GB2269891A - Flue gas terminal. - Google Patents

Abstract

An exhaust gas terminal such as a gas flue terminal 1a, comprises a bottom part 4 having an exhaust gas aperture 5 therein for receiving an exhaust gas duct, a cap 2 overlying the exhaust gas aperture 5, an exhaust gas opening 9 laterally offset from, and in communication with the exhaust gas aperture 5, and a baffle 30 arranged substantially to prevent air flowing, in use, across the terminal 1a from entering the exhaust gas opening 9. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO EXHAUST GAS TERMINALS The present invention relates to the exhausting of gases and more particularly but not exclusively concerns a terminal for exhausting flue gases.

Exhaust gas terminals in the form of gas flue terminals suitable for installing in the ridges of pitched tiled roof structures are well known and have been manufactured by precasting by hand in concrete for many years. A typical such gas flue terminal is shown in Figures 1 and 2 of the accompanying drawings and is designated by the reference 1.

The gas flue terminal 1 is of generally rectangular shape when considered in plan and comprises a cap 2 having a top surface 3 and a bottom part 4 which is adapted to replace a ridge tile in a pitched roof. The bottom part 4 has an elongate exhaust gas aperture 5 in the form of a slot therein which receives an exhaust gas duct that is connected at its lower end by an adaptor to a circular, in cross-section, gas exhaust pipe or flue. The upper end of the exhaust gas duct is provided with an annular flange which rests on the top surface of the bottom part 4 around the exhaust gas aperture 5. The cap 2 and bottom part 4 are fixed rigidly together as by bolts (not shown) which also pass through the annular flange.

The terminal 1 has two exhaust gas openings 9 extending along two opposite sides 10 respectively, with the exhaust gas openings being formed in the cap 2 and being defined between the cap 2 and the bottom part 4 so that the flue gases are constrained to flow out through the openings 9. As will be apparent from Figure 1, the two remaining opposite sides formed by ends 11 of the terminal 1, have exhaust gas openings 12 therein which are also defined between the cap 2 and the bottom part 4 and are of smaller width and overall size than those of the side openings 9. But, it is traditional also to have terminals such as 1, normally for lower internal diameter gas exhaust pipes, in which the ends 11 are closed and are not provided with the openings 12.As the width of the exhaust gas openings 9 and 12 is greater than 16 mm which allows the entry of birds therethrough and into the terminal 1, these openings 9 and 12 are covered with bird mesh 13 or any other suitable protective means such as bars, incorporated in the cap 2 of the terminal 1.

The height 16 of the top 14 of the inner surface of the cap 2 from the highest point 15 of the bottom part 4 is governed by the flow resistance in the exhaust gas pipe leading from the gas appliance from which the gases are being exhausted and thus depends upon the internal diameter of the gas exhaust pipe or flue to provide a suitable flow resistance match. That is to say for a typical gas flue terminal for a gas exhaust pipe having an internal diameter of 5" (125 mm) or 6" (155mm) permit a flue flow rate which will exhaust gases from conventional gas fires where the majority of the heat generated passes into the room the height of 16 would be in the region of 40mm and the width of the exhaust gas openings 9, 50mm.This means that the terminal 1 projects so far beyond the line of the ridge as to present an ugly appearance and destroy the line of the ridge and thus the aesthetic appearance of the roof. I For conventional gas log fires in which a greater proportion of the heat generated is exhausted up the gas exhaust pipe, a higher flue flow rate is required which can be satisfied by the use of a 7" (180mm) or an 8" (205 mm) internal diameter exhaust gas pipe to permit an adequate flue flow rate. In such a case, the height 16 and width of the exhaust gas openings 9 of terminal 1 have to be significantly greater than with 5" and 6" pipes and is thought by those skilled in the art not tp be possible in order to cope with the increased flow rate.

Thus the height of the top of the terminal from the ridge and the aforementioned disadvantages thereof are even greater.

Also, in order further to reduce the flow resistance in the terminal 1, the walls 17 and 18 of the exhaust gas openings 9 are outwardly divergent so that the exhaust gas openings 9 have a greater width at their outer ends than at their inner ends.

