Hi, Hopefully a nice easy one. I am moving a radiator from one wall to another one. What size hole should I be drilling for the pipe to pass up from under the floor to the radiator? I can't look at the old holes because the floorboards have been notched then put in place after the plumbing first fix when the house was built. I just want a nice tidy hole that is not too big or small. 15mm is the obvious answer but no room for expansion if some is needed. Thank in advance.