Dishwashers tout several benefits over handwashing: unless you're a dishwashing ninja, they save time and water; they help to keep sinks and counters clear; and they even sanitize your dishes! It's the last one that really seems to resonate with people: nothing's more important than health and safety, and what good is a dishwasher if the dishes aren't truly clean? But when it comes to health and safety, there's an even more important dishwasher feature that unfortunately isn't a part of every installation: the air gap.
When a dishwasher is installed, its drainage hose is connected to either a garbage disposal, or directly to the sink drain by way of a dishwasher branch tailpiece wye. The path to the drain must be set up in such a way as to prevent drain water or sewage backup from flowing back into the dishwasher and getting all over what you thought were clean dishes. While this isn't a common occurrence, it's entirely possible under the right circumstances without the right safeguards.
Check valves are a built-in safeguard in most dishwashers, but a check valve alone is insufficient protection against backflow into the dishwasher - the amount of food and debris expelled during a wash cycle puts the valve at high risk of getting stuck. Additional safeguards come in the form of an air gap, or a special placement of the drain hose called a "high loop". But only the air gap guarantees backflow protection.
Note: Some municipalities have exempted certain high-end dishwashers from high-loop and air gap requirements, having deemed their designs and check valves sufficient for backflow protection. Despite this, we still urge anyone with a dishwasher to use an air gap.
How It Works
The air gap assembly is installed through the sink or countertop. The bottom of the assembly consists of two branches, each intended for a different length of hose. One length is run between the dishwasher and the air gap, while another runs from the air gap to the sink drain or garbage disposal. The two never meet - they're separated by a literal gap of air. Water draining from the dishwasher travels through the hose, to the air gap, and falls through the gap of air into the second branch, ending up in the drain.
A high loop, on the other hand, does not provide a physical gap between drain and dishwasher. A single length of hose is run from the dishwasher, secured to the highest point possible under/behind the sink, and dropped back down into the drain or disposal. This forces any backflow to push up a good distance before it can reenter the dishwasher. However unlikely that scenario is, the lack of physical separation between dishwasher and drain does mean contamination is possible.
Note: Newer dishwashers often loop the drain hose on the side of the unit, but this is not a substitute for an actual high loop setup under the sink.
A high loop is a minimum requirement in most areas, and in some may even be the only requirement. If that's the case, why do anything more? Why mar that gorgeous sink and counter with a goofy cylinder that will never not scream "look at me!"? You can call it an abundance of caution (or paranoia), but we truly believe in leaving as little as possible to chance when it comes to your home's plumbing. A dishwasher installation lacking an air gap will always be at risk of backflow, even with a code-compliant high loop. That risk may be small, but "perfect storms" can and do happen. The only way to isolate the dishwasher from contamination is to use an air gap.
When It Doesn't Work
The simplicity of the air gap not only makes for an elegantly effective device, it also means there are fewer opportunities for malfunction. But that isn't to say things can't go wrong:
- The most common issue with air gaps is a blockage somewhere along the line, usually due to bits of food and grease accumulating over some time. These kinds of blockages are most likely to be found in the hose between the air gap and drain/disposal (where there's less pressure to move things along). When a blockage becomes large enough, water coming out of the dishwasher will begin to back up, spilling out of the air gap into the sink basin. Try disconnecting the hose and flushing it out.
- If a blockage can't be found in the hoses, there could be something inside the air gap itself - a bottle or pipe brush may be handy in clearing out the unit. If that doesn't solve the problem, it's possible the blockage lies past the hoses: in the the disposal itself, or the trap(s) under the sink. While you would probably notice such a clog well before the dishwasher brought it to your attention, this would be the last obvious place to look. Remove the p-trap and clean out any blockages.
- Ensure that there are no kinks in the hose(s), and that no length is too long; there should be little slack when all is said and done. There should be just enough hose to get to the air gap inlet (or reach the highest point under the counter if using a high loop), and just enough to reach the drain or garbage disposal. The final run of the hose to the drain/disposal needs to be free of any sagging "traps" resulting from excess length, in which debris can accumulate and cause a blockage.
- If the dishwasher is connected to a garbage disposal and constantly spits water from the air gap during draining, it's possible that the knockout plug on the disposal inlet was never knocked out. It happens! Disconnect the power supply, and use a screwdriver and hammer to punch it out. A pair of long-nose pliers will be helpful in fishing out the plastic plug from the disposal.
- Avoid using standard corrugated, plastic hoses - they're easier to damage than higher-quality rubber hoses, and the ridges inside are perfect for accumulating debris, which can lead to a clogged line.
Some find the air gap unnecessary (given check valves and high loops), and a great many more object to its admittedly… lacking appearance (especially in higher-end kitchens). While we do concede the latter, we maintain that aesthetics should always be secondary to safety. That being said, we do offer air gaps with covers in a variety of designer finishes to minimize any disruption to the visual appeal of the sink and counter area. As to the gap's necessity? A high loop may suffice for inspectors in some places, and an expensive dishwasher may not "need" any additional safeguards beyond its check valve, but we believe there's no way to guarantee cleanliness and safety without the humble, homely air gap.