23 May 2011
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United Kingdom
As is (or used to be?) normal, this WC in our downstairs bathroom is connected direct to the drains. No vent pipe. Occasionally the sewer surcharges sufficiently for water in the chamber through which the effluent from this WC passes to rise and cover the inlet into the chamber from the WC. This traps air in the pipe between the chamber and the WC's own outlet. If you flush the WC, or if the water level in the chamber continues rising, the water in the WC pan rises also.

That is all "normal". BUT there is a puzzle. The drain from the WC has a second pipe connected to it (under the ground). This from a gully. I've never had to dig down, so I assume that the connection from the one to the other is an offset tee or a Y connector. The bath and basin in the downstairs bathroom, where the WC is, empty through the wall into this gully. So waste from the gully and waste from the WC end up running along the same pipe into the chamber. (I have verified that this is the case.)

If the sewer surcharges, I would expect the water in the trap of the gully to lift before the water in the trap of the WC starts to rise. In fact, I would expect the water in the WC NOT to rise, and all the back pressure to be relieved via the trap of the gully. The gully is at least 200 mm lower than the WC trap and does not appear to have a deeper trap than the WC's own trap. It also contains considerably less water than the WC's.

But - and this puzzles me very much - if the sewer backfills enough to create back pressure in the shared drain leading into it from both the WC and the gully, ONLY the water in the WC trap rises, NOT the water in the gully!

This plumbing is the age of the house - 51 years. Is it possible that there is a non-return valve in the pipe from the gully, which closes under back air pressure? If there is, I would be surprised if it is still working after so long - especially working well enough to prevent the water level in this gully being affected at all. All the lifting force (compressed air) seems to go exclusively to the WC outlet .

As regards the possible presence of a non-return valve, I don't know if the following is relevant. This gully (uniquely around our house) is not outside. It is in the concrete floor of the (lean-to) garage.
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Seems odd as you say, but nothing surprises me with drainage, you learn to expect the unexpected. It is still possible the Gulley trap has a greater depth of water in it, over a bigger surface area. Gulley will have water in it behind the pot, forming the trap, so the smaller water content of the WC pan may offer less resistance to the air pressure. As with any installation, positive pressure will always find the easiest, not necessarily the lowest, way out.
I don't think that the gully trap contains anything like as much water as the WC trap. The WC pan is a Twyford's "Classic" pan, with a very large, deep trap and the old-fashioned large diameter outlet from trap to soil piper/drain pipe. The pan is a modern slight re-working of a very old design (eg, it no longer has a "horned inlet") and needs 2 gallons of flush water with a good head. When this design of pan was standard it would have been supplied by a high level cistern, to ensure that enough water was supplied to flush it completely, and at a fast enough speed.

Then there's the point that the gully trap is considerably lower than the garage one. The gully is set into the concrete of the garage floor, which is 150 mm lower than the DPC in the house wall on the other side of which is the downstairs bathroom. The DPC is level with the top face of the concrete internal floor of the house. The top of the water in the WC trap is therefore probably at least 300 mm higher than that in the gully.

As you say, though, one should never be surprised at what what may find in a drainage system - especially on old one. But, as you don't mention it, I assume that you rule out the possibility of a non-return valve (especially one still working, and working very well, after 51 years!) in the section of drain leading from the gulley to the point where this "gray water" joins the "black water" from the WC. On reflection, I now wonder whether this is a possibility at all - because the drain pipes connecting our property with the local sewer are not old-fashioned clay, but a material which was "modern" in 1968 and now completely obsolete: 4" moulded bitumen reinforced with fibres such as possibly asbestos ("pitch fibre"). Very few fittings seem to have been available for these pipes. doubt that there was anything so sophisticated as a one-way valve. If there was such a fitting for 4" clay (very unlikely then, and stil unlikely, I would think) it would have needed ingenuity to made a good joint between it and the p.f. pipe. Builders are not noted for ingenuity on things like this (only on cutting corners!).

