WO2025262454A1

WO2025262454A1 - Fuel-fired fluid heating/storage device

Info

Publication number: WO2025262454A1
Application number: PCT/IB2024/055938
Authority: WO
Inventors: Marco Marques; Giuliano SACCONI; Fatjon ZHUKA; Stefano Ferroni; Hong Hu
Original assignee: Ariston SpA
Current assignee: Ariston SpA
Priority date: 2024-06-18
Filing date: 2024-06-18
Publication date: 2025-12-26
Anticipated expiration: 2026-12-18

Abstract

A fuel-fired fluid heating apparatus (1), comprising a tank (2) and a heat exchanger (4) with a downward extending first tube (6) connected to a top wall (8) of the tank (2) and which houses a fuel burner (5) and forms a combustion chamber (10), wherein the first tube (6) forms a first heat exchanging region (12) for exchanging heat between combustion gases (25) generated by the fuel burner (5) and a fluid (3) inside the tank (2), a second tube (13) connected to the first tube (6) and having a lower cup shape baffle portion (15) with a closed bottom wall (16) and a rising pipe (17) connected to the baffle portion (15) and extending from the baffle portion (15) upward, wherein the bottom wall (16) is enlarged with respect to the first tube (6) and the baffle portion (15) and the rising pipe (17) form together a second heat exchanging region (18) for exchanging heat between the combustion gases (25) and the fluid (3) inside the tank (2), a downward extending third tube (26) connected to the rising pipe (17) and forming a continuously downward extending coiled condensing pipe portion (21) extending around both the first tube (6) and the second tube (13),wherein the third tube (26) forms a third heat exchanging region (24) for condensing the combustion gases (25) and for exchanging heat between the combustion gases (25) and the fluid (3) inside the tank (2).

Description

DESCRIPTION "FUEL-FIRED FLUID HEATING/STORAGE DEVICE"

BACKGROUND OF THE INVENTION

The present invention generally relates to fuel-fired fluid heating devices and, in a representatively illustrated embodiment thereof, more particularly provides a fuel-fired water heater having a specially designed condensing type heat exchanger incorporated therein.

Conventional fuel-fired water heaters, especially gas-fired water heaters are typically of a single pass, non-condensing configuration, meaning that the hot combustion gases used to heat the tank-stored water are subjected to only a single pass through a heat exchanger structure (usually a vertical flue) within the tank before being discharged from the water heater to, for example, an external vent structure, and that flue gas condensation does not occur to any appreciable degree in the heat exchanger structure within the water heater tank. In this conventional type of fuel-fired water heater, the overall thermal efficiency is typically limited to about 80-85%. Various proposals have been made to provide fuel-fired water heaters with condensing type single-pass heat exchangers (i.e., in which flue gases condense within the heat exchanger). However, previously proposed single-pass condensing type heat exchange structures incorporated in fuel-fired water heaters typically provide the water heaters with maximum thermal efficiencies limited to the 85-90% range and, due to their particular structure and material, are rather expensive to manufacture, are subject to considerable thermal stress and corrosion, especially at the welds and in regions of sharply varying flue duct sections.

It would be desirable to provide a fuel-fired water heater with a single pass heat exchanger having a higher heat transfer efficiency, e.g. of at least ninety to ninety five percent and preferably greater, and at the same time having features which allow the heat exchanger and the heater to be manufactured at a reduced cost and to have an increased resistance to thermal stress and to corrosion.

SUMMARY OF THE INVENTION

In carrying out principles of the present invention, in accordance with representative embodiments thereof, a fuel-fired fluid heating apparatus is provided which is representatively a water heater but could alternatively be a variety of other types of fuel-fired fluid heating devices without departing from principles of the present invention.

According to a general aspect of the invention, the fuel-fired fluid heating apparatus includes a tank for holding a fluid to be heated, and a single pass heat exchanger (the language “single pass” denotes that the combustion gases go in one end of each tube section and out the other end), preferably of a condensing type, arranged in the interior of the tank. The heat exchanger includes a straight downward extending first tube having a first tube upper end connected to a top wall of the tank, a first tube upper section which houses a fuel burner and forms a combustion chamber, and a first tube lower section extending from the first tube upper section further downward, wherein the first tube forms a first heat exchanging region for exchanging heat between the combustion gas and the fluid to be heated inside the tank.

