Abstract:
This invention consists of a heat exchanger, a heating element, and a return line from a remote faucet to provide a constant circulatory flow of hot water through hot water distribution lines in a building utilizing a tankless water heater, such that hot water is available instantly at remote faucets in the building. The invention will eliminate the waste of water and heat while waiting for hot water to arrive at the outlets. A second benefit is to heat the mass of cold water known as the cold sandwich that results from the time delay for the heating element in the tankless heater to achieve its operating temperature. A third benefit is the convenience of not having to wait for hot water to arrive at remote faucets.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/053,977, filed May 16, 2009, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to providing instant hot water for domestic and small commercial building tankless water heating systems that operate without a water storage tank and its associated heating mechanism. 
         [0003]    In recent years there has been a trend toward the use of in-line, high energy heating elements in place of the traditional 40 or 50-gallon water heaters used in most single-family homes in the USA. The principal benefit of these tankless heaters is their capability to provide continuous hot water, unlimited by the size of a water heater tank. 
         [0004]    These systems use a high-energy heater, normally gas or electricity driven, to heat water as it passes through a heat exchanger on the way to hot water outlets in the building.  FIG. 1  shows a typical water system utilizing an existing technology tankless water heater. 
         [0005]    A difficulty experienced in both conventional and tankless water heating systems is the cooling of water in the hot water distribution line to remote faucets following periods when hot water has not been used. Water in an un-insulated distribution line will cool to ambient in less than one hour, and even quicker in an unheated crawl space, such that it may take up to two minutes or more for hot water to arrive at remote faucets. The wait for hot water in a tankless heater system is somewhat greater than for a conventional water-tank heater due to the fact that the tankless heater itself, in addition to the distribution pipes, is not heated unless water is flowing through it. This heating and cooling may occur many times daily depending upon family size and lifestyle. It has been shown in recent studies that a considerable amount of water is wasted down the drain every day in the average home waiting for hot water to arrive at the faucets. In addition, the wasted water was once heated and since cooled in the line which means that heat energy is being wasted. Sewage costs are often increased since they are normally based upon water consumption. 
         [0006]    Some systems have been developed in an attempt to avoid this waste with the typical tank-type water heater. The most convenient and cost effective approach appears to be a water recirculating system that keeps hot water moving from the heater out to remote faucets, and back to the heater through a return line where all the water is conserved. Most of the heat is recovered since the water returning to the heater has lost only a few degrees in temperature, and the flow rate is extremely low. Some circulating water systems are driven by electric pumps, while others are passive in nature. Passive systems use no electricity or gas and exhibit very high reliability. 
         [0007]    The wait for hot water is even longer in tankless heater designs, since the heater is only hot when water is flowing and an additional time delay exists for the heater to come up to temperature after the faucet is opened. Both the hot water in the line to the faucets and the heater will cool to ambient temperature at other times. 
         [0008]    A secondary problem called “a cold-water-sandwich” has been encountered with tankless heaters. This is a situation where hot water has been used at an outlet, and the heater and distribution piping is hot. When hot water is called for at the outlet after a short interval, water in the pipes is still warm and the user expects it will continue to flow at the same temperature, when a sudden drop in water temperature is encountered, followed shortly again by heated water. This phenomenon is due to a several second delay as the heating element comes up to temperature, and the cool water waiting to enter the heater passes through it before the temperature of the heater rises. The drop in line pressure also reflects the fact that water has started to move the instant the faucet opened, which means that a mass of cold water will move past the heating element and will proceed down the line to the open faucet. 
         [0009]    This cold-water-sandwich also occurs in those installations where a tankless heater is in use and a separate water-tank heater is installed for a source of hot water to circulate to remote faucets for instant hot water. Hot water will always be available at faucets so treated until the water in the circulating portion of the hot water line passes through the faucet. At that time, water waiting in the tankless heater element that has cooled to ambient temperature, has flowed out of the heating element prior to the element reaching its operating temperature and arrives at the remote faucet at ambient temperature. This results in a sudden change in what was a hot water flow to several seconds of what seems like ice-water to a person in a shower. 
         [0010]    With the current emphasis on conservation of water and energy, and attendant increases in the price of these commodities, builders and homeowners are moving toward tankless heaters for domestic water heating, resulting in a significant annual growth in their use. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    A heat exchanger with a thermostatically controlled, heated water jacket and a return line from the farthest outlet on any given hot water branch will provide for a convective hot water flow producing rapid hot water at faucets remote from a tankless water heater at all outlets on the loop. The flow of hot water through the heat exchanger will eliminate the cold-water-sandwich at faucets in recirculating tankless water systems. 
         [0012]    Aspects of embodiments of the invention help to eliminate: waiting for hot water with tankless heaters; waste of water with tankless heaters; waste of heat with tankless heaters; and the cold sandwich in tankless heaters. 
         [0013]    The some advantages of aspects of embodiments of the invention are as follows: 
         [0014]    1. The present invention will serve to provide rapid hot water at remote faucets in a water system utilizing a tankless water heater. 
         [0015]    2. The invention will eliminate the “cold sandwich” in a tankless water heating unit with a hot water circulation system. The sandwich consists initially of hot water due to the circulation system, followed by a several seconds of ambient temperature water, and finally a continuous flow of hot water, 
         [0016]    3. This invention will eliminate two key difficulties associated with the tankless water heater concept with a highly reliable component constructed by ordinary manufacturing processes. 
         [0017]    4. This configuration can be produced by existing machinery and priced competitively. 
         [0018]    5. This invention requires minimum electric power or gas to operate. 
         [0019]    6. Due to the simplicity of this concept, high reliability will provide minimum maintenance and long life. 
         [0020]    7. No new technology is required to be developed or used in this invention. 
         [0021]    8. Installation of this invention is simple and can be accomplished by any qualified installer using standard plumbing tools. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1 . Existing Technology Tankless water heater system schematic diagram 
           [0023]      FIG. 2 . Existing Tankless Instant Hot Water System schematic diagram 
           [0024]      FIG. 3 . Tankless Water Heater System with Instant Hot Water and Elimination of the Cold Sandwich 
           [0025]      FIGS. 4A and 4B . Tankless Heater Instant Hot Water Heat exchanger configuration 
           [0026]      FIG. 5 . Check valve design 
       
