residential interior systems. Cold-water service piping is usually found in two ratings: 160 psi at 73°F and 250 psi at 73°F. Interior tubing carries a rating of 100 psi at 180°F. The most popular interior sizes are 1/2 inch and 3/4 inch. Manifold or "homerun" systems (see "Home-Run Holds Promise," page 41) use 1/2 or 3/8-inch tubing. Cold-water service sizes are available from 3/4 inch to 2 inches, with 1 inch the most popular. Three types of fittings are available in a variety of materials. Insert fittings are made of copper or brass. Heat-fusion fittings are made of PB. Compression fittings may be made from Noryl, a polymer made by GE Plastics, or Udell, also a polymer, made by Amoco Chemical Co. Manifolds that distribute