So what the heck is a Johnson Tee?
Most people are probably familiar with the air gap device that is present at the kitchen sink that prevents drainage water from flowing back into the dishwasher (nice thought). When they are plumbed backwards they make nice little fountains into the sink.
There is another method of venting the dishwasher that, while a little less common, does the same thing—-the “Johnson Tee.” These devices have been around for quite a while—-at least the early 1980’s when I was building houses in the Oswego, NY area. With this method of venting you won’t find the washer drain connected to the disposal or drain under the kitchen sink the way it would be with the countertop air gap. It leaves a much cleaner look and doesn’t use up the extra hole in the sink—which can then be used for something else—like a soap dispenser, a hot water tap or a push button vacuum switch for the disposal—or even a filtered water faucet.
With the Johnson Tee, the drain and washer connections are all behind the dishwasher and inside the wall cavity. The actual vent portion is at the exterior of the home (although sometimes it might be found on the countertop backsplash). This is what the assembly looks like inside the wall before the wall is closed in. At the very bottom of the picture you can see where the drain from the washer will connect.
At the top is where the Tee portion goes through the wall to the exterior and will have a finish cap installed that looks like the one in this next picture.
Like the countertop type air gap device, if you see water or foam coming out of the cap it is an indication of problems with the dishwasher drain and a plumber should be called to make repairs. The biggest problem that the Johnson Tee encounters is that the holes at the exterior get painted over—-which prevents proper function—-so the holes must be maintained open. Another thing that is common is that the test caps get left in place and never properly finished.
This can be remedied by simply drilling a few small holes in the cap or by cutting the cap off and installing the more decorative type pictured above.
So make sure your house is wearing its Johnson Tee or countertop air gap device properly.
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Charles Buell, Seattle Home Inspector
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