Years ago I was hired as a maintenance man in a hotel that had shut down an annex in the winter. They didn't drain the plumbing of water, and had hundreds of breaks. Needless to say I got good at sweating copper fittings together in a hurry. A couple of pointers. Heat the fitting and flux and touch the solder to the metal and let it flow into the joint. Also if you need to work on a closed system flood the area with co2 if you have it or get a couple of pounds of dry ice at the grocery store and put it into the tank. No oxugen no WHOOSH or worse. You can also flood the area with gas out of a MIG bottle. Expensive but effective. I once made a BBQ pit out of a 6 X 2 foot propane tank. I filled the tank with co2 and let it set for a couple of days. I refilled it and cut the door out of the tank with a demo saw. Lots of sparks off the saw, but not a thing from the propane tank. Once I had the opening cut I took a propane weed burner and hosed the inside down with flame. I never got even a little reaction. I still have the BBQ pit to this very day.
