As you can see in these two images, i have a hammer that is too light (200g) and another that is too heavy (750g) for striking wrenches on a copper boiler

I have set to put that right ordering a 12oz (450g) hammer with a wooden handle which i will cut down from the standard 300mm length to a more suitable 200mm length

The process of striking a set wrench to free a fitting on a copper boiler, as opposed to simply pulling harder and harder on the wrench by hand, is critically important if the copper boiler is not be twisted

It is the same process when tightening fittings down onto the copper boiler - dont pull harder and harder; plan to strike the wrench 3 times with the hammer as you feel the fitting begin to tighten on the thread

The first strike will be met with almost no resistance from the wrench; with the second strike you will feel the nut tightening on the thread, and then on the third strike the wrench will turn through a very small angle as it comes up tight, less than one eight of a turn; this is an indicator to you that a fourth strike will land you in trouble by tearing out the threads on the fittings

Far better to use a little thread sealant like loxeal 55-03 to ensure a pressure tight seal than overtighten a fitting and run the risk of damage