The bottom part of the terminal which forms part of the ridge may be bedded into the ridge with concrete between adjacent ridge tiles or secured to the adjacent ridge tiles by any appropriate dry fixing method, e.g. as disclosed in UK Patent No 2141463.

As is known, British Standard 715:1989 specifies that the critical flow resistances for gas flue terminals are 70 pbar at 40 m3/hour flue flow and 280 pbar at 80 m3/hour flue flow.

The known gas flue terminal 1 falls within these critical values for static resistance (no wind) and at low wind speeds at these fluid flows. But with higher wind speeds reaching 10 m/sec, at a flow rate of 40 m3/h, there is failure (the flow resistance exceeds the critical value of 70 pbar for a terminal 1 having a height 16 of 50 mm).In order to bring the flow resistance values down below the critical values for the higher wind speeds, it has been necessary: 1. to raise the terminal cap 2 even higher above the ridge by increasing height 16, which worsens the ugly appearance of the terminal; 2. to increase the width of the exhaust gas openings 9 in the sides 10, which means that some rain penetration into the terminal 1 and thus into the gas exhaust pipe is inevitable; and/or 3. to open up the ends 11 more to increase the width of the openings 12, which also detracts from the weatherproofing of the terminal.

Accordingly, the main object of the present invention is to provide an exhaust gas terminal such as a gas flue terminal which satisfies British Standard 715:1989 and in which the aforesaid disadvantages of the known gas flue terminals are eliminated or at least substantially reduced.

To this end, and from one aspect, the present invention consists in an exhaust gas terminal, such as a gas flue terminal, said terminal comprising a bottom part having an exhaust gas aperture therein for receiving an exhaust gas duct, a cap overlying the exhaust gas aperture, an exhaust gas opening laterally offset from, and in communication with, the exhaust gas aperture, and a baffle arranged substantially to prevent air flowing, in use, across the terminal from entering the exhaust gas opening.

By means of the invention, not only are all the flow resistance values within the critical values specified in British Standard 715:1989 but also the terminal has a minimum height top which maintains the aesthetic appearance of the roof.

Thus, the invention may also be expressed in terms of providing a gas exhaust terminal such as a gas flue terminal with an external baffle which shields an exhaust gas opening against the ingress of air flowing, in use, across the terminal and with a minimum height top, whereby, when the terminal is installed on a roof, the flow resistance values of the terminal are reduced substantially below the critical values for all wind speeds and the terminal is low to the line of the roof and does not detract from the aesthetic appearance of the roof.

From another aspect, the present invention consists in an exhaust gas terminal such as a gas flue terminal, comprising a bottom part having an exhaust gas aperture therein for receiving an exhaust gas duct, a cap overlying the exhaust gas aperture, an exhaust gas opening laterally offset from, and in communication with, the exhaust gas aperture, and a baffle which is integral with the bottom part and which extends upwardly and covers the exhaust gas opening.

Terminals constructed in accordance with the invention are capable of being made all from the same material and in particular from cementitious materials which are heat resistant. Moreover, such terminals may be made in two parts of cementitious mixture for example by casting in moulds by hand, pressing or extrusion. Any suitable cementitious materials may be used such as ordinary concrete, fibrous cement or alumina cement which latter is particularly advantageous because of its enhanced heat resistant properties.

UK Patent Specification No 2,187,222 discloses a ridge tile assembly which includes a concrete ridge tile having a slot for receiving a ventilation duct and a two-part ventilator of plastics or metal for roof spaces but which is stated to be capable of being used as a terminal for a flue. One part of the ventilator is a cap covering a ventilation aperture in the ventilation duct passing through the slot in the concrete ridge tile through which air can pass from the roof space into the ventilator and air flows out of the ventilator through laterally directed openings in the cap.Longitudinally extending external baffles which are integral with the upper end of the ventilation duct are provided for restricting the access of laterally directed wind and/or rain into the ventilation aperture and additional shielding of the ventilation aperture is provided by extending the upper end of the ventilation duct beyond the upper surface of the ridge tile. Whilst the additional shielding provided by ventilation duct may prevent the entry of rain into the ventilation aperture, the same cannot be said for strong winds which would increase the flow resistance if used as a gas flue terminal, the ventilator is not suitable for making in cementitious materials such as concrete and would have to be made of metal if used as a flue for a gas fire.

