Central Heating Glossary

Guide to Terminology & Acronyms

When it comes to central heating systems there are a lot of different terms, phrases and acronyms that are used. Find out what each of them mean in this glossary.



  • A Valve

    a manual gas, water, or air shut-off valve.

  • ABS

    Acrylonitrile Butadiene Styrene - a stiff, plastic pipe and fittings used for drain, waste, and vent lines.

  • AC

    Air Conditioner - an appliance or system that removes heat and humidity from a room or entire building. Air conditioners can be installed in your window or wall and deliver cool air to a room or area of your home without ductwork. Central air conditioning delivers cool air to all rooms in your home from one large, central unit via fans and air ducts.

  • ACH

    Air Changes per Hour - number of times during an hour-long period of time that air is supplied to or removed from a room through both mechanical ventilation and natural ventilation.

  • Aerator

    a device screwed into a tap spout, which mixes air into running water to control flow and reduce splashing.

  • Air Handler

    the indoor component of a heating and cooling system that circulates air throughout a building.

  • Airlock

    a blockage in liquid flow, often a pump or pipe which is caused by an air bubble.

  • Air-to-air heat pump

    another name for an air source heat pump.

  • Air Volume Control

    maintains air charge in a standard water tank.

  • Alkalinity

    the measure of elements in water, which determine its alkaline conditions.

  • Angle Stop

    a shut-off valve between water pipes and a tap, usually used in an emergency repair.

  • Angled Valves

    Angled radiator valves for controlling the flow of liquid or air. They sit as at angle.

  • Anode Rod

    a rod within a heater, which is composed of one or more metals and protects the tank from corrosion.

  • ASHP

    Air Source Heat Pump extracts heat from the air and transfers it in order to raise or lower the temperature of a space.


  • Back Pressure

    pressure that resists the flow of fluid in a piping system.

  • Back Vent

    plumbing fixture’s separate vent in a building drainage system.

  • Back Wash

    the process of cleaning a filter by reversing the flow of water through it – the dirt is sent to the waste disposal.

  • Ball Check Valve

    a valve that uses a ball to seal against a seat to stop flow in one direction.

  • Ball Joint

    spherical assembly allowing pivot rotation for many bathroom and shower accessories.

  • Ballcock

    the fill valve that controls the flow of water from the water supply line into a gravity-operated toilet tank.

  • Barrier-free

    relating to handicap access – e.g., a shower enclosure with minimal bottom track and opening to ease wheelchair access.

  • Base

    a substance that releases hydroxyl ion when dissolved in water.

  • Boiler

    a central heating boiler is used to increase the water temperature in your central heating system. Newer boilers have improvements for energy saving. You can get different types of boilers from combi boilers, condensing boilers to system boilers.

  • Boiler Flue

    is used as a means of escape to the outside for carbon dioxide and other gasses in a central heating system.

  • BTU

    British Thermal Units used to measure the output of heat from a radiator. It is defined as the amount of energy or heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit.

  • Butterfly valve

    a serviceable valve that allows water to flow through it when open and stops the flow when closed.


  • Carbon footprint

    total amount of greenhouse gas emissions created by an individual, organization, event, service, or product – such as a heating system – expressed by carbon dioxide equivalent.

  • Central Heating

    A system for warming a building by heating water or air in one place and circulating it through pipes and radiators or vents.

  • Check Stop

    shut-off valve used with tub/shower tap that is activated by turning a slotted stem with a screwdriver – also known as a screwdriver stop or a service stop.

  • Check Valve

    a type of back flow preventer installed in a pipe that allows water to flow in one direction

  • Cistern

    rainwater storage tank, often underground, also a toilet storage vessel.

  • Closed Loop

    a variety of geothermal heat pump that passes a mixture of water and antifreeze through a closed loop of pipe buried in the ground to collect heat.

  • Closed System

    a central heating system which is closed to the external water supply. It will have a device that will not allow water to expand when heated. Most modern systems are designed this way as it helps prevent oxidisation from fresh water. Most closed systems will have rust inhibitor and boiler noise silencer added to it.

  • Column Radiator

    a more traditionally styled radiator. Popular for its older style and versatility in being sprayed a custom colour.

  • Compact Radiator

    uses fins to effectively distribute hot water throughout the radiator allowing it a larger heat output on a smaller surface area.

  • Combi-Boiler

    Also known as a combination boiler, a combi boiler is a compact, self-contained unit which combines both a central heating water heat exchange and a domestic water heater in one. Unlike a vented heating and hot water system, a combi does not store hot water. Instead, it heats water as and when required either directly from the cold mains (for the domestic hot water) or within a pressurised 'sealed' central heating loop.

