may be better appreciated, there are, of course, additional features of the present invention that will be described herein and will form a part of the subject matter of this specification. 
     PARTICULAR ADVANTAGES OF THE INVENTION 
     The present invention provides an apparatus and method for minimizing variation of the water heater&#39;s predetermined outlet temperature regardless of rapid shifts or fluctuations in water demand by disposing a buffer tank downstream from the primary heat exchanger. Delayed temperature response as well as temperature fluctuations at the water heater&#39;s output is minimized by providing supplemental heat (provided from an energy source other that natural gas) via a secondary heating element. This secondary heating element also provides fine heating modulation. Temperature response delays are also minimized by providing a means to rapidly detect conditions that require additional sources of heat to the water heater. The problem of dead zones (conditions where water flow is below detection using typical prior art sensing means) is solved by utilizing a means to detect trickle, low-level, or low flow followed by activating a secondary heating element associated with a recirculation path, circuit, or watercourse from which the output of the water heater can tap into. In the present invention, detecting trickle flow or low flow rapidly is accomplished by using a differential pressure switch. By controlling outlet temperature to a desired temperature at set time periods in a day, the present invention advantageously avoids both firing and transient delays. The present invention also minimizes the cold sandwich effect in a transient system by mixing the fluid in the buffer tank via recirculation. The present invention optionally includes an antibacterial hot water flush mode that inhibits bacteria growth and/or sanitizes stored water supplies and water system components. This is accomplished by recirculation of outlet flow and maintaining water temperature at an elevated temperature for a predetermine period of time capable of killing off the target microbes, for example, 140 degrees Fahrenheit for 20 minutes. Water freezing hazards are minimized by actively maintaining water at above freezing temperatures by re-circulating and activating a secondary electric heat source and/or pulse firing the burner of the primary heat source (e.g. vacation mode). Transient temperature spikes are reduced by means of controlling a blower that operates independent of the burner and a recirculation pump. There is provided baffles or a barrel-hole style inlet system for creating turbulence and promoting mixing of the incoming water with existing water in the buffer tank. 
     By mounting the flow sensor outside of the recirculation circuit, there is no pressure loss imparted by the flow sensor. This allows the use of a pump with a lower power rating, thereby making the water heater more economical. Additionally, no proprietary pump or flow sensor are required. Flue condensate backup or blockage is detected by using a condensate level sensor system. The present invention optionally includes a feature that enables a user to set or program an automatic temperature rise rate and a high temperature limit. A baffle mixes pre-combusted air in the vicinity of the heat exchanger, thereby promoting heat transfer from the exterior surfaces of the heat exchanger to the pre-combusted air and recovering this absorbed heat of the pre-combusted air by redistributing it to colder portions of the heat exchanger surfaces. 
     User maintenance is facilitated by decoding fault code(s) and optionally providing audible or text speech corrective procedures. Active and continuous monitoring performance and health of the water heater, and automatic adjustment of operating parameters, minimizes unforeseen service shutdowns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by reference to the specification and the drawings, in which like numerals refer to like elements, and wherein: 
         FIG. 1  is one embodiment of a hot water heater system of the present invention; 
         FIG. 2  is a schematic block diagram of a preferred embodiment of a water heater controller of the water heater system of  FIG. 1 ; 
         FIG. 3  is a schematic controls diagram of the water heater controller; 
         FIG. 4  depicts an alternate embodiment of a water heater; 
         FIG. 5  depicts an alternate embodiment of a water heater; 
     
    
    
     The drawings are not to scale, in fact, some aspects have been emphasized for a better illustration and understanding of the written description. For simplicity in representing the complex controls scheme, the diamonds in the block figures schematically represent input and/or output devices. Arrows pointing toward a diamond represent input devices, arrows pointing away from a diamond represent output devices and arrows both pointing toward and away from a diamond represent a dual input-output device. 
     PARTS LIST 
     
         
           2 —hot water heater 
           4 —hot water heater enclosure 
           6 —flow path 
           8 —primary heating system (burner including heat exchanger) 
           9 —primary heating process 
           10 —inlet of water heater 
           11 —inlet flow 
           12 —outlet flow 
           13 —mixed flow for second point of demand 
           14 —thermal insulation for mixing buffer tank 
           15 —mixing buffer tank 
           16 —secondary heating element (electric heater) 
           17 —secondary heating process 
           18 —inlet temperature sensor 
           19 —external auxiliary device circuit 
           21 —mixed flow (for second point of demand) 
           22 —outlet temperature sensor 
           23 —outlet temperature sensor for auxiliary demand 
           24 —flow limiting valve 
           25 —internal recirculating flow circuit 
           26 —flow sensor 
           27 —external recirculating flow circuit 
           28 —recirculation pump 
           29 —blower speed feedback 
           30 —internal recirculation check valve 
           31 —portion of recirculation flow 
           32 —internal recirculating flow 
           33 —external recirculation check valve 
           34 —burner 
           35 —internal recirculation modulating valve 
           36 —blower 
           36   a —blower fan speed control 
           37 —external recirculation modulating valve 
           37   a —external recirculation flow 
           38 —controller 
           39 —temperature sensor (merged flow) 
           40 —flue gas exit 
           41 —merged flow temperature signal 
           42 —expansion tank 
           43 —user interface 
           44 —bypass flow (buffer tank) 
           45 —buffer tank bypass line 
           46 —moisture sensor 
           47 —external auxiliary device modulating valve 
           48 —differential pressure switch 
           50 —condensate level sensor 
           52 —flue gas temperature sensor 
           53 —external gas usage input signal 
           54 —buffer tank flow 
           56 —buffer tank bypass three way valve 
           59 —auxiliary three way valve 
           60 —capillary bypass line 
           61 —distal end of the heat exchanger 
           64 —auxiliary heat sink 
           66 —first demand point 
           67 —cold water point of demand 
           68 —second demand point 
           70 —outlet temperature (T outlet ) 
           72 —inlet temperature (T inlet ) 
           74 —desired temperature (T desired ) 
           76 —temperature difference (T desired −T outlet ) 
           78 —temperature difference (T desired −T inlet ) 
           80 —flow rate 
           82 —flue gas temperature 
           84 —flue gas temperature limit—flue gas temperature 
           90 —feedforward control 
           92 —feedback control 
           94 —main control 
           96 —safety control 
           98 —secondary heat control (electric heat) 
           100 —recirculation control 
           102 —flow limiting valve control 
           104 —differential pressure switch 
           106 —differential pressure signal 
           108 —flue temperature limit 
           112 —available power 
       