Moreover, the ventilator extends high above the roof ridge and is therefore unsightly and detracts from the aesthetic appearance of the roof.

From a still further aspect, the present invention consists in an exhaust gas terminal, such as a gas flue terminal, having external baffling arranged at least substantially to prevent air flowing, in use, across the terminal from entering an exhaust gas opening in the terminal, said terminal being made in two parts which are capable of being made from a cementitious material.

In terminals constructed in accordance with the invention, the cap may be any appropriate width from just covering the exhaust gas aperture to substantially the full width of the terminal. Further, the terminal may be any length consistent with meeting the critical static (no wind) flow resistance value.

Moreover, the cap and bottom part may be of different lengths and the terminal can be provided with baffles to shield exhaust gas openings along only two dpposite sides respectively or exhaust gas openings along all four sides of the terminal.

Whilst the preferred terminal shape is elongate (oblong) when viewed in plan, the terminal could be of any other suitable shape, e.g. square when viewed in plan.

Whilst the terminal is particularly suitable for replacing a single ridge tile in a pitched roof structure it could conceivably replace more than one ridge tile or even all the ridge tiles in a roof if desired or necessary.

From another aspect, the present invention consists in a pitched roof structure having a ridge, eg of ridge tiles, in which at least one of any of the exhaust gas terminals defined hereinabove and constructed in accordance with the invention is incorporated in, and forms part of the ridge, preferably by replacing at least one ridge tile.

The invention also consists in a method of reducing flow resistance in an exhaust gas terminal, in which exhaust gases flow, in use, into the terminal through an exhaust gas aperture and out of the terminal through a laterally offset exhaust gas opening, said method comprising baffling the exhaust gas opening in such manner as to guard against, or substantially to prevent, air flowing across the terminal from entering the exhaust gas opening.

Tests carried out with terminals made in accordance the invention show that: Static (no wind) resistance improves as the height 16 is increased. It is evident that no opening shall cause a restriction to flow. For a 125 mm pipe which has a crosssectional area of 12,275 mm2, with the ends 11 closed (no openings 12) each side opening 9 must have a width of 12,275 mm2 divided by 2 which is 6,137 mm2. So, if the cap is 405 mm in length, the width of the side openings 9 is 6,137 divided by 405 which equals 15 mm. This is a very good value as it is below the maximum width of 16 mm beyond which bird barriers are required.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Figure 1 is a perspective view of a known gas flue terminal for a pitched roof structure having a tiled roof with a ridge forming the roof apex; Figure 2 is a cross section taken along the line II-II of Figure 1; Figure 3 is a perspective view of one embodiment of gas flue terminal constructed in accordance with the invention; Figure 4 is a longitudinal section of the gas flue terminal of Figure 2; Figure 5 is a cross-section taken along the line V-V of Figure 3 and along the line V-V of Figure 6; Figures 6 and 7 are a perspective view and a longitudinal section of another embodiment of the invention, which embodiment also includes the cross-section of Figure 5; ; Figures 8 and 9 are a cross-section and a part longitudinal section of a further embodiment of the invention; Figure 10 is a cross-section of a still further embodiment of the invention; Figure 11 is a perspective view of a pitched roof structure having roof tiles and a ridge of ridge tiles at the apex of the roof, with the gas flue terminal of Figures 5 to 7 installed in the roof ridge; and Figure 12 is a plan view of a gas flue terminal showing wind approach angles in recent tests carried out on a test rig.

In the drawings, the same reference numerals are used to designate the same or similar parts.

The gas flue terminal la illustrated in Figures 3 to 5 differs from that shown in Figures 1 and 2 in that it is of considerably reduced overall height so that it is low to the line of the ridge 60 in Figure 11, and, therefore, maintains the aesthetic appearance of the roof. The reduction in height is due to the presence of side baffles 30 which shield the exhaust gas openings 9 from air flowing, in use, across the terminal la and entering the generally horizontally directed exhaust gas openings 9. Indeed the baffles 30 at least substantially prevent the entry of such air into the exhaust gas openings 9. The baffles 30 are integral with, and extend upwardly from, the bottom part 4 and define with the walls 31 generally vertically extending exhaust gas passages 32. The exhaust gas passages 32 terminate in upwardly directed exit openings 33.The edges 34 of the baffles 32 are bevelled to align with the top surfaces 3 of the wall 31 and of the cap 2 in order to deflect air flowing across the terminal la across the exit openings 33 and away from the exhaust gas openings 9.