  • Compressor

    also referred to as a condenser or outdoor unit, is part of an ASHP or HVAC unit that is located outside. It compresses and pumps refrigerant to meet cooling needs, and contains the condenser coil.

  • Condenser Coil

    a component of air conditioning units that is involved in the basic refrigeration cycle that adds or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump.

  • Convector

    the movement caused within a fluid. Hotter liquids and therefore less dense will cause materials to rise, and colder, denser materials to sink under the influence of gravity, which consequently results in transfer heat via convection.


  • Dam

    the barrier built into a trapway of a toilet that controls the water level in the bowl.

  • Damper

    a device that opens and closes to regulate how much air flows through vents, or ducts in a ducted heat pump system.

  • Dehumidifier

    Equipment that reduces the level of humidity from the air. It works by cooling air to the point where water turns to liquid from vapor form, which is then removed.

  • Deionisation

    process of reducing water to a non-mineral state by passing it over a bed of resins.

  • Diffuser

    placed over HVAC/ASHP ductwork. The diffuser separates air and distributes evenly in different directions.

  • Dip Tube

    tube inside water heater that sends cold water to the bottom of the tank.

  • Direct Vent

    a system that pulls outside air for combustion and vents combustion gases directly outside without a chimney.

  • Directional Tee

    tee fitting that allows direct flow in one direction by use of an internal baffle.

  • Dishwasher Tailpiece

    a flanged adaptor connecting a basket strainer to the drainpipe with a dishwasher inlet.

  • Drain Valve

    device that allows drainage of stored contents from a water heater.

  • Ductwork

    consists of specialized pipes or channels that direct airflow (including supply air, return air and exhaust air) within a home or building.

  • Duovalve

    a twin valve (hot and cold) with a single on/off lever used for washing machines.

  • DWV

    Drainage, Waste and Vent systems, which remove wastewater.

  • Dynamic pressure

    the pressure when the water is flowing.


  • EF

    Energy Factor – a measure of the overall efficiency rating of the water heater based on the model’s recovery, efficiency, stand-by loss, and energy input.

  • Elbow

    sometimes called an ell – a fitting used to change the direction of rigid pipe or tubing.

  • Elevation

    the vertical distance between the level where fluid enters a pipe and the level where it leaves, which must be added to the total discharge head if the inlet is lower than the outlet and subtracted if the inlet is higher.

  • ErP

    This stands for: Energy-related Products. This is a set of regulations that cover boilers, combination boilers, and water heaters

  • Evaporator Coil

    part of the refrigeration cycle that absorbs or adds heat to the system.

  • Expansion Tank

    a safety device that is a tank designed to absorb excess pressure due to thermal expansion. Also known as an expansion vessel.

  • Expansion Valve

    thermostatically controlled valve that controls liquid refrigerant.


  • Fall/flow

    the proper slope or pitch of a pipe for adequate drainage.

  • Feed Pump

    a pump that supplies water to a boiler.

  • Ferrous

    material containing or derived from iron.

  • Fins

    metal components that are fitted to the back of a standard panel radiator to help radiate heat.

  • Flange

    a rib or rim for strength, which is present for guiding or for attachment to another object.

  • Flat Panel Radiator

    a more modern and contemporarily designed radiator. They are efficient and versatile. They use panels to heat a room.

  • Flux

    paste applied to copper pipes and fittings before soldering to help the fusion process and prevent oxidation.

  • Fresh Air Intake

    an opening through which outside air is drawn into your home to replace any air being removed by the ventilation and circulation systems.


  • Galvanise

    the process of applying a coating of zinc to the finished product to provide corrosion protection – the coating can be applied by hot dipping or electrolytic deposition.

  • Gas Cock

    plug valve installed in the main gas line.

  • Gasket

    flat device, usually made of rubber, which is used to provide a watertight seal between metal joints.

  • Gauge

    refers to thickness of stainless steel and is commonly used in reference to quality grades on certain types of lavatories and sinks – for instance, 10 and 20-guage stainless steel sinks have gone through several polishing operations to establish a pristine finish.

  • Geothermal Heat Pumps

    also referred to as ground source, water source, or earth-coupled heat pumps, are a type of heat pump that collects thermal energy from the ground or a water source and transfers that energy inside a building to heat an indoor space.

  • GPF

    Gallons Per Flush – the rate of water flow by which toilets and flush valves are measured and regulated.

  • Greywater

    wastewater filled with contaminants from sinks, showers, and bathtubs, but not from toilets.

  • Guide Arm

    the valve that controls water flow into the toilet tank.


