WO1989000219A1

WO1989000219A1 - Improvements in or relating to sanitary drainage systems

Info

Publication number: WO1989000219A1
Application number: PCT/GB1988/000522
Authority: WO
Inventors: George Molyneux
Original assignee: G. Molyneux (Products) Limited
Priority date: 1987-07-04
Filing date: 1988-07-04
Publication date: 1989-01-12
Also published as: GB8715767D0

Abstract

A sanitary drainage system for a building has one or more system water seals (76; 92; 96) (i.e. water seals in the outlets from items such as washbasins, baths or water closets). To prevent penetration of a system seal which could occur due to foul air bubbling through the seal if positive pressure occurs on the foul air side of the system seal, or if the system seal is destroyed due to negative pressure on the foul air side, a vent water seal is provided of lower head than the system water seal and communicating with an outlet (48; 84) on the outside of the building. The vent water seal may utilise the water in the cistern (14; 62) of a water closet, the interior of the cistern communicating with the outside of the building through, for example, an overflow pipe (30).

Description

Improvements in or relating to Sanitary Drainage Systems

Most sanitary drainage systems in buildings, and especially in dwelling houses, incorporate water seals between the waste outlet (e.g. water closet pan, or the outlet from a bath, wash basin or sink) and the drains. The purpose of the water seal is to prevent odours from the drains entering the building. For present purposes, such a water seal will be referred to as a system water seal and it will be appreciated that the downstream side of a system water seal is the foul air side. It will be understood, that the system for a typical dwelling house bathroom will include three water seals, namely one in the water closet pan, one in the wash basin, and one in the bath, though all the drainpipes will connect into a single draining system.

At certain times, a negative pressure (i.e. less than ambient atmospheric pressure) occurs on the foul air side of the system water seal or seals. Typically, this arises when a water closet is flushed: the water flowing out of the water closet pan tends to form an annular curtain around the inside of the wall of a vertical soil stack and this causes the "core" of air in the soil pipe to be pulled along with the water, changing some of the static pressure of the air- to kinetic pressure and thereby reducing the static pressure, thus creating the negative pressure tendency. This effect is particularly pronounced where there is a considerable vertical stretch of soil pipe. The negative pressure draws some of the water from any system water seals out of those seals into the drain, thereby reducing the depth of the system water seals.

However, because of the usual orientation of the bathroom appliances along with the design and layout of the sanitary drainage system, it is frequently the wash basin water seal which is destroyed by the action of negative drain pressures and without it being replenished. Building regulations, and good building practice, require the provision of vents into the foul air side of a sanitary drainage system to prevent the occurrence of negative pressure in the system. The most common form of vent is a pipe open at the top and extending either up the outside wall of the building or through the roof of the building. In either case, it is a requirement in the

United Kingdom at the present time, that the vent outlet

(or strictly speaking inlet, having regard to its function of preventing the occμrrence of negative pressure in the system) must be at least 1 metre above the level of any opening into the building (door or window) within three metres of the vent pipe.

Vent pipes are effective, but usually spoil the appearance of the building. They can also be quite expensive. To avoid the use of vent pipes, air admittance valves are connected to the foul air side of the system water seal(s). An air admittance valve is simply a non¬ return valve designed to yield to allow air to enter the foul air side of the system when a negative pressure tendency occurs, to prevent the creation of a negative pressure of a magnitude which would draw water from a water seal into the system.

Air admittance valves are subject to mechanical failure, but a more serious disadvantage is that they cannot deal with positive pressure surges in the drainage system. A common cause of positive pressure surge is so- called "surcharging". This occurs when the discharged water from a system cannot immediately escape through an underground drain serving several systems, because one of the other systems is already using the drain. This occurs for instance if two water closets in houses connected to the same underground drain are flushed simultaneously or nearly so. The discharged water which cannot escape freely "backs up" in the branch drain pipe and creates a back pressure surge - positive pressure - on the foul air side of the system water seal(s). Now the pressure required to break a system water seal of 40mm or 50mm head is not large (approximately 5 millibars for a 50mm seal) and therefore a positive pressure surge will relieve itself by bubbling through the shallowest system seal in the system. Very often this is the wash basin seal. The result of this is the emission of a foul air bubble into the room in which the penetrated seal is located, giving rise to a foul smell in that room with no apparent cause.