For a 5" (125 mm) or 6" (155 mm) internal diameter gas pipe 8, the height 16 can range from about 6mm and the width of the exhaust gas openings 9 can be of any suitable value up to 16 mm within this range. This means that a bird barrier means is not required as it is for exhaust gas openings having widths greater than 16 mm (the width of the openings 9 is substantially the same as the height 16). For 7" (180 mm) or 8" (205 mm) internal diameter gas pipes, the heights 16 and widths of the exhaust gas openings 9 can be up to 40 mm.

It may be necessary to increase the height 16 and width of the exhaust gas openings 9 to obtain optimum results, in particular better static resistance values for the 5" or 6" gas pipes in which case a width and height value of 25 mm may be appropriate. In such a case a bird barrier means is required.

The walls 17 and 18 of the exhaust gas openings 9 need not diverge as in the known gas flue terminal of Figures 1 and 2 but can be parallel.

The embodiment of Figures 5 to 7 differs from that of Figures 3 to 5 in that the remaining two opposite sides (ends) of a terminal 1b shown in Figures 6 and 7 are opened to provide exhaust gas openings 12 which are covered by baffles 40. The height 16 is the same as that of the exhaust gas openings 9 and the width of the exhaust gas openings 12 is also the same as that of the openings 9. This embodiment provides for reduced flow resistance without increasing the height 16 or the width of the exhaust gas openings 9.

As the angle of the roof and thus of the ridge increases, so does the angle of the bottom side portions 51 of the bottom part 4, as will be apparent from Figs. 8 and 9. In the embodiment of Figs 8 and 9, the bottom side portions 51 are disposed outside the terminal lc and outwardly of the baffles 50 in contrast to the corresponding bottom side portions of the bottom parts 4 in the embodiments of Figures 3 to 7. The edges of the baffles 50 extend upwardly beyond the outer edges of the walls 31. The height 16 is a minimum of about 6mm and the terminal lc has closed ends 11 as shown in Figure 9.

Whilst in the embodiments of Figures 3 to 7 the cap 2 has a width which is slightly less than the width of the bottom part 4, in the embodiments of Figures 8, 9 and 10 the width of the cap 2 is substantially less than that of the bottom part 4.

This is taken to an extreme in the terminal ld of Figure 10, in which the cap 2 only just covers the exhaust gas aperture 5. In this embodiment, the height 16 is greater than 15 mm.

The embodiments of Figs. 8 to 11 have an advantage over those of Figs. 3 to 7 in that the visible leading edges 80 correspond to those of the adjacent ridge tiles, unlike the eqivalent leading edges 81, as will be apparent from Fig. 5.

Figure 11 shows the terminal ib of Figures 5 to 7 installed in the ridge 60 of a pitched tiled roof structure having roof tiles 61 and replacing one of the ridge tiles 62. A separate exhaust gas duct 70 extends through the exhaust gas aperture 5 into the terminal lb where its upper end carries an integral annular flange (not visible) which rests on the top of the bottom part 4 surrounding the exhaust gas aperture 5. The exhaust gas duct 70 is connected to an exhaust gas pipe or flue 71 leading from a gas appliance by means of an adaptor 72.

Reference is now made to the accompanying Tables which give the results of comparative tests with the known terminal 1 of Figures 1 and 2 and terminals la and lb constructed in accordance with the invention and as illustrated in Figures 3 to 7. All of the tests were carried out with the wind approach angles indicated in the Tables and will better be understood by reference to the wind approach angles illustrated diagrammatically in Fig. 12.

Table 1 - known terminal 1 of Figures 1 and 2 designated "High Top - Open Sides and Ends".

Table 2 - the terminal la of Figures 3 to 5 with a height 16 of 12 mm designated "Low Top - Open Sides & Closed ends".

Table 3 - the terminal la of Figures 3 to 5 with a height 16 of 16 mm designated "Low Top - Open sides & Closed Ends".

