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Various threads about this, but still puzzled about my problem.
Kitchen sink with dishwasher and overflow connected into waste above trap. Trap leads to vertical downpipe running out beneath floor.
The problem is that if water is run or dishwasher drains, it backs into the sink. If I release the pipework after the trap, air is forced out and the water drains rapidly away. It will drain if left, but only very slowly.
If I disconnect all the waste pipework, I can direct water from a garden hose down the vertical pipe at full pressure, with no backing up at all.
I've tried fitting an air admittance valve onto a 'T' at the top of the pipework, but with no effect. The air wants to come out not in.
All this in 40mm plastic pipe. I've tried swapping from the U bend to a bottle trap, but no difference.
Probably just coincidence?, but the problem appeared when we swapped to a new dishwasher - the waste is plumbed in above the trap exactly as the old one.
Does anybody have any suggestions? - I can only think it's some weird external drain problem, but kitchen backs straight onto garden, and I can't see where there's an appropriate access point.
Kitchen sink with dishwasher and overflow connected into waste above trap. Trap leads to vertical downpipe running out beneath floor.
The problem is that if water is run or dishwasher drains, it backs into the sink. If I release the pipework after the trap, air is forced out and the water drains rapidly away. It will drain if left, but only very slowly.
If I disconnect all the waste pipework, I can direct water from a garden hose down the vertical pipe at full pressure, with no backing up at all.
I've tried fitting an air admittance valve onto a 'T' at the top of the pipework, but with no effect. The air wants to come out not in.
All this in 40mm plastic pipe. I've tried swapping from the U bend to a bottle trap, but no difference.
Probably just coincidence?, but the problem appeared when we swapped to a new dishwasher - the waste is plumbed in above the trap exactly as the old one.
Does anybody have any suggestions? - I can only think it's some weird external drain problem, but kitchen backs straight onto garden, and I can't see where there's an appropriate access point.