  • Hard Water

    natural water containing impurities in various proportions – traditional hardness is a measure of calcium or dissolved solids in a solution, measured in parts per million.

  • Heat Exchanger

    a heat transfer system.

  • Heat Output

    the amount of thermal energy a heat pump releases into to warm a space. It is measured in BTUs.

  • Heat Transfer

    occurs when heat is moved from one area to another, heating or cooling an indoor area.

  • Heat Trap

    restriction of heat loss through water connections to a tank.

  • Heat Pump

    this is a heat source and cooling device that can be used as both central heating and air conditioning. It uses the outside air to both provide heat and / or cold air when required. These devices can be good for energy efficiency.

  • Heating Coil

    a part of the HVAC system that conducts heat within the appliance.

  • Hot Water Storage Tank

    this is a tank in some central heating systems that contains hot water that is used for taps and showers in the home. May also be known as a hot water cylinder.

  • HVAC

    Heating, Ventilation and Air Conditioning systems. More modern HVAC systems will also include air cleaning and moisture control features.

  • Hydraulic Pressure

    pressure that exists in a system containing non-compressible liquid.

  • Hydrolysis

    the reaction of a salt with water to form an acid and a base.

  • Hydronic

    system of forced hot water.

  • Hydrojet

    cold inlet tube designed to reduce sediment build-up and increase efficiency in a water heater.

  • Hydroxyl

    the chemical group of ions, which is neutral or positively charged.


  • I.D.

    Inside Diameter – all pipes are sized according to their inside diameter.

  • Impeller

    rotating wheel with vanes found inside a centrifugal pump as it spins at high speed it draws fluids in and thrusts them under pressure to the discharge outlet.

  • Influent

    the stream entering a unit, stream, or process, such as the hard water entering an ion exchange water softener.

  • Instantaneous Water Heater

    a type of water heater, which heats supply as it passes through with no storage tank necessary – also known as a tankless heater.

  • Ion

    an atom or group of atoms that function as a unit and have a positive or negative electrical charge due to the gain or loss of one or more electrons.

  • Ion exchange

    a reversible process in which ions are released from an insoluble permanent material in exchange for other ions in a surrounding solution; the direction of the exchange depends upon the affinities of the ion exchange for the ions present and the concentrations of the ions in the solution.

  • Ionisation

    the process in which atoms gain or lose electrons – sometimes used as synonymous with dissociation; the separation of molecules in to charged ions in solutions.

  • IPS

    Iron Pipe Size – standard pipe threads (the female IPS refers to internal pipe threads and the male IPS refers to external pipe threads).

  • Interceptor

    device for separating grease and oil from drainage systems.


  • LDO

    Lint, Dust and Oil.

  • LP

    Liquid Propane – a fuel for gas water heaters.

  • Lateral Sewage Line

    a sewage line that connects one sewage pipe with another.

  • Lav Tube

    water supply tube for a lavatory.

  • Leakage

    loss of fluid, usually due to mechanical breakdown or gasket wear.

  • Leader Pipe

    a pipe that carries rainwater to the ground or sewer.

  • Light Duty

    small commercial applications.

  • Lime

    the substance that settles on the bottom of a water tank – also known as sediment.

  • Line-set

    a pair of copper tubes that connect a condenser to an evaporator so refrigerant can move between the two.

  • Load Calculation

    determines the size of heat pump necessary to provide proper temperature control in a desired area. Load calculations analyse factors including air volume of the desired area and the level of insulation in your home.

  • Lock Nut

    a nut fitted into a piece of pipe and screwed onto another pipe to join the two pieces.

  • Lock Shield

    a radiator valve which is used to balance the system by restricting the flow of water.

  • Low Consumption Toilet

    a class of toilet designed to flush using 1.6 gallons of water or less – also known as a water-saving toilet.


  • M Tubing

    an industry standard for copper tubing defined by the tube wall thickness and identified by a red stripe.

  • MHT

    Male Hose Threads – the threads on the outside of a connection.

  • Mini Split

    A mini split is a ductless heat pump system in which an outdoor compressor unit is combined with multiple air handlers inside, usually one in every high-use room. It allows you to control the thermostat/ temperature in different areas of the home.

  • MIPC

    Male Iron Pipe Connection – standard external threads on pipe and fittings.

  • Macerator

    a device installed in a drain line between a toilet and the soil stack to reduce solids to liquid form.

  • Magnesium

    an element in the Earth’s crust that makes water hard when dissolved, causing scale and insoluble soap curds.

  • Main

    the primary artery of water supply in a drain system.

  • Male Fitting

    fitting that is inserted into another fitting.

  • Malleable Fittings

    fittings made of cast iron, a metal which is soft and pliable.