Table 4 - the terminal 1b of Figures 5 to 7 with a height 16 of 16 mm, designated "Low Top - Open Sides & Ends", and Table 5 - summary of test results for the most onerous condition in Tables 1 to 4.

TABLE 1 HIGH TOP - OPEN SIDES & ENDS (15mm) FOR RIDGE TERMINALS DESIGNED TO TAKE A 125mm INTERNAL DIAMETER FLUE RIPE The static pressure measured in ptar.

WIND FLUE FLOW RATE FLUE FLOW RATE APPROACH OF 40m /h OF 80# /h ANGLE, RELATIVE WIND SPEED IN m/s WIND SPEED IN m/s TOO ROOF IN HORIZONTAL PLANE 0 3.0 6.0 10.0 0 3.0 6.0 10.0 0 /180 3 25 62 127 24 40 80 240 30 /150 3 20 52 101 24 40 95 163 60 /120 3 15 23 25 24 32 41 63 90 3 12 9 -10 24 26 30 35 MAXIMUM 70 70 70 70 280 280 280 280 TABLE 2 LOW TOP - OPEN SIDES & CLOSED ENDS (TOP/BOTTOM HEIGHT 12mm) SIDE BAFFLES ONLY.

FOR RIDGE TERMINALS DESIGNED TO TAKE A 125mm INTERNAL DIAMETER FLUE PIPE The static pressure measured in pbar.

WIND FLUE FLOW RATE FLUE FLOW RATE APPROACH OF 40# /h OF 80# /h ANGLE, RELATIVE WIND SPEED IN m/s WIND SPEED IN m/s TOO ROOF IN HORIZONTAL PLANE 0 3.0 6.0 10.0 0 3.0 6.0 10.0 0 /180 30 15 12 10 120 119 118 117 30 /150 30 16 15 6 120 116 115 99 60 /120 30 16 -15 -120 120 111 58 -52 90 30 15 10 20 120 120 93 57 MAXIMUM 70 70 70 70 280 280 280 280 TABLE 3 LOW TOP - OPEN SIDES & CLOSED ENDS (TOP/BOTTOM HEIGHT 16mm) SIDE BAFFLES ONLY, FOR RIDGE TERMINALS DESIGNED TO MAKE A 125mm INTERNAL DIAMETER FLUE PIPE The static pressure measured in bar

WIND FLUE FLOW RATE FLUE FLOW RATE APPROACH OF 40m /h, OF 80m /h ANGLE, RELATIVE WIND SPEED IN m/s WIND SPEED IN M/s TO ROOF IN HORIZONTAL PLANE 0 3.0 6.0 10.0 0 3.0 6.0 10.0 0 /180 10 5 3 0 78 76 75 59 30 /150 10 5 3 -2 78 73 72 49 60 /120 10 5 -20 -140 78 68 15 -71 90 10 5 3 5 78 77 50 18 MAXIMUM 70 70 70 70 280 280 280 280 TABLE 4 LOW TOP - OPEN SIDES & ENDS (TOP/BOTTOM HEIGHT 16mm) SIDE/END BAFFLES.

FOR RIDGE TERMINALS DESIGNED TO MAKE A 125mm INTERNAL DIAMETER FLUE PIPE The static pressure neasured in bar.

WIND FLUE FLOW RATE FLUE FLOW RATE APPROACH OF 40m /h OF 80m /h ANGLE.

RELATIVE WIND SPEED IN m/s WIND SPEED IN m/s TO ROOF IN HORIZONTAL PLANE 0 3.0 6.0 10.0 0 3.0 6.0 10.0 0 / 180 10 2 -5 -15 78 70 55 40 30 / 150 10 0 -5 -40 78 65 50 40 60 / 120 10 0 -30 -140 78 60 5 -100 90 10 0 -10 -60 78 65 40 15 MAXIMUM 70 70 70 70 280 280 280 280 TABLE 5 MOST ONEROUS CONDITION SUMMARY WIND SPEED 10 M/SEC

