EP0445889B1

EP0445889B1 - Combined central-heating and water-heating boiler

Info

Publication number: EP0445889B1
Application number: EP91200472A
Authority: EP
Inventors: J.H. Deckers
Original assignee: DEJATECH BV
Current assignee: Robert Bosch GmbH
Priority date: 1990-03-06
Filing date: 1991-03-05
Publication date: 1994-08-31
Anticipated expiration: 2011-03-05
Also published as: EP0445889A1; DE69103650T2; ES2063433T3; DK0445889T3; DE69103650D1; ATE110839T1; NL9000511A

Abstract

A gas-fired central-heating and water-heating boiler comprising a water tank (13) which is part of a closed central-heating water circuit, a U-shaped compact unit formed from one piece of aluminum alloy, comprising a burner chamber (22) and a heat exchanger (23) being submerged in the tank (13) and a water mains-connected tapping coil (19) for hot water for domestic use extending in the tank (13), the ratio of the water mass, in liters, that surrounds the submerged U-shaped unit (22,23) in the tank (13), to the mass of the submerged U-shaped unit (22,23), in kilograms, being greater than 1. Mounted at the top of the leg of the U where the burner chamber is provided, are means (26,27) for forcing a combustible gas/air mixture downwards into the burner.(22) and at the lowest point of the U an element (29) is arranged, of a material of poor thermal conductivity compared with the Al alloy of the heat exchanger, condensation occurring on this element (29). <IMAGE>

Description

This invention relates to a gas-fired central-heating and water-heating boiler, comprising the features recited in the pre-characterizing part of claim 1 and as disclosed in DE-A- 21 59 827.
A combined central-heating/hot-water appliance comprising the same features and described in GB-A-2 220 052, is designed for installation in boats or caravans and accordingly has a limited capacity regarding both heating and the supply of hot domestic water. This appears among other things from the small diameter of the heat exchanger tube which forms a flue gas duct arranged helically around the combustion chamber of the burner.
This invention relates to a combined central-heating/hot-water appliance designed for dwelling houses and similar residences, where different requirements apply with respect to its capacity and especially its efficiency.
In known combined central-heating/hot-water appliances for dwelling houses and the like, in some manner or other a water tank (boiler) has been added to a central heating boiler, the contents of the former being heated by water that has been heated in the central heating boiler and circulates in a closed circuit. In principle, there are two possible constructions, namely:

a tapping or through-flow boiler of the type shown in fig.1 of DE-A- 21 59 827, which is completely filled with water from the closed circuit of the central heating boiler and in which a coil is connected at one end to the water mains and at the other end hot water for domestic use can be tapped. This is a compact construction which can be supplied in size 30x30x100 cm. Its operation is comparable to that of a (gas) water heater;
a storage boiler, in which a large quantity of water for domestic use is stored, which is heated by heated central heating water passing through a coil. This construction is comparable to an (electric) boiler. The dimensions of the storage boilers are generally larger, for instance 60x60x100 cm.

