We highly recommend using Pex Al Pex pipe when installing a outdoor furnace for many reasons. To start regular pex has a very high expansion ratio; 1.1 inch per 100 feet per 10° F. Which means Expansion should also be taken into account. 100 feet of underground pex heated up to 180°F will expand 6 to 7 inches lengthwise. This puts pressure on fittings and may cause leaks or failures. Pex-al-pex lines have expansion properties similar to copper. There are no special considerations for expansion during the installation. When used with hot water the expansion is minimal. Lines maintain their shape and underground runs do not exert 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. Pex Al Pex also has a layer of aluminum in between two layers of plastic which acts as a oxygen barrier. Sold by the foot