    
     Also it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
     DEFINITIONS OF TERMS USED IN THIS SPECIFICATION 
     The term hybrid tankless water heating system shall have equivalent nomenclature including: the hybrid water heater, the heater, the device, the present invention, or the invention. As used in this specification, the following terms shall be defined as follows: 
     Hybrid water heater shall mean a water heater that combines two heating means, via a primary heating element and a secondary heating element, such as a gas burner and an electric immersion coil. 
     Usage is defined as units of “damage.” 
     “Damage” is a quantity of usage as seen in a water heater. “Damage” can include the effects of scaling, wear, burner cycles, amount of water delivered, etc. An increased usage of a water heater causes a corresponding increase in the amount of “damage.” 
     The efficiency of a water heating system is defined as the amount of converted thermal energy in the delivered water per unit of energy input. The efficiency of a water heater system is typically nonlinear with respect to flow rate. 
     Additionally, the term “exemplary” shall possess only one meaning in this disclosure; wherein the term “exemplary” shall mean: serving as an example, instance, or illustration. 
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
       FIG. 1  illustrates one embodiment of a novel hybrid tankless water heater  2  (“water heater”) and the relative placement of various components of the system. Briefly described, the water heater  2  comprises an enclosure  4  having a thermally insulated mixing buffer tank  15  with baffles disposed downstream from a heat exchanger of a primary heating system  8 . There is further provided a secondary heating element  16  to supply supplemental heat, preferably disposed in mixing buffer tank  15 . The combination of two heating systems (such as gas and immersion electric) thus renders the present invention a “hybrid” water heater. 
     Optionally there is further provided a differential pressure switch  48  that functions to rapidly detect a need to turn on primary  8  or secondary  16  heating elements by detecting trickle or low flow. There is also provided an internal recirculating system  25  and an inverted burner system in primary heating system  8 . The differential pressure switch is an important feature of the present invention in that it provides a means for fine controlling of output water temperature, a means for expedient temperature response to a demand, and a means for detecting leakage and activating associated alarms or alerts. 
     Other embodiments may include advantageous features described in greater detail below, including a moisture sensor in the enclosure, an integrated buffer and expansion tank or an independent expansion tank, a condensate level sensor, a capillary bypass line, a blower that operates independent of the burner and recirculation pump, and a feature that allows the user to set an automatic temperature rise rate and a high temperature limit. Yet other additional advantageous features are provided and described herein, including features to enhance user ability for self-maintenance of the water heater. 
     A demand at first demand point  66  causes a fluid flow to enter the water heater  2  at inlet  10  and exit water heater  2  at outlet  12 . In the illustrative embodiment, water at first demand point  66  is not mixed with cold incoming water upon receiving heat from system heat sources (e.g., primary heating element  9  and secondary heating element  16 ), however, it is contemplated that water at first demand point  66  may be mixed with a cold water supply via a three way valve connection or other similar mechanism. 
     Continuing to refer to  FIG. 1 , water heater  2  includes a flow limiting valve  24  for restricting inlet flow  11 . The flow limiting valve  24  controls the flow rate of water entering the water heater  2 . Water heater  2  further includes an inlet water temperature sensor  18  to sense the inlet water temperature. 
     The internal recirculating flow circuit  25  facilitates an internal recirculating flow  32  that is merged with the inlet flow  11 . This internal recirculating flow circuit  25  includes an internal recirculation modulating valve  35  and a check valve  30 . Check valve  30  permits recirculation from outlet  12  end of water heater  2  to inlet  10  end and stops any flow from inlet  10  end to outlet  12  end of water heater  2  while the internal recirculation modulating valve  35  modulates the magnitude of internal recirculation flow  32  in a predetermined fashion. 
     The external recirculating flow circuit  27  comprises an external recirculation modulating valve  37  that modulates the magnitude of external recirculation flow  37   a . Flow circuit  27  includes an external recirculation check valve which permits only the flow of water from the heated outlet via the external cold water supply line back to the inlet of the heated, and a cold water point of demand  67  attached thereon. 
     Continuing to refer to  FIG. 1 , the inlet water temperature sensor  18  is placed upstream of the point where internal recirculating flow  32  and water flow from inlet  10  converge. The water heater  2  preferably includes a flow sensor  26  that is disposed upstream from the point where the internal recirculating flow  32  and water flow from inlet  10  converge. Temperature sensor  39  is placed at this point of convergence. 
     The water heater  2  has a primary heating element  9 . In the embodiment depicted in  FIG. 1 , primary heating system  8  comprises a burner  34 , a blower  36 , a fuel supply valve, and a heat exchanger. The heat exchanger transfers heat from the burner  34  to the water flow. Preferably, the blower  36  is used in conjunction with the burner  34  to enhance mixing of the fuel with ambient air prior to combustion. The blower  36  can also enhance convectional heat transfer by forcing the hot flue gas to distal end of the heat exchanger  61  from burner  34 . 
     Upon passing through the heat exchanger, the heated water enters a mixing buffer tank  15 . The mixing buffer tank  15  serves as a reserve supply of warm water to ease excessively cold or warm water during transience. Preferably, mixing buffer tank  15  contains a secondary heating system. An immersion heating element is beneficial in this application. In the embodiment depicted, an electrical heating coil is used as a secondary heating element  16 . When water flow at first demand point  66  exists, the flow from mixing buffer tank  15  can exit outlet  12  and/or it can recirculate. The water heater  2  can include outlet temperature sensor  22  at outlet  12  of the water heater  2 . A decrease in temperature as indicated by outlet temperature sensor  22  over time can be used to detect a small flow. 
     A typical flow sensor requires a minimum flow in order to start registering a flow rate. However, a particular advantage is realized when a differential pressure switch  48  is utilized to detect a low flow condition such that it effectively covers the range of flow the flow sensor  26  is unable to detect. Specifically, flows greater than 0.005 gpm can be detected with this arrangement. It is to be understood that differential pressure sensors that detect pressure and flows greater or lower than 0.005 gpm can be suitably used with the present invention. A detected flow causes primary heating element  9  and secondary heating element  16  to turn on and water to be recirculated in order to maintain the desired outlet temperature. 
     Recirculation alone, or recirculation with heating, also prevents freeze and may be activated with or without the detection of a flow. Recirculation and heating may also be programmed to start at a user-specified regular interval for a user-specified duration using a timer, or the like. Alternatively, the program may be set according to learned interval and duration. In a learning system, water usage data such as the time stamp and duration of a demand and the volume of water demanded is collected over time and analyzed. As a result, recirculation and heating can be automatically turned on to anticipate periods of high demand. In addition to providing freeze prevention, automatic activation of recirculation and heating minimizes delays of temperature response to a demand. 
     When a large amount of hot water is abruptly demanded, buffer tank  15  may not be capable of supplying the amount of hot water demanded. A buffer tank bypass three way valve  56  is provided to give water heater  2  the capability of supplying heated water directly to the point of use at the desired temperature. When the use of buffer tank  15  is desired, buffer tank bypass three way valve  56  is controlled such that bypass flow  44  is ceased. Conversely, if bypass is desired, the buffer tank flow  54  is ceased. 
     Continuing to refer to  FIG. 1 , water heater  2  further comprises an expansion tank  42 . Fluid expands as it is heated, causing pressure in the water heater flow system to rise. Inclusion of expansion tank  42  provides a particular advantage in accommodating this fluid expansion and may be mounted any location in the fluid flow system which experiences fluid expansion. In another embodiment not shown, the function of an expansion tank is incorporated into the buffer tank  15 , eliminating the need of an independent expansion tank. In the illustrative embodiment, expansion tank  42  is mounted at inlet  10  end of water heater  2 . 
       FIG. 2  is a generalized block diagram of the water heater controller of the water heater  2  of  FIG. 1  depicting the inputs and outputs to controller  38 . At the heart of the water heater controller is general purpose controller  38 , the comprehensive unit capable of receiving electrical signals, for example from sensors and switches, performing computations based on the signals and outputting electrical control signals as a result of the computations to actuate certain electrical or electro-mechanical components. In the present invention, controller  38  receives a plurality of sensor inputs and outputs a plurality of control signals to perform water heating control. 
     Referring to  FIG. 2 , user interface  43  is depicted as a bi-directional communication tool for a user to enter preferences manually or automatically provide control inputs from a second control device. User interface  43  also functions as a display for pertinent water heater  2  information or to provide control output to a second control device. Flow sensor  26  provides flow rate to controller  38  indicating user hot water demand from first demand point  66 . In a system with multiple demand points, this flow rate could indicate the total flow rate of all of the demand points. 
     Outlet temperature sensor  22  provides a signal corresponding to the water temperature at outlet  12  of water heater  2  of a water pipeline leading to first demand point  66 . In a system with multiple demand points, there is provided an outlet temperature sensor  22  for each outlet. 
