PEX water pipe is an extremely flexible polyethylene pipe that is installed using the "trunk and branch" system if you're not using a manifold hub -- a central supply stem that feeds multiple individual hot and cold pipes to plumbing fixtures throughout the house and requires much more piping. The trunk and branch system is the traditional plumbing method; it uses larger supply pipes to deliver water through the building to smaller branch pipes, which in turn deliver the water to individual plumbing fixtures. Installing a PEX trunk and branch system requires knowledge of some basic features of this method.
Main Trunk Lines
Main trunk lines are the primary water supply pipes that typically run the length of the building. In a smaller residential setting, these pipes are usually ¾ inch in diameter. In larger houses with several bathrooms and multiple stories, many plumbers install 1-inch main lines. The greater the usage demand and the farther the water needs to flow, the larger the main trunks need to be. PEX pipe needs to be supported by attaching pipe hangers to the floor joists or studs at 16-inch intervals, because PEX will sag easier than metal pipe.
Branch supply pipes use tee fittings to connect an individual plumbing fixture such as a sink, bathtub or toilet to the main water lines. These branch lines are usually ½ inch in diameter but could be 3/8 inch if running to a low-water-demand fixture such as a lavatory sink or even a toilet. The branch lines connect to the fixtures using PEX fittings.
Pipe connections are made with specially made PEX fittings and crimp rings. PEX fittings are available in plastic, copper or brass. Plastic fittings are cheaper, but brass or copper fittings can be more durable. Crimp rings compress over the pipe and fitting to make a watertight seal. The two primary styles of crimp rings are stainless steel and copper. Each style of crimp rings requires the appropriate installation tool to crimp the bands into place.
PEX Types and Building Codes
PEX is available in rigid or flexible pipe. The rigid type needs 90-degree elbows to go around sharp corners but can be easily bent to turn gentle broad offsets, thus minimizing the need for fittings. Flexible or coiled PEX is designed to be installed with few or no elbows. Both rigid and coiled are interchangeable and will connect to each other. Check with local building codes to verify whether PEX is approved for your area.
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