Connecting a washing machine to a plumber's clean-out pipe that goes to the sink from outside is not recommended and may not be up to code in many places. Plumber's clean-out pipes are designed for access to the plumbing system for maintenance and cleaning purposes, not for connecting appliances like washing machines.
Washing machines require a dedicated drain line with proper venting to ensure efficient drainage and to prevent issues like backflow and odors. The clean-out pipe is not designed to handle the volume of water discharged by a washing machine, and it may cause drainage problems and potential damage to your plumbing system.
When installing a washing machine, it is crucial to follow local building codes and plumbing regulations. In most cases, a washing machine should be connected to a standpipe or a dedicated drain line with a P-trap to prevent sewer gases from entering your home. The standpipe should be properly sized and have an air gap to avoid water siphoning back into the machine.
To ensure a safe and compliant installation, it is best to hire a professional plumber or a qualified appliance installer. They can properly assess your plumbing system, recommend the appropriate setup, and ensure that all connections are done correctly to prevent any issues in the future. Attempting to connect the washing machine to the clean-out pipe yourself could lead to plumbing problems and potential violations of building codes.