Domestic water system

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Domestic water supply or system (DWS) is a comprehensive term for the potable water supply systems in residential, commercial, institutional, and industrial buildings. Potable water is drinking water, but is used in more quantities for operating plumbing fixtures that are not intended for drinking or cooking. This article addresses the supply side of plumbing systems, where traps, drains, and vents, rainwater, surface, and subsurface water drainage, fire sprinklers, and other topics are addressed in related articles.

Potable water supply

This supply may come from several possible sources.

Domestic water systems have been evolved since the first thinking man located his home near a running water supply, e.g. a stream or river. The water flow also allowed sending waste water away from his domicile.

Modern indoor plumbing delivers clean, safe, potable water to each service point in the distribution system. It is imperative that the clean water not be contaminated by the waste water (disposal) side of the process system. Historically, this contamination of drinking water has been the largest killer of humans.[1]

Cold water supply

Most modern western water systems are directly fed from a municipal water system by a high-pressure pipe, usually located under the road or street. A water meter is installed to allow the supplier to charge appropriately for the water usage. Many houses in rural areas still use a cistern or a well when a convenient water supply is not available; a pump and pressure tanks are used to create and maintain system pressure needed for operating the plumbing fixtures.

Any external water supply is almost always a 'cold' unheated or cooled water supply. The cold water supply system may include filter or water softener appliances. This cold water is then fed to plumbing fixtures that require cold water. The largest users of cold water are water closets (toilets) and outdoor hose bibbs, but cold potable water is needed at lavatories, sinks, bathtubs, showers, water fountains, humidifiers, and ice-makers too, for example. Cold water is also supplied to water heaters, if a building is so equipped.

Hot water supply

Domestic hot water is provided by means of water heater appliances, or through district heating. The hot water from these units is then piped to the various fixtures and appliances that require hot water, such as lavatories, sinks, bathtubs, showers, washing machines, and dishwashers.

Fixtures and appliances

Everything in a building that uses water falls under one of two categories; Fixture or Appliance. As the consumption points above perform their function, most produce waste/sewage components that will require removal by the waste/sewage side of the system.

Fixtures are devices that use water without an additional source of power. They include, for example:

  • Hose bibbs, colloquially known as 'taps' or 'faucets'
  • Water closets, colloquially known as toilets or loos
  • Urinals
  • Bidets
  • Lavatories, sinks, and washbasins
  • Bathtubs and showers
  • Drinking fountains (uncooled or unheated)

Appliances are devices that use water coupled with an additional source of power. connection to these appliances incorporates a backflow prevention principle of some form -- the minimum is an air gap. See cross connection control & backflow prevention for an overview of backflow prevention methods and devices currently in use, both through the use of mechanical and physical principles. Appliances include, for example:

Pipe materials

In old construction, lead plumbing was common. It was generally eclipsed toward the end of the 1800s by galvanized iron water pipes which were attached with threaded pipe fittings. Higher durability, and cost, systems were made with brass pipe and fittings. Copper with soldered fittings became popular around 1950, though it had been used as early as 1900. Plastic supply pipes have become increasingly common since about 1970, with a variety of materials and fittings employed. Plumbing codes define which materials may be used, and all materials must be proven by ASTM, UL, and/or NFPA testing.


Galvanized steel supply pipes are commonly found with interior diameters from 1/2" to 2", though most single family homes' systems won't require any supply pipes larger than 3/4". Pipes have National Pipe Thread (NPT) standard male threads, which connect with female threads on elbows, tees, couplers, valves, and other fittings. Galvanized steel (often known simply as "galv" or "iron" in the plumbing trade) is relatively expensive, difficult to work with due to weight and requirement of a pipe threader, and suffers from a tendency to obstruction due to mineral deposits forming on the inside of the pipe. It remains common for repair of existing "galv" systems and to satisfy building code non-combustibility requirements typically found in hotels, apartment buildings and other commercial applications. It is also extremely durable. Black lacquered steel pipe is the most widely used pipe material for fire sprinklers.


Tubing made of copper was introduced in about 1900, but didn't become popular until approximately 1950, depending on local building code adoption. Common wall-thicknesses of copper tubing are "Type K", "Type L" and "Type M";[2] Type "M" are relatively thin-walled and generally suitable for condensate and other drains, but generally illegal for pressure applications, Type "L" has a thicker pipe wall section, and is used in residential and commercial water supply and pressure applications, Type "K" has the thickest wall section of the three types of pressure rated tubing and is commonly used for deep underground burial such as under sidewalks and streets, with a suitable corrosion protection coating or continuous polyethylene sleeve as required by code. Types "K" and "L" are generally available in both hard drawn "sticks" and in rolls of soft annealed tubing, Type "M" is usually only available in hard drawn "sticks". Thin-walled types used to be relatively inexpensive, but since 2002 copper prices have risen considerably due to rising global demand and a stagnant supply.