The heat exchanger further includes a second tube connected to a first tube side wall of the first tube and having a lower cup shape baffle portion with a closed bottom wall and a rising pipe connected to the baffle portion and extending from the baffle portion upward, wherein second tube receives downward flowing combustion gas from the first tube and the bottom wall is enlarged with respect to the first tube and acts as a baffle for deflecting the combustion gas upward, and wherein the cup shape baffle portion and the rising pipe form together a second heat exchanging region for exchanging heat between the combustion gas and the fluid to be heated inside the tank.

The heat exchanger further includes a downward extending third tube having a third tube upper end portion connected to a rising pipe upper end portion of the rising pipe, a continuously downward extending condensing pipe portion forming multiple coils extending around both the first tube and the second tube, and a third tube lower outlet portion exiting the tank, wherein the rising pipe upper end portion and the third tube upper end portion deflect the combustion gas downward, and wherein the third tube forms a third heat exchanging region for at least partially condensing the combustion gas and for exchanging heat between the combustion gas and the fluid to be heated inside the tank.

In a preferred embodiment thereof, the apparatus is a fuel-fired (e.g. gas-fired) commercial water heater.

The heat exchanger has:

- a primary, non-condensing section defined by the combustion chamber arranged in a top portion of the tank, the cup shape baffle portion and the rising pipe, and

- a secondary, condensing section defined by the coiled condensing pipe portion extending throughout a lower interior portion of the tank and the lower outlet portion exiting the tank.

According to other preferred aspects of the invention, the non-condensing but highly thermally stressed first tube of the heat exchanger, and the non-condensing second tube of the heat exchanger are formed from only externally coated or enameled or porcelainized metal material, e.g. steel, whereas the condensing third tube of the heat exchanger is formed from an acid resistant, externally and internally coated or enameled or porcelainized metal material, e.g. steel. The tank is formed from only internally coated or enameled or porcelainized metal material, e.g. steel.

Accordingly, an initial, non-condensing tract of the heat exchanger has a heat transfer rate per heat exchange area unit substantially greater than the heat transfer rate per heat exchange area unit of the final, condensing tract of the heat exchanger, and the expected and experimentally verified heat exchange efficiency can reach more than 90% or even more than 95%. The heat exchange efficiency values referred to in the present disclosure are calculated e.g. according to the ANSI Z21 .10.3-2019 / CSA 4.3-2019 standard.

According to yet other preferred aspects of the invention, each individual part of the heat exchanger or the entire heat exchanger and/or the tank is made from stainless steel material. The shape configuration of the heat exchanger allows manufacturing of individual tube sections having reduced geometric complexity and tube connection by welding in advantageous weld positions with regard to thermal stress, galvanic corrosion resistance, exposure to acids contained in combustion fume condensate and accessibility during enamel coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 schematically illustrates, in cross-sectional view, a fuel-fired water heater incorporating therein a single-pass condensing type heat exchanger embodying principles of the present invention,

FIGURE 2 schematically illustrates, in perspective view, a single-pass condensing type heat exchanger according to an embodiment of the invention,

FIGURE 3 schematically illustrates, in cross-sectional view, a single-pass condensing type heat exchanger according to a first embodiment of the invention,

FIGURES 3A, 3B schematically illustrate, in cross-sectional view (Figure 3A) and in top view (figure 3B), a detail of a single-pass condensing type heat exchanger according to a variation of the first embodiment of the invention,

FIGURE 4 schematically illustrates, in cross-sectional view, a detail of single-pass condensing type heat exchanger according to an embodiment of the invention,

FIGURE 5 schematically illustrates, in cross-sectional view, a single-pass condensing type heat exchanger according to a second embodiment of the invention,

FIGURE 6 schematically illustrates, in perspective view, a single-pass condensing type heat exchanger according to a third embodiment of the invention,

FIGURE 7 schematically illustrates, in cross-sectional view, the single-pass condensing type heat exchanger in figure 6,

FIGURE 8 schematically illustrates, in cross-sectional view, a single-pass condensing type heat exchanger according to a fourth embodiment of the invention,

FIGURE 9 schematically illustrates, in perspective view, a fluid tank or water tank of a fuel- fired water heater according to an embodiment of the invention.