Both can be arranged adjacent to the central heating boiler and in principle in both cases elsewhere (in the central heating boiler) water is heated for a specific purpose, namely the heating of space, and this water also heats water that is being supplied for a different purpose (domestic use).
As shown in fig.1 of DE-A- 21 59 827 a cylindrical combustion chamber extends concentrically into a boiler vessel, a hot water tapping coil surrounds the combustion chamber in the boiler vessel.
The known conventional combined central-heating/domestic water boilers have the drawback that the capacity of the domestic water appliance is often on the short side. The reason is that, at a given boiler capacity, it is required that in any event the heat necessary for maintaining a living space at a given temperature be supplied. Another important drawback of the known combined boilers is the long so-called convenience waiting time, i.e. it takes a long time (0.5-0.75 min) for hot water to flow from the tap after the hot-water tap has been turned on.
The invention is based on the idea that houses are increasingly better insulated and that accordingly the ratio of central heating water to domestic water can be adjusted in favour of domestic water. It is therefore an object of the invention to provide a novel central heating boiler which, while being of compact construction and of environmentally friendly design, is suitable for normal heating of houses and has a comparatively large capacity regarding the supply of water for domestic use.
The basic principle of the invention is that the burner chamber and the heat exchanger are formed as a compact U-shaped unit cast in one piece from an aluminum alloy, with the ratio of the water mass, in litres, that surrounds the submerged U-shaped unit (22,23) in the tank, to the mass of the submerged U-shaped unit, in kilograms, being greater than 1 and the heat exchanger being provided with fins enlarging the heating surface.
When such a construction and ratio are used, at a conventional burner capacity of 25 kW (minimum 18 kW), in the tank a water temperature of 87° (maximum 97°) can be maintained, which means that using a mixing valve in the tap, 7 liters of hot water of 60° per minute can be tapped continuously without noticeable temperature fluctuations.
Said ratio of water in the tank to the aluminum mass submerged therein, is important.
The fact is that when the water mass is too small relative to the heat exchanger mass, upon the burner being switched off, the hot heat exchanger material will continue for some time to transfer heat to the water in the tank. As a result, when the water mass is too small, the temperature may run up to 160°C, which heat is carried off into the central heating system. This phenomenon is called "HOT SHOT" and is clearly undesirable. When, however, the above ratio of water to aluminum mass is employed, given a minimum water store of 35 litres, the convenience quantity of 7 1/min at 60°C can be guaranteed continuously without Hot Shot phenomena. In the case of greater quantities of water, an even more uniform hot water delivery can be accomplished.
With a view to a compact construction of the burner-and-heat-exchanger unit, it is a U-shape construction, being mounted at the top of one leg of the U a burner with means for forcing a combustible gas/air mixture downwards into the burner and the lower end of said leg, and the other leg of the U being formed as heat exchanger for exchange with the water mass which the assembly is submerged in.
The advantage of this construction is that in a combined boiler of limited dimensions, a sufficient amount of water can be maintained at temperature to provide for the instantaneous availability - accordingly involving a very short convenience waiting time - of hot domestic water. Its capacity for supplying domestic water is comparable to that of a much larger storage boiler with a source of heat of its own.
The U-shape of the assembly and the good conditions that have been created for optimum heat exchange with the surrounding water mass, require measures to prevent condensation in the bend of the U, particularly when the boiler is being started up and at a low temperature of the water mass in the tank.
To ensure that the temperature at the lowest point of the U is at all times well above the evaporation temperature (150°), it is proposed to arrange an element there of poor thermal conductivity compared with the aluminum alloy of the heat exchanger.
Upon passage of flue gases, the temperature of this element will rise rapidly and remain high because it does not readily give off the heat it received to its surroundings.
In further elaboration of the invention the element may be a plug of slight mass in the wall of the U, which plug may also be used as core support when casting the U-tube with lost core.
By way of an example, one embodiment of the combined boiler will now be described with reference to the accompanying drawings, in which:

Figs. 1 and 2 schematically illustrate the principle of known combined boiler systems;
Fig. 3 is a schematic longitudinal sectional view of the combined boiler according to the invention;
Fig. 4 is a sectional view taken on arrows IV-IV of Fig. 3; and
Fig. 5 is a detailed sectional view of the element designated by arrow V in Fig. 3.