The present invention is concerned with the venting of sanitary drainage systems in a manner which will deal with positive as well as negative pressure surges without having to resort to the use of the traditional vent pipe. The invention can be used in substitution for or in addition to the use of an air admittance valve.

According to a first aspect of this invention a sanitary drainage system for a building which incorporates at least one system water seal is vented on the foul air side of the system water seal through a vent water seal of lower head than the system seal or any of them to an outlet on the outside of the building. With an arrangement in accordance with the invention, any positive or negative pressure surge will be relieved through the vent water seal, since that will offer less resistance than the or any system seal. In the case of a positive pressure surge the foul air thus released from the system is vented through the outlet to the outside of the building. Even if this outlet is nearer to a door or window opening than would be permitted for the outlet of a vent pipe, it gives an improvement over the presently used air admittance valve systems wherein all the foul air is released into the room. It will be appreciated that positive pressure will only normally occur when an air admittance valve is used instead of a vent pipe, though it can also occur in installations where a stub soil sack is used without any means of ventilation - this is permissible in the United Kingdom, providing the crown of the water closet trap is no more than 1.5 metres above the invert level of the underground drain.

The vent water seal can be provided simply by locating an open end of a vent conduit below the level of a reservoir of water in a container, the head of the vent water seal being determined by the vertical distance between the water level and the open end of the conduit. It is preferred however to provide a trap in the conduit /

/ since otherwise the foul air space would be in unrestricted communication with the space within which the reservoir is located if the level of water in the reservoir falls below the open end of the vent conduit.

In a preferred arrangement, the vent conduit comprises a U-shaped portion located with its bottom portion below the level of water in the reservoir and both arms of the U extending above the level of water in the reservoir, there being a hole of smaller cross- sectional area than the bore of the conduit, formed through the wall of the U-shaped portion of the conduit below the water level in the reservoir, to permit water to enter the U-shaped portion from the reservoir to 'form the vent water seal but to restrict the flow of water into the conduit when water begins to flow out of it so as to prevent full bore flow through the conduit and thereby prevent the formation of a syphon effect through the conduit.

Preferably the vent water seal is of an automatically replenished kind. It may, for example, be formed in a water cistern provided with an automatic water supply e.g. a water supply controlled by a ball float valve.

It is further preferred that the vent water seal is provided in a sealed container and that the outlet is through a conduit feeding from the interior of the container to the outside of the building. In a preferred arrangement the outlet conduit also forms at least part of an overflow for the container.

According to a further preferred arrangement, a vent conduit from the foul air side of the system water seal to the vent water seal is so arranged that it forms an overflow arrangement for the container passing from the interior of the container through the vent water seal into the foul air side of the drainage system.

A convenient arrangement in accordance with the invention utilises the water cistern of a water closet to provide the vent water seal. In that case, it is only necessary to provide a vent conduit leading from the vent water seal in the cistern into part of the drainage system on the foul air side of the system water seal(s), so long as an air outlet is provided from the interior of the cistern to the outside of the building. In most instances an air outlet is already available through the ordinary overflow pipe of the cistern.

It is further preferred that the vent conduit passes through part of the outlet pipe of the cistern, the outlet pipe being open into the cistern around the conduit, sealing on to the conduit where the latter leaves the outlet pipe and having a branch leading to an overflow outlet. This particular arrangement presents two advantages, viz:-

(i) Both the overflow pipe and the vent conduit enter the cistern through a single opening in the wall of the cistern, so that it is not necessary^" to provide an extra hole in the cistern wall to receive the vent conduit: (ii) In the case of an overflow, part of the overflowing water flows out of the cistern through the vent conduit into the drain. This reduces the rate of flow through the overflow pipe itself to the outside of the building. Although the water escaping on the outside of the building will still create a nuisance (as required by the United Kingdom Building Regulations) it will, in the case of a prolonged or serious overflow, effect less damage than would be the case if all the overflowing water were to escape via the overflow pipe.

