“Pex,” the generic name of cross linked polyethylene tubing, is a modern plastic pipe. It is safe enough to supply either hot or cold drinking water, and it is commonly used for hydronic underground heating systems. Because it is flexible in all temperature ranges, it is one of the best replacements for cast iron pipes. It never corrodes and never rots, it can be bowed to weave its way over, under and through obstacles, and it can be installed using any regular Pex-specific compression pipe fitting.
Turn off the water supply to the pipes that are going to be removed.
Unscrew the cast iron pipe joints with a pipe wrench so the original female fitting on the feeder pipes remain to remove all the cast iron pipes. For rusty pipes, spray some petroleum-based lubricant on the joint and allow it to soak in for a few minutes. Tapping on the rusty joint will also loosen the rust for easier removal. Stubbornly joined pipes should be liberally sprayed with lubricant and allowed to sit for 15 minutes or longer. Use two pipe wrenches, turning each in the opposite direction to remove the pipe.
Lay out your Pex tubing. Start at the water supply and work your way to all the connections that the cast iron pipes connected to. You will be able to hang Pex over beams, go through holes where cast iron pipe couldn’t, and wind it around areas in one piece where a cast iron pipe would have needed an angle connector
Hang Pex tubing permanently with hangers or straps. There are many ways to attach Pex to beams and overhead joists. Hangers are thick metal wires that wrap around the Pex tubing. The ends are pounded into a wooden surface to hold the tubing up and out of the way. A Pex strap is a half circle that loops over the Pex tube, and the attached nail is pounded in with a hammer to secure the tubing.
Screw in a Pex male fitting at the water supply with an adjustable wrench. There are two types. One type is a standard compression fitting made especially for Pex tubing. The other is a professional-style crimp fitting. For the average DIY person, a compression fitting works fine. If you plan on doing a lot of Pex installation in the future, it will be better to get a professional style crimper and crimp rings. They are faster and will save time when doing multiple installations.
Attach the Pex tubing to the water-supply fitting. Use an adjustable wrench for a compression fitting or a crimper and a ring for a professional crimp fitting.
Cut the Pex tubing at the next connection point with a utility knife. Because Pex is essentially thick plastic, a utility knife will do the job. The cut does not have to be perfectly straight, just close.
Attach the Pex tubing at the first connection point in the same manner as you did at the water supply pipe. From there, move to the next connection point. Screw in a male Pex fitting of your choice, cut the tubing to length, and secure it with a compression fitting or a crimp fitting.
Add a tee fitting, where needed, to go in different directions. Cut the Pex with your utility knife, splice in a tee fitting, your choice of either compression or crimp, and secure the fitting in line.