Slow drain from bathroom waste

[SithLordHood]

Six months ago i re-did our main bathroom, moved the bath, sink and toilet and re-plumbed to the main soil stack, see image. Was all working fine until recently.

The sink continues to always drain no problems, but the bath now drains slow as does the toilet. Occasionally when the toilet is flushed the bath trap gurgles.
When flushed, the toilet always goes down after 20 seconds or so, flushing multiple times does the same thing, back-up but then drains slow-ish.

Lifted the inspection hatch in the garden and i see no blockages at all. Whilst watching this pipe, when the bath is drained and/or the toilet flushes they both drain normally (not slow).

If I turn on the sink tap and then turn on the bath, it seems to drain ok, i turn off the sink and the bath continues to drain ok. Wondering if its something to do with the fact that i have activated the small air admittance valve fitted to the sink trap?

Sink, bath and toilet wastes all have recommended slope applied into the soil stack. House built in 2006, soil stack air admittance valve is the original unit. I seem to recall some years ago being able to unscrew the very top of it and i cleaned the bits i could disassemble. The previous bathroom layout would occasionally gargle the bath and shower traps when the toilet was flushed so thought the AAV was playing up back then too..

Am i right in thinking the air admittance valve at the top of the stack is the most likely cause?
It's solvent welded to the top of the stack, right in the corner of the loft which has lots of crap in the way to get to. Any ideas on how to slice the valve off in a confined area? I dont have a picture of it but might be able to get one at the weekend and see i i can buy another and transplant the internals over?

 

[cross thread]

AAV is a good starting point

 

[Hugh Jaleak]

Air admittance valve does exactly what it says on the tin, it'll let air in, it doesn't (shouldn't) let air out. I don't think it is your issue here, I suspect you have a blockage between the stack and the Inspection Chamber. Undo/remove the Basin Trap leaving the end of the waste pipe open temporarily, flush the WC, and see if that changes anything.

 

[SithLordHood]

So I removed the basin trap, this improved things slightly, the toilet and bath both drain better. I then placed my hand against the open basin soil pipe to block it up and flushed the toilet. As the toilet slowly drains I can feel air wanting to escape the basin soil pipe into the room.

So i am guessing thet you are right, there is some kind of restriction between the stack and inspection chamber.
Its wierd as looking at the inspection chamber point when bath or toilet drains the water quantity seems to be running ok...

 

[Madrab]

Main issue I can see here is rodding the stack if there is a restriction as it must be below the level of the branch. Are there any rodding points on the vertical either above or below? If not then the only other option would be to remove the AAV and rod it vertically.

Depending on how high the stack is in the attic will determine if you can cut it below the AAV and then extend it to make it easier to work on. Is there any reason the stack wasn't taken up and out or was it purely roof line aesthetics? If it's stuck away in a corner then some temporary flooring would be advisable if not there already.

 

[Hugh Jaleak]

Quite possible there is a 'belly' or dip in the underground pipework, which may be acting as a plug, and causing air to be trapped when you flush/discharge a bath. The equivalent volume of water is being displaced, hence why it looks like things are flowing normally.

Simple test, put some food colouring in the WC Pan, and Flush the WC, see how long the coloured water takes to appear at the manhole.

 

[SithLordHood]

Thanks for the replies guys.. Had another chance to look at this today.

I put food colouring down the pan and with the inspection chamber opened observed the flow. After a very brief amount of clear water i see the coloured water come straight out. Doing the same with toilet tissue in the pan i get the same result, i even tried flushing a leaf down the pan to make sure what i was seeing at the inspection chamber was a result of that particular flush, it was.

I then used a hose to dump water into the outlet pipe from the inspection chamber and ran it for a good 10 minutes to see if the water would start backing up (thinking there is some kind of restriction at the output from the inspection chamber to the sewer) but the water flowed away just fine. Blasted water the other way (toward the bathroom, as best i could) in case there was anything obstructing the pipe but it did not make any difference.

Put the inspection chamber cover back on and after a flush or two I see the same issue, slow draining toilet.

For now I have left the inspection chamber cover on but not screwed down (as to engage the lid on the rubber seal). This helps slightly and the toilet drains down a bit better.

I was wondering if the air admittance valve opens by detecting outward pressure before then drawing air in? or does the mechanism open by suction?
I also thought about fitting a 40mm air admittance valve on the sink waste (i can access this easily - its a right pain to get to the AAV in the loft, and its cemented on so will need cutting off..). The AAV that is already on the sink waste is a small one in the u-bend.

Any other ideas?

 

[Madrab]

An AAV is just a one way rubber diaphragm, as the negative pressure (vacuum) builds in the soil pipe, that pulls the seal up, opening up the airways and allowing air in, once that vacuum is released or alternatively positive pressure is applied then that pushes on the diaphragm closing it.



Take the AAV off and see if it still does it.