05 May 2017

"Dishwasher air gap" explained

A dishwasher air gap is an egg-sized device that gets mounted above the sink to help prevent contaminated water from draining back into the dishwasher...

Because these air gaps are a fairly new requirement, we've come across a lot of air gap devices that aren't installed properly. To understand these defects, it's helpful to understand how these devices function... [diagram and discussion at the link]

Plumbers don't like them, inspectors don't like them, and homeowners hate them... Oh, and for the record, dishwasher air gaps are definitely not something that we recommend installing during our home inspections.
Excerpted from Reuben Saltzman's incomparable The Home Inspector blog.


  1. Oh, so that's what the thing in our new rental is. I thought it was a placeholder for a soap dispenser. Good to know.

  2. We've had those for decades in California. Seems like a good idea to me. I've seen it in action once. When sewer was backed up, the dishwasher emptied into the sink instead of itself.

  3. It's funny, the whole article rants about these things, but barely mentions what their purpose is, and how they ended up in the code. If nobody wants them, who lobbied for them for what reason?

    And yes, there things are common in VA. Not blowing up water hoses because something gets stuck seems a good idea to me.

  4. I don't have one. Will something bad happen?

    1. From what I read in his blog post, it's not necessary to have one. It's when they are hooked up wrong that problems occur.

  5. The purpose of the air gap is that if your water pressure drops, the drain line can have a higher pressure causing brown or black water (waste water) to flow back into the supply line and contaminate it. The air gap ensures that during a pressure loss on the supply side, only air will flow back into the supply line. It is a requirement in restaurants and other food service industries.

  6. I recall reading an article in Fine Homebuilding that discussed air gaps among other ddetails involving dishwasher installs. the writer- a fellow with a fair bit of experience with them- mentioned a problem scenario . olive pit gets pushed out with the waste water. gets up into the air gap. clogs unit. this common food debris- at least among some households- has the perfect shape to clog the unit.


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