Dishwasher venting can be somewhat confusing for the do-it-yourselfer. There are two things to consider when planning your dishwasher venting installation:
- What can be done to vent the dishwasher?
- What the local building code requires for dishwasher venting.
Some local codes will specifically require the use of an air gap when installing a dishwasher while in other areas the codes have designated a high loop in the drain line as an approved alternative.
It is important to check local building and plumbing codes before installing a dishwasher. In many cases, you can look on the website for your county’s building and safety department to find the code requirements. If you can’t find the information online or if you have any doubts call them and ask. It is also a good idea to read the manufacturer’s recommendations for installation.
Dishwasher Venting and Draining Methods
Air Gap Method
Connecting the drain line to an air gap is one method for installing a dishwasher. In some areas, the building codes require that a dishwasher be connected to an air gap installed on the sink. Air gaps are there to keep any contaminants in the drain line from entering the potable water system. An air gap creates an unobstructed vertical space between the flood rim of the fixture and the drain outlet. Without an air gap there is the possibility, however slim, to have drain water siphon back into the dishwasher and into the water supply if you had a complete loss of pressure due to a break in the water main.
Air gaps will also prevent any drain water from backing up into the dishwasher, which can happen if the sink is clogged and even sometimes when you run the garbage disposer. An air gap will take up a mounting hole in your sink, but they are available in almost any finish to match your faucet and other sink top accessories.
High Loop Method
In the high loop method, you take the drain line of the dishwasher and attach it as high as possible under the counter rim of the kitchen sink before connecting the end to the drain. In some areas, the high loop method is an approved option for dishwasher drain installation. The loop will help prevent a backed up drain or a large volume of water in the sink from siphoning back into the dishwasher. The high loop method is nowhere near as effective as an air gap, but it can help to prevent contaminated water from going back into the dishwasher.
I have found that, where it is allowed, more and more people are using this method simply to do away with the sink top air gap. The high loop method frees up a sink mounting hole that can be used for the faucet, soap dispenser, water filtration, or instant hot water dispenser.
Without A Garbage Disposer
When there is no garbage disposer under the kitchen sink a y-branch tailpiece or dishwasher branch tailpiece needs to be installed onto the sink drain line. This will allow a place for the dishwasher to drain into. You can either go from the air gap into the y-branch (always preferred) or from the dishwasher into the y-branch tailpiece using the high loop method (if your local codes allow).
Note that the y-branch needs to be installed before the p-trap.
With A Garbage Disposer
When there is a garbage disposer installed under the sink the dishwasher drain line will tie into the drain through the disposer. You can either go from the dishwasher to the air gap and then to the garbage disposer or from the dishwasher into the disposer using a high loop.