Why does my sink gurgle when the washing machine drains?

A washing machine doesn't drain into a fixture with its own trap the way a sink does. Its hose goes into a standpipe, a vertical pipe with a trap below it, or into a laundry sink. The plumbing code says so: "The waste from an automatic clothes washer shall discharge through an air break into a standpipe in accordance with Section 802.4.3 or into a laundry sink." The standpipe's trap and drain also have to be sized for the job, since "The trap and fixture drain for an automatic clothes washer standpipe shall be not less than 2 inches (51 mm) in diameter."

Then the machine's pump pushes a big volume through that pipe in a short burst. That water needs air behind it. Oatey, a plumbing manufacturer, describes how the vents handle this: "As water drains, the vents allow pressures to remain neutral throughout the system." If the vent is blocked, or a partial clog slows the line, the surge can't push air along the normal path. The air takes the next route, back up through the closest trap, and you hear a gurgle at the sink. Oatey's summary is that "when a blockage closes off the pipe completely, the air will escape via the drain instead. And that's what's causing the gurgling."

So a gurgle at the sink during the washer's drain cycle says one thing: the drain or vent serving that group of fixtures is restricted. Which part is what you sort out next.

Because the washer is the trigger, check the part you can reach first.

The drain hose fit. Whirlpool's installation checklist says the hose must not be sealed in: "Do not seal the drain hose into the drainpipe with tape." It also says "There must be a gap between the drain hose and the drain pipe to allow airflow." A hose jammed tightly or sealed with tape leaves no air path, which is one way a washer's drain surge ends up gurgling at a sink. The same checklist warns that the hose shouldn't extend too far into the drainpipe and points to the U-shaped hose form to set the right length. Follow your washer's manual for the exact depth.

The standpipe height. The code's rule for standpipes is that they "shall extend not less than 18 inches (457 mm) but not greater than 42 inches (1067 mm) above the trap weir." Whirlpool adds a check for its machines: the top of the drain should be at least 39 inches from the floor, because below that "the water will start to flow out of the machine due to gravity." A standpipe that's too low is a different fault than a gurgling sink, but it's on the same checklist.

The standpipe trap and the line beyond it. Lint, hair and soap scum can build up in the trap under the standpipe. A partly blocked trap slows the washer's water, and the surge backs up into nearby drains. If the standpipe itself fills up during the drain cycle, see what to do when a washing machine drain overflows.

If the hose and standpipe check out, the trouble is downstream.

1. 1Partial clog in the branch. The laundry drain can join a sink or bathroom line. A partial clog there passes slow trickles but chokes under a washer's surge. The gurgle shows up at the fixture closest to the clog.
2. 2Blocked or restricted vent. A vent that's blocked or an air admittance valve that's stuck lets the surge pull air through a trap. What a plumbing vent does explains why, and what an air admittance valve is covers the valve.
3. 3A developing main line clog. If a second fixture, such as a toilet or a tub, also gurgles or slows when the washer drains, the blockage is probably in the main drain, not the branch. Read main line versus branch clog to compare the signs.

A quick test tells you a lot. Run the washer's drain or spin cycle by itself, with no one using another fixture, and watch each drain in the house. A gurgle at only one sink points to that branch. A gurgle at several fixtures points toward the main line. If a toilet also gurgles, see why a toilet gurgles, and if flushing the toilet is what sets off the sink, that's covered in why a sink gurgles when you flush.

Start with the hose, then the trap, then the sink. Each step is cheap and reversible.

Work through this list in order.

1. 1Pull the washer out and check the drain hose: no tape sealing it in, no kinks, and the U-shaped form in place. Slide it back to the right depth.
2. 2Clean the standpipe trap if you can reach it, or have it cleaned.
3. 3Clear the sink drain that's gurgling. Remove the stopper and pull any hair and gunk, and clean the trap.
4. 4Try the washer again on a drain-only cycle.

If it still gurgles, don't keep running loads. A partially clogged line can back up water into a sink or floor drain in the middle of a cycle, and that's a lot of soapy water. Our page on what not to put down the drain helps you keep the line clear afterward. Laundry lint, soap buildup and grease residue all add up over time.

Call a plumber if the gurgle continues after you've fixed the hose, if water rises in a sink, tub or floor drain during a cycle, if more than one fixture gurgles, or if you smell sewer gas. Those point to a vent problem or a main line clog that need a cable, a camera or roof access to fix.

Our team at HQ Plumbing & Air is licensed and insured, offers free estimates and gives you the price up front, before we start. When a drain is backing up into the house, that's an emergency, and you can reach us 24/7.

Oatey describes what a full blockage does downstream: "If you have a full blockage further down the line, you might experience backups from other fixtures in the home." That's why a gurgle during the washer's drain cycle is worth acting on early, when the fix is a cleaned trap or a cleared branch and not a flooded laundry room.

A few habits keep the laundry drain moving. Use the drain hose form and the depth your washer's manual gives. Keep the standpipe clear of lint and soap buildup. Don't wrap or tape the hose into the pipe. If the machine has ever overflowed, treat it as a warning and have the line checked. The washer didn't cause the problem. It just moves enough water to show you where the drain is struggling.

For other causes of gurgling drains, see why is my drain gurgling.