  • Mixing Valve

    mixes hot and cold water to achieve a specified delivery temperature.

  • Multi-port Valve

    rotary type backwash valve, which can replace up to six regular gate valves.

  • Multi-stage Pump

    a pump that has more than one impeller.


  • NOM

    nominal – usually refers to the inside diameter of trade sizes of copper pipe.

  • Natural Gas

    a colourless, odourless fuel (primarily consisting of methane) derived from the Earth – odours are added to aid in leak detection.

  • Negative Pressure

    less than atmospheric pressure.

  • Nipple

    a short length of pipe installed between couplings or other fittings.

  • Nipple Extractor

    a tool used to unscrew sections of piping.

  • Non-Ferrous

    does not contain iron.

  • Non-Potable

    unsuitable for drinking.


  • O-Ring

    round rubber washer used to create a watertight seal.

  • OD

    Outside Diameter – the diameter of a pipe measured from the outside edge.

  • Oil Powered

    a water heater that uses oil for its fuel source.

  • Open Loop

    a geothermal heat pump that collects thermal energy from groundwater or water from another nearby source. It takes water from its source, moves it through a loop of pipe to collect the thermal energy, and transfers it back into the source.

  • Operating Cost

    the cost of running a water heater for a given period of time.

  • Outlet

    the opening through which water exits the pump.

  • Overflow Tube

    the vertical tube inside a toilet tank that directs water into the bowl in case the ballcock malfunctions.


  • Packaged Unit

    an air-handling unit, defined as either recirculating or once-through design, created for outdoor installation. They most often include, internally, their own heating and cooling devices.

  • PB

    Polybutylene – a flexible type of tubing used in water supply systems.

  • PP

    Polypropylene Pipe.

  • PPM

    Parts Per Million – a unit of measure for impurities in water

  • PR

    Pressure Regulator – usually required if water pressure exceeds 80 PSI (pounds per square inch).

  • Packing nut

    the nut that holds the stem of a tap in position.

  • Pilot

    a small burner used to ignite the main burner.

  • Plumb

    precisely vertical.

  • Poppet

    valve that rises perpendicularly to or from its seat.

  • Pressure

    the pressure or flow rate of water tunning through a system. Important to maintain the correct level of pressure required for your system to keep pipes clear of debris and running efficiently. Mains water pressure is also a phrase that is used to describe how much pressure is delivered from the water company to the home through its pipework.

  • Pressure Head

    pressure in a plumbing system – the unit of measure which is the vertical force exerted by water at a depth of one foot.

  • Primary Air

    air which is mixed with gas before the gas leaves a burner port to burn – the ideal burning condition is generally 10 cubic feet of air per one cubic foot of gas.

  • Pumping Level

    the lowest water level reached during pumping operation.


  • Quick Closing Valve

    a valve or tap that closes automatically when released manually or that is controlled by mechanical means for fast action closing.


  • Radiator

    a device which is used to heat a room. Liquid is pushed through exposed pipes and heated to warm a room. Available in many different styles, sizes and outputs.

  • Raw Water

    water supplied to the plant before any treatment.

  • Red Water

    water that is red or brown in colour due to the presence of precipitated iron and/or iron bacteria.

  • Reducer

    a fitting that connects pipes of different sizes together.

  • Refrigeration Cycle

    transfers thermal energy from a colder space to a warmer space. This cycle is a reverse thermodynamic cycle that performs the opposite of the type of energy transfer that happens naturally without intervention from a system such as a heat pump or HVAC unit.

  • Refill Tube

    the tube that directs water from a ballcock into the overflow tube to refill bowl after a siphon break.

  • Relief Valve

    a valve that opens to relieve excess temperature.

  • Rim Holes

    a series of small holes in the underside of a toilet rim through which water flows to wash over the entire inner surface of the bowl.

  • Rust Inhibitor

    a solution added to central heating systems to minimise the risk of rusting of metal components.


  • Safety Shutoff Valve

    a device on a gas appliance that shuts off the gas supply to prevent a hazardous situation.

  • Scale

    a thin layer of calcium on the interior of a tank that may prevent heat transfer.

  • Sediment

    the substance that settles on the bottom of a tank, also known as lime

  • SEER

    Seasonal Energy Efficiency Ratio measures the energy efficiency of your air conditioning system. It measures the total cooling of your air conditioner or heat pump in BTUs compared with the energy output (in watt hours) used within the same period. The higher the SEER rating on your air conditioner, the more energy efficient it is, meaning lower energy costs to run it.

  • Siphon

    a pipe connecting two canals, through which water flows over a high point of gravity.

