PEX is an acronym for polyethylene, crosslinked. It is made to last about 200 years, but if you live for another 200 years and the PEX you buy is still in the floor and working - please give us a call - as soon as possible. PEX pipe is, however, the material you'll find all over Europe. The Europeans have been doing hydronic radiant floor heating with PEX since the early Seventies and there's a lot more of it there than there is here. PEX Tubing has a great track record.
PEX Tubing manufacturers have been taking the time to spell out the differences between their products. And there are differences in the way different companies make PEX piping. Knowing this can help you make the right decision when you're shopping for a PEX supplier for your radiant floor heating or plumbing system.
Most manufacturers start with a material called HDPE, which stands for High-Density Polyethylene. HDPE evolved from polyethylene, a material discovered by accident by a group of British scientists during the 1930s.
PEX differs from HDPE in that PEX has this special three-dimensional link between the molecules. It's this network of macromolecules that gives PEX such a fine memory for its original shape. If you kink PEX pipe, you can just heat it up and it will always return to the shape in which it was first crosslinked.
The linking of the molecules happens during the manufacturing process, and how a manufacturer chooses to make that happen affects the properties of the final product. The manufacturer can't link all the molecules together because that would make the PEX pipe too brittle. On the other hand, if they link too few molecules the material won't be any better than HDPE, from whence it came. They have to find just the right combination of linked and non-linked molecules.
There are different ways to get where they need to go, and from what I've learned, some ways seem better than others. Here are the principal methods manufacturers are using today to make PEX:
Engel-method PEX (also known as PEX-A) This is the stuff Tomas Engel brought into the world. Engel was the guy who invented PEX. He is a European scientist and he had nothing to do with the radiant heating industry when he invented PEX. What's important for you to know is that Engel-method PEX gets crosslinked while it's still in its melted form. The manufacturers do this by adding peroxide to the mix and then applying a tremendous amount of pressure and temperature to the liquid. What comes squirting out of the machine is PEX that's as clear as glass. As it cools, it takes on a darker color. The Engel method gives the pipe an essentially uniform distribution of the crosslinking sites throughout the material. This is the best way to make PEX pipe. It takes a while longer to do it this way, and this sort of PEX may cost a bit more. HousePEX PEX-A Tubing is made with the Engle-Method - thus HousePEX PEX-A PEX is "PEX-A".
Irradiation-method PEX (also known as PEX-C) Irradiated PEX starts out as straight polyethylene tubing. As with other methods of making straight polyethylene, the tubing takes on a definite form during the extrusion process. With the irradiation method, the crosslinking takes place in a second process when the manufacturer runs the tubing through an electron beam cannon. The beam gets the polyethylene molecules so excited that they crosslink. This method can sometimes result in a less uniform crosslinking of the material. If it's not done properly, the outer layer of irradiated PEX tube can become brittle, but this is not a concern in a well-controlled process. HousePEX PEX-C Tubing is made with the Irradiation-method - thus HousePEX PEX-C PEX is "PEX-C".
Silane-method PEX (also known as PEX-B) The big difference between this method and the other two methods is that with the Engel and Irradiation methods, the crosslinking consists of a bond between carbon molecules. With the Silane method the crosslinking takes place across silicon and oxygen molecules. These links are weaker than the carbon-carbon links that result from the other methods, and this may have an effect on the long-term chemical stability of the material. If we live long enough, we'll probably find out.
It's good for you to know the differences between these materials even though they all go under the name PEX. Let's face it, you're the one who's going to be putting the tubing under the floor.
In fairness, though, I have to tell you that none of these methods is a bad method; The American Society for Testing and Materials (ASTM) approves all three methods. However, in a world among "equals," it seems to me that some methods are more "equal" than others. Don't think of PEX as a commodity item. There is a difference.