Plumbing: The Main Drain & Water Supply System

sewer layout

Even if you don’t plan on doing the work yourself, it is a good idea to have working knowledge of the plumbing system in a home and the strict code requirements that apply.

Most plumbing work depends on common sense, and simple repairs have probably introduced you to the concepts that are fundamental to the system as a whole.

A home plumbing system has 4 components:  the building sewer, the drain-waste-vent-system (DWV), the water supply system, and the fixtures.

Different functions, different rules, and different plumbing techniques characterize each component.

The Main Drain

The house sewer.  All the drains in the house connect to the main drain, which is outside the foundation.  The main drain connects to a septic tank or to a public sewer stub at the property line.  Most local codes specify the size of the connecting pipe.

The pipe size for a single family dwelling is usually 4 inches ID (inside diameter).  Codes also specify the type of pipe — usually vitrified clay, cast iron, plastic, or bituminized fiber.  Clay pipe must join with no hub bands or compression.

Mortar joints, which were common in older homes, no longer meet code specifications.

The house sewer must be at least 10 feet away from the water supply pipe or 12 inches below it if the two are in the same trench.  The depth of the trench depends on the climate and the location of the septic tank inlet or public sewer stub.  The sewer pipe must slope 1/4 inch per foot.

The DWV system.  Sometimes called the sanitary system, the DWV system includes all the drains and waste pipes in and under the house as well as the vents.

The DWV system is completely independent of the water supply system and contractors usually install it first.  The DWV system is not pressurized; water and waste move because of gravity.

house plumbing diagram

For this reason the DWV system requires careful installation.  “Upstream” and “downstream” are important locations; keep your position in mind.  The regulations for the DWV system are strict.

Precise local codes protect public health.  All the requirements concerning pipe size, fitting orientation, trap location, slope, and fixture height are strategies that keep contaminants — liquid, solid, or gas — out of the house.

Water Supply System

The water supply system brings cold water to the house, heats some of it, and distributes the water to various fixtures.  The supply system is pressurized so pipes can run directly and do not slope or have vents.

Normal “street pressure” is 40 to 55 psi (pounds per square inch), but may range as low as 30 psi or as high as 80 psi.  If the street pressure is above normal, install a pressure reducer near the main valve.

The main shut-off valve should be near the foundation line.  Supply systems in cold climates should include a bleeder valve that allows the homeowner to drain the whole system.

The size of most water pipes is 1/2 inch or 3/4 inch.  You will find a larger main if street pressure is lower than normal or if the house is taller than 2 stories.

Use a 3/4-inch line to feed the water heater and for all runs that feed more than one fixture.  If a water softener is included in the system, the pipes should be one size larger than normal to make up for the drop in pressure it causes.

If you install a water softener in an existing system, you may be required by the local building code to run a bypass around it to maintain pressure.

All fixtures except dishwashers and toilets have both hot- and cold-water supply pipes. (In humid areas the toilet may have both to prevent condensation).

Cold water is on the right and hot water is on the left.  The pipes stub out and terminate at valves, called stops, which are located under the fixture.

Sometimes pipes bang and chatter when you turn off a faucet; the noise is called water hammer.

To prevent it, many codes require air chambers.  Air chambers are short, capped-off pipes above the supply stub tees of a fixture.  They are usually 12 inches long and one size larger than the supply pipes.

The chambers trap air and cushion the shock of water hammer.  Dishwashers and Watch the Videowashing machines need them because they use electric valves that snap shut.  In addition, install air chambers for the kitchen sink and for the highest fixture in the bathroom.

Strapping the pipes and nailing strapping to the joists will also prevent water hammer.

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