     Inlet water temperature sensor  18  provides a signal corresponding to the water temperature at water heater inlet  10  of a water source. Preferably, moisture sensor  46  is mounted in the cavity of water heater enclosure  4 , and provides a signal corresponding to the humidity in the cavity of water heater enclosure  4 . This provides a significant advantage over the prior art in that a means for leak detection in the enclosure is provided. A differential pressure switch  48  provides a signal indicating the presence of a small demand from first demand point  66  in the water heater  2 . A condensate level sensor  50  provides a signal indicating whether the condensate resulting from the condensing heat exchanger is draining properly. This provides a significant advantage over the prior art in that it provides a means for corrosion control and preventing overflow and the resulting mess. A flue gas temperature sensor  52  provides a signal corresponding to the flue gas temperature. An excessive flue gas temperature or temperature rise rate causes the controller to de-rate the burner to avoid potentially unsafe operation due to fire hazards and damage to the water heater  2 . The means of detecting excessive heat is a significant advantage over the prior art by providing a manner to enhance safety and prevent equipment damage. 
     A control output is provided to control the water flow rate through the water heater  2  via a flow limiting valve  24 . A control output is provided to control pump  28  speed. In one embodiment, the pump is a single speed pump. A control output is provided to control fuel flow rate of burner  34 . A burner  34  is immediately lit when fuel is admitted at the burner  34 . If the fuel valve is opened and a flame is not lit within a predetermined period of time (e.g., due to failed sparking or absence of fuel flow), the fuel valve will be shut off and retried after a predetermined period. A control output is provided to control a blower  36  fan speed. The blower  36  fan is used in conjunction with burner  34  in order to cause maximum heat transfer from burner  34  to the external surfaces of the heat exchanger. Blower  36  fan may also be turned on independent of burner  34  such as in the case where heat loss is desired of the heat exchanger tubes. Blower  36  speed control is further enhanced by the use of blower speed feedback  29 . 
     A control output is provided to control the power output of a secondary heating element  16 , in this case, an electric heating element. This electric heating element is used when the burner  34  is incapable of achieving the heating response desired independently or when burner  34  is incapable of providing a low heat rate. In one embodiment, the burner shutoff switch and the fuel shut off switch are a single integral switch unit. During low flow, the primary heating element (the burner) does not engage. When the flow is slightly above the level the secondary heating element (electric coil) can handle alone, pulse firing of the primary heating element will commence. Once the flow reaches a predetermined limit, the blower will be modulated to correspond to the flow demand. During periods of transience in flow, where the blower may be slow to respond, the secondary heating element will provide instantaneous, but limited, heat to the water. 
     A control output is provided to modulate the amount of fluid flowing through buffer tank  15 . A buffer tank bypass three way valve  56  is used to divert flow from buffer tank  15  when the demand for hot water cannot be met by the water in buffer tank  15 . The diverted water flows directly from the heat exchanger to the point of demand, avoiding cooling that occurs upon mixing with cooler water stored in buffer tank  15 . A control output is provided to modulate the recirculation flow rate and the flow rate through an auxiliary heat sink such as in the case of a radiant floor heating. In situations where water is in an overheated condition, an auxiliary heat sink can help achieve a specific water temperature output by diverting excess thermal energy to an auxiliary heat sink. 
     A control output is provided to modulate the flow magnitude through the internal and/or external recirculating flow circuits  25 ,  27 . The placement of pump  28  in the main flow path enables one or both of the recirculating flows. 
       FIG. 3  depicts a controls diagram for the current invention. It is to be understood that even though not all inputs are shown for each control portion, it is the intent of the inventors that one or more of the omitted inputs may be used in the control system. The current diagram as it is shown represents a simplified controls diagram depicting major control inputs affecting each control portion. The rate of change of each input is also omitted since time domain is inherent in any control systems. 
     Referring generally to  FIGS. 1 and 2 , and more particularly to  FIG. 3 , the outlet water temperature is controlled by varying the output of flow limiting valve  24 , primary heating element  9 , secondary heating process  17 , pump  28  and three way valve  56 . Primary heating element  9  represents the heat source provided by the combination of a blower  36  and a burner  34 . Controller  38  comprises several distinct control portions. Each control function  90 ,  92 ,  94 ,  96 ,  98 ,  100 ,  102  is depicted in a distinct block diagram in  FIG. 3 . Each controller  38  within a control portion represents a control methodology responsible for driving one or more hardware components. Each control methodology can include a control method such as a Proportional Integral Derivative (PID) control or a component of this control method such as the Proportional portion, the Integral portion, the Derivative portion or any combination of these components. Each control methodology can also include a fuzzy logic control. The output of each control portion contributes to the water heating rate of the water heater  2  and therefore the outlet temperature  70 . 
     In the exemplary embodiments, the control system includes main control  94  which is responsible for driving outlet temperature  70  quickly towards desired temperature  74 . It can be thought of as the control portion that brings outlet temperature  70  to desired temperature  74  at steady state. Main control  94  compares inlet temperature  72  and desired temperature  74  and calculates a corresponding heating control output based on the difference  78  in the inlet temperature  72  and desired temperature  74 . Main control  94  also receives a flow rate  80  and calculates a corresponding heating control output based on flow rate  80 . In addition, main control  94  takes the differential pressure signal  106  between the inlet and outlet flow as an indication of the presence of a flow. The magnitude of a demand is indicated by the combination of both flow rate  80  and differential pressure signal  106 . A small demand is indicated by a differential pressure only, while a large demand is indicated by a combination of a differential pressure as registered on differential pressure switch  104  and a flow rate as registered by flow sensor  26 . As flow rate increases, the intensity of burner  34  is increases. Conversely, as flow rate decreases, the intensity of burner  34  is also decreased. Under certain conditions, burner  34  can be turned off. Recirculation is provided and functions to remove residual heat from the heat exchanger. 
     The control system further includes feedback control  92 , which is responsible for mitigating transience and also ensuring that outlet temperature  70  is driven towards desired temperature  74  and meets desired temperature  74 . Feedback control  92  compares the desired temperature  74  and outlet temperature  70  and calculates a corresponding heating control output based on temperature difference  76  between the desired temperature and the outlet temperature. 
     The control system further includes feedforward control  90  which is responsible for mitigating transience and also ensuring that outlet temperature  70  is driven towards desired temperature  74  by applying a heating control output based on outlet temperature  70 . Feedforward control  90  receives the outlet temperature  70  and calculates a corresponding heating control output based on the magnitude of outlet temperature  70 . 
     The control system further includes recirculation control  100  which comprises an internal recirculation control and external recirculation control features. Internal recirculation control is primarily used for mitigating transience and aids in mitigating freeze hazards. External recirculation control eliminates the issue related to the cold column trapped between the heater and a point of demand. Additionally, external recirculation functions to reduce the delays associated with supplying hot water from the heater to a point of demand. Recirculation is provided by a pump  28  in combination with either internal modulating valve  35  or external  37  modulating valve, and is used when a buffer tank  15  flow exists as permitted by buffer tank bypass three way valve  56 . When outlet temperature  70  approaches substantially the merged flow temperature  41  as measured by temperature sensor  39 , recirculation is turned on in conjunction with buffer tank  15  to avoid potential overshoot in the situations where flow rate is suddenly decreased or stopped. 
     The control system  102  further includes a flow limiting valve and an available power  112  calculator, the calculator provides an estimate of available power based on the sum total of the power ratings of each of the system&#39;s heat sources. The power requirement is the power required to take the water with a flow rate  80  from an input temperature  72  to a desired temperature  74 . If the power requirement exceeds that of the available power calculated by available power  112  calculator; controller  38  calculates a flow rate which would result in a power requirement meeting the available power  112  and controls the flow limiting valve to provide this flow rate. 
     The control system further includes a buffer tank bypass three way valve  56 , wherein the valve provides the ability to meet a user demand in the situation where the temperature at buffer tank  15  or at the outlet is not sufficiently hot. Controller  38  determines whether outlet temperature  70  is high enough to meet a user demand by comparing the desired temperature to the outlet temperature  70  and buffer tank  15  temperature. If the desired temperature is higher than either the outlet or buffer tank  15  temperature, the controller further determines whether buffer tank  15  flow needs to be reduced. If the outlet temperature is larger than buffer tank  15  temperature, the valve port connected to buffer tank  15  will be closed or reduced effectively closing or reducing buffer tank  15  flow and the valve port connected to buffer tank bypass line  45  will be opened or increased in order to divert all or allow larger flow through buffer tank bypass line  45 . By bypassing buffer tank  15 , all heated water is delivered directly and expediently to first demand point  66  without having been mixed with cooler water in buffer tank  15 . As a demand persists, bypass flow  44  will be gradually decreased and buffer tank flow  54  increased so as to increase the ability of the heating system to handle transience as buffer tank  15  will store sufficient heated water to buffer any temperature fluctuations due to a sudden increase or decrease in demand. 