According to a second aspect of the invention an overflow arrangement for a cistern comprises two or more outlet conduits, the cross-sections of which in total provide the required minimum overflow cross-section, at least one having an open end which will create an overflow drip if the overflow water level in the cistern is exceeded, and at least another leading into a drain.

Preferably the conduit which leads into the drain extends through part of another conduit, where the two conduits pass through the wall of the cistern. In this manner, both conduits pass through the same hole in the wall of the cistern and it is not necessary to provide an extra hole for .the drain-connected conduit. In. the preferred construction part of the drain-connected conduit is located coaxially within the other conduit, the other conduit having an open end around the drain conduit inside the cistern, being sealed to the outside of the drain conduit where the latter passes out of the other conduit and having a branch which leads to an open outlet end.

According to a third aspect of the invention an overflow fitting for a cistern comprises inner and outer concentric pipes, the inner pipe being open at both ends and the outer pipe having means for sealingly securing it in a hole in a cistern wall when the second pipe extends through that wall, sealing means sealing between the inside of the outer pipe and the outside of the inner pipe and a branch pipe leading from the outer pipe at a position between the means for securing that pipe to the cistern wall and the seal between the inner and outer pipes.

Preferably a dip tube open at the lower end is attached to the end of the inner pipe which in use is inside the cistern. There may be a U-bend formed on the dip tube to ^' provide a water trap therein. In a refinement, a hole of small cross-sectional area than the cross-sectional area of the dip tube bore is formed in the U-bend of the dip tube.

The invention in all three aspects, will be better understood from the following description of a specific arrangement embodying all three aspects of the invention, and certain modifications thereof. This specific description is given by way of example only of the invention.

In the accompanying drawings:- Figure 1 is a diagrammatic perspective view of a water closet installation in a building, Figure 2 is a diagrammatic cross-section through an overflow fitting.

Figure 3 is a perspective view of part of the overflow fitting illustrated in Figure 2,

Figure 4 is a diagram illustrating a venting arrangement in accordance with the invention connected to a system which incorporates a water closet and a wash basin. Figure 5 is a view similar to Figure 4, but illustrating the condition when the water closet is being flushed. Figure 6 is a view similar to Figure 4 but illustrating the occurrence of a positive pressure surge in the system.

Figure 7 is a diagram illustrating a system in a bathroom comprising a water closet, a wash basin and a bath and incorporating a vent system in accordance with the invention.

Figure 8 shows a modification of the overflow fitting illustrated in Figure 2 to provide a water trap, and Figure 9 illustrates a further modified overflow fitting in position in a cistern with provision to avoid a syphoning effect. in Figure 1 , there is shown a water closet 10, comprising a pan 12 and a water cistern 14 of conventional design. The cistern 14 holds a reservoir of water 16 and the level of water is automatically maintained by a ball float inlet valve 18 which is also of conventional design and requires no further description. The cistern is also fitted with a syphonic discharge mechanism illustrated generally at 20. The pan 12 is formed with the conventional water trap, which is not visible in Figure 1 , and the soil pipe 22 from the outside of the water trap, leads into a vertical soil stack 24. Other water closets at different levels may be connected by soil pipes into the stack 24.

