Back when modern radiant floor systems were first installed, copper or steel pipes were used. They were embedded in concrete under the floor and hot water ran through them to heat the building. It doesn't take a genius to see where the problems came from:

• The pipes expanded and contracted more than the surrounding concrete, causing them to crack and burst.
• The concrete cracked and, in turn, cracked the pipes.
• Concrete corrodes copper.

So Pex tubing was invented in Northern Europe, specifically to answer the radiant floor need. Pex is flexible but very strong. It does not corrode. It is nearly inert. It can withstand the moderately hot temperatures needed for a low-heat space heating system like radiant floors. Technically, Pex is a cross-linked polyethylene but, to solar thermal installations, it's an answer to all our needs!

Radiant floors use Pex tubes with an internal diameter of a half inch. Some people use 5/8 inch, which reduces flow resistance. Both sizes work well. Length-wise, you should count between 1 and 1.25 feet of Pex for every square foot of floor area in the heating bed.

Preparing Pex

Pex is supplied on great big rolls, usually with at least 300 feet of coiled tube. It is inflexible when cold, so you should store it somewhere warm for at least six hours before you try to unroll it.

Pex cannot be used out of the box – if it isn't laid flat first, it will just spring up into unwieldy coils that are impossible to use. You must uncoil the entire length of tubing before you use it, so you'll need a lot of space to lay it out. There are three main ways of uncoiling Pex:

• One person holds the starting end of the Pex tube and the other rolls the coil along like a wheel, laying the tube flat as it goes. Be careful to lay the tubing flat – if it runs away from you, it will spring up and tangle.
• One person holds the coil with their arms through the middle like an axle. The other person takes the leading end and walks away with it, while the coil-holder flips the coil on their arms to help it unravel. This takes a bit of practice but works well and looks very professional!
• Use an uncoiler, which is a flat platform that turns horizontally like a Lazy Susan. One person holds the coil on the uncoiler and helps it unravel while the other walks with the leading end of the tube.

Pex should be left lying flat for at least an hour. The warmer the tubing gets, the more flexible it becomes, so it's best to do this on a sunny day when the most heat is available. But don't leave it out there too long – several hours shouldn't harm Pex but leaving it in the sun for days could affect it. Most manufacturers recommend covering the tubing with a tarp if you intend to store it outside or leave it uncovered for extended periods. Check the documentation if you