Abstract:
Apparatus and method for providing heat to a hot water storage system in which water from the storage system is circulated through a heat exchanger in the flue of a combustion heat source and circulatory flow is provided by a circulating pump in a secondary circulatory system including a heat exchanger having greater heat transfer capacity than the heat exchanger in the flue so as to dissipate excess heat at a controllable rate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to heating apparatus and particularly to heating apparatus that obtains heat by circulating water through a heat exchanger in the flue of an independent combustion heat source. 
     2. Description of the Prior Art 
     A common hot water heater of some years back used a storage tank with a separate gas-fired combustion unit containing a helix of copper tubing. The bottom of the tank was connected to the bottom of the helix while the top of the helix was connected to the top of the tank. Cold water from the bottom of the tank would rise through the helix as it was heated by the gas flame and pass into the top of the tank. The gas would be turned off when a predetermined temperature was reached. Today domestic hot water heaters are most commonly one of three types: Storage tank with integral gas burner, storage tank with integral electric heating elements and heat exchanger connected into a central heating furnace of the water or steam types with or without storage tank. 
     Domestic hot water heaters operating off central heating furnaces conventionally absorb their heat from the water in the furnace rather than directly from the flue gasses. This is necessary so that the domestic hot water does not reach a dangerously high temperature while at the same time avoiding the necessity of turning off the furnace merely because the domestic hot water is too hot. This is the same reason that domestic hot water is seldom drawn from hot air furnaces. The hot air readily rises far above the boiling point of water and would require shutting down the central heating system when the domestic hot water approached a hazardous temperature level. 
     Besides the above limitations, it has always been a disadvantage of using a central heating furnace for domestic hot water that in the summer it results in operating an inefficiently large unit for providing a small amount of heat. On the other hand, in cold weather when the central heating furnace is functioning anyway, the central heating furnace becomes a much cheaper and more efficient source than the electricity or gas integral units in integrated hot water heaters. 
     SUMMARY OF THE INVENTION 
     Now in accordance with the present invention a method and apparatus are provided for providing heat to a hot water storage unit from a heat exchanger in the flue of an independent combustion heating unit without the water in the storage unit ever reaching a temperature that would require shutting down the combustion heating unit. In use for domestic hot water, integral gas or electric heating is used with the storage tank to maintain water temperature when the independent combustion heating unit is not operating. To accomplish this, the flue mounted heat exchanger is connected in a circulatory loop with the storage tank and a secondary circulatory loop is connected in the line of the primary loop going from the tank to the flue for dissipating excess heat. A circulating pump in the secondary loop provides circulation in both loops. A thermostatic valve in the line from the flue to the tank prevents circulation until the water in the flue mounted heat exchanger reaches a minimum temperature while a second thermostatic valve located at a juncture between the primary and secondary circulatory loops controls the volume of water flowing in the secondary loop so that flow increases as water temperature exceeds a predetermined level. 
     Thus it is an object of the invention to provide a flue mounted heat exchanger connected in a circulatory loop with a hot water storage system to provide auxiliary hot water heating from a combustion heating unit otherwise independent of the hot water storage system. 
     Further objects and features of the invention will become more fully understood upon reading the following description together with the Drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a diagram, partially schematic and partially block, depicting a hot water heating and storage system according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention is designed to take heat from the flue of a central heating furnace to provide auxiliary heat to a domestic hot water heater during operation of the furnace. Overheating is prevented by a unique dissipating arrangement in the line going to the flue from the domestic hot water heater. Since the furnace is only operated in cold weather, the heat dissipating arrangement can be installed in a room or other space requiring heat. 
     As depicted in the drawing, the main components of the system are: flue mounted heat exchanger 10 in stack 15 of central heating furnace 11, water tank 12 and external radiator 14. Heat exchanger 10 is suitably a helix of copper tubing mounted in stack 15 of furnace 11 between the furnace and a chimney. Furnace 11 may be fired by gas, oil or other combustible material and may be of the hot water, steam or hot air type. Furnace 11 is operated in accordance with the demands of the central heating system without regard to the operation of the present water heating system. Water tank 12 is depicted as a hot water storage tank with integral electric heating. An integral gas or oil-fired unit may be in water tank 12 instead of electric heating elements 13. Primary loop 16 for water circulation connects heat exchanger 10 with water tank 12. Secondary loop 17 for water circulation is interconnected with primary loop 16. Radiator 14, suitably made of finned copper tubing conventional for radiation from hot water heating systems, is connected in secondary loop 17. 
