Caught in a Trap

I have spent a few days putting in both the copper water pipes to the bathroom fittings, and the waste pipes to connect in with the main pipe installed a couple of weeks ago by the builder. There have been no major problems, though I don't yet know whether it is all water-tight. For the waste pipes I have used solvent weld elbows and unions which will hopefully give a permanent water tight structure that can be lost behind the ceiling and never thought about again !

My main pre-occupation has been the trap for the shower. You will remember from the previous post that the builder-installed waste pipe to meet the shower trap has ended up too low. Instead of the trap that I bought prior to that installation, I have had to seek out an alternative with a shallower "U". This has lead me through some more education, more building regulations, and more problems.

The trap, or U-bend, is designed to prevent foul air from the soil pipes getting into the bathroom via the waste pipes. The trap holds a volume of water effectively blocking the pipe to air ingress. However, there is potentially a problem when waste pipes from multiple fittings are joined into a single outlet, as they are here. The movement of water through an adjacent pipe can exert a negative pressure on a trap and cause the water in the U-bend to be syphoned out. There are anti-syphon traps available (which allow air to be sucked in from outside to counteract this negative pressure) but these are only recommended to be used on basins. On our configuration, I believe the biggest potential for syphoning is the waste from the bath joining the waste from the shower only 18" or so down-stream from the shower trap.

To counter the possibility of syphoning within traps, the building regulations stipulate a minimum "trap seal". You will see this described differently on various web-sites, but the definition that makes most sense to me is that this is the depth of water in the U-bend, measured from the lower edge of the outlet to the upper-edge of the bottom of the U. i.e. if the trap contains any less water than this depth, the seal will be broken, and foul air ingress will result. Building regulations recommend a trap seal of 75mm, and stipulate a minimum of 50mm for baths and showers that use a "branched" configuration into a single outlet. The really shallow bath traps that you may have seen in your local B&Q should only be used if they are stand-alone (not connected to any other waste pipes) and empty into an open gully or hopper. These generally have a trap seal of only 20mm and hence are very susceptible to the syphon effect. They are principally sold for fitting a trap under a bath without having to hack a hole in the floor.

Anyway, back to my problem. Due to the low entry point into the waste pipe from the shower I am going to have to ditch the 75mm telescopic trap that I started out with and opt for a 50mm alternative. However, you would not believe how difficult this has been. So far I have purchased two more traps and still can't achieve what I want. This picture shows the three traps I have to choose from :

The original 75mm trap seal telescoping is on the right. The telescopic bit allows you to extend the trap up to meet the bottom of the shower outlet. As I have described though, the tall elbow is what gives this it's 75mm seal, and also what causes the bottom of the U to project below the bottom of the joists.

My next purchase was the 50mm trap seal unit on the left. This solves the problem of the deep U, but unfortunately is not telescopic, and is too short to meet the shower outlet. I have a selection of different sized straight extenders, but all are too long.

You might imagine that it would be a simple case of substituting the short elbow from the non-telescopic trap onto the original. As you can see here though, the fittings are not the same :

While both have a threaded elbow, and the locking ring on the main body of the trap, the rubber seal and the way the two parts mesh together are very different. I actually have a fourth trap which is destined for the bath, and this is different again, with a threaded bottom half, and the locking ring on the elbow. There seems to be no concept of standardisation in this marketplace !

So, the third purchase was the one that was supposed to solve this problem. From the same supplier as the second I bought another 75mm telescopic trap. This is the middle one of the first picture. I made the supplier test that the two elbows were interchangeable before he mailed it to me. However, this doesn't work either. You can see from the picture that the telescopic section is much taller than the telescopic section of my original trap, on the right. In fact it is too tall which, when connected to the shower outlet, pushes the whole thing down below the bottom of the joists again. Back to square one.

In fact, today, Dad and Jan have used some ingenuity, and copious amounts of sawing and filing, to make the elbow swap shown in the second picture actually work. They have cut off one of the flanges, fitted two rubber seals, some narrower section pipe inside and got it to fit together. The mark 1 version leaked, but mark 2 has passed the static cold water test. It is now in position waiting for completion of the rest of the waste pipes in order to undergo the full hot water flush through test. For backup I have a tub of mastic putty to gunge everything up with, or alternatively, spread solvent weld on the threads of the elbow to fix it permanently in position.