So this brings us back to the different resistance to back pressure of the two traps. If, as you suggest, that is the explanation (and it does seem to be the only possible one), then it still seems very odd that the back pressure not only lifts the water in the WC trap at all, but that it lifts ONLY the water in this trap. As this water lifts, the opposing head which it exerts on the back pressure air increases, so wouldn't you expect at least SOME lifting of the water in the gully - when the increasing opposing head of the rising water in the WC becomes greater than the opposing head in the gully?
Ah, now you throw a completely different light on matters. Pitch fibre was widely used in it's day, but quickly proved to be bloody awful stuff. Now makes me wonder if there isn't a partial collapse or other issue with the run to the gulley, which it itself may be holding water, thus giving the effect of a second trap, hence the WC is affected rather than the gulley.

Due to the fact this run only ever carries water, it's not manifested as an issue, as there's no solids going that way that could block it. Water is getting through without too much hindrance. I would think it extremely unlikely there was any type of NRV fitted on the gulley run alone, if it was needed then I would expect it to be near the boundary where the sewer leaves the property, to prevent surcharging beyond that point on the property.
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The size or amount of the water is not relevant, the height will be lifted will be the same everywhere. That's why air pressure is measured in inches of mercury and manometer are using centimetres of water but don't need to be a standard bore. It's force per area.
What may be happening is there's a leak on the gulley run so the additional air pressure is escaping. The wc must be holding the pressure.
clay gullies are usually cracked.
You should just put that in your signature, although ours was not at the back and it was almost 100 years old when it came out. Same for next door, both of theirs appear to be okay
Many thanks for all the responses. May I cover them all here, please?:-


Yes, this does not have a good reputation. Like plastic drain pipe (but even more so), it is sensitive to ground pressure, so needs good ballasting - not just laying straight into sometimes wet/sometimes dry clay.

Certainly some of my neighbours have had to have their pitch fibre connections to the sewer feeder branch (4" clay), which runs under some of our gardens, parallel with the boundary, re-made in recent years - because of repeated blocking. Camera inspection showed squashing-type collapse. The flattening of the sections of pipe removed was very striking. Our connection, which is particularly long, was inspected by camera and declared "fine". It has never blocked (dare I say that?!), and works fine - except, obviously, on the rare occasions when the whole local system gets surcharged by surface water ingress.

The length of pipe between the outlet of the gully in the garage and the first inspection chamber into which it runs is only about 6 feet, so I doubt that the pipe has sunk enough for form an inline trap. However, wouldn't a pocket of water like this simply get pushed along above the involuntary inline trap? If so, wouldn't the block of air in the pipe being pressurized by the water rise in the chamber just move this plug of water along with it, pressuring the air ahead of it?


I don't believe that the gully is cracked. If it were, the ground under the concrete floor would be wet, so the surface of the surrounding concrete, which has no DPM (it's "only a garage floor"), would be damp. In any case, if there were a leak large enough to discharge back pressure air, or air plus water, into the subfloor/clay, surely that would act as a pressure relief point/vent for the shared pipework serving both traps, so the water in NEITHER trap would rise, or would rise temporarily by only a small amount? I also point out that, when the WC pan filled up quite high after a 2-gallon flush the water level did not drop noticeably at all until the sewer cleared. If the gully had been leaking pressurized air/water into the ground, wouldn't the pressure holding the plug of water out of level in the WC trap have been dissipated, and the WC water have fallen back to the bottom of the partition between the pan and the back section of the trap?


Yes, I was wrong to think that the relative weights of the quantities of water in the gully and the WC trap are a factor in determining the resistances of the water in these two tracks (opposing heads) to back pressure.

So do you agree with the following. It's the head that matters, not the quantity of water. In a vented tank vessel with an outlet, such as a gully or WC trap, pressure is exerted per square unit of outlet area. The two outlet areas are about the same (is that relevant?). The heads are different, the surface of the water in the WC trap being at least 300 mm higher than in the gully. So the head in the gully, the one with the less resistance to lifting by the trapped air under pressure in the shared connection between WC drain and gully drain to the chamber, might be expected to rise up to the pressure equilibrium point, and ultimately to have air expelled through it. This would act as a pressure relief valve, so that reverse pressure in the drain pipe would never rise high enough to lift the water in the WC trap.

