Plumbing query (lack of venting?)

[diy_fun_uk]

A few years back, I replaced the plumbing under mum's kitchen sink with a kit. For reference she lives in a high-rise. Looking right to left, there's the connection for washing machine, main sink (with no overflow) the smaller half sink (with no overflow) and then the drainer section that has a small drain connected to the half sink waste. You can see the plumbing in the pic below.

Something that's always annoyed us is this. You can run the tap in either sink forever and a day and it keeps draining away no problem. However if mum tries to empty a basin of water too quickly the main sink backs up and some water also appears in the half sink although just above the drain waste. So she has to empty it more slowly. Also, when the washing machine starts to drain, for the first few secs you hear it draining fine, then there's a change in the noise/tone and you can hear the water starting to back up, then sure enough some water will appear in the main or sometimes both sinks ... then slowly clear after the machine stops pumping.

I'm wondering if it's a venting issue i.e. air getting trapped? It doesn't seem a coincidence that when mum tries to pour water down too quickly, or when the washing machine is pumping water out, this is when it backs up. I would have thought the drainer drain would give some venting, however maybe not enough? And maybe the horizontal pipe from the half sink is getting blocked by water coming out the washing machine at pressure, thus stopping the venting? Sometimes i.e. maybe once/twice a year, the sink will just completely block, however I suspect that's more to do with issues in the main plumbing of the building that I obviously can't get to.

Could it be a venting issue? Or would it be resolved by making the two vertical pieces of pipe that come down from the basin wastes shorter i.e. so the whole thing is 'higher up' with greater distance from the u bend?

 

[Gasguru]

I'd suggest either the trap or waste pipework's blocked.....what's the pipework like to the stack? Correct gradient, length, what material?
I've had many blockages at the stack connection especially after 50 years or so.

 

[diy_fun_uk]

I'd suggest either the trap or waste pipework's blocked.....what's the pipework like to the stack? Correct gradient, length, what material?
I've had many blockages at the stack connection especially after 50 years or so.

Thanks for the reply. Nothing's blocked this side of the kitchen cabinet. Behind the kitchen cabinet, I have no (easy) way to check the gradient etc without starting to take the kitchen apart! I'd love to see the 'behind the scenes' plumbing but no access. Funnily enough the shower is also prone to slow draining which leads to me think there could be a recurring issue in the block plumbing. However the sink does drain okay, like I say, you can run the tap minute after minute at quite a rate and it'll keep on draining. It's only when water is put down at a greater volume that it seems to block.

 

[John the plumber]

Is there no ridding access to waste pipe outside?

 

[oldbuffer]

I'd suggest there are three possibilities, two as Gasguru suggests plus another:
1. The pipework to the soil stack is partially blocked. It can cope with a certain flow, but when this exceeded (whole bowl emptied, washing machine pump) it can't handle the extra and back fills the sink.
2. The pipework doesn't have sufficient fall from the trap to the soil stack. Effects would be the same as 1. above.
3. The soil pipe isn't venting properly. A large volume of waste water going down it is creating something of a vacuum and partially stopping the flow.
4. The soil pipe itself is partially blocked.
Problems 1. or 2. would only affect your mother's property.
Problems 3 and 4. might affect others on the same soil stack. Worth asking neighbours if they have similar problems. Does the lavatory flush properly and the pan clear quickly?
I'm afraid the most likely cause is 1. or 2. You could try getting someone to "snake" the pipework through to the stack. If that doesn't resolve the issue you are going to either put up with it or start cutting inspection panels to check the gradient of the waste pipe on its way to the stack.
Altering the lengths of pipework visible under the sink won't do anything.

 

[Gasguru]

How old is the block?

 

[diy_fun_uk]

Thanks for the replies. There's no straightforward access to anything behind the walls. To the right of the kitchen sink is a cupboard housing the water cylinder. In that cupboard on the back wall is a large access hatch installed by contractors a few years back when the entire block was receiving upgrades. The main run of pipework will be behind that. However when I say 'large hatch' I mean large. It's held in place by multiple screws however it kind of goes behind the water cylinder with very tight access so again wouldn't be the easiest thing to remove.

The toilet flushes ok and doesn't back up, never has. The bathroom sink occasionally blocks and as I say the shower is slow draining. I reckon you could be right Oldbuffer i.e. pipework partially blocked somewhere where I can't get to. I did try a snake type thing with small brush on the end a few years back. Entered from the back of the kitchen cupboard area with all the plumbing kit removed. Might try that again. Things like checking gradients etc are a no no due to no access. I will however ask mum to ask her neighbours if theirs is ever slow to drain and/or blocked.

 

[diy_fun_uk]

How old is the block?

Put up in the late 60's.

 

[Gasguru]

So that's plenty of time for the soil stack connection to become blocked, especially if you have a copper waste pipe where hair etc can catch on the burred pipe edge.
I'd gain access or at least shove a drain snake down the pipe to get an idea.