Conventionally, the water cistern 14 would be provided with an overflow arrangement in the form of an overflow pipe passing through a hole in an end wall of the cistern and out through the wall 26 of the building in which the water closet is located. The outlet end of the overflow pipe is open, and United Kingdom Building Regulations at the present time, require that the outlet from the overflow shall be situated in a position where any water dripping from the overflow will create a nuisance, so that occupants of the building will take speedy action to prevent the overflow. This is a water conservation measure. The arrangement which is illustrated in Figure 1 has a special overflow fitting 28 which is more clearly illustrated in Figures 2 and 3. A short outer pipe 30 extends through a clearance hole 32 in an end wall 34 of the cistern 14 and this outer pipe 30 resembles conventional overflow pipes, in that it is provided with a flange 36 and a screw threaded portion 38 on which a sealing washer 40 and a clamping nut 42 are received. When the outer pipe 30 is fitted to the cistern, the flange 36 engages with the outer face of the wall 34 and the washer 40 is nipped onto the inner face of that wall by tightening the nut 42.

The outer pipe 30 differs from conventional overflow pipes, in that it is formed with a branch connection 44 (see Figure 3) and the end opposite the screw threaded inlet end, is provided with a rubber 0- ring seal 46.

When the overflow fitting is in position on the cistern 14, an outlet pipe 48 is pressed into the open end of the branch connection 44, and this outlet pipe is then taken through the outer wall of the building as illustrated in Figure 1 , to an open end (not shown) which will be located in a position where water dripping from it will create a nuisance. In other words, the conventional overflow system is via the outer pipe 30, the branch connection 44 and the outlet pipe 48. An inner pipe or vent conduit 50 of smaller bore than the outer pipe 30, is located concentrically within the outer pipe, and is a force fit through the O-ring seal 46, so that the latter forms a watertight seal between the outer and inner pipes, at the position where the vent pipe 50 leaves the outer pipe 30. (It is to be understood, that the expression "concentric" used herein, simply means that one pipe is located within the other, and there is no requirement for precise geometric concentricity.) It will be appreciated, that this arrangement provides an annular passage through the outer pipe 30 from the open inlet end of the pipe 30 which is inside the cistern, into the connector 44 and the outlet pipe 48. Obviously, the cross-sectional area of this annular passage in the pipe 30 is appreciably smaller than the cross-section of the pipe 30 itself.

On the inside of the cistern 14, the vent conduit 50 is fitted with a dip tube 52 having an open lower end. It will be appreciated, that the hole 32 for the overflow fitting is relatively high in the side wall of the cistern, since it must be above the normal water level. However, the length of the dip tube 52 is such that the open lower end will always be below the water level which is maintained in the cistern by the ball float valve 18. The location of the lower end of the dip tube 52, is illustrated in Figure 6 in relation to the normal water level within the cistern. It is an important feature of the invention, that the head of water between the normal water level in the cistern and the open lower end of the dip tube 52 is less than (and preferably appreciably less than) the head of water in the weakest water seal of the system. In the arrangement illustrated in Figure 1 , there is only one system water seal, that being the water trap provided in the water closet pan. If the head of water above the lower end of the dip tube 52 is say 40mm, then that should be appreciably less than the head of water in the water closet trap which is of a minimum 50mm depth in the United Kingdom.

The other end of the vent pipe 50 is simply connected into the soil pipe 24 as illustrated in Figure 1. Hence, any water escaping fr.om the cistern through the vent pipe 50, will simply discharge into the drainage system. It is also important to notice, that the inside of the soil stack 24 is on the foul air side of any system water seals such as that in the water closet pan 12. However, the connection which is provided from this foul air zone through the vent pipe 50 is normally sealed by the water seal created around the dip tube 52 by the reservoir of water 16 within the cistern 14.

The arrangement in accordance with the invention will deal with both positive and negative pressure surges on the foul air side of the system water seals. Problems of "surcharging" are more likely to occur, where a wash basin forms part of a system, and such a system is illustrated in Figures 4, 5 and 6 which will now be described.

A water closet 60 has a cistern 62 and a pan 64. The soil pipe 66 from the water closet connects into a soil stack 68. This is an arrangement which is permissible in low level buildings, wherein there is no venting of the foul air system at all. A stub soil stack 70 extends above the main part of the soil stack 68, and is closed by a sealed cap 72.

