A return bend is a pipe fitting which allows a pipe to make a 180 degree change in direction. Return bends are U-shaped, and could be considered two L-bends joined together. This type of fitting can be molded, cast, or made by heating and bending existing pipe. Most stores which stock plumbing supplies and equipment carry return bends in a variety of materials and sizes and it is usually possible to order custom fittings for special circumstances. Plumbers can also make their own U-bends if they have the equipment necessary for working pipes.
The return bend fitting is usually threaded so that it can connect with other pipes. When two pipes run parallel to each other and a connection needs to be made between them to allow fluid moving down one pipe to be returned up the other, a return bend is the fitting most commonly used. There are a range of settings in which such fittings can be useful. For example, in a solar hot water heater, a grid of parallel pipes linked to each other with U-bends can be used to maximize exposure to the sun so that water heats as it moves through the array of pipes.
The materials used to make a return bend can be important. Some options include various plastics, copper, and cast iron. Different materials have strengths and weaknesses which must be weighed when installing pipe fittings. Copper, for example, is often prized for being quieter for household plumbing. Plastics, however, can be easier to work and install. Other considerations can include the climate in the area where the plumbing is being installed, and what is being moved through the pipes.
Bends can become problem areas in plumbing, as materials other than fluids when end up in the plumbing can clog in the bends and cause problems. Like other types of bent pipe fittings, a return bend is designed carefully to reduce the risks that solids in the pipes will become stuck in it. Plumbers also usually design systems to make it difficult for solids to enter at all, and include traps and filters which can be used to catch solids before they get deep into the plumbing and become difficult to remove.
When selecting a return bend for a plumbing project, it is important to select a fitting which is of the appropriate size for the project, and which uses the right material. People who are not experienced at plumbing tasks may want to solicit advice from a staff member of the hardware store, or consider consulting a plumber to find out which fitting would be most suitable for a given project.