In the plumbing trade the size of copper tubing is measured by its nominal diameter (average inside diameter). Some trades, heating and cooling technicians for instance, use the outside diameter (OD) to designate copper tube sizes. The OD of copper tube is always 1/8th inch larger than its nominal size. Therefore, 1" nominal copper tube and 1-1/8th" inch ACR tube are exactly the same tube with different size designations. The wall thickness of the tube, as mentioned above, never affects the sizing of the tube. Type K 1/2" nominal tube, is the same size as Type L 1/2" nominal tube (5/8" ACR).

Copper Tubing Sizes (CTS) for Plumbing
Nominal Size OD in inches ID in inches
Type K Type L Type M
3/8 1/2 0.402 0.430 0.450
1/2 5/8 0.528 0.545 0.569
5/8 3/4 0.652 0.668 0.690
3/4 7/8 0.745 0.785 0.811
1 1-1/8 0.995 1.025 1.055
1-¼ 1-3/8 1.245 1.265 1.291
1-½ 1-5/8 1.481 1.505 1.527
2 2-1/8 1.959 1.985 2.009
2-½ 2-5/8 2.435 2.465 2.495
3 3-1/8 2.907 2.945 2.981

Generally, copper tubes are soldered directly into copper or brass fittings, although compression, crimp, or flare fittings are also used. Formerly, concerns with copper supply tubes included the lead used in the solder at joints (50% tin and 50% lead). Some studies have shown significant "leaching" of the lead into the potable water stream, particularly after long periods of low usage, followed by peak demand periods. In hard water applications, shortly after installation, the interior of the pipes will be coated with the deposited minerals, which had been dissolved in the water and therefore the vast majority of exposed lead would be prevented from entering the potable water. Building codes now require lead-free solder. Building Codes throughout the U.S. require the use of virtually "lead-free" (<.2% lead) solder or filler metals in plumbing fittings and appliances as well.


Plastic pipe is in wide use for domestic water supply and drainage, waste, and vent (DWV) pipe. For example, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polypropylene (PP), polybutlyene (PB), and polyethylene (PE) may be allowed by code for certain uses. Some examples of plastics in water supply systems are:

  • PVC/CPVC - rigid plastic pipes similar to PVC drain pipes but with thicker walls to deal with municipal water pressure, introduced around 1970. PVC should be used for cold water only, or venting. CPVC can be used for hot and cold potable water supply. Connections are made with primers and solvent cements as required by code.
  • PBT - flexible (usually gray or blue) plastic pipe which is attached to barbed fittings and secured in place with a copper crimp ring. The primary manufacturer of PBT tubing and fittings was driven into bankruptcy by a class-action lawsuit over failures of this system. However, PB and PBT tubing has returned to the market and codes, typically first for 'exposed locations' such as risers.
  • PEX - cross linked polyethylene system with mechanically joined fittings employing barbs and crimped steel or copper fittings.
  • Polytanks - plastic polyethylene cisterns, underground water tanks, above ground water tanks, are made of linear polyethylene suitable as a potable water storage tank, provided in white, black or green, approved by NSF and made of FDA approved materials.
  • Aqua - known as PEX-Al-PEX, for its PEX/aluminum sandwich - aluminum pipe sandwiched between layers of PEX and connected with brass compression fittings. In 2005, a large number of their fittings were recalled.

Fittings and valves

Potable water supply systems require not only pipe, but also many fittings and valves which add considerably to their functionality as well as cost. The Piping and plumbing fittings and Valves articles discuss them further.

Regulation and compliance

Before a water supply system is constructed or modified, the designer and contractor need to consult the local plumbing code and obtain a building permits prior to construction.[3][4] Even replacing an existing water heater may require a permit and inspection of the work. National and local fire codes should be integrated in the design phase of the water system too to prevent "failure comply with regulations" notices. Some areas of the United States require on-site water reserves of potable and fire water by law.

Waste water

The waste water from the various appliances, fixtures, and taps is transferred to the waste and sewage removal system via the sewage drain system. This system consists of larger diameter piping, water traps, and is well vented to prevent toxic gases from entering the living space. The plumbing drains and vents article discusses the topic further, and introduces sewage treatment.

See also


  1. Plumbing: the Arteries of Civilization, Modern Marvels video series, The History Channel, AAE-42223, A&E Television, 1996
  2. Copper Tube Handbook, the Copper Development Association, New York, USA, 2006
  3. Uniform Plumbing Code, IAPMO
  4. International Plumbing Code, ICC
  • ASTM B75-02 Specification for Seamless Copper Tube
  • ASTM B42-02e1 Standard Specification for Seamless Copper Pipe, Standard Sizes
  • ASTM B88-03 Standard Specification for Seamless Copper Water Tube

External links