DETAILED DESCRIPTION

[001 1] Schematically illustrated in cross-section in FIGURE 1 is a fuel-fired fluid heating apparatus 1 , representatively a commercial gas-fired water heater. The heating apparatus 1 comprises:

- a tank 2 for holding a fluid 3, e.g. domestic sanitary water, to be heated, - a single pass heat exchanger 4 (the language “single pass” denotes that the combustion gases go in one end of each tube section and out the other end) of a condensing type, arranged in an interior of the tank 2,

- a fuel burner 5, wherein the heat exchanger 4 comprises:

- a straight downward extending first tube 6 having a first tube upper end 7 connected to a top wall 8 of the tank 2, a first tube upper section 9 which houses part of the fuel burner 5 and forms a combustion chamber 10, and a first tube lower section 1 1 extending from the first tube upper section 9 further downward, wherein the first tube 6 forms a first heat exchanging region

12 for exchanging heat between combustion gases 25 generated by the fuel burner 5 and the fluid 3 inside the tank 2,

- a second tube 13 connected to a first tube side wall 14 of the first tube 6 and having a lower cup shape baffle portion 15 with a closed bottom wall 16 and a rising pipe 17 connected to the baffle portion 15 and extending from the baffle portion 15 upward, wherein the bottom wall 16 is enlarged with respect to the first tube and the baffle portion 15 and the rising pipe 17 form together a second heat exchanging region 18 for exchanging heat between the combustion gases 25 and the fluid 3 inside the tank 2,

- a downward (in the use position of the apparatus) extending third tube 26 having a third tube upper end portion 19 connected to a rising pipe upper end portion 20 of the rising pipe 17, a continuously downward extending condensing pipe portion 21 forming multiple coils 22 extending around both the first tube 6 and the second tube 13, and a third tube lower outlet portion 23 exiting the tank 3, wherein the third tube 26 forms a third heat exchanging region 24 for at least partially condensing the combustion gases 25 and for exchanging heat between the combustion gases 25 and the fluid 3 inside the tank 2.

During operation of the apparatus 1 , the combustion gases 25 flow from the fuel burner 5, arranged at a top region of the tank 2, through the heat exchanger 4, wherein the second tube

13 receives downward flowing combustion gases 25 from the first tube 6 and the bottom wall 16 acts as a baffle for deflecting the combustion gases 25 upward into the rising pipe 17, wherein the upward flowing combustion gases 25 from the rising pipe 17 are deflected downward when flowing through the rising pipe upper end portion 20 and the third tube upper end portion 19, wherein the combustion gases 25 flow continuously downward through the condensing pipe portion 21 and third tube lower outlet portion 23.

The alternating upward and downward deflections of the combustion gases 25 flow create turbulences in the combustion gases and allow to significantly increase the heat exchange length, thereby advantageously increasing (by the turbulences) the heat exchange between the combustion gases 25 and the heat exchanger tube walls and increasing the overall heat exchange surface area. The heat exchanger 4 has:

- a primary, non-condensing section defined by the combustion chamber 10 arranged in a top portion 27 of the tank 2, the baffle portion 15 and the rising pipe 17, and

- a secondary, condensing section defined by the coiled condensing pipe portion 21 extending throughout a lower interior portion 29 of the tank 2, and the lower outlet portion 23 exiting the tank.

The apparatus 1 also has a fluid (or water) inlet 28 disposed on the tank 2, at a lower interior portion 29 of the tank 2, for admitting fluid (or water) directly into the lower interior portion 29 of the tank 2, and a fluid (or water) outlet 30 disposed on the tank 2, at an upper interior tank portion 31 , for withdrawing heated fluid (or water) from the tank 2.

According to embodiments, the first tube 6 and the second tube 13 are formed from (only) externally coated or enameled or porcelainized metal material, e.g. steel. The third tube 26 is formed from an internally acid resistant - coated or enameled or porcelainized, and additionally externally coated or enameled or porcelainized metal material, e.g. steel. With regard to the heat exchanger 4 “internally” means the side of the tube wall exposed to the combustion gases 25, and “externally” means the side of the tube wall exposed to the fluid (e.g. water) inside the tank 2.

