Tuesday, November 3, 2009

Pex vs. Pex-al-pex

Within the outdoor wood furnace industry there are mainly two types of pipe used, pex and pex-al-pex.

The basic pex pipe is cross-linked polyethylene. Most commonly, crimp rings are used to pinch the pipe around the brass fittings. Compression fittings are also available.

Pex-al-pex (also known as pex-al, pexal or p.a.p.) is a five layer pipe composed of plastic, adhesive and aluminum. Crimp and compression fittings are both available for pex-al-pex pipe.

When used with water at high temperatures, such as outdoor wood furnace applications, expansion is to be expected. Regular pex has a very high expansion ratio. This expansion creates dramatic droops in the installation. A nice, neat set of lines looks like limp noodles. Aesthetics aside, this expansion can also cause problems.

When pex line is used in underground pipe the expansion becomes compressed. The extra line puts pressure on the pump and fittings attached at either end.

Pex-al-pex lines have expansion properties similar to copper. When they are running hot water the expansion is minimal. Lines maintain their shape and underground runs do not put extra pressure on pumps or fittings.

Pex-al-pex lines also have a full 1 inch inside diameter (ID) compared to 1 inch pex lines that have an ID slightly less than 7/8 inches. This difference in ID makes a big difference when transferring BTU with hot water.

Transferring the proper BTU through the water lines is crucial to the performance of the heat exchangers used in an outdoor wood furnace system. Limit the ID and the BTU transfer is reduced significantly.

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