Abstract: A plumbing installation is provided which includes an outlet box, at least one water supply pipe extending into the box and a torque inhibiting connection between the pipe and a boiler drain valve positioned in the interior of the box. A compression nut is screwed over a male threaded lower shank of the boiler drain valve to compress a ferrule around the upper end of the water supply pipe. A male threaded cylindrical adapter is connected to, and extends downwardly from, the compression nut through a water supply hole in a bottom wall of the outlet box. The adapter surrounds the water supply pipe. A lock nut is screwed over the adapter and squeezes the bottom wall between the compression nut and the lock nut. When the water supply pipe is made of CPVC this installation prevents lateral movement and/or torquing of the pipe which could weaken or fracture chemical welds between a lower end of the CPVC pipe and a CPVC fitting.