The tank 2 is formed from (only) internally coated or enameled or porcelainized metal material, e.g. steel. With regard to the tank 2 “internally” means the side of the tank wall exposed to the fluid (e.g. water) inside the tank 2.

According to alternative embodiments, each individual one of first, second and third tubes 6, 13, 26 or the entire heat exchanger 4 and/or the tank 2 is made from stainless steel material, possibly without any surface coating or enamel coating.

DETAILED EMBODIMENT DESCRIPTION OF THE FIRST TUBE 6

In accordance with an embodiment, the first tube 6 has a straight longitudinal extension parallel to a longitudinal extension of a tank side wall 32 of the tank 2, the longitudinal extensions being vertical in an operating position of the apparatus 1 . The first tube 6 extends from the tank top wall 8 towards a tank bottom wall 33 for a first tube length 35 of at least 2/3 or at least 3/4 or at least 7/9 of a maximum internal tank height 34, i.e. a maximum distance between the tank top wall 8 and the tank bottom wall 33, without however reaching the tank bottom wall 33.

The first tube 6 has a constant cross-section throughout its longitudinal extension, preferably a circular cross-section, alternatively an oval cross-section.

DETAILED EMBODIMENT DESCRIPTION OF THE SECOND TUBE 13

In accordance with embodiments (Figures 3, 5), the bottom wall 16 of the baffle portion 15 has an internally concave (inverted) dome shape which improves the guidance of the combustion gases 25 during their upward deflection and increases the heat exchange surface area and the resistance to thermal stress of the baffle portion 15. The baffle portion 15 has a tubular side wall 36 with a lower end connected, e.g. by a ring weld, to the bottom wall 16 and an upper end connected, e.g. by a ring weld, to the first tube 6 and tapered from the lower end to the upper end. In a preferred embodiment, the tubular side wall 36 has a truncated cone shape. A connection region between the side wall 36 and the bottom wall 16 is continuously curved without corners. A connection region between the side wall 36 and the first tube 6 includes a circumferential ring weld and bordering tube wall sections defining, in a radial cross-section plane of the first tube 6, an external connection angle 37 greater than 135° or greater than 160°.

In accordance with embodiments (figures 3A, 3B), the heat exchanger 4 comprises a flow deviation wall 48 extending inside the baffle portion 15 above a height at which the rising pipe 17 connects to the baffle portion 15. The flow deviation wall protrudes from a same rising pipe attachment side 49 at which the rising pipe 17 connects to the baffle portion 15, possibly with an inclination towards the bottom wall 16 of the baffle portion 15.

The flow deviation wall 48 protrudes, e.g. from an internal surface:

- of a first tube side wall 14 of the first tube 6 or

- of a baffle portion side wall 36 of the baffle portion 15, into an inner space of the baffle portion 15, possibly with an inclination towards the bottom wall 16 of the baffle portion 15.

The flow deviation wall 48 increases a length of a flow path of combustion fumes entering the baffle portion 15 up to reaching the rising pipe 17, thereby increasing an effectively used heat exchange surface.

The combustion fumes flow firstly downward along the baffle portion side wall 36 opposite the rising pipe attachment side 49, then along the baffle portion bottom wall 16 towards the rising pipe attachment side 49 and again upward along the baffle portion side wall 36 on the rising pipe attachment side 49 until reaching the rising pipe 17 inlet (see arrow in figure 3A).

In accordance with embodiments (Figures 7, 9), the bottom wall 16 of the baffle portion 15 has a flat disk shape and the baffle portion 15 further includes a tubular side wall 36 having a constant cross-section or a cylindrical shape with a lower end connected, e.g. by a ring weld, to the bottom wall 16 and an upper end connected, e.g. by a ring weld, to a flat ring wall 39 which in turn is connected by a further ring weld to an external surface of the first tube 6.

In this embodiment the connection regions involve rectangular corners between bordering steel sheet material, which are generally less preferable in view of thermal stress and strain, but which can be economically and very reliably manufactured by industrial fillet welding processes.

