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Question DetailsAsked on 11/22/2015

utility sink backs up into bathroom sink

The drain pipe for our downstairs bathroom is connected to the drainpipe for the utility sink prior to going into the sewer drain. The pipe is at an angle so there is downward drainage. Every time the utility sink fills with water it backs up into the bathroom sink. There doesn't seem to be a clog in the drain pipe because it does drain into sewer drain. Could it have something to do with the lack of an air admittance vent on the bathroom sink?

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1 Answer


Was this following almost identical link your question too - has been answered now -

Lack of an air supply to the drain lines can result in flows through the pipe pulling a partial vacuum and drawing the water out of the trap, letting in sewer gas from the drain pipes below. It does NOT cause backups - that is due to a flow restriction, or rarely because the slope or amount of pipe drop before hitting the sewer line or too small a pipe size on the backup source pipe, or wrong type of wye (like a Tee or upside-down wye rather than a true angle wye or street wye installed right side up) was used, so the fluid prefers to at least partly back up into an adjacent drain rather than go down the sewer. IF this has been a chronic issue or started only after getting a new washer (which might put out higher flows), one of those could be the problem, but if a new problem with not other changes in the system has to be a partial blockage in the pipes somewhere, as in answer above - or possibly an overfull septic tank, which adds up to it still constituting a blockage.

Remember - the washine machine discharge is, with the possible exception of emptying a full tub, your largest flow volume in the sewer line, so when you have a partial blockage or restriction other drainage, including flushing the toilet (about 2-5 gallons) may hit the blockage and back up some in the pipes, but not drain enough water to back up in the pipe to the point of coming out the lowest "upstream" drain. The washing machine, on full load, commonly pumps out the water fast enough (faster than shower or many tubs emptying) and has a large enough amount of water (commonly 10-20 gallons per cycle for top-loaders) that when it empties into a partly obstructed pipe, even though some of the water makes it past the obstruction, it will fill the sewer pipes up as it backs up at the obstruction until it overflows at the lowest elevation drain "upstream" of the blockage.

This is commonly a floor drain if the blockage is in the underground piping, or the lowest elevation laundry tub, bathtub or shower generally if in above-ground piping or there are no floor drains "upstream" of the blockage. Commonly starts with gurgling in the drain that it is backing up into during the emptying cycle, then eventually soap suds foaming out of it or a bit of water sloshing out of the drain (which you might not even notice if not there when it happens, and all you see is a damp spot or a bit of residue around the drain), and eventually (which might be at next toilet flush, or can be many months down the road if due to a gradual reduction in sewer pipe diameter from fiber and soap and grease buildup) will commonly build up to major backup in a drain - or total blockage without warning - so not something to let go unremedied as in first response.

Answered 5 years ago by LCD

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