No vent

[Rekord]

I have an upstairs toilet that gets blocked about twice a year, on examining the system there doesn’t seem to be a vent or AAV anywhere. First question, is this the cause of the blockages. If so can I put a T in the horizontal section, going outside and then up and above the roof. See attached drawing.

 

[Wayners]

I think yes to all your questions

 

[strasin]

I've dealt with similar issues, and a vent can really help with drainage. Your idea to run it outside and above the roof should improve airflow.

geometry arrow​

 

[Rekord]

Thanks for the replies, does it make any difference where on the horizontal I put the T? Toilet end or the other end?

 

[Hugh Jaleak]

When the WC blocks, how easy is it to get it clear? This may suggest how close the blockage is to the pan.

Vent is unlikely to make any difference, it's blocking because solids are building up somewhere in the pipework. By all means, install a Vent, (it can do no harm), Ideally Junction )Tee) would be stack end, but its not vital. What I would be doing is checking and adjusting the fall on the 'horizontal' section, as I suspect his may be where your problem lies. I'd be looking of a minimum of 50mm, ideally around 70mm of fall between the back of the Pan and the stack.

It is rare for Stacks to blocks in the vertical section, unless there is an obstruction holding solids back,, so I'd also look at any wastes connected to the stack, and ensure the pipework hasn't been pushed too far in, providing a convenient 'perch' for material to collect on. Lower down, the momentum of the falling waste and water is usually enough to push everything through into the drain, but it is also not unknown for there to be issues with a damaged rest bend at the foot of the stack, causing solids to catch and build up.

 

[Wayners]

No AAV or a broken AAV will cause a vacuum and toilet will fill from pressure when there is a flood or heavy rain.

Mother in law called waterboard out several times because toilet was blocked.
I punched through boxing in bathroom and swapped AAV and all her problems has gone. Been over 12 months with normal flooding and heavy rain and toilet has been fine.
Can't say I understand why the toilet would not drain even with pressure.
I think it was a mistake to fit an AAV and would of been better to fix the flashing around soil pipe vent on roof when they moved in in 2001 instead of fitting a AAV

 

[Hugh Jaleak]

Can't say I understand why the toilet would not drain even with pressure.

Simple answer is, because if the drain is blocked, and an AAV is fitted at the top, the WC is trying to discharge into what is essentially a sealed section of pipework. In this scenario, the flush needs to displace the equivalent volume of air, if it cant then the water/waste has nowhere to go, and instead fills the pan.

You will find the air will take the path of least resistance to escape, so usually vents through a bath/basin/shower etc, if none are connected to the stack then the weight of water in the pan puts up somewhat more resistance.