In accordance with embodiments (Figures 3, 5, 9) the rising pipe 17 has a curved lower end portion 38 connected, by a ring weld, to a side wall 36 of the baffle portion 15, and a straight rising portion 39 extending parallel to the first tube 6 and at a distance thereof without enveloping the first tube 6. The rising pipe 17 has a constant cross-section throughout a longitudinal extension thereof.

In accordance with an embodiment (Figure 7) the rising pipe 17 is externally inserted over the first tube 6 thereby defining an annular space 40 between an external surface of the first tube 6 and an internal surface of the rising pipe 17. In a preferred exemplary embodiment, the rising pipe 17 is formed by an upward extension of the side wall 36 of the baffle portion 15, and preferably as a single piece with the side wall 36. The entire rising pipe 17 is preferably straight and has a constant cross-section or is cylindrical and an upper end of the rising pipe 17 can be connected, e.g. by a ring weld, to a flat ring wall 39 which in turn is connected by a further ring weld to an external surface of the first tube 6.

The bottom wall 16 of the baffle portion 15 is arranged at a distance from the tank bottom wall 33.

DETAILED EMBODIMENT DESCRIPTION OF THE THIRD TUBE 26

In accordance with embodiments (Figures 3, 4, 5, 8), the third tube 26 forms at least one (Figures 4, 5, 8) or more (Figures 2, 3) upper coil/s 22’ between the upper end portion 19 and a group of lower coils 22” of said multiple coils 22, wherein the one or more upper coil/s 22’ is/are arranged at a greater vertical pitch 41 , i.e. vertical coil distance, to a directly subsequent underlying coil 22 than a vertical pitch between two directly subsequent lower coil/s 22. This provides a greater space for the fluid to be heated between the hotter upper coil/s 22’ than between the cooler lower coils 22.

For the same purpose, the upper coil/s 22’ have a greater downward inclination than the lower coils 22”.

The third tube 26 has a constant cross-section and a continuous curvature without corners or steps.

The condensing pipe portion 21 forms a plurality of (e.g. from 2 to 15, preferably from 7 to 10) windings, at least a group of which windings extend with a constant helix radius about a helix axis parallel or coaxial with a central axis of the first tube 6, as shown in the exemplary embodiments in the figures. An uppermost (starting) winding and/or a lowermost (final) winding may have a non-constant helix radius or a smaller helix radius than intermediate windings of the condensing pipe portion 21 .

In accordance with embodiments (Figures 3, 5, 9), a flow section of the first tube 6 is greater than a flow section of the rising pipe 17 and the flow section of the rising pipe 17 is equal to a flow section of the third tube.

The heat exchanger 4, as illustrated, has a vertical, down-fired orientation in use.

DETAILED EMBODIMENT DESCRIPTION OF THE TANK 2

In accordance with an embodiment, the tank 2 has a cylindrical tank side wall 32 extending vertically in a use position of the apparatus 1 , an internally concave (inverted) dome shaped tank bottom wall 33 and an externally concave dome shaped tank top wall 8.

The fuel burner 5 and the combustion chamber 10 are arranged in a down-firing position at the tank top wall 8 with part of the fuel burner 5 arranged in a burner mounting space 42 defined by an external surface 43 of the tank top wall 8 within the longitudinal extension of the tank side wall 32.

The heat exchanger 4 extends downward beyond the extension of the tank side wall 32 without reaching the tank bottom wall 33 and the bottom wall 16 of the baffle portion 15 of the heat exchanger 4 is arranged at least partly within a lower dome space 44 defined by an internal surface 45 of the tank bottom wall 33 outside the longitudinal extension of the tank side wall 32.

The described shape of the tank 2 and the tank shape correlation with the arrangement of the fuel burner 5 and the heat exchanger 4 increases internal tank volume while reducing additional equipment space requirement.

