Digital shower unit  - A sophisticated thermostatic hot and cold water supply unit for a shower. The unit is fully electronically controlled to give accurate water temperature and flow of the blended hot and cold water.

Diverter  - A device used to control the direction of water to various outlets.

Double Check Valve - A device designed to prevent back syphonage. Consists of two Check Valves in line with each other (in series).

Dual control - In a shower control two entirely separate and independent control knobs are provided to adjust flow and temperature. Unlike concentric controls. 

Dual flow taps also see mixer taps

Dual flow taps are used where the hot and cold water must remain separate for hygiene or health reasons or where the hot and cold water pressure are unequal.

Dual Flush - A full flush of 6 litres maximum or short flush of 4 litres maximum. Some eco-friendly dual flush WCs can flush with less water than this and still conform with the regulations.

Dynamic Pressure - Water pressure in a pipe or fitting under flow conditions. Sometimes known as maintained pressure. Not Static pressure.


Electric instantaneous shower - This type of shower unit has a built-in water heater, rather like the element in a kettle. Incoming cold mains water is heated instantly, and the appliance only heats the water as it is used. It is independent of the domestic hot water system and is therefore is very efficient. Some models have a built in thermostatic mixer. Electric showers have a kW rating - the higher the rating then the better the flow rate will be. 10.8 kW is the maximum. An electric shower requires its own electric supply cables. Electric showers must always be installed by a competent electrician. 

Enamel - A glass-like surface applied to steel or cast iron baths and fired in an oven to bake it hard. Forms and attractive and easily cleaned high gloss surface which is resistant to burns, scratching and most household chemicals. Also known as Porcelain Enamel.


Flat access Entry - To and from the showering area within a wet room. The showering area is level with the rest of the floor.

Float Operated Valve - A valve used to control flow of water into a cistern.

Flow rate - The measurement of the amount of water delivered through a tap valve. 

Flow - This describes the amount of water passing through a pipe. Not to be confused with pressure.

Flow Regulator ( or Limiter) - A device with moving and sometimes adjustable parts which responds to inlet pressure to control flow to a reasonably constant rate.

Flow Restrictor - A device with no moving parts used to restrict flow but unlike a regulator it doesn't’t maintain constant flow when the supply pressure varies. 

Flushing device - A mechanism inside a cistern used to flush a WC. Available in two designs:

  1. the traditional lever-operated waste water preventer called the siphon
  2. the push button flush valve

Four hole mixer - Usually only found on baths where the hot supply, cold supply, filler spout (nozzle) and shower hose are all separate and mounted in individual holes in the bath’s rim. 


Gravity fed plumbing system - see plumbing system

GRP Glass Reinforced Polyester - Resin-bonded glass fibre strands sprayed onto the underside of an acrylic bath. When the resin has cured it sets solid to give the bath improved strength and rigidity.