The automatic washing machine is one of the great conveniences of the modern age. No one wants to return to the days of washtubs and scrub boards and lye soap. However, when the washing machine won't drain properly, and water backs up into the laundry room or, worse, into the kitchen sink, this modern convenience can be something of a headache. There are several conditions that can cause the washing machine drain water to back up, and each cause has a different possible remedy.
Use a plumbing snake or other pipe-clearing device to remove any accumulated lint, clogs or roots to ensure conclusively that the problem is an air lock and not simply an obstruction of some kind in the piping.
Increase the size of the washing machine drain pipe, particularly if the washing machine is fairly new and the drain pipe is fairly old. The newer washing machines may use more water and exhaust it faster than the existing washing machine drain pipe was designed for---a 1 1/2- to 2-inch pipe installed where a 2 1/2-inch pipe is now needed.
Install an Air Admittance Pipe (AAP) if it is inconvenient or too expensive to install a regular vent pipe. Check with the plumbing codes in your area to see if this compromise remedy is permitted.
Install a regular vent pipe for the washing machine. The air trapped in the washing machine drain pipe has to go somewhere, and when it tries to come back up the drain pipe, an air lock can result.
If the current washing machine is a tub-type, consider replacing it with a front-loader type. The front-loader uses one-third less water, and one-third less water in means one-third less water to evacuate out. If you install an AAP, make sure that it is installed in a place convenient for replacement.