WIND FLUE FLOW RATE FLUE FLOW RATE APPROACH OF 40m /h OF 80m /h RELATIVE FLUE TYPE FLUE TYPE TO ROOF IN HORIZONTAL PLANE 1 2 3 4 1 2 3 4 0/180 127 10 0 -15 240 117 59 40 30/150 101 6 -2 -40 163 99 49 40 60/120 25 -120 -140 -140 63 -52 -71 -100 90 -10 20 5 -60 35 57 18 15 MAXIMUM 70 70 70 70 280 280 280 280 1 = TRADITIONAL HIGH TOP OPEN ENDS (Needs additional bird barrier) 2 = LOW TOP (12mm above base) END CLOSED BAFFLES ON SIDE ONLY 3 = LOW TOP (16mm above base) END CLOSED BAFFLES ON SIDE ONLY 4 = LOW TOP (16mm above base) END OPEN BAFFLES ON SIDE ENDS It will be seen when comparing the test results for the most onerous condition as illustrated in Table 5 that the flow resistance values for terminals la and lb are much less than those.. from terminal 1 at the highest measured wind speed.

It will be appreciated that various modifications may be made without departing from the scope of the invention. For example instead of being used as a gas flue terminal, the terminals la to id may be used to exhaust air from a lift shaft. Also, instead of making the terminals la to id of a cementitious material, which is preferred, a suitable metal may be used.

Claims (39)

1. An exhaust gas terminal, such as a gas flue terminal, said terminal comprising a bottom part having an exhaust gas aperture therein for receiving an exhaust gas duct, a cap overlying the exhaust gas aperture, an exhaust gas opening laterally offset from, and in communication with, the exhaust gas aperture, and a baffle arranged substantially to prevent air flowing, in use, across the terminal from entering the exhaust gas opening.

2. An exhaust gas terminal, such as a gas flue terminal, said terminal comprising a bottom part which has an exhaust gas aperture therein for receiving an exhaust gas duct, a cap overlying the exhaust gas aperture, an exhaust gas opening laterally offset from, and in communication with, the exhaust gas aperture, and a baffle which is integral with the bottom part and which extends upwardly and shields the exhaust gas opening.

3. An exhaust gas terminal according to claim 1 or claim 2, and including a wall which, with the baffle, defines an exhaust gas passage connecting the exhaust gas opening to an exit opening.

4. An exhaust gas terminal according to claim 3, wherein the baffle has an edge, associated with the exit opening, shaped to deflect air flowing across the terminal across the exit opening and away from the exhaust gas opening.

5. An exhaust gas terminal according to claim 4, wherein the baffle aligns with or extends beyond the outer edge of the wall.

6. An exhaust gas terminal according to any of claims 3 to 5, wherein the exit opening is generally upwardly directed.

7. An exhaust gas terminal according to any of claims 3 to 6, wherein the exhaust gas passage extends substantially vertically.

8. An exhaust gas terminal according to any preceding claim, wherein the exhaust gas opening is generally horizontally directed.

9. An exhaust gas terminal according to any of claims 3 to 8, wherein the baffle and the wall are elongate substantially parallel members defining an elongate exhaust gas passage.

10. An exhaust gas terminal according to any of claims 3 to 9, wherein the wall and the cap are integral with one another.

11. An exhaust gas terminal according to any preceding claim, wherein the cap just covers the exhaust gas aperture and has a width which is substantially less than that of the bottom part.

12. An exhaust gas terminal according to any of claims 1 to 10, wherein the cap has a width which is slightly less than that of the bottom part.

13. An exhaust gas terminal according to any preceding claim, wherein the terminal is of generally rectangular shape when considered in plan, and wherein there are two such exhaust gas openings extending along two opposite sides respectively of the terminal and two such baffles each associated with a respective one of the two exhaust gas openings, and wherein the remaining two opposite sides are closed.

14. An exhaust gas terminal according to claim 13, wherein the remaining two opposite sides of the terminal are not closed but also have two such exhaust gas openings extending therealong and two such baffles, each associated with a respective one of these exhaust gas openings.

15. An exhaust gas terminal according to claim 13 or claim 14, wherein the exhaust gas openings each have a width which does not exceed about 16mm and which is preferably about 15mm.

16. An exhaust gas terminal according to claim 15, and suitable for use with an exhaust gas pipe of up to about 6 inches internal diameter.