       FIG. 4  depicts an alternate embodiment of water heater  2  configured to provide hot water for a variety of applications. In this embodiment, water heater  2  is connected to a second demand point  68  via an auxiliary three way valve  59 . A second outlet temperature sensor  23  provides water temperature at second demand point  68 . In this configuration, heated flow  13  is mixed with inlet flow cold water to achieve a desired temperature specified for second demand point  68 . Mixed flow  21  to second demand point  68  is a merged flow of heated  13  and cold water inlet flow. Controller  38  determines the positions of three way valve  59  ports for mixing the correct amounts of heated and cold water to achieve the desired temperature. The need for having an additional water outlet at distinct temperatures is solved by providing a three way mixing valve and a temperature sensor operably connected to water heater controller  38 . The temperature to each water outlet is user definable. For example, the water line connected a dishwasher could be set to 140 degrees Fahrenheit while the water line connected to a shower could be set at 105 degrees Fahrenheit and the water line connected to a hydronic heating system could be set at 160 degrees Fahrenheit. 
     Referring again to  FIG. 4 , an external auxiliary device circuit  19  is provided. In this embodiment, auxiliary heat sink  64  (e.g. a radiant heat coil), incorporates a modulating valve  47  wherein modulated heated flow  13  is externally recirculated with the aid of pump  28  and returned via internal recirculation flow  32 . In cases where cold water column at first demand point  66  is a concern, such external configuration may also be used to recirculate heated water to first demand point  66  placed at great distance from water heater  2 . The problem of long delays to get hot water to the first demand point  66  due to cold water being present in the lengths of pipe between the water heater and first demand point  66  is solved by providing pump  28  to enable external recirculation. Additionally, external auxiliary device circuit  19  can be used to introduce heated water into the hybrid tankless water heating system derived from alternate energy sources such as solar energy, geothermal energy, microwave energy, electric energy, or the like. 
       FIG. 5  depicts another way a heat sink may be connected. In this embodiment, a heat sink such as a radiant heat coil is connected to internal recirculating flow circuit  25 . This embodiment provides less flexibility in controlling the heat sink flow since auxiliary heat sink  64  flow is also internal recirculation flow  32 . 
     Now that an exemplary embodiment of the present invention has been described, focus will be turned to a discussion of the novel features and advantages provided. Many drawbacks and limitations of the prior art have been overcome by the present invention. 
     By placing buffer tank  15  downstream from the primary heat exchanger variations of actual outlet temperature from a desired outlet temperature can be reduced. Referring to  FIG. 1 , buffer tank  15  receives heated water at its inlet from the exit port of the heat exchanger that is receiving heat from a burner  34 . In one mode of operation, the water flowing through buffer tank  15  is delivered to first demand point  66 . In another mode of operation, a portion of the water flowing through buffer tank  15  is delivered to first demand point  66  and the remaining portion is recirculated to the inlet of the water heater. The recirculated water is merged with incoming cold water at the inlet, resulting in a flow into the heat exchanger that is at a higher temperature than the incoming cold water. The water temperature exiting the heat exchanger can fluctuate due to variable heat rate provided by burner  34  during operation. When the water flow with fluctuating temperature enters the buffer tank, the incoming flow mixes with the existing buffer tank  15  water, thereby lessening the impact of any sudden changes in temperature of the incoming flow and resulting in water flow that is more uniform over time. 
     The current buffer tank  15  placement differs from prior art water heater systems where buffer tank  15  is mounted upstream of the heat exchanger. In the prior art configuration, a forced recirculation is necessary to effectively mix a heated flow with the cold incoming water, necessitating the utilization of a recirculation pump. Since buffer tank  15  is placed downstream from the heat exchanger in the current invention, no forced recirculation is necessary to realize the benefit of buffer tank  15  during portions of the water heater operation. Thus, the problem of minimizing variation of actual outlet temperature from a desired outlet temperature in a fluid control system is solved by disposing a buffer tank  15  downstream of the heat exchanger in a fluid heating system. 
     Another drawback with current on demand water heaters is an undesirable and inconvenient delay for the user to receive water in the desired temperature range. The delay associated with obtaining the hot water at the desired temperature is related to the temperature of the cold water input feeding the water heater. The delay can be attributed to the corresponding heat transfer from the burner to the contained water located within the heat exchanger tubes. A byproduct of the delay results in a user avoiding the cold initial water flow resulting in waste of both time and water. By way of illustration, most users taking a shower will wait to commence use of the water until the temperature reaches (increases to) a certain comfortable range. The flowing water, as well as the user&#39;s time, is wasted as the cooler water flows down the drain waiting for the flowing water to reach desired or target temperature. 
     A second heating source, when placed inside a reservoir of water such as the stored water in buffer tank  15 , enables the water heater to quickly meet the sudden increase of a sizable hot water demand by quickly raising the water temperature in buffer tank  15  so that water can be delivered to the user at desired temperature. In one embodiment, an immersion electric heater is used as secondary heating element  16 . A secondary heating element  16 , e.g. an electrical heating element, transfers heat by conduction to the water surrounding its coil. Unlike a burner-heat exchanger arrangement, the heating source of secondary heating element  16  comes in direct contact with the water and can therefore transfer heat more efficiently to the water. 
     A second heating source also provides fine heating modulation which is lacking in a primary heating source. Fine tuning the outlet fluid temperature is achieved by providing primary heating element for coarse adjustment to the inlet fluid temperature and further providing a secondary heating element for fine adjustment to the inlet fluid temperature. 
     Overshoot in outlet temperature response is minimized by providing a secondary heating element  16  with variable heating power at low heating rate, and/or a mixing valve and/or a capillary bypass line  60 . By way of illustration, the secondary heating element  16  (e.g., electric heating coil) is used where ambient temperature is over 70 degrees F. Primary heating element  9  (e.g., burner) provides coarse adjustment and secondary heating element  16  provides fine adjustment to the inlet temperature. A decrease in demand causes excess heat stored in the heat exchanger tubes to be transmitted to the fluid due to the temperature gradient favoring heat transfer from the tubings to the fluid. Buffer tank  15  holds ample amount of water to be recirculated in order to dissipate the excess heat once heating has ceased. A mixing valve mixes overly heated fluid with fluid at a lower temperature, quickly reducing a potential overshoot in the outlet temperature to a safe temperature. 
     Capillary bypass line  60  operably connecting the cold fluid inlet and the hot fluid outlet allows fluid flow from the high pressure side (i.e., cold side) to the low pressure side (i.e., hot side) causing the overly heated fluid to be mixed with the colder fluid. 
     Delayed temperature response is also minimized by proactively maintaining the outlet temperature during pre-set periods of water usage or predicted periods of water usage. In the present invention, there are set time periods in a day at which the outlet temperature will be controlled to the desired temperature to avoid both firing and transient delays. During these periods, the water will be heated, stored in the buffer tank and recirculated. The user may preprogram periods of a day in which high water usage is expected via user interface  43 . The user may also select a mode where the controller, through collection and analysis of water usage data over a period of time, automatically determines periods of high usage and maintain outlet water temperature at desired temperature. 
     When a water demand detected, delayed temperature response is solved by proactively recirculating fluid for initiating the primary heating element. The rapid detection of a growing demand by differential pressure switch  104  causes burner  34  to turn on even when the flow (say between 0.005 and 0.5 gpm) has not risen above the flow sensor  26  detection threshold (typically 0.5 gpm), thereby shortening the time it takes to achieve the desired outlet temperature. 
     Current tankless water heaters start controlling to desired temperature by detecting a flow condition. Typically, a minimum flow threshold of 0.5 GPM is required to turn on the burner. If a demand fluctuates above and below the minimum threshold, the water will be heated only during portions of the demand resulting in water temperature not meeting the desired temperature. In order to detect a small flow, a prior art method uses a difference in temperature between the inlet and outlet ports of a buffer tank  15  or the rate of drop of the inlet port temperature and outlet port temperature to indicate a flow condition. The use of this indication may be erroneous since the temperature differential between the inlet port and outlet port of buffer tank  15  does not necessarily indicate a flow demand. Another prior art method to detect that a demand exists in a buffer tank involves sensing the temperature drop in either the inlet port or outlet port. This method often results in erroneous demand indication. Therefore, utilizing these indicators may cause the water heater to warm the tank water unnecessarily. 
     Typically, a minimum flow threshold of 0.5 GPM is required to indicate a demand to cause the burner to turn on. In prior art tankless water heaters, since water is heated only when the minimum flow threshold is met, there will be no heating when the flow demand does not rise above the minimum threshold. This flow condition below the minimum threshold is called the dead zone. In the present invention, a small flow is detected using a differential pressure switch  48 , wherein the switch is capable of detecting a flow greater than 0.005 GPM. The detection of a small flow causes secondary heating element  16  to turn on, thereby keeping the outlet water warm. Thus, the problem of dead zones is solved by utilizing a differential pressure switch capable of detecting a small flow for initiating a heating element. 