A wash basin 74 is provided with the usual water trap 76, but it has been found that the wash basin water seal which is vulnerable to destruction by negative pressure, or through which foul air sometimes escapes due to positive pressure. The drainpipe 78 from the wash basin 74 is connected into the soil stack 68. The arrangement illustrated in Figures 4, 5 and 6 incorporates a special overflow fitting 80, similar to the overflow fitting 28 described with reference to Figures 1 to 3, and the vent conduit 82, which is equivalent to the conduit 50 shown in Figure 2 is taken through a sealed opening in the cap 72, into the upper end of the stub stack 70. Here again therefore, the vent conduit 82 enters the foul air side of the system. An outlet pipe 84 which is equivalent to the outlet pipe 48 is taken through the wall of the building to a drip position.

Figure 5 illustrates what happens when the water closet 60 is flushed. Water escaping from the pan 64 flows through the soil pipe 66, but does not fill the bore of that pipe. It also flows into the soil stack 68 on its way into the underground drainage system, but in passing down the vertical section of the stack 68, it tends to form an annular curtain around the inside of the wall of the stack. The core of air within this annular curtain of water tends to be drawn downwardly as illustrated by the arrowheads in Figure 5, and this creates a negative pressure surge in the foul air system. This negative air surge may well be sufficient to draw a small quantity of water out of the water trap 76 of the wash basin 74, but with low level systems, the negative air surge is normally so small, that it has no appreciable effect on the water trap 76, and this is why it is not necessary in such a system to fit an air admittance valve. As illustrated in Figure 5, if a negative pressure occurs in the vertical soil stack 70, when the water closet is being flushed, air will be drawn through the pipe 82 from the open end of the dip tube 52 from inside the cistern, which is in communication with the atmosphere on the outside of the building through the outlet pipe 84 and the overflow fitting.

However, if the end of the dip tube 52 is submerged below the water in the cistern 14 when a negative pressure occurs in the vertical soil stack 70, then water will be drawn into the foul air system from the cistern 14 and this will prevent the negative pressure from pulling the water out of the washbasin trap 76 because the head of water forming the vent seal in the cistern is less than the head of water in the washbasin trap. The loss of water from inside the cistern 14 is of no consequence because the water reservoir in the cistern is automatically replenished, and this replenishment would only be necessary for the very short period -of time that the vertical soil stack contained discharging closet water. This is because normal atmospheric pressure would be restored in the soil stack when the discharging closet water reached the underground horizontal drain. However, if a negative pressure does occur in the soil stack 70 when the dip tube 52 is submerged below the water in the cistern 14, it will more likely have been caused by induced siphonage due to some appliance other than the closet 60 discharging into the sanitary drainage system and this could for example be a bath as shown in Figure 7. Even with systems such as that illustrated in Figures 4, 5 and 6, it is possible however to have positive pressure surges for instance, if two water closets of houses connected into the same drainage system are flushed more or less simultaneously. What happens in the case of a positive pressure surge is illustrated in Figure 6. The positive air surge is illustrated within the soil stack 68 and the stub stack 70 by arrowheads, and providing this surge is of sufficient strength, it can be relieved by the water seal provided within the cistern 62. Since the water seal within the cistern is of lower head than the water seal 76 (or any other system seal within the system) and therefore, the water seal within the cistern 62 represents the weakest seal in the system it will release air from the foul air side of the system seals before air could be released from any of those system seals. Consequently, the bubble of foul air is released into the closed cistern 62. From the interior of the cistern 62 the foul air can escape along the outlet pipe 84 to the outside of the building. This prevents the emission of foul smelling air into the room in which the water closet and wash basin are situated. t will now be appreciated, that the single unit system illustrated in Figure 1 operates on exactly the same principle as that described with reference to Figures 4 to 6, ..insofar as the release of foul smelling air on the occurrence of . a positive pressure surge is concerned.

