A Kitchen Sink Backing Up When the Washing Machine Drains
A washing machine backing up into the kitchen sink can be caused by either a clogged drain or a vent problem.
When a kitchen sink is clogged and backing up the problem can often be solved with a plunger. But when the water backing up into the sink is coming from the washing machine, the problem is a little more complicated.
Location of the blockage
All plumbing fixture drains in a home, including the washing machine drain, must connect to a 3- or 4-inch main drain pipe that leads outside the home to a sewer or septic tank. This configuration is similar to a main highway with a series of on-ramps leading onto it. For a washing machine to drain into a kitchen sink, the kitchen sink drain must tie into to the washing machine drain pipe via a wye fitting before the connection with the main drain pipe. The clog is at some point after the kitchen sink drain connection but before the connection to other drains. If the clog was before the kitchen sink, the water would simply back up from the washing machine drain. If the clog was after the connection with other fixture drains, it would back up into the lowest fixture, usually a shower or toilet.
Washing machines pump water into the drain at rates of 15 gallons per minute or more, much faster than any other fixture or appliance. A drain pipe of 1 1/2 inches was considered sufficient to handle older washing machines, but in many areas a washing machine drain pipe size of at least 2 inches is now required by code for proper drainage. Kitchen-sink drain lines develop clogs more often than other plumbing fixtures because of the grease and food often poured into them. Because of the amount of water pumped from the washing machine, even a partial drain blockage in the shared drain pipe from the kitchen sink can be enough to cause the washing machine to back up into the sink.
If there is a gurgling sound from the sink drain when the washing machine drains, it's possible that a venting issue is causing the backup. If the washing machine or kitchen vent is run incorrectly or undersized, this will cause drainage issues. Likewise if the vent is plugged. Think of a straw with liquid in it. If one end of the straw is plugged, the liquid will not drain out. A plumbing system works the same way. If there is gurgling coming from the kitchen sink, the sink is actually being used as a vent.
Fixing the Problem
If the sink is gurgling, the vent can be checked visually for blockages, including leaves, nests or ice. Another way to ensure the vents are clear is to run water from a hose into the vents on the roof – a vent that backs up is definitely plugged – though given the danger inherent in accessing a roof, especially on a multistory home, you may want to leave this to professional plumbers. A drain pipe clog is likely too far away to be unplugged by plunging. A plumber's snake is usually required to clear the pipe.
Gary Sprague is a retired master plumber who now works as a writer. He lives with his family in Maine.