     Variance between desired outlet fluid temperature and actual output fluid temperature is reduced by providing supplemental heat via a secondary heating element  16 . The need to respond quickly to a hot water demand causes the controller to fire the burner at high rate in order to meet a heating demand. Firing the burner at high rate causes water to warm up quickly. However, the heat exchanger tubing material can store excess heat and reach a high temperature. When the desired temperature is about to be reached, the burner is turned off to avoid or reduce outlet temperature overshoot. If the burner is turned off too early, the outlet water temperature will not reach the desired temperature quickly. If the burner is turned off too late, the risk of overheating the water is greater. Even after the burner is turned off, the excess heat stored in the heat exchanger continues to cause a positive temperature gradient between the heat exchanger tubes and the water flowing through them, thereby causing the heat exchanger to maintain the previously induced burner heating rate even while the burner has now been turned off. If the outlet water temperature can no longer meet the desired temperature, the burner is again turned on. This frequent turn-on and turn-off of the burner causes the undesirable phenomenon called hunting, i.e., the outlet temperature fluctuates about the desired temperature even in steady state. With the use of secondary heating element  16  (e.g. electric heating element) in conjunction with burner  34  (primary heating system  8 ), water heater  2  can react quickly to a water demand by firing burner  34  to heat the inlet water quickly while reducing overshoot. Secondary heating element  16  is capable of providing a small heat rate that complements the heat rate produced with normal burner  34  operation and pulse firing of burner  34 . Secondary heating element  16  minimizes the need for burner  34  to take the outlet temperature very close to the desired temperature before turning off and thereby overcomes temperature overshoot. The problem of excessive heating control hunting especially during periods of trickle or low flows is solved by providing a secondary heating element  16  or pulse firing of primary heating element  9  for fine control of outlet fluid temperature and a means for selecting whether to initiate primary heating element  9 , a secondary heating element  16 , or combinations thereof. 
     In prior art water heater systems, a cold sandwich effect occurs when a user briefly turns off a faucet that has been running water of desired temperature for an extended period of time. This brief cessation of demand creates a column of scalding hot water resulting from excess heat transferred to the small quantity of water remaining in the water heater from the heat exchanger thermal mass upon shutting down burner  34 . Responding to this high outlet temperature, the controller ceases to provide more heat to the water causing a trailing column of water to be under heated. Eventually when the outlet temperature sensor starts detecting this cold water column due to a resuming demand, heating resumes and a steady state flow with desired temperature is achieved. The problem of the cold sandwich effect in a transient system is solved by recirculating the small quantity of water remaining in the heat exchanger upon cessation of a demand via the buffer tank. Recirculation causes excess heat to be dissipated and distributed more uniformly throughout the remaining volume of water in the water heater. When a demand restarts, recirculating via buffer tank  15  causes the colder incoming water to be mixed with existing warmer water in the buffer tank to yield water at more uniform temperature over time. 
     Legionnaire bacteria grow well in stagnant water of from 80 to 120 degree Fahrenheit. Recirculation of outlet flow and maintaining water temperature at elevated temperature of 140 deg Fahrenheit aids in eliminating this dangerous bacteria. In the current invention, the water heater can be programmed to turn on periodically. Recirculation causes heat sources to turn on in order to maintain the outlet temperature at the desired temperature. Thus, the problem of bacteria growth in stored water supplies and water system components is solved by recirculation of outlet flow and maintaining water temperature at elevated temperature of 140 degrees Fahrenheit. 
     In the current invention, an unconventional heat exchanger is used. The burner  34  is inverted and placed above the heat exchanger tubes in a burner cavity. When the fuel supply line is opened, the blower blows air-fuel mixture down towards the heat exchanger disposed below the blower nozzle. As such, the flue gas has the tendency to rise and it must be forced down towards the heat exchanger by a blower. The heat exchanger is an array of tubes where the heat provided by burner  34  is transferred to the water flowing in the tubes. The heat exchanger is configured such that the exiting water flow portion is disposed closest to burner  34  and the incoming water flow portion is disposed most distant from burner  34 . 
     Prior art tankless water heater systems do not provide for an alternate power source during primary power outage. In the current invention, an alternate power source is provided and automatically sourced should a primary power loss to the water heater system occur. Thus, a preferred embodiment provides and automatically sources a secondary onboard energy source such as a backup battery or an inline hydro generator capable of generating electrical power from hydraulic power. By continuously providing power to the water heater in the event of a primary power loss, freeze protection is provided, giving enough time for the user to either drain the water heater system or provide additional and continuous backup power. In addition, continuous operation of the water heater system also allows for temporary hot water usage. 
     The problem of overheating (temperature spikes) of water during a rapid reduction (step down and completely off) in demand is solved by actively re-circulating the water, cutting down heat input, and in some cases by turning the blower to a maximum speed without flame. This is achieved by detecting a change in flow demand, starting the recirculating pump, lowering or shutting off firing of the burner and turning on the blower. Thus, transient temperature spikes are reduced by means of controlling a blower that can optionally operate independent from the burner and a recirculation pump. 
     In the current invention, detection of a leakage between the water inlet and water outlet of the water heater is accomplished with a moisture sensor that alarms when excessive moisture is detected in the cavity of the fluid heater inside the water heater enclosure. This sensor is capable of sensing a leak that occurs within the water heater enclosure. 
     In the current invention, leakage from external plumbing operably connected to the water outlet is detected and/or ceased by providing a differential pressure switch operably connected to the inlet end and outlet end of the water heater. This differential pressure switch detects a small flow condition by registering a pressure differential between the two ends of the water heater over a period of time. If this period exceeds a preset leak time threshold, a leak condition is raised and a warning is issued to the user and/or the flow control valve is moved to the closed position. 
     In the current invention, a small leak or an open faucet for long periods of time will be detected via a pressure difference as registered by a pressure differential switch over an extended period of time as specified by the user. For example, a continuous detection of a pressure difference over a period of 30 minutes may indicate a leak. 
     In the current invention, detecting leakage downstream of the flow sensor is achieved by providing a flow sensor to detect the actual amount of flow and compare such actual amount of flow to a known quantity of water programmed to be delivered such that when the difference exceeds a predetermined fault threshold, a leak condition is raised. Upon detecting a leak condition, a warning is issued to the user and the flow limiting valve is shut to stop further water loss. 
     Freeze hazards are minimized in the present invention by periodically maintaining water temperature at a level higher than the freezing point. The current invention periodically recirculates flow even without an external user demand. This recirculation may occur at preprogrammed or learned usage periods and monitoring the heater inlet and outlet temperature or their rate of change such that when the outlet or inlet temperature drops below a certain threshold, or if the rate of temperature drop exceeds a certain threshold, recirculation and/or a secondary heating element  16  is initiated. Similarly, when the outlet or inlet temperature drops below a certain threshold or if the rate of temperature drop exceeds a certain threshold, a secondary heating element  16 , combined with pulse firing of primary heating element  9 , as well as recirculation, are initiated. 
     Detecting trickle flow or low flow at any given time is accomplished by using a differential pressure switch. A differential pressure switch is operably connected to inlet  10  and outlet  12  ends of the water heater. The switch is in the off position when there is no flow in the water heater. A flow in the water heater creates a pressure differential between inlet  10  and outlet  12  ends, thereby moving the differential pressure switch to the on position. The pressure differential switch is preferably capable of detecting flow greater than 0.005 gpm or a range of flow not detectable in a flow sensor. 
     Prior art buffer tank  15  temperature based water heater control systems rely on the temperature difference between buffer tank  15  inlet temperature and buffer tank  15  outlet temperature to indicate a need to turn on a heat source. In the present invention, the entire range of flow demand is detectable by using the differential pressure switch and the flow sensor. Since the temperature difference between buffer tank  15  inlet and outlet is not required, only one temperature sensor is required for buffer tank  15 , provided that inlet buffer tank  15  flow is mixed well to yield a uniform water temperature throughout buffer tank  15 . This provides an advantage over the prior art using multiple temperature sensors in the controls scheme. Temperature variations within buffer tank  15  are minimized by having baffles or a barrel-hole style inlet system for creating turbulence which promotes mixing of the incoming water with existing water in the tank, enabling the use of a single temperature sensor representative of the entire buffer tank  15  temperature. 
     In the prior art, the need for a forced recirculation is indicated by a drop in the buffer tank inlet temperature. In this prior art configuration, heating is tightly coupled with the presence of a recirculating flow and heating would not occur until force recirculation has been activated. In the present invention, no forced recirculation is necessary to initiate heating. When a demand exists as indicated by the differential pressure switch and the flow sensor, a high firing rate of burner  34  combined with recirculation, meets the hot water demand in a short time without having to unnecessarily warm water stored in buffer tank  15  for an extended period of time. Thus, the problem of long duration necessary to heat up water to the desired temperature is solved by actively recirculating the water and by deliberately increasing the heat input (higher firing rate) for a predetermined duration upon detecting a flow demand. This provides a significant advantage over the prior art since temperature control is enhanced during transience and startup flow conditions. 
