Compression fitting with copper nut and plastic pipe keeps leaking

Question

I have a problem with a compression fitting leaking for the water supply of my new humidifier. The connection includes a copper nut and ferrule and plastic tubing. There is a slow drip at the connection that I cannot get to stop. I’m afraid of over tightening. After the first try did the same thing and I DID over tighten, I cut the plastic tube square, used a new ferrule and tried again tightening very slowly. At this point I’m not sure I’d i need to continue tightening or if there’s another problem. Help!

Answer 1

With plastic pipe compression fittings usually come with an insert made of metal or hard plastic. The compression nut and ferrule (generally plastic for plastic pipe) is placed over the pipe, then the insert is pushed into the end of the pipe. When the joint is tightened the insert prevents the plastic pipe from collapsing as the ferrule bites in. Without the insert it is very difficult to make a compression connection in plastic watertight.

See the second question in the FAQ on this page: https://www.plumbingsupply.com/ed-compression.html