  • Sleeve

    pipe that is passed through a wall for the purpose of inserting another pipe through it.

  • Slip Joint

    a connection made with compression fittings.

  • Soft Water

    water that has a low calcium and magnesium content.

  • Straight Cross

    fitting that connects four pipes of the same diameter.

  • Straight Valves

    used on radiators with underside connections where the pips are coming up from the floor.

  • Supply Stop

    the valve that controls water supply to a toilet.

  • Sweep

    a drain ell fitting with a long radius that allows for smooth passage of waste.


  • Tap

    an exposed plumbing fitting used to control the flow of water to a basin.

  • TBE

    Threaded Both Ends.

  • T&C

    Threaded and Coupled.

  • Tails/Sleeving Kits

    copper pipes that are connected to the radiator.

  • Tap Adapter

    fitting with one plain end and one threaded female end.

  • Tappings

    the thread drilled into the designer radiator where the element or valves can easily fit into.

  • Test Plug

    a rubber plug used to seal off sections of pipe to test for leaks.

  • Thermostat

    a device that is used to control the amount of heat or temperature of a central heating system. TRV valves (thermostatic radiator valves) are devices that are used to control the room temperature in an individual room by controlling an individual radiator. Smart thermostats can also be installed and used as room thermostats in modern homes.

  • Thermostatic

    heat that is regulated to reach and maintain a desired temperature, usually using valves.

  • Towel Radiators

    also referred to as towel warmers or towel rails. Often used in bathrooms as a specific place to warm towels and the room.

  • Trap

    curved section of drain line that prevents sewer odours from escaping into the atmosphere – all features that have drains must have a P-trap installed, whilst toilets will have S-traps.

  • Trap Arm

    the waste arm portion of a drainage trap.

  • Trap Dip

    the U-bend portion of a drainage trap.

  • Trap Drain

    a flex adapter to connect drainpipes.

  • Trap Seal

    height of water in a toilet bowl at rest, providing a water seal to prevent sewer gases entering the home.

  • TRV

    Thermostatic Radiator Valve – can automatically open and close necessary to achieve the pre-set room air temperature.

  • Type M tube

    an industry standard for copper tubing defined by the tube wall thickness and identified by a red stripe.


  • U-bend

    a section of a waste pipe that is shaped like the letter U, which stops smells coming back into the property.

  • ULF

    Ultra Low Flush – a widely used description of toilets that use 1.6 gallons per flush or less.

  • Urinal Strainers

    domed metal piece fastened to the bottom of a urinal to prevent debris from blocking the trapway of the unit.

  • Usable Storage

    percentage of hot water that can be drawn from a tank before the temperature drops to a point that it is no longer considered hot.


  • Valve

    a device that regulates the flow of fluid.

  • Valve Seat

    the non-moving part of a valve – water flow is stopped when the movable element meets the valve seat.

  • VAV System

    A Variable Air Volume System is an HVAC system that provides a stable supply-air and temperature. It varies the airflow rate to meet the temperature requirements. They conserve energy by operating at lower fan speeds during times of lower temperature control demand.

  • Vent

    a pipe that allows air into a drain system to balance air pressure and to prevent water in the traps from being siphoned off.

  • Vent Header

    a vent pipe into which several vents connect – this leads to the vent stack and out of the building.

  • Vent Stack

    upper portion of the soil stack above the top fixture, through which gases and odours escape.

  • Volatile

    capable of vaporisation at a relatively low temperature.

  • Volumetric

    refers to measurement by volume rather than weight.

  • Voral Tube

    flexible steel hose used for handheld shower sprays.


  • Warranty

    can cover any issues with radiators, an type of legal consumer promise to repair or replace products if broken or faulty within a set time frame.

  • Waste Arm Drain

    an extension pipe usually used to extend a sink drain into a wall.

  • Waste and Overflow Drain

    an assembly for a bathtub – the outlet at the top removes overflow water during tub filling and the drain at the bottom removes wastewater when the tub is drained.

  • Waste Plug

    drain stopper on a bathtub drain.

  • Waste Shoe

    bathtub drain assembly.

  • Waste Stack

    main vertical pipe in a drainage system, extending one or more stories.

  • Water Service Pipe

    pipe from the water main.

  • Water Storage Tank

    these are cold water storage tanks that are often found in the lofts of houses. They hold cold water that is used to feed a central heating system.

  • Watt

    a unit of power used to quantify the rate of energy transfer. Electricity is measured in Watts. It is equal to one joule of work per second.

  • Wet vent

    drain or waste pipe that also acts for one or more fixtures on the same line.

  • Working pressure

    maximum pressure of the operating system permissible.