Reference numerals fluid heating apparatus 1 tank 2 fluid 3 heat exchanger 4 fuel burner 5, first tube 6 first tube upper end 7 tank top wall 8 first tube upper section 9 combustion chamber 10 first tube lower section 11 first heat exchanging region 12 second tube 13 first tube side wall 14 of the first tube 6 lower cup shape baffle portion 15 bottom wall 16 rising pipe 17 second heat exchanging region 18 third tube upper end portion 19 rising pipe upper end portion 20 condensing pipe portion 21 coils 22 upper coil/s 22’ lower coil/s 22” third tube lower outlet portion 23 third heat exchanging region 24 combustion gases 25 third tube 26 tank top portion 27 water inlet 28 lower interior tank portion 29 water outlet 30 upper interior tank portion 31 tank side wall 32 tank bottom wall 33 maximum tank height 34 first tube length 35 side wall 36 of baffle portion 15 external connection angle 37 lower end portion 38 of rising pipe 17 ring wall 39 annular space 40 vertical pitch 41 between underlying coils burner mounting space 42 tank top wall external surface 43 lower dome space 44 internal tank surface 45 deviation wall 48 rising pipe attachment side 49

Claims

1 . A fuel-fired fluid heating apparatus (1 ), comprising:

- a tank (2) for holding a fluid (3) to be heated,

- a single pass heat exchanger (4) of a condensing type, arranged in an interior of the tank (2),

- a fuel burner (5), wherein the heat exchanger (4) comprises:

- a downward extending first tube (6) having a first tube upper end (7) connected to a top wall (8) of the tank (2), a first tube upper section (9) which houses part of the fuel burner (5) and forms a combustion chamber (10), and a first tube lower section (1 1 ) extending from the first tube upper section (9) further downward, wherein the first tube (6) forms a first heat exchanging region (12) for exchanging heat between combustion gases (25) generated by the fuel burner (5) and the fluid (3) inside the tank (2),

- a second tube (13) connected to a first tube side wall (14) of the first tube (6) and having a lower cup shape baffle portion (15) with a closed bottom wall (16) and a rising pipe (17) connected to the baffle portion (15) and extending from the baffle portion (15) upward, wherein the bottom wall (16) is enlarged with respect to the first tube (6) and the baffle portion (15) and the rising pipe (17) form together a second heat exchanging region (18) for exchanging heat between the combustion gases (25) and the fluid (3) inside the tank (2),

- a downward extending third tube (26) having a third tube upper end portion (19) connected to a rising pipe upper end portion (20) of the rising pipe (17), a continuously downward extending condensing pipe portion (21 ) forming multiple coils (22) extending around both the first tube (6) and the second tube (13), and a third tube lower outlet portion (23) exiting the tank (3), wherein the third tube (26) forms a third heat exchanging region (24) for condensing the combustion gases (25) and for exchanging heat between the combustion gases (25) and the fluid (3) inside the tank (2).

2. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the first tube (6) and the second tube (13) are formed from externally enameled metal material, the third tube (26) is formed from metal material which is internally acid resistant enameled and additionally externally enameled, and the tank (2) is formed from internally enameled metal material.

3. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein:

- each individual one of the first, second and third tubes (6, 13, 26) or

- the entire heat exchanger (4) and the tank (2), is/are made from stainless steel material without surface coating.

4. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the first tube (6) has a straight longitudinal extension parallel to a longitudinal extension of a tank side wall (32) of the tank (2), the longitudinal extensions being vertical in an operating position of the apparatus (1 ), and the first tube (6) extends from the tank top wall (8) towards a tank bottom wall (33) for a first tube length (35) of at least from 2/3 to 7/9 of a maximum internal tank height (34), without however reaching the tank bottom wall (33).

5. The fuel-fired fluid heating apparatus (1 ) according to claim 4, wherein the first tube (6) has a constant cross-section throughout its longitudinal extension.

6. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the bottom wall (16) of the baffle portion (15) has an internally concave dome shape.

7. The fuel-fired fluid heating apparatus (1 ) according to claim 6, wherein the baffle portion (15) has a tubular side wall (36) with a lower end connected the bottom wall (16) and an upper end connected to the first tube (6) and tapered from the lower end to the upper end.

