Bung in the tank had always worked for me, if it’s open vent central heating you are dealing with, then I use a speedfit stopend on the vent pipe in preference to a bung ( as it can’t fall off). Although once you start to drain down the vacuum which forms helps to hold the bung anyway.
Remember to only have one open joint/ cut pipe at a time - otherwise you lose the vacuum and you are in big trouble!
Remember to only have one open joint/ cut pipe at a time - otherwise you lose the vacuum and you are in big trouble!