Because a dishwasher drains into your kitchen's plumbing system, there is a slight possibility of dirty drain water siphoning back into the dishwasher and contaminating clean dishes or being drawn back into the fresh water supply. To prevent this, building codes require some method of creating an air space in the dishwasher drain hose. There are two common ways to do this:
- Looping the dishwasher drain hose up under the sink cabinet. This connection method is known as a high loop.
- Routing the drain pipe through an air gap fitting that mounts to the top of the countertop or sink.
Dishwasher venting can be somewhat confusing for the do-it-yourselfer. The best strategy is to follow whatever local code guidelines require for dishwasher venting. Some local codes will specifically require the use of an air gap when installing a dishwasher, while in other areas the high loop method is 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.
The Air Gap Method
Connecting the drain line to an air gap is one method that can be used when installing a dishwasher, and in some areas, it is a mandated requirement.
An air gap is a small metal or plastic fitting that is installed on the countertop or sink, with two hose fittings on the underside. The dishwasher drain hose connects to one fitting on the bottom of the air gap, while a secondary hose runs from a second air gap fitting to the house's drain system.
The purpose of the air gap is to prevent vacuum pressure from allowing drain water to be drawn back through the drain hoses into the dishwasher chamber. It works by allowing fresh air into the drain hose when any negative pressure occurs, eliminating any suction force.
Without an air gap, there is the possibility, however slim, for drain water to siphon back into the dishwasher. This dirty water can contaminate clean dishes, and it can potentially be drawn back into the fresh water supply under certain conditions.
The air gap fitting usually fits into an existing mounting hole on the top of the sink, and they are available in a variety of finishes to match your faucet and sink.
The High Loop Method
In the high loop method, the drain line of the dishwasher is looped up as high as possible under the sink countertop before it runs back down to connect to the drain system. In some areas, this high loop method is an allowable option. Because the top of the loop is located above the flood level of the dishwasher, it makes it unlikely that drain water can be siphoned 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 being drawn back into the dishwasher.
Dishwasher Drain Connections With and Without a Garbage Disposal
When there is a garbage disposal installed under the sink, the dishwasher drain line should run up through the air gap or high loop BEFORE it is connected to the nipple on the garbage disposal. Do not bypass the garbage disposal if it is present.
When there is no garbage disposal present, the dishwasher hose should run up to the air gap or high loop first, then run down to the sink drain. Normally the hose connects to the sink drain by means of a Y-branch fitting on the sink's drain tailpiece.