The supply and drainage subsystems are two distinct operations, with no overlapping between them. There are bridges between the two, however, and the bridges are what make the plumbing system worth having. In plumbing jargon, any bridge between the supply and drainage systems is a fixture.
Toilets, sinks, and tubs are fixtures. In addition, an outside faucet is a fixture and so is a washing machine. All devices that draw freshwater and discharge wastewater are fixtures, and all are designed to keep the supply and drainage systems strictly segregated.
Some fixtures have individual supply shutoff valves so you don't need to close the main shutoff to repair them. It's a good idea to make sure everyone in the family knows the location of the main shutoff valve in your house as well as how to use it. You may want to tag the main shutoff valve so anyone can easily find it.
Before you embark on any plumbing repairs, always turn off the water supply to the fixture or the main shutoff. In addition, check with your local plumbing code official before you add or change any pipe in your house. You will learn what is allowed and what is prohibited and whether or not a homeowner is allowed to do his or her own work. If you get the green light, you can save yourself a lot of money by doing your own repairs.
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