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We're having trouble stopping our kitchen sink drain from backing up. The sink now backs up nearly instantly (after a few seconds), previously it took a bit longer to backup.

We've tried on multiple times with different methods to fix it ourselves including with a plunger, alkaline drain un-blocker as well as caustic soda.

At one point, the bathroom sink (close to kitchen) also backed up. From that we deduced where a blockage might be and unscrewed the pipe and cleared out a lot of white sediment that had built up along the bottom side of the pipe. This "solved" the problem for around a day, but then the problem returned.

After this, we researched another method that involved pushing a 3m spring/screw down the pipe in order to try and pull up a blockage. We did this from the sink end and from the other end of a near 6m inaccessible stretch of pipe that ends near a junction of piping (see diagram below). Nothing was recovered from this, it seems as if there may be no physical blockage.

We've all but run out of ideas at this point, but some research on the internet suggests that the problem might be related to air/suction, it seems to fit with (1) the blockage occuring nearly instantly, (2) not finding a physical blockage & (3) we've unscrewed the pipes at the sink and a junction enough times that we could easily have introduced a problem without realising it, not being plumbers. I've also confirmed that there is no water backed up near where the pipes all join up before going underground.

We have a dishwasher that also feeds into the drain straight after the kitchen sink. This has some sort of non-return valve which we've also unscrewed a couple of times and at one point it leaked causing us to screw it up very tightly, which could have introduced a problem for all we know. We're not sure how it works, or whether we should have screwed it up tightly, but the problem appears to remain even when unscrewed and holding a hand over the opening.

I've included some photos and a diagram, on the off chance that it helps, it's a problem that's been with us for a while. The diagram shows one of the difficulties with diagnosing the problem, in that a near 6m stretch of pipe after the sink is not accessible.

If no physical blockage is present, what would be causing the sink to backup? Is there some kind of air-trap or pressure problem that could be the cause?

Any general advice for how to investigate/fix is welcome, particularly as the covid situation makes this a bad/impossible time to get a professional to investigate.

Update I doubt it matters, but just in case, we're in the UK. Also, there are no visible drain vents anywhere along the system we're discussing. There is however a central heating system feeding into the same drain and the seal is not air tight, with two 1.5cm pipes feeding into the drain.

Also, all of this plumbing is on the groundfloor.

Diagram

diagram of system

Photos

Under kitchen sink

Pipes under sink showing dishwasher junction

Drainage junctions

Drainage junctions

Specifics of what solved it

Obviously all of the advice on here was sound, but I wanted to highlight some of the specifics that helped solve this, in case it helps someone else.

The problem turned out to be a simple blockage, a build up of sludge, but our diagnostics turned out to be flawed. In the end on advice from this question, we bought a 6m snake - https://www.amazon.co.uk/gp/product/B000O55KJC/ref=ppx_yo_dt_b_asin_title_o01_s01?ie=UTF8&psc=1

The above worked well, and far better than our 3m snake without the blue attachment.

The specific technique that helped was to time how long it took to back up. From the volume of the pipe and from how much water your tap flows per second you can then work out where the blockage must start. From that we could tell the problem wasn't the piping around the sink, it was the inaccessible part.

We then proceeded to snake it, two techniques helped, turning the coil when a blockage was struck, but also sliding it backwards and forwards. We undid the bathroom sink junction at the opposite end to see if the snake was through and when we came out the other side. We then put the garden hose pipe down the sink and turned it on, it still backed up, so we continued snaking and looking for the blockage. Eventually a lot of sludge came out and we had an idea we had made progress. It's not been long, but the sink now drains. And if it backs up again we have a fair idea that the same procedure will help.

Thanks again for all that helped us fix this!

take off all the kitchen piping

  • How far did you snake the line from the kitchen end? Did it reach the end of the inaccessible stretch of pipe? On the second picture, where does the inaccessible stretch of pipe connect in? – Programmer66 Apr 10 at 14:50
  • Is it possible that's just trapped air, and there's a vent problem? – Duston Apr 10 at 14:57
  • On another note, the corrugated sections is not approved for use in drain pipes, the walls must be smooth. In the second photo, is this a 'DIY' set up and proper vent lines were installed? – Programmer66 Apr 10 at 15:00
  • Those slopes don't seem steep enough to drain solids properly. Somewhere else is your soil stack, and without an air inlet to let air in you will have problems draining. Perhaps that backed up and sealed. Finding it will be hard though. – J.Hirsch Apr 10 at 15:10
  • @Programmer66 we can't be sure, but we think we covered the full length of the inaccessible pipe by going in from both ends. The inaccessible pipe connects just behind the vertical section in the picture. – goose Apr 10 at 15:14
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As suggested in the comment by @Duston, If the vent pipe is completely blocked, a vacuum can form which is holding the water in the drain pipe. The snake is only traversing through the water.

Another reasons based on new information, is that not enough vent pipe were installed or installed at proper locations in the drainage system.

Add additional information on drain vents
Below are pictures of drain vents, roof and wall. The wall shows two new vents, the small is the air vent for the sink, the larger is the vent for the range hood. Hope this helps. Roof drain vent

Wall Drain View w Range hood vent

| improve this answer | |
  • Thanks Programmer66 - what would the vent pipe look like? – goose Apr 10 at 15:17
  • The air vent pipe looks like a drain pipe and normally from the drain goes straight up to the Roof. The vent pipes are those strange pipes in the roof that people wondered what the pipes are for. LOL. Sometimes the vent pipes may exit out of the side of the house, but there are codes that limit where it may exit, example a certain distance from any window, etc. – Programmer66 Apr 10 at 15:26
  • I can't see proper vents in the way described, but couldn't the bathroom sink / bath act as a vent? Also, the central heating system waste flow runs into the same drainage (I've only just spotted it) and it's not sealed properly. The ventilation sounds like the right cause, but I'm wondering whether the problem developed at the dishwasher join. It was quite loose before it leaked. I screwed up tightly thinking that was best, now I'm not so sure. – goose Apr 10 at 15:45
  • Sink -bath, etc. have traps, purpose of traps is to prevent air from venting through them. This may be the time to call in the plumber if it is a vent issue. If this is a two story house, a TEMPORARY quick-fix to try. On a 2nd floor basin, open up one, and put a shop vac with hose attached to outlet (blower), insert into the drain pipe and see if it will generate enough offsetting pressure. Use towels to seal it as much as you can. You will be trying to build up pressure in the drain line. Even if this works, it is NOT solving the root cause of the original blockage. – Programmer66 Apr 10 at 16:25
  • I would look for vents in the in areas above the "inaccessible" area of the drain pipe. If a DIY installation, maybe no vents were installed. – Programmer66 Apr 10 at 16:29

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