     Radiator 14 is designed to provide greater heat transfer to ambient air, from circulating water at the predetermined holding temperature for water tank 12, than the heat transfer through heat exchanger 10 at the maximum operating temperature in stack 15. 
     Feed portion 20 of primary loop 16 connects the bottom of water tank 12 to heat exchanger 10. Return portion 21 of primary loop 16 connects heat exchanger 10 to the top of water tank 12. Thermostatic valve 22, that opens on rising temperature exceeding a predetermined level, is connected in the circulation path of feed portion 20. Thermostatic valve 22 is also connected in the circulation path of secondary loop 17. Thus feed portion 20 of primary loop 16 connects to secondary loop 17 at common junctures 24 and 25 on either side of valve 22, juncture 25 being on the side of valve 22 closer to heat exchanger 10. 
     Secondary loop 17 contains circulating pump 26 in series with radiator 14. Pump 26 is of the centrifugal or other conventional circulating type providing a flow which varies inversely with back pressure. 
     Return portion 21 of primary loop 16 is thermally coupled to thermostatic switch 27 and has a second thermostatic valve 28 in its flow path. Switch 27 closes when rising temperature reaches a predetermined level while valve 28 opens on rising temperature exceeding a predetermined level. Both switch 27 and valve 28 are positioned as close as convenient to stack 15 so that rising water temperature in heat exchanger 10 is sensed with a negligible delay. Thermostatic switch 27 is connected in series with electric supply line 31 to controller 30 providing power to pump 26. Thermostatic valves 22 and 28 are both of conventional type such as used in automotive cooling systems set to open and close at the desired temperatures. 
     The pipe size in secondary loop 17, the size of pump 26, the valve orifice in valve 22 and the ports in junctures 24 and 25 interconnecting primary and secondary loops 16 and 17 and connecting them to valve 22 are all selected to provide an increase in water flow through secondary loop 17 relative to flow in primary loop 16 as valve 22 opens. 
     Connection from a water supply such as a well or city water main is suitably made in feed portion 20 as depicted by connecting tee 34. Connection to the household hot water piping faucets is suitably made in return portion 21 as depicted by connecting tee 35. 
     When furnace 11 is not operating, as in summer, the integral heating unit for tank 12 provides the hot water heating source while valve 28 prevents circulation between tank 12 and heat exchanger 10. When furnace 11 fires, flue gases heat water in heat exchanger 10 so that heat transmitted to switch 27 and valve 28 closes and opens them respectively. Closing of switch 27 starts pump 26 which then circulates water along the path from tank 12 through juncture 24, radiator 14, pump 26, juncture 25, heat exchanger 10, valve 28 and back to tank 12. All flow goes through primary and secondary loops 16 and 17 serially with no passage through valve 22 which remains closed. Valve 22 is set to open at or near the desired holding temperature in tank 12. Water going to valve 22 comes either from the water supply or the bottom of tank 12. 
     When no water is being drawn and the tank temperature reaches the holding level, valve 22 opens. Valve 22 then provides a reduced back pressure recirculation path in secondary loop 17. Under these conditions, water flow in secondary loop 17 and thus radiator 14 exceeds water flow through primary loop 16 and thus heat exchanger 10. This increased flow in radiator 14 increases heat dissipation in radiator 14. At the same time the raised water temperature increases the temperature differential at radiator 14 while decreasing the temperature differential at heat exchanger 10. Temperature stabilization is reached at the desired point and it is believed this is fully due to the theory of heat transfer described above. However the theory of heat transfer is not part of the invention and the system as described provides the desired temperature stabilization in actual practice. 
     While the invention has been described with relation to a specific embodiment, many variations are obvious within the inventive concept and the invention is useful for heating water in connection with other systems than domestic hot water and with storage tanks with or without integral separate heating sources. Thus it is intended to cover the invention with the full scope of the appended claims.