     In systems relying on the water temperature in buffer tank  15  as a leading indicator to trigger heating, much of the heating is wasted when there is no actual demand. However, with a system relying on the differential fluid pressure between the inlet and the outlet ends and flow sensor to indicate demand, heating would commence only when there is an actual flow unless it is programmed to do so otherwise. Also, temperature is a lagging indicator in that it takes longer for temperature to change to indicate a change in demand whereas a heating system based on the presence or magnitude of a flow indicates a change in demand immediately so that this change can be acted upon immediately thereby improving the heating response time. Thus, the goal of maintaining outlet temperature is achieved by using a differential pressure and a flow rate as leading indicators in the present invention. 
     When hot water is first requested, a motorized three way valve closes the flow path to the buffer tank, thereby bypassing it and diverting all heated water to the point of use. As the burner keeps up, the three way valve slowly diverts the flow through to buffer tank  15  and maintains a portion of the flow in the bypass line. Thus, the long duration necessary to heat water to the desired temperature is eliminated by bypassing the buffer tank  15  during startup and increased demands so that undiluted hot water can be delivered to the point of use without having been mixed with cooler water in the buffer tank. 
     Water hammer occurs when a user suddenly ceases water demand by closing a valve creating pressure surges. A capillary bypass line  60  can absorb the pressure surges, mitigating the damaging effects of water hammer. Thus, the problem of high pressure spikes is minimized by using a dedicated capillary bypass line  60  operably connected to the inlet and outlet ends of the water heater. 
     The minimum power output of burner  34  is typically 20,000 BTU/hour (with a modulation range of 10:1 (turn down ratio) and a burner size of 200,000 BTU/hour). In order to achieve a lower average power output, pulse firing is used. In pulse firing, the burner power is modulated such that in a pulse cycle, burner  34  is turned on for a preset duration and turned off or turned down to a lower setting for another preset duration. The average power is the average power of each cycle. Thus, the problem of inadequate low firing rate or minimum firing rate constraint is solved by pulse firing burner  34  (with constant blower operations). 
     As the water heater ages, various components of water heater  2  may break down and require repair or replacement to ensure proper operation of water heater  2  and to avoid safety hazards. Scaling may develop in the interior surfaces of the flow tubes of water heater  2 . Inadequate recirculation can be a sign of pump breakdown. The ability to detect a failed pump is provided by detecting excessive flue temperature and flue temperature rise rate. This provides a significant advantage over the prior art. A service alert can be issued when such a condition is detected. Thus, pump failure or inadequate re-circulation flow is detected by monitoring the flue out temperature and its rate of change to proactively managing the heat input (e.g., powering off the unit). Additionally, the problem of degraded heat exchanger performance is detected by monitoring the flue out temperature and its rate of change, buffer tank or outlet water temperature to proactively inform user to intervene. Under normal operation, the heat provided by the burner is properly transferred to and absorbed by the water flowing through the water heater. Since a good portion of provided heat is recovered, the flue gas temperature should not be excessively high. The flue vent material is protected by monitoring the flue gas temperature and its rate of change to proactively manage heat input. 
     Most modern water heaters are designed without providing the user or service personnel serviceability. Poorly maintained water heater may need to be replaced prematurely. In the present invention, serviceability is facilitated. Hard water causes unwanted mineral deposits (scaling) on the fluid contact surfaces of the water heater system. Severe scaling can cause severe drop in the water heater efficiency and life span. Scale deposits in the interior surfaces of heat exchanger tubes can reduce the heat exchanger efficiency as the scale deposits reduce heat transfer rate from the exterior to the interior surfaces of the heat exchanger tubes. Therefore, more heat would be required to raise each degree of water temperature. Excessive scale deposits, or any other like issues, that cause reduced heat exchanger efficiency, can lead to overheating of the exterior surfaces of a heat exchanger resulting in a shortened heat exchanger service life. In addition to resulting in damage to the heat exchanger, overheating of the heat exchanger exterior surfaces leads to undue energy loss. The problem of overheat and heat loss of the exterior surfaces of heat exchanger is solved by providing at least a baffle or a structure capable of swirling or mixing pre-combusted air (as provided by blower  36 ) in the vicinity of the heat exchanger, thereby promoting heat transfer from the exterior surfaces of the heat exchanger to the pre-combusted air and recovering this absorbed heat of the pre-combusted air by redistributing it to colder portions of the heat exchanger surfaces. 
     There is provided a service access to the hot water system such that a cleaning agent may be introduced in the recirculation circuit and a dedicated service mode such that the recirculation pump may be activated without turning on any heat sources. The prior art does not provide end-user direct access such as this, rather, requires a trained maintenance person to perform such tasks by at least partially taking apart the water heater. 
     When a problem occurs, a typical water heater diagnostic system provides rudimentary information in the form of fault codes which require further decoding for corrective instructions. A novel feature of the current invention enables an end user of the current water heater to take appropriate steps by providing not only fault codes but also corrective instructions associated with the fault codes. As such, the end user is capable of taking corrective actions without having to wait for service personnel or resort to an instruction manual. In one embodiment, a fault code is decoded by providing speech capability to the water heater, a means of transmitting a spoken fault code by phone to a fault code database, a means for identifying the problem source and its associated corrective procedures based on the transmitted fault code and a means for notifying the user of the corrective instructions. In another embodiment, a fault code is decoded by automatically transmitting a fault code via internet to an off-site database, retrieving corrective instructions based on this fault code, and notifying the user of corrective instructions with text or audible speech instructions. 
     In the current invention, active and continuous monitoring performance and health of the water heater minimize unforeseen service outages. In one aspect, the controls provide for automatic adjustment of operating parameters based on measured performance. Performance is determined by logging blower rpm, power output and comparing them to nominal values and the water heater&#39;s historical data. Additionally or alternatively, prognostic feedback is provided to the user so that a water heater problem can be responded to before a break down can occur. In the event a problem is so severe that it cannot be corrected automatically, the user is timely alerted. 
     In a condensing heat exchanger, a drip collection pan is used to collect the condensate which is in turn drained with or without pressure assist via a drainage tube to a sump. A blockage in the drainage tube can cause backup or overflow of the condensate and cause corrosion in water heater components exposed to this condensate. A condensate level sensor  50  provides alert and calls attention of the user that condensate drainage is blocked and requires attention. A small pump may also be used to aid condensate drainage. In the case a small pump is used, the need for gravity drainage is unnecessary and therefore the need to mount the water heater at an elevated position to create this gravity drainage is eased. Thus, the problem of flue condensate backup or blockage is detected by using condensate level sensor  50 . 
     Flow sensor  26  as used in this invention is connected to the inlet of the water heater and therefore does not experience elevated temperature. As such, it does not require a more expensive high temperature grade flow sensor. By mounting the flow sensor outside of the recirculation circuit, there is no pressure loss imparted by the flow sensor. This allows the use of a pump with a lower power rating, thereby making the water heater more economical. Additionally, no proprietary valves are required. 
     When a user first enters a shower, the user may be initially satisfied with a water temperature that is higher than the ambient air temperature. The user will likely demand progressively hotter water as the shower progresses due to user temperature acclimation. Normally, a user demands hotter water by manually increasing the valve controlling the hot water supply. The present invention optionally includes a feature that allows the user to set an automatic temperature rise rate and a high temperature limit that aligns with the user&#39;s temperature rate of change profile or acclimation profile; thereby providing a varying temperature service. For example, the initial requested temperature is 90 degrees. The user may choose to increase the water temperature by 1 degree per minute. If the demand is left untouched, the water temperature would be increased to 95 degrees in 5 minutes of usage. If the high temperature limit is set at 92 degrees, the water temperature would be increased only to 92 degrees in 2 minutes and remains at that temperature throughout the rest of the duration of the demand. 
     Automatic faucets have been in widespread use for some time. In an automatic faucet system, a proximity sensor is used to detect the presence of a demand. When a user approaches the faucet with his or her hands extended into the field of view of the proximity sensor, the faucet is automatically turned on and water is automatically discharged. This will trigger external or even internal recirculation, thus “preparing” the water heater to be ready for anticipated water use. Proximity sensors here are used to detect the presence of a person in the bathroom, sending a signal to the heater and preparing the heater for hot water use. 
     Natural gas is an energy source or fuel commonly used in residential homes, businesses, as well as industrial settings, appliances and systems that operate on natural gas include HVAC systems, kitchen stoves, clothes dryers, water heaters, and the like. The number of natural gas devices that can be powered simultaneously is a function of the utility natural gas service line(s) maximum capacity compared to the sum of the gas usage rates of the individual devices. A natural gas over-demand situation occurs when the cumulative sum of usage rates from each of the operating natural gas appliances or devices is greater than the maximum capacity of the incoming natural gas service utility line or lines. During such a natural gas over-demand situations, it&#39;s common to engage a device&#39;s fail-safe system. System fail-safes include flameout or other type of natural gas shut down type routines resulting from inadequate natural gas supply or gas starving. 
