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Question DetailsAsked on 7/31/2016

Water only leaks on cement slab beneath sink cabinet when dishwasher drains. New dishwasher. New DW drain hose.

No leaks at fittings for dishwasher drain into disposal. No leak under dishwasher. No leak when using disposal, water purifying faucet or regular faucet. No leak on cabinet floor. Just installed dishwasher and new corregated drain line this week. Still leaking from same spot under cabinet. Looks to be coming from drain pipe in block wall. Also snaked out roof vent and no stoppage found. Why would it only leak when dishwasher drains?

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1 Answer


Run paper towel all along the water supply line to the dishwasher, and along the flex drain hose to find any wet areas after/while using the dishwasher - leak will be near the top (highest point) of the wet spot.

Since under sink cabinet, I am assuming you have verified it is not leaking to there from under the dishwasher itself (like due to leaking pump or hose or fitting).

Unless the disposal hose barb (where drain hose goes in to the dishwasher) is leaking or hose is cracked, most likely place is the air vent - typically looks like this -

Commonly on top of sink back rim so if it overflows it goes into the sink, but sometimes a checkball type are mounted under the sink - prevents backflow in the line from the garbage disposal to the dishwasher, and if line from it to garbage disposal or the vent itself (center part usually pops out to expose the two hose ends inside) is plugged up at all it can overflow.

If sink top mounted and overflows, if the mounting gasket is not tight it can also drain down inside the vent through the hole in the sink into the cabinet area underneath, or sometimes if the sink overhangs the back of the cabinet or countertop drains that way, out onto the countertop behind the sink or down behind the cabinet to the floor/subfloor.

Another less common source - the water feed line or valve supplying hot water to the dishwasher leaking only when the pressure surges of the dishwasher turning on and off causing a leak at the crack in the line or a fitting or valve.

Another really odd one, which took me days to figure out on ours - on some faucet models the flex hose spray nozzles are pressurized all the time - and when the dishwasher kicks its flow on the pressure slacks off in that line for a moment due to the surge of flow to the dishwasher, so if the gasket/seal is old could be causing a bit of a dribble before it reseals, which can run down the nozzle and hose through the hose hole to the undersink area.

However, you said leaks when it drains - not when running, so above two not likely the cause in that case - I would be looking at the drain hose connections and fittings, and at the air vent midway along the drain hose.

If you had this installed by a contractor, sounds like time to call HIM back to find and fix the leak, since pretty evidently due to the new installation.

Since you mentioned looks like coming from drain pipe at back wall, could be there is a crack or bad joint there leaking only when there is a large discharge flow volume - which I would think would also leak (probably worse) if you filled the sink with water and let it all empty at once - but if you let the dishwasher do all the cleaning maybe you never do that.

Anyway, use the paper towel or bare hand method - running along tubing and pipes from low up toward high point checking as you go for wetness. Paper towel works better on metal because the hand does not detect the difference between dampness and cold metal very well. Could also dry floor, then put paper towel down and watch (while dishwasher runs) where the drip lands on it, then trace upwards to where it is falling off from.

Answered 4 years ago by LCD

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