Does a graywater drain to grade need a vent?

Question

Do I need to vent a wash machine drain that does not enter a sewage system but flows to a trench in the pasture. Will it damage my washer if there is no vent

Answer 1

Venting allows air to flow into the pipe from a location other than the drain itself. This helps keep the trap filled with water (otherwise it may pull the water out of the trap). Having water in the trap prevents drain flies, et. al. from traveling through the pipe and emerging from your drain. An air-admittance valve (AAV) is sufficient for this purpose.

Venting can also allow air to vent out of the pipe. This can help prevent smells from material decaying in the pipe from entering the house. Also, if there is a clog restricting flow in the pipe, allowing the air out may allow water to enter the pipe quickly and drain out slowly. An AAV will not do this; you need a real vent for this purpose. A vent anywhere along the length of the pipe will help with this. Ideally you want the vent between the trap and the potential clog (because after the clog, the clog restricts all flow). The closer to the trap it is, the more likely that will be true.

Only the possibility of restricted flow would put extra stress on the washer. Many washers do fine with just an AAV, which would not help with that. Oversizing the pipe and having a good cleanout in a convenient location may be more important than the vent. Also maintaining sufficient slope in the pipe to support gravity drainage. And of course you don't want the pipe to freeze.

Answer 2

If the pipe just carries the wash water and doesn't join any other pipes on its way outside, you don't need a trap, and so you don't need a vent. You can think of the pipe as just a long hole in the wall.

If you want a trap, perhaps to limit (somewhat) the entry of cold air and vermin, you need a vent. The vent isn't subject to all the usual regulations. You're defining your own problem so can build the solution with a little creativity. Just do the minimum to prevent the trap from being siphoned out without creating a new path into the house from outside.

Your vent could attach after the trap, rise up higher than the top of the washer, and then exit to the outside and terminate with a screen. You don't have to go over the roof or worry about venting sewer gas. You do have to follow the sloping rules to make sure water anywhere in the vent or drain will drain out.