Turning now to Figure 7, there is illustrated a typical bathroom layout, comprising a bath 90 with a water trap 92 connected to its outlet; a wash basin 94 with a water trap 96, and a water closet 98. The system includes a soil stack 100 into which the soil pipe 102 of the water closet 98 leads. Further, the system includes a drainpipe 104 from the water trap 92 of the bath 90, into the soil stack 100 below the soil pipe 102, there being a branch connection 106 from the water trap 96 of the wash basin 94 into the drainpipe 104. The top end of the soil stack 100 may be simply sealed at 108 or closed by an air admittance valve or alternatively, it may be taken up to an extension 110, which provides a conventional vent pipe arrangement.

For present purposes, it is assumed that the conventional vent pipe is omitted, and the top of the soil stack 100 is simply sealed . Supposing that the bath 90 is discharged, resulting in full bore flow through the drainpipe 104. The flow of water in the pipe 104 would pull air from the washbasin connection pipe 106 and the negative pressure in the pipe 106 would tend to induce siphonage of water from the wash basin trap 96. In some instances this would destroy the water seal formed by the trap 96, with the result that, once the bath had been discharged, foul smells could enter the building through the wash basin outlet.

In any case, however, the conventional system is not capable of dealing with positive pressure surges in the system comprising the soil stack 100, the drainpipe 104 and the branch pipe 106. Positive surges are quite likely to occur with such a system, and are especially prone to occur in the soil stack 100 and be transmitted along pipes 104 and 106 resulting in the release of. foul smelling air into the bathroom through either the wash basin water seal 96 or the bath water seal 92.

A special overflow fitting 112 similar to that illustrated at 28 in Figures 1, 2 & 3, is fitted to the cistern of the water closet, and the vent conduit 114 from this fitting leads into the branch pipe 106.

Now if a negative pressure occurs in the system (such as that described above tending to destroy the wash basin water seal) the negative pressure is relieved by water being drawn from the cistern of the water closet through the special overflow fitting 112 and the air vent pipe 114. Therefore there would be no tendency to destroy the water seal at the wash basin trap 96. Water would be drawn from the cistern only so long as the bath discharge continued at the most, and in practice, probably only for a part of this period; but in any event this would be immaterial because of the automatic replenishment of the water in the cistern. If on the other hand there is a positive pressure surge in the branch pipe 106, it can be released through the vent seal provided in the cistern of the water closet, and thence through the outlet pipe (not shown) to atmosphere outside the building. It will be appreciated that if a positive pressure begins to build up in the drainpipe 104, the build up will also occur in the branch pipe 106, and will still be relieved through the water seal vent in the cistern.

A disadvantage of the cistern illustrated in Figures 4 to 6 can be seen by reference to Figure 5. At the time when most of the water in the cistern has discharged through the pan of the water closet, the water level in the cistern is very low, and certainly below the open lower end of the dip tube 52. In this condition therefore, the foul air side of the system is actually open through the vent conduit 82 and the dip tube to the inside of the cistern. This in itself should not be a disadvantage, since the cistern provides an adequate seal, and there is venting of the space in the cistern above the water level, through the outlet pipe 84.

However, as a means of preventing the opening of the foul air system into the cistern, a short U-bend 120 (see

Figure 8) can be fitted to the lower end of the dip tube

52. The effective water seal is still from the water level in the cistern to the invert level of the U-bend

120, and the overflow fitting functions to release positive air pressure surges in the system through the water seal in the cistern, exactly as previously described. However, when water is discharged from the cistern on flushing the water closet, a water trap is left in the U-bend 120, which therefore continues to seal the foul air side of the system from the air inside the cistern.

Another possible problem with the system is dealt with by the arrangement illustrated in Figure 9. It will be seen that the overflow fitting is generally similar to that described with reference to Figures 2 and 3, in that there is an inner vent conduit or pipe 50 and an outer pipe 30, with a dip tube 52. In this arrangement however, the dip tube forms one stem of a U-tube, the other stem 122 of which has its open top end 124 well above the ordinary water level in the cistern 14. Also a small bore hole 126 is formed through the wall of the branch 124 of the dip tube close to the U-bend at the bottom of that tube. It is an essential feature of this arrangement, that the cross-sectional area of the hole 126 shall be appreciably smaller than the cross-sectional area of the bore of the dip tube itself.

