Preventing siphoning in new shower waste

26 Aug 2015
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United Kingdom
I am completely relaying out my bathroom, ie gutting and moving all fittings to different locations. Many of the waste runs have to go under the tiled floor, so I will be using solvent weld wherever possible.

I have three alternatives for my shower waste.

1) To run it into an old push-fit boss on the soil stack within the floor/ceiling void. The old boss looks OK, but the seal is old and not replaceable and I’m worried it may leak.

2) To tee it into a new 4 metre run of 50mm pipe going from the bath to the stack, teeing in approx 1 metre from the stack. The downside of this is obviously the possibility of siphoning the shower trap when emptying the bath. As far as I can see there is no feasible location for an AAV, due to the need to keep it high and accessible.

3) To tee it into the 50mm run from the bath as in option 2 above, and then run a vent pipe from the shower branch to the old pushfit boss in option 1, thereby venting the shower branch to the stack.

Would option 3 be enough to stop the shower trap siphoning, and would it be considered good practice. The old pushfit boss (vent) would be approx 200mm above the new 50mm boss.

I have pics and a sketch if they would help.

Thanks in advance for any help or advice.
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Industry has gone to the dogs.....bring back the anti syphon the good ole days self/induced syphonage wasn't heard of.....anti vac traps along with extension traps , Frankie fookin waste systems etc didn't exist.....No stinking kitchen wastes , no foul smells......a proper job done..

Building inspector would fill a basin and remove plug , when he stuck his pencil down the waste he would expect the trap to maintain a 3" seal......if not then waste run was excessive and had to be corrected...

Industry gone to the dogs.
Steelmasons : my industry has gone to the dogs too, which is why I can't afford to pay a plumber and end up struggling to do it myself :) .

I sympathise with your industry problems, but am not sure whether it's your industry that is making you weep or the suggestions of this particular DIYer! If the latter, then please explain why - I'm trying to learn!

So that you know where I am, I have an engineering background, and have been doing my own DIY stuff on house and car for 40 years, so I am reasonably good at what I do understand. But I am certainly not an expert in plumbing. Just learning as I go along.

My guess is that my suggested option 3 will prevent siphoning, because it will let air into the shower line when the bath water rushes by. What I was hoping to get from the experts here is whether venting to the stack may be seen for some reason as bad practice, or be against regs for some reason that I have not thought of. I have tried googling but can't find anything.
Can you get an in line AAV into the 50mm bath waste run? Is the old boss the highest entry in the stack?

With option 3, you would be creating a ventilated branch but if you wish to stay within the building regs (Part H) the top of the vent pipe needs to be above the spillover level of the highest appliance connected to the branch.

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Thanks Madrab, I think we're getting close, but not quite there.

Re the AAV suggestion. To keep things simple I haven't yet mentioned there will be a basin on another tee between the bath and the shower tee. I plan to fit that with an anti-syphon trap to prevent it (the basin) being pulled by the bath waste. But to prevent the shower siphoning, my understanding is that the AAV or vent pipe protecting the shower trap would need to join the run between the shower tee and the shower trap. I don't understand how admitting air further up the bath run will stop the bath waste pulling the shower trap as it passes the end of the shower run. Remember I'm a learner - please put me right if I've got this wrong.

Re Option 3, I’ve managed to find that reg – thanks! Since my last post I have realised I can get the vent pipe into the stack 500mm higher which would be a much better solution. Presumably the spillover level referred to in the regs would be the shower tray, as that is the appliance served by that branch. If it’s the spillover level of the bath or basin connected upstream and on separate branches I’m knackered again. Will have difficulty maintaining the constant incline though.
It all comes down to correct design and considering worst case scenario. Is worst case that all 3 drain at the same time? Therefore is it achievable to drain 3 appliances into the same branch without inducing syphoning in any of the traps?
When it comes to ventilated branches all traps should be vented, if it's liable to pull on the shower trap then it is liable to pull on any of the traps, that can include self syphoning of the bath trap.
Best way to think about it is, after all the branches can a plug of water fill the pipe, creating a vacuum behind it, therefore drawing on the traps inducing syphoning.
Look at using a HepVo valve on the shower. These dont contain water so there is nothing to syphon. It will also prevent any unwanted reappearance of the contents of the bath in the shower tray, should the waste begin to block downstream of the tee in future.
Guys, thanks for all your input. I thought you might like to know what I have eventually gone with.

Given the space and access I have available and the some of the issues raised by Madrab (especially spillover levels and the constant incline) I have dropped my own idea of a vent pipe and gone instead with a waterless Hepvo type trap for the shower, as suggested by Hugh Jaleak. I wasn’t keen on these originally because people say they block, but my thinking is that it will be a lot easier to unblock one of these if I do have problems than it would be to correct a dodgy vent pipe installation.

Hepvo imply their trap never blocks, but that’s not what some people say. So I will probably go with the Wirquin one available from Screwfix, because that is built in to the waste fitting and can be unblocked from inside the shower tray rather than by removing the fitting from underneath. So worst case I could unblock it on a fairly regular basis without too much hassle, and with the tray on risers I can even swap it out completely if it packs in for some reason. And who knows, it may never block at all.

Thanks again for helping me work this through.

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