17. An exhaust gas terminal according to any of claims 1 to 14, and suitable for use with an exhaust gas pipe of up to about 8 inches internal diameter.

18. An exhaust gas terminal as claimed in claim 17, wherein the height of the top of the inner surface of the cap from the highest point of the bottom part is about 25mm.

19. An exhaust gas terminal according to any of claims 1 to 16, wherein the height of the top of the inner surface of the cap from the highest part of the bottom part is up to about 16mm.

20. An exhaust gas terminal according to claim 19, wherein the height of the top of the inner surface of the cap from the highest part of the bottom part is about 6mm at least.

21. An exhaust gas terminal according to claim 20, wherein the height of the top of the inner surface of the cap from the highest point of the bottom part is about 16mm.

22. An exhaust gas terminal according to any preceding claim, wherein the or each exhaust gas opening is of a size to prevent the entry of birds into the terminal.

23. An exhaust gas terminal according to any preceding claim, wherein the or each exhaust gas aperture has a width which is substantially the same as the height of the top of the inner surface of the cap from the highest point of the upper surface of the bottom part.

24. An exhaust gas terminal according to any preceding claim, wherein the height of the top of the inner surface of the cap is substantially the same as the width of the or each exhaust gas opening.

25. An exhaust gas terminal according to any preceding claim, and suitable for use in a pitched roof structure having a ridge, wherein the bottom part is adapted to take the place of a part of the ridge.

26. An exhaust gas terminal according to claim 25, wherein the bottom part is adapted to take the place of a ridge tile.

27. An exhaust gas terminal such as a gas flue terminal and provided with an external baffle which shields an exhaust gas opening against the ingress of air flowing, in use, across the terminal, and with a minimum height top whereby the terminal does not detract from the aesthetic appearance of the roof.

28. An exhaust gas terminal substantially as hereinbefore described with reference to Figs. 3 to 5, or Figs. 5 to 7, or Figs. 8 and 9, or Fig. 10 of the accompanying drawings.

29. A pitched roof structure having a ridge forming the apex of the roof and having at least one exhaust gas terminal as claimed in any preceding claim incorporated in, and forming part of, the ridge.

30. A pitched roof structure according to claim 30, wherein the ridge is formed from ridge tiles and wherein the bottom part of the or each exhaust gas terminal replaces at least one of the ridge tiles.

31. A pitched roof structure according to claim 29 or claim 30, wherein the baffles permit a reduction in the width of the or each exhaust gas opening whereby the or each exhaust gas terminal is of minimum height in relation to the general line of, and lies low in relation to, the ridge, thereby to maintain the aesthetic appearance of the roof.

32. A pitched roof structure according to any of claims 29 to 31 wherein, as the angle of the ridge increases, the width of the cap decreases.

33. A pitched roof structure as claimed in claim 30, or any claim dependent thereon, wherein the terminal has leading edges which are visible and of which each has a thickness which corresponds to that of each of the leading edges of adjacent ridge tiles.'

34. A pitched roof structure substantially as hereinbefore described with reference to, or as illustrated in, Fig. 11 of the accompanying drawings.

35. A method of reducing flow resistance in an exhaust gas terminal installed in a roof, in which exhaust gases flow, in use, into the terminal through an exhaust gas aperture and out of the terminal through a laterally offset exhaust gas opening, said method comprising baffling the exhaust gas opening in such manner as substantially to prevent air flowing across the terminal from entering the exhaust gas opening.

36. A method of reducing flow resistance in an exhaust gas terminal installed in a roof, in which exhaust gases flow, in use, into the terminal through an exhaust gas aperture and out of the terminal through a laterally offset exhaust gas opening, said method comprising baffling the exhaust gas opening in such manner as to guard against air flowing across the terminal from entering the exhaust gas opening.

37. A method as claimed in claim 35 or 36, wherein the baffling is integral with a bottom part of the terminal, which bottom part is adapted to replace a ridge tile in a ridge of a roof.

38. Method of reducing flow resistance in an exhaust gas terminal installed in a roof 1 substantially as hereinbefore described with reference to, or as illustrated in, the accompanying drawings.

39. A method of reducing flow resistance in an exhaust gas terminal installed in a roof, substantially as hereinbefore described with reference to any of the accompanying tables.