Figs 1 and 2, respectively, show a hot water tank 1 of a tapping or through-flow boiler and a hot water tank 2 of a storage boiler. In the through-flow boiler according to Fig. 1, the tank 1 is filled with water of the central heating circuit which is supplied via pipe 3 and is discharged via pipe 4 by the circulation pump (not shown) of the central heating boiler. Extending helically through the tank 1 is a tube 5 which at the end 6 is connected to the water mains and at the other end is provided with a tap 7. In the storage boiler according to Fig. 2, the tank 2 is filled with water from the mains that is supplied via the pipe 8 and the tank is further connected to a tap 9. The mains water in the tank 2 is heated by a coil 10 which is connected to the central heating circuit via pipes 11 and 12.
In both cases, heat is circulated from the central heating boiler via pipes 3,4, and 11,12, respectively, in a tank arranged in spaced relationship to the source of heat, namely the heat exchanger of the central heating boiler.
In the combined boiler according to the invention, which is of the through-flow type, a tank 13 is included in the closed central heating water circuit, namely via a circulation pump 14 connected to the return pipe 15, 15a and via a threeway valve 16 to the pressure pipe 17a,17 leading to radiators or similar space-heating elements. The threeway valve 16 is connected to the delivery side of the pump 14 via a bypass 18.
Extending in the tank 13 is a tube coil 19 which via conduits 20 and 21 (via a mixing valve M) is connected to the water mains system and to a tap (neither being shown). Submerged in the tank 13 is a one-piece assembly of a burner 22 and a U-shaped heat exchanger 23. The heat exchanger 23 is connected to a flue 24 for combustion gases.
The burner 22 is included in a combustion chamber 25 provided at the top of one leg of the U-shaped heat exchanger 23. The burner is fed with a gas/air mixture by a fan 26 and a gas valve 27.
The combustion gases are forced through the U-shaped heat exchanger towards the flue 24. During this process the heat transfer to the water in the tank 13 is promoted by virtue of the fact that fins 28 have been formed on the heat exchanger, which enlarge the heating surface.
Screwed into the lowest point of the U is a plug 29 of a material of poor thermal conductivity in comparison with the aluminum from which the heat exchanger is made.
The operation of the combined boiler according to the invention is as follows:
Return water from the central heating circuit is forced from the return pipe 15 through the circulation pump 14 and the extended return pipe 15a into the tank 13 and returned via the standpipe 17a and the supply pipe 17 into the central heating circuit. In the tank 13 the central heating water is heated by the assemby of heat exchanger 23 and burner 22 directly submerged therein. The hot water in the tank 13 forms at the same time the source of heat for domestic water in the coil 19. When comparatively much hot domestic water is required, the heat transfer to the living space via the supply pipe 17 can be temporarily interrrupted by virtue of the threeway valve 16 being switched to short-circuiting the delivery side of the circulation pump 14 and the supply pipe 17 via the bypass 18. In that event, the central heating water will be recirculated through the radiators, bypassing the boiler. Under those conditions, all heat produced by the burner 22 is directly transferred to the coil 19 via the quantity of water that has been isolated in the tank 13. Since during this process there is no circulation in the tank 13 as a result of any pumping action, there must be provided for ready heat transfer from flue gases to the quantity of water closed off in the tank. For that reason, in the combined boiler according to the invention, use is made of comparatively large fins 28 enlarging the heating surface, for instance of a height of 4-5 cm on the flue gas side and a height of 1-1.5 cm on the water side.
Condensation in the heat exchanger 23 is prevented by virtue of the fact that during normal operation the plug 29 of material of comparatively poor thermal conductivity, has acquired a high temperature and this plug will still have a high temperature in excess of 150° when the temperature in the heat exchanger 23 decreases, so that at the lowest and hence coldest point of the U-shaped heat exchanger, any condensate that has formed will be evaporated immediately.
In a combined boiler according to the invention, with a burner capacity of approx. 25 kW, a gross tank volume of 35 litres, while the burner/heat exchanger unit has a water displacement of 16 litres, when domestic water is being tapped, during the first two minutes, water will be tapped of a temperature which decreases from approx. 85° to approx. 68°C and in the mixing valve M, cold mains water can be added so as to maintain the desired tapping temperature of 60°C. During the next 12 minutes, the tapping temperature decreases to approx. 60°C and gradually a lesser quantity of cold water can be added. Tapping at 60°C can be continued indefinitely.
After an interruption of the tapping, the temperature in the tapping coil rises again to approx. 85°C and when tapping is renewed, the above temperature course repeats itself.

Claims

A gas-fired central-heating and water-heating boiler, wherein a burner and a heat exchanger are submerged in a water tank which is part of the closed central-heating water circuit and wherein a water mains-connected tapping coil for hot water for domestic use extends in the water tank, characterized in that the burner chamber (22) and the heat exchanger (23) are formed as a compact U-shaped unit (22, 23) cast in one piece from an aluminum alloy, with the ratio of the water mass, in litres, that surrounds the submerged U-shaped unit (22,23) in the tank (13), to the mass of the submerged U-shaped unit (22,23), in kilograms, being greater than 1, the heat exchanger (23) being provided with fins (28) enlarging the heating surface.
A combined boiler according to claim 1, characterized in that there being mounted at the top of one leg of the U a burner (22) with means (26,27) for forcing a combustible gas/air mixture downwards into the burner (22) and the lower end of said leg and the other leg of the U being formed as heat exchanger (23) for exchange with the water mass which the assembly is submerged in.
A combined boiler according to claim 1, characterized in that at the lowest point of the U an element (29) is arranged, of a material of poor thermal conductivity compared with the Al alloy of the heat exchanger.
A combined boiler according to any one of the preceding claims, characterized in that the return pipe (15) and the supply pipe (17) which connect the boiler with space-heating elements, are connected to the tank (13) via a pump (14) and a threeway valve (16), respectively, and the threeway valve (16) is connected to the delivery side of the pump by means of a bypass (18).