Plastic pipe will not be suitable for joining to all metal or other pipes of different materials. You should proceed with caution because some plastics will melt and spoil if the job is not done correctly. It will also cause weak points and leaks.
Step 1 – Preparation
Heat the soldering iron to its minimum setting. Leave it to heat while you sand the copper and plastic piping. Sand the edges and around an inch along the pipe and wipe it down. This will help the flux adhere. Sand the copper pipe in the same way and wipe.
Step 2 – Compression Adapter
Fit the compression adapter on to the plastic piping and use the flux and soldering iron to melt and hold the solder. Use Teflon tape inside the joint before applying the flux.
Step 3 – Copper Piping
Once you have fitted the adapter to the plastic pipe, fix the copper pipe to the other end of the adapter and solder the two pieces together. Use Teflon tape on the side of the thread. The two parts should be left to cool properly and then you should check for air leaks before fitting the piece and using water through them.