    
    
       [0027]    Appendix A. Heat Exchanger Test 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    A system schematic of the present invention installed in a Tankless Water Heating system is shown in  FIG. 3 . An element of this invention is a heat exchanger unit ( 20 ) as shown in  FIG. 4B  installed with its input coupled to the output of a tankless water heater ( 21 ), and its output connected to the hot water distribution line ( 22 ) supplying hot water to the various hot water faucets ( 23 ) in the building. A second element, also shown in  FIG. 3 , is one or more return lines ( 24 ) for the purpose of creating a loop for water to flow to all points of the hot water distribution system, and back to the unit through the return lines. The illustrated embodiment of the system is through the use of a convective circulation loop to keep hot water at remote faucets at all times. The electric circulation pump ( 25 ) shown in  FIG. 3  is optional for those installations where the physical arrangement of the building will not allow the necessary height to induce convective circulation. Check valves ( 26 ) in the return loop will prevent reverse flow in the return lines. For purposes of describing the unit, one embodiment is shown to describe the operational concept, however other embodiments are also possible. 
         [0029]    The heat exchanger shown in  FIG. 4A  ( 27 ) is comprised of an outer housing ( 28 ), a water tank ( 29 ), a heat tube ( 30 ), and a heating element ( 31 ). A water jacket ( 32 ) is created in the space between the tank and the heat tube, to provide efficient heat transfer from heated water in the water jacket to water flowing through the heat tube. Return line check valves ( 33 ) will be attached to the tank and fluidly couple the return lines to the interior of the tank. Thermal insulation ( 34 ) in the space between the outer housing and the tank will minimize loss of water jacket heat. All operational components are capable of withstanding water at domestic water pressures without leakage or damage. 
         [0030]    The outer housing ( 28 ) of the heat exchanger will serve as a container to package all elements of the heat exchanger, including provisions for mounting, and will have access points for water inlet ( 35 ) and outlet fittings ( 36 ) and one or more return line check valves ( 33 ). Access for an energy connection, either electric or gas, will be compatible with standard connections in use today. The housing will also serve to present an attractive external configuration in keeping with installation in a modern building, and to protect the unit from dust or damage. 
         [0031]    The water tank ( 29 ) is a water-tight, preferably made of stainless steel although other materials may be substituted, element that will have openings to accommodate sealably attaching each end of the heat tube, a check valve, a thermostat ( 37 ), and heater element fittings. The tank in the embodiment shown has a cylindrical shape, however any geometric configuration capable of containing the necessary internal parts and compatible with the outer housing design will suffice. One convenient embodiment of the tank may be formed with a rolled sheet metal cylindrical body with a bottom and top cap welded on during unit assembly, at the discretion of the designer. The required volume of the water in the tank surrounding the heat tube is determined by the heat capacity necessary to heat the cold sandwich considering the heat balance and efficiency of the heat transfer from the water in the tank to the water in the heat tube. 
         [0032]    The heat tube ( 30 ) is a second water-tight element, preferably made of stainless steel although other materials may be substituted, located internal to the tank ( 29 ), that will include a water inlet ( 35 ) and a water outlet ( 36 ) capable of connection to standard water pipes. A portion of the heat tube will have a larger cross section that will increase its surface area inside the tank to better expose water flowing through it to the hot water in the water jacket, and to slow the velocity of the water flowing through it to allow more time for heat transfer. In this embodiment, the heat tube is shown as a cylindrical tube, with the larger diameter portion of the heat tube will include serrations ( 37 ) as shown in  FIG. 4B , as a further expansion of its surface area. The increase of heat transfer area may also be accomplished through the addition of fins to the outside and inside surfaces of the heat tube. A small hole ( 38 ) in the upper end of the heat tube will allow water from the heat tube to fill the water jacket, and allow a continuous flow of water to circulate from the water jacket to the remote faucets through the hot water distribution lines, and return to the water jacket through the return lines. The heat tube will be contained inside the tank and each end of the heat tube will be sealably attached to openings in the ends of the tank. 
