You did not state an age of the plumbing. Half inch is all you need
unless it has become calcified which is less likely with copper than
iron pipe.

I would bet on the mixer valve or defective seats for a Delta style
faucet. If it has always been that way it could be a trapped solder
ball inside the fixture.

I would splurge and spend the $20 to buy a new mixing valve and seats
before I ripped out pipe.


The existing plumbing is about ten years old. We also have very soft
slightly alkaline water here. No deposits have every built up anywhere that
I'm aware of. Toilet tanks are clean enough to drink out of. Pipes that I
have had apart show no sign of deposits.

I already have a new valve ( I hate the one that is in there - no volume
control, just temp control only). I bought it a long time ago, just never
had a good enough of a reason to install it. What is definitely going to
happen is that I'm going to rip out the wall and replace the valve. What is
uncertain is how much plastic I replace with copper.

To be honest, I'm leaning towards just biting the bullet and getting rid of
the plastic. I have the time and money to do so, and I hate the idea of
plastic pipe in my walls.