Synthetic Rubber Hose
Synthetic rubber hose is very flexible and highly temperature resistant, but it is less durable than PEX tubing and has a low pressure rating. A number of different rubbers have been used to produce synthetic rubber hose for use in hydronic systems. Cross-linked EPDM is one of the most popular types.
Composite Tubing
Some manufacturers are producing composite tubing for hydronic heating systems. Composite construction commonly involves the combination of aluminum sandwiched between layers of plastic (PEX) or synthetic rubber.
PAX tubing is a typical three-layer composite tubing consisting of an inner layer of PEX tubing, a middle layer of aluminum, and an outer layer of PEX tubing. The aluminum middle layer, which provides an effective barrier to oxygen penetration through the tub- ing walls, is bonded to the inner PEX layer. PEX tubing is very flex- ible and, because of its aluminum middle layer, will retain its shape after bending much more easily than standard PEX tubing or BPEX tubing. PAX tubing is recommended for use in hydronic floor panel heating systems.
Brass tube and brass pipe should also be distinguished from one another. Brass tubing is generally manufactured from yellow brass (about 65 percent copper, 35 percent zinc). Brass piping is more frequently a red brass (about 85 percent copper, 15 percent zinc) and is much stronger than the tubing. As a result, brass pipe is sometimes used in heating, ventilating, and air-conditioning sys- tems, particularly when it is necessary to use a pipe material that strongly resists corrosion.
Pipe Fittings
Pipe cannot be obtained in unlimited lengths. In most pipe installa- tions, it is frequently necessary to join together two or more shorter lengths of pipe in order to create the longer one required by the blueprints. Furthermore, in practically all pipe installations there are numerous changes in directions and branches that require join- ing the pipes together in special arrangements. Pipe fittings have been devised for the necessary connections.
A pipe fitting may therefore be defined as any piece attached to pipes in order to lengthen a pipe, to alter its direction, to connect a branch to a main, to connect two pipes of different sizes, or to close an end.
Classification of Pipe Fittings
The pipe fittings used in heating, ventilating, and air-conditioning installations are most commonly either screwed or flanged types. Screwed pipe fittings use a male and female thread combination, which tightens together to form the joint. Screwed pipe fittings are designated as either male or female, depending on the location of the thread. A female thread is an internal thread, and a male thread is an external one.
A flanged pipe fitting has a lip or extension projecting at a right angle to its surface. This lip is bolted to the facing lip of the adja- cent fitting for additional strength. As a result, flanged fittings are generally recommended for 4-inch pipe and above.
Screwed and flanged pipe fittings are used to make temporary joints. If soldering, brazing, or welding is used in joining the separate lengths of pipe, the joint is considered a permanent one. The advantage of a so-called temporary joint is that it can be easily dis- assembled for repair.
The great multiplicity of pipe fittings can be divided on the basis of their functions into the following six general classes:
• Extension or joining fittings
• Reducing or enlarging fittings
• Directional fittings
• Branching fittings
• Union or makeup fittings
• Shutoff or closing fittings