28 Mar 2020
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United Kingdom
Slow drain basin – no block

I have a bathroom basin with a really slow drain. I have taken apart the bottle trap cleaned and tested all works fine. I have used a hosepipe To blast water down the waste pipe without bottle trap connected and water can be heard training in the soil stack with no problems. I have also done this with the hoes seals in the waste as I thought it could be pressure build up. Still drained. I have also slow poured water in the waste, still fine. I bought a anti siphon bottle trap to test but still same issue.

Can anyone suggest something that I’ve not tried? I’m pulling my hair out here. Thank you very much for your help!
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What type of plug in the basin ? Can the plug be removed totally or is it captive ?
It’s removed so no restriction. All clean, you can see bottom of trap.
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Does the drainage pipework from the basin fall all the way to the soil stack? If not, could be offering resistance to low pressure flow of waste, but be happy to clear a hose pipe under pressure.
Wouldn’t that effect drainage from hose pipe? How ever I get water into the water waste pipe with trap disconnected it drains fine...
Yes from the basin under the floor 2m straight into the stack outside. Doesn’t share waste pipe.
Try a different trap , s trap or p trap which are better flowing than bottle traps . I don't know how slow your basin drains ,but from what you have told us it can't be that bad ??
I’ve tired a standard bottle trap and an anti syphon bottle trap, both same issue. If I connect trap to basin but not to waste and turn on tap it flows out of trap perfectly...
Is the soil stack open vented or does it terminate in an Air Admittance Valve? Secondly, does the WC connected that stack flush ok, or does the level rise and slowly drop.
Only thing I can think of is there's some kind of pressure build up that's preventing the flow from getting away as it should. Often find this with a blocked sewer or stack, that's also capped off by an AAV, the air cannot be displaced by the water so has to find an alternatively route out. Other thing to check, has the waste pipe been pushed too far into the boss on the stack, and is touching the opposite side of the stack internally?
Only way to sort it is to get access to the full length of pipe and section it and check each of the sections. Has it always done this or is it a more recent issue?
I know water can escape freely as tested with hose... it’s a head scratcher ;)

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