     In one embodiment of the present invention, the undesirable gas over-demand type situations are avoided or reduced in frequency in hybrid tankless water heating systems by utilizing a modulating gas burner where the natural gas consumption rate is reduced, or de-rated. This reduction is accomplished via the modulation of a modulating gas valve and cooperating blower. Controlling the heater in such a manner, results in the reduction of natural gas usage or consumption rate and associated reduction in blower speed by predetermined levels dictated by a controller. The typical hybrid tankless water heating system includes a blower, a burner, and a buffer tank having an auxiliary heating means having an energy source other than natural gas along with supporting hardware. Supporting hardware includes at least one natural gas pressure sensor, can be a hardwired element as well as a wireless version, that is located upstream or in the gas feed line of the hybrid tankless water heating system, and a controller to enable cooperation among the various system elements. Alternatively, a network of gas powered devices can relay their status including usage rates to a controller thereby creating an alternate embodiment without the need for a natural gas pressure sensor (since available gas capacity can be calculated). 
     In a typical configuration, the natural gas pressure sensor is used to determine the real time natural gas available to the hybrid tankless water heating system, and in conjunction with a controller, is used to detect a natural gas over-demand situation. If such a natural gas over-demand situation is detected or predicted, the controller reacts by de-rating the burner and blower subsystem of the hybrid tankless water heating system. Given a particular natural gas input line geometry (e.g. round pipe inner diameter); maximum flow rates are easily calculated along with corresponding heat output. 
     Another variation of the present embodiment under discussion is the use of two or more external gas usage sensors or monitors each having an input signal to the controller that provides an indication as to whether or not the gas supply is shared among gas consuming appliances, enabling the calculation of the real time gas supply rate available. If it is determined that the utility natural gas service line is shared by one or more gas consuming appliance, the water heater burner will be de-rated if a natural gas over-demand situation is detected or predicted by limiting the gas valve opening to align with the actual gas supply available at that moment. By having a more accurate burner heat output prediction, the controller is better able to determine whether supplemental heat via the buffer tank to compensate for the reduction of natural gas should be used. In one embodiment, the external gas usage input receives gas pressure sensor measurement from the gas supply line. In another embodiment, the controller wirelessly receives one or more signals indicating the corresponding pressure drop, usage rates, or the like of the other appliance(s) sharing the same gas supply line. 
     The typical method for reducing natural gas consumption in a hybrid tankless water heating system to avoid a natural gas over-demand situation comprises continuous monitoring of available natural gas pressure along one or more portions of the natural gas distribution network monitoring for potential natural gas over-demand situations. If such a natural gas over-demand situation is detected, a controller reduces the burner natural gas consumption rate and speed of the corresponding blower by predetermined levels such that natural gas consumption rate of the hybrid tankless water heating system avoids the undesirable natural gas over-demand situation. 
     The following example provides a more detailed analysis of one embodiment. The embodiment includes a modulating gas burner wherein the supply of gas is modulated by adjusting a modulating gas valve connected to the modulating gas burner. Referring again to  FIG. 2 , there is further provided an external gas usage input signal  53  which corresponds to the rate at which the gas supply is used by external gas consuming appliances such as a furnace. In a typical operating environment, there may be multiple gas consuming appliances, such as a gas furnace, radiant floor heater and water heater burner which source gas from the same gas supply. If all appliances are operating at their respective high or maximum capacities, a condition may exist where the gas supply is incapable of supplying gas to all appliances at normal operating pressure. Under such condition, one or more appliance is said to be starving for gas wherein the actual gas supply rate is below the expected amount. There exists a need in modulating gas supply to the current burner such that the heat output corresponding to the gas supply is quantifiable. The external gas usage input signal  53  provides an indication as to whether or not the gas supply is shared amongst gas consuming appliances and/or what gas supply rate is available. In one embodiment, such indication is provided by a gas pressure reading that is lower than a predetermined level. Upon determination of a shared gas supply, the water heater burner will be derated by limiting the gas valve opening to better reflect the true heat output corresponding to the gas supply. By having a more accurate burner heat output prediction, the controller is better able to determine whether supplemental heat is necessary. In one embodiment, the external gas usage input receives gas pressure sensor measurement from the gas supply line. In another embodiment, the external gas usage input receives a signal indicating the rate or the presence of gas usage of a second appliance sharing the same gas supply. In one embodiment, the external gas usage input is received wirelessly. 
     When a hybrid tankless water heating system is initially turned on or when a water demand is abruptly increased, there is an initial warm up period where the burner-blower subsystem has to get the heat exchanger up to a temperature that will yield output water flow at the target predetermined temperature. During this warm-up period, if hot water is requested, the temperature of the output water flow will be lower than the desired target predetermined temperature, thus creating undesirable cold water transient. 
     In one embodiment of the present invention, the length of time or duration associated with undesirable cold water transient is reduced by the incorporation of a water flow limiting valve and a controller functionally connected to said water said water flow limiting valve. This is accomplished by restricting the water flow through the hybrid tankless water heating system or water heater, thereby enabling the delivery of water at the predetermined temperature at a reduced flow rate. Supporting hardware includes at a minimum, an output temperature sensor located at the output of the hybrid tankless water heater for measuring an output temperature, an input temperature sensor located at the input of the hybrid tankless water heating system for measuring an input temperature, a flow sensor for measuring a flow rate requested of said hybrid tankless water heating system and a controller for determining an operating capacity corresponding to the difference between the predetermined temperature and the output temperature, the difference between the output temperature and the input temperature and the flow rate. As a demand capacity increases abruptly, the flow limiting valve is restricted to reduce flow rate such that the output water predetermined temperature is quickly achieved. As the water heater&#39;s operating capacity becomes more readily available, the setting of the water flow limiting valve is adjusted such that higher flow is permitted while the output water is maintained at the predetermined temperature. 
     An example of the aforementioned flow rate follower function is described in the following sample scenario having a given input or inlet water pressure. In this example the target water flow is 5 gpm at predetermined target temperature. Initially, the water flow limiting valve is set to a predetermined position that is less than its maximum setting, to enable a predetermined flow rate of 3 gpm. As the water heater output temperature nears or attains a steady state condition, the water flow limiting valve gradually opens allowing additional water flow. This occurs as a result of the burner-blower subsystem heating the heat exchanger up to a temperature, or thermal capacity that is capable of delivering output water at a higher flow rate at the predetermined target temperature. An initial flow represented by t=0 is set at 3 gpm. Then at time t=1, the valve opens further to allow flow up to 4 gpm (valve restriction is decreased to allow more flow). Finally at time t=2, the flow is increased to 5 gpm (valve restriction ceases allowing full flow). It is understood that steady state conditions are achieved at both t=1 and t=2 time intervals. This allows for the heat exchanger-burner blower system to catch-up to the heat demand while maintaining the desired outlet temperature since the burner cannot instantly achieve the desired thermal output. 
     When a hybrid tankless water heating system is initially turned on, the system selects one of a number of default operating points; the operating points include burner heat output values and associated optimum blower speeds that are optimized for specific operating environments. The default schedule of operating points is constructed with the typical environment in mind. Situations exist where the environment of a specific water heater installation cause operating points to deviate from their default values. In such situations, it would be advantageous to update the schedule of default operating points with values better suited to the given installation environment. The default value or values will only be added to, replaced and/or initiate a modification of the schedule of operating points when a new operating point satisfies a schedule of operating points update criteria. 
     An automatic blower adjustment is a valuable feature due to installation variations. Each installation would have specific air intake, exhaust restriction, gas pressure variation. The blower formula included from the factory could be more than 10% off. Auto calibration type software would use the burner heat output (mdot×Cp×delta T) and blower speed parameters to confirm or update the factory map to each specific install, wherein mdot is mass flow rate of water, Cp is the specific heat capacity of water, and delta T is the difference between Tinlet and Toutlet. This would improve the performance of a tankless water heating system, which includes improvements in transient time delays and time to reach steady state temperature conditions. 
     A method for updating a default schedule of operating points would utilize water heater operating parameters including mass flow rate, Tinlet, Toutlet, and blower speed (rpm) to run the predetermined computations and create a map or an updated schedule of operating points. Other conditions required to enable an update to the default schedule of operating points would include the attainment of steady state conditions as it pertains to water output temperature, water flow or rate, and blower speed. 
     Below is an example of a factory set default schedule of operating points relating burner heat output to blower speed. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Burner Heat Output (kBTU/hr) 
                 Speed (RPM) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 10 
                 900 
               
