Washing machine drains must be able to handle strong currents of water, as modern machines have powerful pumps for efficient cleaning. All components of a washing machine drain should be installed before the wall and floor finishes are applied. When retrofitting a drain, you will likely need to remove the wall covering over the old drain, make the repairs or changes, and put new wall coverings in place.
The outlet box is the central connection point for the washing machine hoses. A typical box includes an opening where the standpipe is inserted and two removable covers where the hot and cold supply valves are mounted. The box has four tabs, two on each side, that are fastened to the wall framing to hold it securely in place.
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A standpipe extends from the outlet box to the trap below. The washing machine's drain hose is inserted into the top of the standpipe where it fits into the outlet box. Building codes specify how high this pipe must rise above the finished floor. In many jurisdictions, that height is 36 to 42 inches. The pipe must be 1 1/2 or 2 inches in diameter, depending upon local building and plumbing codes. In most, 2 inches is the standard.
The cleanout fitting is often installed just below the outlet box on the standpipe. This is a plumbing fitting called a tee, which has a threaded plug that can be removed to clean the line. The drywall is finished to the threaded plug housing to make it easy to remove the plug when the line needs to be cleaned.
Commonly referred to as the P trap, this S-shaped group of fittings creates a barrier of water to prevent sewer gasses from rising through the plumbing lines. Some building codes specify how high above the finished floor the P-trap must be positioned, while others allow the trap to be placed below the floor. A typical requirement is for the trap to be at least 6 inches above the finished floor.
This piece of pipe connects the trap either to a feeder line or directly to the building's main drain line. A drain line carries washing machine drain water from the trap to the main drain.
Vents provide air so the drain lines don't become vacuum-locked. The vent will usually rise straight out of the sanitary tee that the P-trap is connected to. In some installations it may connect directly to the drain line near the trap. The vent may also serve as a vent to nearby fixtures. A good working vent is especially important for washing machines, since they discharge so much water at one time.