Plumbing Definitions, Fixtures, and Fixture Fittings
Plumbing Definitions, Fixtures, and Fixture Fittings. This domain tests precise vocabulary and the rules for fixtures, fixture fittings, and their installation. Expect definitions of fixture, fixture fitting, fixture unit, accessible, readily accessible, trap, and backflow, plus requirements for clearance, support, materials, and replacement parts.
Core rules
- A fixture is a receptacle or device that receives water or waste and discharges it to the drainage system. A fixture fitting is a device such as a faucet, valve, or flushometer that controls water flow to a fixture.
- Drainage fixture units (DFU) rate fixtures for drainage load; water supply fixture units (WSFU) rate them for supply demand. They are different scales and are not added together.
- Fixtures must be accessible for cleaning and repair, supported, connected through an approved trap, and installed with approved materials and backflow protection.
- Replacement parts must be approved for the fixture and must not create a cross-connection.
Worked example
A plan lists a lavatory, water closet, and drinking fountain. For drainage sizing, count each fixture's DFU from the code table and add them as drainage load. For water supply sizing, count WSFU and apply the supply demand method. The reasoning is that drainage and supply use separate load scales and separate sizing procedures. A water heater is an appliance, not a fixture, so it is not counted as a DFU.
Common traps
Do not call every plumbing product a fixture. Do not treat a faucet as a fixture; it is a fixture fitting. Do not use DFU values for water-pipe sizing. Accessible does not always mean readily accessible; readily accessible allows direct service without removing permanent construction.