Nowadays, most suppliers deal with some sort of PEX tubing. PEX is an acronym for polyethylene, crosslinked. It's very good stuff from what I've seen. It's also made to last about 200 years, but you and I will never know for sure if that's true because we will both have joined the ranks of the Dead Men by then. PEX is, however, the material you'll find all over Europe. The Europeans have been doing hydronic radiant heating with PEX since the early Seventies and there's a lot more of it there than there is here. PEX has a great track record and has reached the point where it's almost considered a commodity item by many heating contractors and plumbers.

To avoid having it become a commodity (which is the kiss of death for any manufacturer), PEX 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. Knowing this can help you make the right decision when you're shopping for a PEX supplier. Here's the deal.

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. Polyethylene played a huge part in World War II because it gave the British the ability to carry radar on their airplanes for the first time in history. This was during the Battle of Britain. The German pilots didn't have radar (and they didn't know that the British did) so the British were able to kick the stuffing out of the Nazis by shooting at them from behind the clouds. But that's a story for another day.

PEX differs from HDPE in that PEX has this special three-dimensional link between the molecules. Scientists call this stuff a "macro-molecule." What that means is that if you hold a 200' coil of PEX pipe in your hands, you're actually holding only a few molecules.

Pretty strange, eh? 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 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. I heard him speak at a meeting in Sweden in 1990 and was fascinated by his story. But that, too, is a story for another day. 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.