Hi everyone. I am new at this so bear with me...I am trying to replace my tub faucet with a new one because the diverter is not working properly. (Not enough water is being diverted to the shower head). I think i have plastic pipe behind the wall. I started to twist it off, thinking it is screwed on, and it felt as if i were turning the pipe behind the wall. Whats up? How would it be attached if it were plastic piping back there? There doesn't appear to be any set screws on the existing faucet. Thanks. Any help would be appreciated.