How Residential Water Supply Works
The water supply for any home comes from one of three places. Most homes receive their water from a water utility, the rest get their water from wells or storage tanks. The municipal supply provides the water pressure, but other homes may rely on gravity or pumps to produce water pressure. After that, the residential water supply system is about the same in all homes.
Most homes in the U.S. receive municipal water and so the first part of the system is the water main, typically located at the edge of the property. The water main provides a main valve that allows the shut off of the entire water supply for a home. It is important to know the location of this valve and how to operate it in the event of a burst pipe or other plumbing emergency that requires you to stop the flow of water. The water main typically also includes a water meter, with which the utility company can monitor your usage and bill you accordingly.
After the meter, a water line of 3/4" or larger pipe goes toward the home. The pipe is buried and in areas that have freezing temperatures, the depth to which it is buried is set by the local depth of the frost line. Supply lines to the house can be plastic, copper or galvanized iron pipe. Copper is the preferred material and also the most expensive.
Before the water supply line enters the home, or just after in colder climates, the supply will often have a back-flow preventer installed, to prevent low pressure from allowing contaminated water being drawn into the main water supply. Also at this point, the cold water supply may have a second main shut off valve. If your home has a fire sprinkler system, the water supply may bypass the water meter, may have its own supply line or it may split out before or after the back-flow device. Another device you may find here is a pressure reducer to reduce the water pressure down to safe levels, generally around 60 psi for residential systems. Plumbing fixtures can be damaged if subjected to the high pressure of unregulated water supplies. After all these devices, the water line may split off to a hose bib or your yard's irrigation system.
Once inside the home, the water line typically goes directly to the home's water heater, to ensure adequate water pressure to the hot water supply. The cold water line splits here, between the water heater and for the rest of the house. The cold water line that enters the water heater then becomes the beginning of the hot water supply system. The other line or lines that split off will go to locations throughout the house.
Cold water supply lines will often reduce to smaller sized pipes as the supply lines split off. It is best to run the larger size pipes to the area of the home where they will be used and then only switch to smaller diameters for the risers that lead to each service.
Cold water service lines will run to every toilet, sink, tub, shower and water using appliance such as a washer, dishwasher, ice maker, instant hot water or any other plumbed device. It is best practice to install local shut off valves at each of these locations to allow service and repair without having to shut off water to the entire home while work is completed.