Updated: Aug 17, 2020
A residential home inspection should include a thorough assessment underneath the homes sinks, most notably in the kitchen area. An inspector is looking for signs of leaking water (current and past), staining, mold growth, and general piping connections.
A common observation made during home the home inspection, is poorly connected dishwasher apparatus to the waste disposal system. This blog will discuss the deficiency and illustrate the correct method for dishwasher connection to disposal.
First, we want to prevent nasty sewer gases from traveling up through the pipes and into the dishwasher, and obviously the general breathing space of the home. Second, we want the dishwasher to discharge waste water properly (either to the main pipe or to the food grinder) and, we don't want contaminated water from the grinder or kitchen drain pipe, back-flowing to the dishwasher.
Back to the "sewer" gas issue - To stop unwanted gases from entering the dishwasher and other areas of the home, a P-trap should be installed underneath the sink. Note in the illustration to the left, the P-trap (the "kink" in the pipe - left and down from the dishwasher high loop drain tube). The ideal purpose of this trapped area is to hold water all the time - to create an impenetrable barrier for the sewer gas wanting to find it's way into the home. When the faucet valves are off (no water running), standing water should be present in the trapped area of the P-trap. Therefore, to drain the dishwasher properly and prevent unwanted gases, the dishwasher drain hose should be tapped into the "clean" side or non-sewer-side of the P-trap. This is shown correctly in the photograph above.
Now we need to ensure that faucet water and waste water (food-water, or "sewer" water) does not enter the dishwasher unit via it's drain tube, when things start getting clogged and all mucked up inside the main pipes / grinder. To do this, we either install a high loop or air gap. The high loop is just 180 degree loop in the dishwasher drain tube, that is setup high - like in the photograph above. It should be set up as high as possible, physically touching the underside of the counter top. For this set-up, it would take a pressure-driven situation to overcome gravity and create backflow issues into the dishwasher.
An even better method, which has now been adopted my many state and local coding agencies (including both Northern California and Minnesota), is to create an air gap. The air gap is that knob-like-thing that sits above your sink, as shown in the photograph below.
In California, the following is stated regarding your dishwashers drainage code compliance:
807.3 Domestic Dishwashing Machine
No domestic dish washing machine shall be directly connected to a drainage system or food waste disposer without the use of an approved dishwasher air gap fitting on the discharge side of the dishwashing machine. Listed air gaps shall be installed with the flood-level (FL) marking at or above the flood level of the sink or drainboard, whichever is higher.
In Minnesota, the following is stated regarding your dishwashers draining code compliance:
2015 Minnesota Plumbing Code, Chapter 4714, Section 807.4
No domestic dishwashing machines shall be directly connected to drainage system or food waste disposer without the use of an approved dishwasher air gap fitting on the discharge side of the dishwashing machine. Listed air gaps shall be installed with the flood-level marking at or above the flood level of the sink or drain-board, whichever is higher. Hammer arresters shall also be installed (2015 Minnesota Plumbing Code, Chapter 4714, Section 609.10).
The largest difference between the air-gap method and high loop method is the air gap is dis-connected inside the metal chamber. Therefore, any backflow from the sink or grinder should be assumed to spew out of the air gap if this situation were to occur. If your home doesn't have one of these, that's ok - check under the kitchen sink to see if you are set up with a high loop (non air-gap style). They are still good, but may not be to code anymore.
If your home isn't setup like this, you should hire a handyman or plumber to remediate this area of the home and make it code compliant. It's cleaner, healthier, and one less thing for you to worry about if you sell. You definitely don't want to deal with it during a home inspection, pre-sale scenario.
Our home inspections often times reveal this condition.
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