Plumbing Reassurance Needed

[a twig]

Hopefully a quick sanity check:

Having some problems with air admittance in the plumbing system. Set up is roughly as follows:

Subject to further investigation with the camera, the airvent seems clear (was up on roof today, plumbers snake disappeared down to its full 3m length, and several watering cans of water disappeared down quite happily.

The bathroom at the top of the stack is rarely ever used - the bathroom on the left of the sketch (downstairs) is the main one. When particularly large "debris" is passed, or a lot of toilet paper is used, there is a good old gurgle from the sink and shower pipes, but enough water remains in the traps to avoid smells. Seems to be the simple answer that just an air admittance valve (AAV) installed in the pipework for the downstairs bathroom would be the answer.

However, two things are giving me a slight pause for thought:
[li=1]
[*] When that situation happens, the water level in the upstairs bathroom also gets pulled down so that the upstairs starts to smell of sewer - this would imply that the upstairs vent is blocked, which it doesn't seem to be.
[*] After one particularly "impressive" flush, air came up the downstairs sink so violently as to create a mini fountain in the downstairs sink for a second or two :lol:
[/li]

Only change to the plumbing system is that the mechanism in the downstairs toilet has been replaced with a modern one, which has a considerably more powerful flush action. It might be that the more extreme symptoms were just the new flush clearing some previous partial blockages and normality may ensue?

Am I now just overthinking and need to get on a whack an AAV in - or are there more pressing issues?

 

[Feltwell]

That's a very long "horizontal" run at 6m. Sounds like whatever is passing down that pipe is creating bit of a vacuum behind it, which I reckon fitting an AAV would cure.

What's odd is why you're getting the vacuum created to pull down the upstairs trap level down. As you say, that would imply a blocked or partially blocked roof vent. What do you mean by a plumbers snake? I'd be tempted to get some drain rods down there with a disc on the end - make sure it's totally clear - the water from your watering can doesn't prove a huge amount, it would have flowed past a partial blockage by the time you've brought the next can up, and if the plumbers snake is what I think it is then it won't necessarily dislodge a partial blockage.

A set of rods is fairly cheap at least.

 

[a twig]

Grand - right first stop will be AAV, god knows where I'll route that pipework - especially as that is the "finished" bathroom, the wife is going to kill me.

I've seen some teed in under the sink before - as long as it is above the trap it should be ok? :shock:

I really don't want to start cutting into our nice timber flooring and running pipe up into the loft and boxing it in...

Main vent - roof access is a little bit tricky for the vent, hence watering cans and plumbers snake, not my proper rods.

I have a 10m long USB borescope - when the wife gets in from work tomorrow I can get her to sit inside the bathroom with the laptop and pass me the cable through the window, and do a video inspection. I'm currently several stone heavier than the last time I was skipping about on the roof, and the roofing ladders I knocked up were long since burnt, so rodding would be a bit more of a major operation.

It is only ~2.5m from top of vent to where toilet joins stack, so fingers crossed should be able to get an answer fairly easily...

 

[a twig]

Local place has 32mm trap with built in AAV for sale, so as a cheap and non-invasive first pass, going to whack that in and see what difference that makes. Fingers crossed

 

[Bogart]

As you can get access of roof to the stack should not be too hard to ascertain status. Shine good torch down it of drop a light on a string rope whatever and have a good look. Will also reassure you that pipe from downstairs is ok by amount exiting it. 6m is a reasonable run but with a decent drop should not cause a problem in itself.