         [0033]    Since the heat tube is a heat exchanger, various other configurations may be used. One very effective alternative is a tube/fin device similar to an automobile radiator that will reside inside the water tank in the same fashion as the heat tube described above. It will be a water-tight unit with an inlet and an outlet and will transfer heat from the heated water in the tank to the ambient temperature water leaving the main heater element in the tankless heater immediately after a faucet is opened. Another version of a fin/tube heat exchanger is a water tube having thermal conductors attached to its exterior so as to provide additional area exposed to the hot water in the tank, allowing heat to be conducted through the conductors to the tube, and the water therein. 
         [0034]    An electric heater element ( 31 ) is shown in one embodiment and is positioned internal to the tank in close relation to the heat tube. The heater element is controlled by a thermostat ( 37 ) exposed thermally to the water in the water jacket and will close the electrical heater circuit when water temperature in the jacket falls below a preset value. The thermostat set temperature will be such that it is sufficient for the specified mass of water to heat the mass of cold sandwich water to a temperature acceptable as hot water. The jacket temperature will be maintained within a given limit cycle by the thermostat controlling the electric heater. Other heating concepts are possible in other embodiments. 
         [0035]    The water jacket ( 32 ) is a volume defined by the exterior of the heat tube and the interior walls of the tank. When filled with hot water it will serve as a heat sink to provide heat to the cold sandwich water as it passes through the heat tube following opening of a faucet in the hot water distribution system. 
         [0036]    A return line ( 24 ) will emanate from the hot water distribution line proximate the faucet farthest from the heat exchanger in each branch to be provided with instant hot water. This line will form a circulation loop from the heat exchanger to the remote faucets and back to the heat exchanger. Some portion of the circulation loop is required to be above the heat exchanger for the convective embodiment to function. 
         [0037]    The unit will also contain a check valve, as described in  FIG. 5 , to prevent reverse flow in the convective circulation loop. The valve will utilize a neutral buoyancy poppet ( 39 ) to respond reliably to the low pressure differential in a convective system. 
         [0038]    Insulation ( 34 ) in the space between the tank and the outer housing, to minimize heat loss of the water jacket to surroundings, can be any of several commercial products currently available. Fiberglass pads or pour-in wool-type insulation will suffice. Another embodiment may also evacuate air from this space to eliminate convective heat transfer. 
         [0039]    An alternative embodiment that incorporates an optional electric pump in the return line to support instant hot water circulation for installations where sufficient difference in elevation to induce a passive, convective circulation flow does not exist is shown in  FIG. 3 . All other elements of this alternative are similar to the preferred embodiment. 
         [0040]    An additional embodiment considers the use of other heat sources to keep the water in the water jacket at the required temperature. A gas burner, associated with the primary tankless burner could be designed into the system with little impact upon the total system complexity and cost. As fuel cell technology improves a small fuel cell may be employed to provide heat to the water in the jacket. Catalytic heaters, similar to those used in hand warmers, could also function to provide the low level of heat necessary to keep the water jacket up to temperature. Another heat source uses the electrical impedance of water as its resistive element may also be incorporated into this invention as a highly reliable and simple heating mechanism. 
         [0041]    Still another alternative embodiment, as shown in  FIG. 6 , is the Integral Tankless Heater concept that incorporates the instant hot water heat exchanger unit within the tankless heater housing. The heat exchanger unit is similar to the above unit shown in  FIG. 4A , The storage container includes a water inlet, an outlet, and a circulation return line inlet. The return line inlet also contains a check valve to prevent reverse flow in the return line. Heat for the heat exchanger is provided by a small unit ( 40 ) of the same general style as the main heating element for the tankless heater, using the same energy supply, and exhaust systems. A separate control system will maintain the water in the heat exchanger at a constant elevated temperature. The heat exchanger unit operates in conjunction with a return line to create a hot water circulation loop comprised of the normal hot water distribution pipe, the return line connected to the hot water pipe at a point near a remote faucet, and the instant hot water heat exchanger as shown in  FIG. 6 . 
