I installed an under-sink RO system on November 9, 2015. This system fed a drinking water tap at the sink, and also filled my 20-gallon RO water tank in the orchid room.
About a month after installation, we noticed that when the RO system was running, water was leaking out from behind the kitchen sink. Extensive use of towels, mopping up twice a week (when the tank needed to be refilled), and repeated adventures under the sink convinced me that it was not the RO system that was leaking, but something in the plumbing system of the house.
So, we brought in the plumbers! They were also stumped on how the RO system could cause a leak when running, but not when idle. Finally, they decided (and I concurred) that it was a slab leak in the cold-water side, and that we had two options: re-plumbing/re-routing the cold water system, or jack-hammering up the foundation where the leak may be. We opted for re-routing, which had been done previously in our house after a slab leak on the hot water side.
So, they started re-routing the cold water side, which necessitated the cutting of approximately 10 holes in our drywall (Three of these holes shown in the picture.)
At the first testing of the new system, the RO system turned on, and the LEAK WAS STILL THERE. This necessitated much head-scratching and investigating, and eventual discovery of the leak!
We found a small crack in the kitchen-sink drain pipe, inside the wall behind the sink where the drain pipe went into the foundation. It was on the “house side” of the drain pipe, and as the output of the RO system was dribbling water into the drain, this small amount of water was trickling down the house side of the drain pipe, catching the crack and leaking out. Normal drainage from the sink or the dishwasher was of sufficient volume that the water largely ran to the back of the drain pipe, missing the crack. There is a 90 degree bend in the pipe after the drain enters the wall behind the sink. In a larger flow, the water is naturally pushed to the back of the vertical pipe inside the wall. It was the small amount of water discharged from the RO system that exposed the leak.
So now the leak has been discovered — and fixed! We only now must get some drywall patched, textured, and painted.