               
                   
                 15 
                 950 
               
               
                   
                 200 
                 5700 
               
               
                   
                   
               
            
           
         
       
     
     When a new data point that satisfies the point update criteria is identified, the controller would modify the default schedule of operating points in light of the new information or new operating point; e.g. add a new data point to the default schedule or replace an existing data point. 
     In one embodiment of the present invention, the schedule of default operating points updating system is accomplished in a water heating system having one temperature sensor for measuring water output, one temperature sensor for measuring water input, a blower monitor for determining blower speed, and a controller functionally connected to each of said temperature sensors and said blower monitor. The controller samples the temperature sensors and monitors blower operation in search for an improved operating point(s) that satisfy the schedule of operating points update criteria. Once an operating point(s) meets or satisfies the update criteria, the new improved operating point will be incorporated into the default schedule of operating points. The following is an example of a schedule of operating points update criteria algorithm: 
     Heat output X at a particular blower speed Y, is determined by using the following formula:
 
 X=mdot×Cp ×( T outlet− T inlet),
 
where
 
mdot is the water mass flow rate,
 
Cp is the specific heat capacity,
 
Toutlet is the temperature of heated outgoing water, and
 
Tinlet is the temperature of cold incoming water.
 
     X is determined using the above formula while Y is obtained by using a blower monitor. X is typically expressed in kBTU/hr units, while blower speed is typically expressed in rpm. The following is a table representing five operating points of heat output versus blower speed. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 X1 Y1 
               
               
                   
                 X2 Y2 
               
               
                   
                 X3 Y3 
               
               
                   
                 X4 Y4 
               
               
                   
                 X5 Y5 
               
               
                   
                   
               
            
           
         
       
     
     Any new X Y pair (new operating point) is only considered for incorporation into the above table if the point reaches a predetermined steady state condition. Steady state condition is achieved or satisfied when Toutlet, Tinlet, flow rate and blower speed have been in a substantially steady condition for a predetermined period of time. The term K is defined as the variance threshold below which a new operating point is used for updating the factory default table. 
     EXAMPLES 
     Example 1 
     While water heater is in a steady state condition with conditions X1&lt;X&lt;X2 and Y1&lt;Y&lt;Y2 being satisfied, and if the absolute difference between ratio (X−X1)/(X2−X1) and ratio (Y−Y1)/(Y2−Y1) is greater than K, then the proposed new operating point or pair X, Y will be discarded. If the difference is less than K, then the proposed new operating point or pair X, Y will be incorporated into the factory default table. X1, Y1 and X2, Y2 will be retained. 
     Example 2 
     While water heater is in a steady state condition, with conditions X2&lt;X&lt;X3 and Y1≦Y≦Y2 being satisfied, and if the absolute difference between ratio (X−X1)/(X3−X1) and ratio (Y−Y1)/(Y3−Y1) is greater than K, and then the proposed new operating point or pair X, Y will be discarded. However, if the difference is less than K, then the proposed new operating point or pair X, Y will be incorporated into the lookup table. However, in this example, the existing X2, Y2 operating point will be replaced with the new operating point or pair X, Y. 
     It is to be understood that a plurality of methods may be employed in determining whether an X, Y pair should be adopted. 
     Thus, having broadly outlined the more important features of the present invention in order that the detailed description thereof may be better understood, and that the present contribution to the art may be better appreciated, there are, of course, additional features of the present invention that will be described herein and will form a part of the subject matter of this specification. 
     As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent construction insofar as they do not depart from the spirit and scope of the conception regarded as the present invention.