It is also to be noted, that the open top end 124 of the dip tube is located below the bottom of the overflow pipe 30.

Water in the cistern 14 fills the bottom part of the U-tube 52 up to the-, water level in the cistern. Thus, if there is a positive pressure surge on the foul air side of the system seals, it can be released through the water seal provided within the U-tube 52, and the strength of the seal is determined by the depth of the water seal, between the water level in the cistern and the invert of the U-bend at the lower end of the U-tube 52. From the point of view of releasing positive pressure surges, the arrangement illustrated in Figure 9 operates in the same manner as the venting arrangements previously described.

When the water closet is flushed, and the water level in the cistern falls rapidly, water will eventually begin to issue through the small bore hole 126, but the arrangement is such, that before the water seal provided in the U-bend 52 has been broken, the water rising within the cistern reaches the level of the hole 126 and once again begins to fill the U-bend. In other words, there is always a water seal between the foul air side of the system and the interior of the cistern 14.

The problem which could arise with the fitting illustrated in Figures 2 and 3 is that when a negative pressure occurs in the wash basin waste pipe (e.g. the branch pipe 106 in Figure 7) water will be sucked through the dip tube 52 into the drainage system. It is possible that this will set up a syphon from the water reservoir in the cistern 14 through the vent pipe into the drainage system, and consequently water will continuously drain from the cistern into the drainage system. This problem will only occur in a situation where the connection from the vent conduit 50 to the drainage system is below the water level in the cistern, as for example in the Figure 7 arrangement, but it is quite likely that an arrangement such as that shown in Figure 7 will be commonly used. However, if water begins to drain out of the U- bend 52 illustrated in Figure 9, it can only be replenished through the small bore hole 126. Hence, a full bore flow cannot be established in the vent conduit 50, and air will be admitted via the open end 124 of the tube 52. The admission of this air will break any syphon set up in the conduit 50, and this will prevent continuous discharge of water from the cistern into the drainage system.

Reverting to Figure 2, the operation of the overflow fitting when an overflow actually occurs will now be described. As the water level in the cistern rises above the correct water level, it will eventually reach the level of the bottom of the outer pipe 30. The water will then flow through the bottom part of the annular space between the pipes 30 and 50, and will trickle out through the branch connection 44 and the outlet pipe 48, appearing as a drip on the outside of the building in a position where it will create a nuisance. In this way, the usual indication is given that an overflow is occurring. if this drip is not sufficient to cope with the inflow of water into the cistern, the water level will eventually build up to that at which water begins to flow through the inner pipe 50. However, this water is simply discharged into the drainage system, where it will cause no harm. There is therefore a division of the overflowing water between the two pipes, and except in the case of a very small trickle, ^* this will always have the effect of reducing the quantity of water which is being delivered on the outside of the building. This is an advantage, because the water escaping on the outside of the building can damage the structure of the building.

It will be appreciated, that this improvement in the overflow function is achieved as a by-product of the provision of the venting for positive pressure surges. It would in fact be possible to achieve this improved overflow arrangement, by having completely separate overflow pipes, one leading to the outside of the building and the other leading into the drainage system so long as the required vertical relationship between the pipe inlets is maintained. The advantage of the concentric arrangement illustrated in Figure 2 is that it is only necessary to utilise the conventional single clearance hole 32 through the wall of the cistern, and it is not necessary to provide an additional hole. it should be mentioned that in some arrangements, the cistern overflow pipe extends vertically upwards through the base of the cistern to an open top end above the normal water level. Such an overflow could be constructed substantially as previously described herein excepting that the outer pipe would terminate at the overflow level, whereas the inner pipe would be bent through 180° and the end portion formed into a dip tube dipping into the water in the cistern to form the vent seal.

Claims

CLAIMS :

1. A sanitary drainage system for a building incorporating at least one system water seal vented on the foul air side of the system water seal through a vent water seal of lower head than the system seal or any of them to an outlet on the outside of the building.

