PEX is a relatively new plumbing product that is becoming more popular and incorporated into building codes for use in old and new construction. PEX is a flexible tubular plumbing system joined with compression, metal or plastic insert fittings or other mechanical methods that do not require glue or heat. CPVC is a rigid plastic plumbing system joined with fittings that are glued together. In short, no, you cannot glue PEX to CPVC because solvent-based glue will soften the PEX tubing, and the joint will not hold.
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Compatibility Between Plumbing Fittings
The most commonly used PEX plumbing line fittings use a special tool to crimp the tubing to the fitting. Other types of fittings use a compression ring and nut to seal the tubing to the fitting. CPVC pipe fittings are glued to the CPVC using a solvent-based glue to fuse the pipes to the fittings. The solvent-based glue will soften the PEX tubing and the joint will not hold. CPVC is a rigid plastic that is not compatible with PEX fittings, since the rigid plastic cannot be crimped into position for a crimp system and will not spread to work with a compression ring and nut for a compression fitting.
However, push fittings are compatible with almost all types of plumbing pipes, including PEX and CPVC. The fittings use an O-ring to hold pipes against an internal gasket, similar to a quick-release fitting on a water or air hose. Simply push the PEX tubing into one side of the fitting and push the CPVC into the other side of the fitting and the connection is made without the glue required for CPVC or the special tools required for PEX plumbing systems.
How to Join PEX and CPVC Pipes
Push fittings are becoming a more popular choice for plumbing fittings. There are a variety of manufacturers of this type of fitting and the fittings are now available in regular home improvement centers. The fittings are expensive, but much less costly than repeated mistakes or having to switch entire plumbing lines to another system for compatibility. The fittings are re-usable, so if a mistake is made or the system changes, the fittings are a one-time purchase that can be moved as needed without damage to the fittings.
Merge PEX and CPVC pipes by cleaning off the end of both types of pipe. Check for burrs, nicks, cracks or damage to each pipe and remove with a razor blade for PEX or sandpaper on the CPVC side. Push fittings seal best with a nice, smooth surface on the end of whatever type of pipe is inserted into the fitting. Position the end of the push fitting onto the end of the CPVC pipe and press onto the pipe firmly until the pipe cannot go into the fitting any further. (The CPVC is fitted first, since it is the more stationary pipe.) Move the end of the PEX tubing onto other side of the push fitting and press the tubing into the fitting in a secure and fluid push to seat the PEX tubing into the push fitting. Turn on the water and test the fitting.