         [0042]    The heat exchanger is used to provide a low-level hot water circulation flow to remote outlets for the purpose of providing instant hot water to remote faucets, and to eliminate the cold-water sandwich. All water exiting the main heating element shall pass through the circulation heat exchanger. Circulation may be driven by convective forces or through the use of an electric circulation pump. The circulation heater shall contain a sufficient quantity of heated water and residual heat to warm the cold sandwich as it passes through the storage tank. 
         [0043]    In yet another embodiment, as the cold water sandwich moves down the distribution line toward the hot water outlets, it encounters a separate, secondary, high intensity, heating element that is spaced at an appropriate distance from the main tankless heater to allow it to reach its operating temperature and heat the cold sandwich as it passes through. The secondary heating element is initiated at the time the main heater is started, which occurs when a hot water outlet is opened. This allows the secondary heater element to be up to temperature when the cold-water sandwich arrives. In another version of this same embodiment, rather than using another heating element to heat the cold sandwich, the distribution line is rerouted back through a separate loop in the main heating element, such that the cold water has traveled an appropriate distance for its arrival back at the heating element to coincide with the element reaching operating temperature. 
         [0044]    In the standard tankless water heater, during periods of no water use, the water will normally be at ambient temperature since the heating element is powered only when water is flowing. Water in the distribution lines and the heater itself will cool to ambient temperature in one hour or less. When a hot water faucet is opened the pressure in the distribution line will drop, and water will flow from the supply through the tankless heater, and through the hot water distribution line to the faucet. The drop in pressure also signals a control circuit in the tankless heater to heat the water flowing through it. This heating is implemented by igniting a combustion burner or providing power to an electric element. Since the water in the distribution pipe is at ambient temperature, a delay in hot water arriving at the faucet will be experienced until all cool water in the distribution pipe is expended. 
         [0045]    Since a delay of several seconds is experienced for the heating elements in the tankless heater to reach their operating temperature, a volume of ambient temperature water that was inside, and about to enter the heater, will proceed down the distribution line toward the faucet without being heated causing an additional delay for hot water to arrive at remote faucets. 
         [0046]    Using the elements identified in this invention, the heating element in the tank will maintain the water in the water jacket at a temperature in the range of 130 degrees F. that will transmit heat into the water in the heat tube bringing it to essentially the same temperature. Water will continually circulate from the heat exchanger through the hot water distribution lines to remote faucets and back through the check valve to the heat exchanger due to convective forces. Heated water from the distribution lines, and the heat tube, is available immediately at the remote faucet. As the cold sandwich flows through the heat tube, it is heated by the hot water in the water jacket. The check valve in the return line will prevent hot water in the tank from flowing up the return line toward the faucet, which would deprive the water in the cold sandwich of the heat in the jacket. Thus, with this invention hot water would be availability immediately to the remote faucets and the cold sandwich would be eliminated. 
         [0047]    It can be recognized from the previous descriptions that the principal difficulties with tankless water heaters can be overcome with the present invention. Savings in water and heat, and the convenience of instant hot water will appeal to those who have or will purchase a tankless water heater. Due to the simplicity of the invention that directly attacks the issues with a minimum of components, the present invention will yield a product that will solve a known problem, will prove to be extremely reliable, and mesh well into the conservation movement. 
       APPENDIX A 
     Heat Exchanger Test 
       [0048]    Test Objective: Determine the effectiveness of a heat exchanger to eliminate the cold sandwich encountered with a tankless water heater. This assumes the system is using a water heat sink to support a circulation flow to the remote faucets for instant hot water.
 
