What kind of solder should I be using when sweating copper piping for a forced hot water system? The changes I will be making are near the boiler. I'm curious because this is the first time I have needed to solder together any water piping on a system like this. I've heard guys using 95/5 solder, instead of the normal lead free plumbing solder.
My understanding is that you only need to use lead-free solder for potable water supply piping. For every other kind of piping, you can use lead solder, and that would include the copper pipe in hot water heating systems.
Since the heating system is a closed system, when soldering, make sure you allow some way for the heated air inside the pipe to escape. Otherwise you build up air pressure inside the pipe that works against capillary pressure trying to suck the molten solder into the joint. The result can be joints that won't take solder.
NO WATER in any pipes or isolated away from where your soldring........95/5 is fine with flux and steel wool to clean all the fittings.the most important connections are to the exsisting....no water,or black water residue when you solder there.