Determining if your water shut off valve is a solder or compression union will not involve a complicated home-improvement algorithm but rather one simple step.
Two types of valves exist. The first is old-school soldered in place. Previous to compression piping systems (around the 1980’s) plumbers ran copper tubing that required a torch and solder. The plumber would first clean, connect and then heat the pipe and fitting until solder would melt and fill their union.
Compression systems can utilize copper or plastic piping. Terminations here are only the compression variety. The plumber inserts the water valve onto either copper or plastic pipe and then tightens the compression nut. As this nut is tightened a ring between the valve and nut compresses around the pipe making the joint water tight.
Inspect the Shut Off Valve
Flashlight in hand, inspect the water shut off valve’s connection to your home’s water pipe. A plastic water pipe is a certain indicator that your shut off valve is compression.
A copper pipe requires that you look for a compression nut. This nut will have angles, like a stop sign, to accommodate a wrench. The presence of this nut indicates the shut off valve is compression.
Copper to copper terminations that do not have a visible nut but rather solder at its edges indicates that it is a non-compression fitting.