Ease of installation and maintenance is just one reason why plumbers love cross-linked polyethylene (PEX) pipes and increasingly use them to retrofit existing copper pipe systems. PEX pipes are flexible, which means fewer fittings and fewer burst pipes in freezing temperatures. Also, you don't need glue or solder to join them. And when the pipe installation is complete, there's no need to flush out residual flux. If you want to add PEX pipes to your existing copper system, you can do it with a push fitting, a solder-on adapter that fits either a pipe or a fitting, such as an elbow, or a threaded adapter.

Repairing a hot water copper pipe
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How to Connect Pex to Copper

Installing Push Fittings

Copper-to-PEX push fittings are available under the brand name Shark Bite. They are made of brass, which won't corrode copper, and are manufactured with internal tines that hold the pipes against an internal O-ring to make a watertight seal. As the name suggests, you simply push these fittings onto the pipes. The pipes need to be fully inserted into the fitting for this to work, so it's important to measure the insertion depth on each pipe and mark a line. When this line is flush with the edge of the fitting, the pipe is fully inserted.

Before putting the PEX pipe in the fitting, insert a support sleeve into the pipe. This plastic tube fits snugly inside and prevents the walls from collapsing, which would break the seal. Place the sleeve all the way into the pipe, then push the pipe all the way into the the brass connector. The copper pipe won't collapse, so it doesn't need a support sleeve, but it does need to be clean. Brush off the oxidation layer with a wire brush, then insert the pipe all the way into the other side of the fitting and the connection is complete.

Installing Solder-On Connections

Solder-on connections are also made of brass. They come in two varieties: The female sweat adapter has a slip connection that fits over a pipe, while the male sweat adapter slips inside a fitting. You would use the male fitting one if you want to connect PEX tubing directly to an existing elbow or tee. The other end of each of these connectors is a barbed tube that fits inside the PEX pipe.

Solder the fitting onto the pipe before pushing the PEX tubing onto the connector. Once the metal cools, push the PEX pipe onto the barbed connector until it is fully seated. Secure the connection by crimping a clamp around the PEX pipe with a crimping tool.

Threaded Connectors

You can connect PEX tubing to a threaded copper connection by using a threaded adapter. You can buy these with both male and female threads. Screw the adapter onto the copper fitting, using plumbing tape and a wrench to make a tight, leak-proof connections. Push the PEX pipe onto the barbed connector on the other end of the fitting and secure the PEX pipe with a crimp ring.