8. The fuel-fired fluid heating apparatus (1 ) according to claim 7, wherein the tubular side wall (36) of the baffle portion (15) has a truncated cone shape, wherein a connection region between the tubular side wall (36) and the bottom wall (16) is continuously curved without corners, wherein a connection region between the tubular side wall (36) and the first tube (6) includes a circumferential ring weld and bordering tube wall sections defining, in a radial cross-section plane of the first tube (6), an external connection angle (37) greater than 135°.

9. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the bottom wall (16) of the baffle portion (15) has a flat disk shape, wherein the baffle portion (15) further includes a tubular side wall (36) having a constant crosssection and a lower end connected to the bottom wall (16) and an upper end connected to a flat ring wall (39) which in turn is connected by a further ring weld to an external surface of the first tube (6).

10. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the rising pipe (17) has a curved lower end portion (38) connected, by a ring weld, to a side wall (36) of the baffle portion (15), and a straight rising portion (39) extending parallel to the first tube (6) and at a distance thereof without enveloping the first tube (6), the rising pipe (17) having a constant cross-section throughout a longitudinal extension thereof.

1 1 . The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the rising pipe (17) is externally inserted over the first tube (6) thereby defining an annular space (40) between an external surface of the first tube (6) and an internal surface of the rising pipe (17).

12. The fuel-fired fluid heating apparatus (1 ) according to claim 11 , wherein the rising pipe (17) is formed by an upward extension of a side wall (36) of the baffle portion (15) and as a single piece with the side wall (36).

13. The fuel-fired fluid heating apparatus (1 ) according to claim 12, wherein the rising pipe (17) is straight and has a constant cross-section and an upper end of the rising pipe (17) is connected by a ring weld to a flat ring wall (39) which in turn is connected by a further ring weld to an external surface of the first tube (6).

14. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the bottom wall (16) of the baffle portion (15) is arranged at a distance from the tank bottom wall (33).

15. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the third tube (26) forms at least one upper coil (22’) between the upper end portion (19) and a group of lower coils (22”) of said multiple coils (22), wherein the at least one upper coil (22’) is arranged at a greater vertical pitch (41 ) to a directly subsequent underlying coil (22) than a vertical pitch between two directly subsequent lower coil/s (22).

16. The fuel-fired fluid heating apparatus (1 ) according to claim 15, wherein the at least one upper coil (22’) has a greater downward inclination than the lower coils (22”).

17. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the third tube (26) has a constant cross-section and a continuous curvature without corners or steps.

18. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the condensing pipe portion (21 ) forms a plurality of windings that develop with a constant helix radius about a helix axis parallel to a central axis of the first tube (6).

19. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein a first tube flow section of the first tube (6) is greater than a rising pipe flow section of the rising pipe (17) and the rising pipe flow section is equal to a third tube flow section of the third tube (26).

20. The fuel-fired fluid heating apparatus (1 ) according to claim 1 , wherein the tank (2) has a tank side wall (32) extending vertically in a use position of the apparatus (1 ), an internally concave dome shaped tank bottom wall (33) and an externally concave dome shaped tank top wall (8).

21 . The fuel-fired fluid heating apparatus (1 ) according to claim 20, wherein the fuel burner (5) and the combustion chamber (10) are arranged in a down-firing position at the tank top wall (8) with part of the fuel burner (5) arranged in a burner mounting space (42) defined by an external surface (43) of the tank top wall (8) within an extension of the tank side wall (32).

22. The fuel-fired fluid heating apparatus (1 ) according to claim 1 or claim 20, wherein the heat exchanger (4) extends downward beyond an extension of the tank side wall (32) without reaching the tank bottom wall (33) and wherein the bottom wall (16) of the baffle portion (15) of the heat exchanger (4) is arranged at least partially within a lower dome space (44) defined by an internal surface (45) of the tank bottom wall (33) outside the extension of the tank side wall 32.

23. The fuel-fired fluid heating apparatus (1 ) according to claim 1 or 7, wherein the heat exchanger (4) comprises a flow deviation wall (48) extending inside the baffle portion (15) above a height at which the rising pipe (17) connects to the baffle portion (15), wherein the flow deviation wall (48) protrudes from a same rising pipe attachment side (49) at which the rising pipe (17) connects to the baffle portion (15), and has an inclination towards the bottom wall (16) of the baffle portion (15).