Approach: The approach to satisfy the objective of the test is to simulate a heat exchanger stabilized in a 125-130 Deg F. water heat sink, in series downstream of a tankless water heater, and to inject a five second volume of ambient temp (72 Deg F.) water into the exchanger. Measure the temperature of water as it exits the exchanger. Specifically, record the lowest temperature observed at the outlet immediately after the five second flow of ambient water into the exchanger.
 
Test Article: Finned transmission cooler, 0.75×5×10 inch, 0.525 inch round copper tube, aluminum fins. Fin volume=37.5 cubic in, area=50 in sq. 15 fins/inch.
 
Test Set-Up: Place the heat exchanger in the bottom of a fibre-glass utility sink. Rubber hoses connect the exchanger inlet to the faucet. Install thermocouples in the heat sink water and the exchanger outlet. Pre-test flow rate measured at 1.1 Gal/Min.
 
Instrumentation: Ten channel electronic I-C thermometer, electronic timer.
 
Procedure: Fill sink with water at heater outlet temp, (125 Deg F.) to cover the exchanger by about two inches. Fill the heat exchanger with hot water. Set the hot and cold faucets to yield water at a temp of 72 Deg F., by running it into a bucket so that it does not cool the water in the sink. Connect the hose from the faucet to the exchanger inlet for 5 seconds, observing the water temperature at the exchanger outlet. Record the water temperature in the heat sink, and the temperature at the exchanger outlet.
 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Run 
                 Sink Temp 
                 Low outlet temp 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Test Results: 
                 1 
                 123 
                 91 
               
               
                   
                   
                 2 
                 125 
                 93 
               
               
                   
                   
               
             
          
         
       
     
       CONCLUSIONS 
       [0000]    
       
         
           
             1. The cold sandwich temp dropped to ˜90 Deg F. which may be OK since the entire pipe to the remote faucet will be at ˜125 Deg F. and will continue to warm the sandwich. Ideally, the temp would not drop by more than 5 Deg F. 
             2. Additional heat exchanger area should and higher heat sink temp be tested. 
             3. Flow rate is low. Should be at ˜3.5 Gal/Min. 
             4. Concept will work with improved heat transfer. 
             5. An electric immersion heater in the pipe, 5 seconds downstream, would solve the cold sandwich issue, but would not provide instant hot water.