Please explain if I'm wrong, and, if you'd be so patient, where, because, all other things being equal (eg, no significant involuntary inline trap in the section of pipe serving only the gully, if that might be a relevant consideration, almost certainly no NRV, and apparently no leak in the gully pot large enough to discharge the back pressure), it is a the fact that the water in the higher WC trap, and not in the lower gully trap rose, and stayed at the raised level until the sewer cleared.
Maybe the first clay collar after the gully is slightly open and let’s a bit of air out under pressure?
Thanks, Ian,

Your argument is interesting, especially as the joint you refer to is between a clay gully outlet stub and the collared end of a pitch fibre drain pipe. Such a joint might well become leaky over 51 years due to the different expansion and contraction rates of the improvised means needed to seal it (ordinary mortar, I guess, rather than a proper rubber seal as for a connection between two lengths of PF pipe).

The gully is in what used to be the rear section of the original garage, long ago separated off as my workshop. This now a semi-indoors environment, as the garage is a lean-to and there is access to the workshop from the utility room (the sink in the workshop is also occasionally used for domestic purposes).

If there is a leak where you suspect, not large enough for this much-used* gully to make the floor surface immediately surrounding it damp, but capable of dissipating air pressure in its outlet pipe, then, whilst this could explain why the level in the gully did not rise during the recent surcharging of our sewer, it does not explain why the level in just the WC did rise (a lot). (The two empty into a common pipe leading into the adjacent chamber.)

I now don't think I am going to find an explanation which clearly identifies the cause of this "strange behaviour". I know I could stop the WC back-filling by digging up a lot of my workshop floor, eliminating the gully, and running the outlet from a WC with a P-trap through the wall into a vented soil pipe. The three gray water wastes would empty into the vent pipe via a manifold. All this would not of itself stop the WC backfilling. For that, the vent pipe would have to be taken up through the roof to atmospheric pressure and left open-topped. Alternatively, for less disruption, the pipe could terminate within my workshop but would have to be fitted with an air admittance valve. If the water level in the WC was high I would have to remove the valve to relieve the smelly back pressure air until the sewer cleared.

The would mean a lot of very disruptive work, with changes to the floor and tiling in the bathroom and a new WC pan, as well as a lot of digging in my workshop. As with so many good ideas, it sounds out of proportion to the problem, which is very occasional, and doesn't usually last long.

And it would not solve the basic problem - the very occasional backfilling of the sewer. It would just mean that we could continue to use the downstairs WC during the sort of moderate sewer surcharging that we sometimes experience in the same way that we were able to continue using the upstairs WC during the recent 24 hours or so during which the sewer surcharge make it inadvisable to use the downstairs one.

* Downstairs bathroom bath and basin, both of which are used quite a lot, and the much-used sink in my workshop.

Cheers, and many thanks to all who have tried to help!


Such a leak would let small amounts of water into the ground below the concrete slab of the garage floor whenever the gully is used (it is for the basin and bath in the bathroom and the much used sink in my workshop, which is the back of the original garage), but arguably not enough to wet the floor slab enough for it to show on the surface. However, it might relieve pressure which would otherwise
if there were a leak large enough to discharge back pressure air, or air plus water, into the subfloor/clay, surely that would act as a pressure relief point/vent for the shared pipework serving both traps, so the water in NEITHER trap would rise, or would rise temporarily by only a small amount
Not totally sure but i think it would act as two separate drains. In ours the downstairs toilet braches off the main stack below ground.
If the drains are full, the toilet bubbles as the level rises, but the main stack everything is fine in terms of bubbling traps.
Just to add to that, if the water level in the drain was rising then it would fill to the same level in both branches regardless of whether one branch was open to the air or not.

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