2. A sanitary drainage system for a building as claimed in Claim 1, in whichthevent seal is provided by locating an open end of a vent conduit below the level of a reservoir of water in a container, the head of the vent water seal being determined by the vertical distance between the water level and the open end of the conduit.

3. A sanitary drainage system for a building as claimed in Claim 2, in which a water trap is provided in the conduit between the foul air side of the system seal(s) and the open end of the conduit.

4. A sanitary drainage system for a building as claimed in Claim 3, in which the vent conduit comprises a U-shaped portion located with its bottom portion below the level of water in the reservoir and both arms of the U extending above the level of water in the reservoir, there being a hole of smaller cross-sectional area than the bore of the conduit, formed through the wall of the U-shaped portion of the conduit below the water level in the reservoir, to pexmit water to enter the U-shaped portion from the reservoir to form the vent water seal but to restrict the flow of water into the conduit when water begins to flow out of it so as to prevent full bore flow through the conduit and thereby prevent the formation of a syphon effect through the conduit.

5. A sanitary drainage system for a building as claimed in any one of Claims 1 to 4, in which the vent water seal is of an automatically replenished kind.

6. A sanitarydrainage system for a building as claimed in Claim 5, in which the vent water seal is formed in a water cistern provided with an automatic water supply.

7. A sanitary drainage system for a building as claimed in any one of Claims 1 to 6, in which the vent water seal is provided in a sealed container and the outlet is through a conduit feeding from the interior of the container to the outside of the building.

8. A sanitary drainage system for a building as claimed in Claim 7, in which the outlet conduit also forms at least part of an overflow for the container.

9. A sanitary drainage system for a building as claimed in Claim 8, in which a vent conduit from the foul air side of the system water seal to the vent water seal is so arranged that it forms an overflow arrangement for the container passing from the interior of the container through the vent water seal into the foul air side of the drainage system.

10. A sanitary drainage system for a building as claimed in any one of Claims 1 to 9, in which the vent water seal is provided by the water cistern of a water closet.

11. A sanitary drainage system for a building as claimed in Claim 10, wherein the water cistern is provided with an overflow pipe and in which the vent conduit passes through part of the overflow pipe, the overflow pipe being open into the cistern around the conduit, sealing on to the conduit where the latter leaves the overflow pipe and having a branch leading to an overlfow outlet.

12. An overflow arrangement for a cistern comprising two or more outlet conduits, the cross-sections of which in total provide the required minimum overflow cross-section, at least one having an open end which will create an overflow drip if the overflow water level in the cistern is exceeded, and at least another leading into a drain.

13. An overflow arrangement for a cistern as claimed in Claim 12, in which the conduit which leads into the drain extends through part of another conduit where the two conduits pass through the wall of the cistern.

14. An overflow arrangement for a cistern as claimed in Claim 13, in which part of the drain connected conduit is located coaxially within the other conduit, the other conduit having an open end around the drain conduit inside the cistern, being sealed to the outside of the drain conduit where the latter passes out of the other conduit and having a branch which leads to an open outlet end.

15. An overflow fitting for a cistern comprising inner and outer concentric pipes, the inner pipe being open at both ends and the outer pipe having means for sealingly securing it in a^"hole in a cistern wall when the second pipe extends through the wall, sealing means sealing between the inside of the outer pipe and the outside of the inner pipe and a branch pipe leading from the outer pipe at a position between the mans for securing that pipe to the cistern wall and the seal between the inner and outer pipes.

16. An overflow fitting for a cistern as claimed in Claim 15, in which a dip tube open at the lower end is attached to the end of the inner pipe which in use is inside the cistern.

17. An overflow fitting for a cistern as claimed in

Claim 16, in which a U-bend formed on the dip tube provides a water trap therein.

18. An overflow fitting for a cistern as claimed in Claim 17, in which a hole of smaller cross-sectional area than the cross-sectional area of the dip tube bore is formed in the U-bend of the dip tube.