Hmm!
Basically compression fittings are nuts just like any nut on a thread. Look around and make sure nothing can knock them or shake the nut loose. Trying the system hot is a good idea as the pipes move as they expand and contract and so could loosen the nut.
But compression fittings are used nearly everywhere and don't seem to cause any problems at all. If you look at your radiator valves they are compression usually. But make sure you get them full socket and do not over tighten. In other words use a compression spanner or one that fits the fittings back nut if you can rather than a giant wrench, thinking the tighter I get it the better it will be.
What you usually do by over tightening, is make the ring dent or mishape the pipe, buckle the ring or the ring can come out of the backnut making the fitting leak or even strip the threads. Its hand tight and about a quarter to half turn more with a spanner, medium sized wrench or similar.
I must admit I have not counted how far exactly I turn teh back nut after getting it hand tight, probably having done so many over the years its kinda instinctive as to how tight it should be.
Its just till you can feel a bit of resistance on the back nut to your torque pressure.
As to using PTFE well that is a choice really, I personally prefer jointing paste watching it doesn't go inside the pipe, but then I was trained in the days of bos white and hemp, not PTFE.
The idea of using PTFE or jointing paste is to make up for any imperfections in the metal to metal seating faces between the olive/ring and the body of the fitting and back nut. But many makers now say you do not need any jointing materials at all. I must admit I instinctively use it and don't feel the fitting is properly dressed without any. But then I am an old school guy.
