Well, given you've asked a plumbing question in the electrical forum then I am the ideal person to reply, given that I am an IT guy and most certainly not an electrician.
I think you are planning to replace your radiator valves ('taps'). First things first, I assume you
(a) know you have to drain down the system before removing the existing valves from the copper pipes (then refill it after you're done); or
(b) already have some sort of local isolation valves on the pipework leading up to your radiator valves, so you can stop all the mucky central heating water weeing out everywhere; or
(c) trust those spray pipe freezing kits.
The connection between the new radiator valves and your existing copper pipe work is normally a compression fitting (in fact, I've never seen anything else) with an olive (copper or brass ring) that goes on the pipe between the nut and the fixing.
NEVER EVER EVER EVER EVER EVEN SO MUCH AS CONTEMPLATE - EVEN IN THE NAME OF HUMOUR - putting PTFE tape, jointing compound, or other gunk on the thread of a compression fitting.
Compression fittings don't seal on the thread. They seal on the join between the olive and the inside face of the fitting. The olive is crushed to a water tight fit as you do up the joint.
Instead of reaching for gunk, just do the fitting up properly. I tend to go for hand tight plus about half a turn, then repressurise and nip up at quarter of a turn, wait and see, quarter of a turn, ... if it is still weeping.
If you want to be belt and braces and use something to bung up the joint (or, if you're re-using an old olive and nut that's already on a pipe) then wind a few turns of PTFE tape around the face of the olive that goes in to the fitting. Again, though, should not be necessary on a new compression joint if you've seated it properly and not gone in all gung-ho and over tightened it.
