A washing machine drain vent serves a very important purpose. Although not always mandated by building codes, it’s a good idea to put one in when you install a washing machine.
If the drain of your washing machine fits tightly into the drain, you definitely need a washing machine drain vent. It allows air in which prevents the possibility of a vacuum forming that could affect the water draining from your washing machine.
You should also install a vent if your washing machine is more than 4 feet from the vent stack, as this will make sure there’s ample room for the washing machine to drain properly. You also need to be aware of the building codes where you live. In some areas, you legally have to install a washing machine drain air vent, but in other areas, you don’t.
Unless mandated by building codes, if there’s a gap around the drain tube for the washing machine where it enters the drain, you probably won’t need to install a vent, as there will be ample air to prevent the formation of a vacuum. Whether you vent or not, make sure you always use a drain trap.