Boiler Glossary

Here are some of the most commonly used terms, words and acronyms used by people in the boiler and general plumbing trade.

A | B | C | D | E | F | G| H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Adjustable Damper

An adjustable damper in a boiler regulates the temperature in the room by controlling heat emissions.

Air Release Point

A place to intentionally release air from the heating system.

Air Separator

This device removes any air bubbles in the system water, reducing corrosion and circulation noise.


This occurs when an air bubble gets trapped in the pipe, stopping or decreasing the flow.

Aluminium Extrusion

A metal component made of aluminium.

Automatic On-Off Switches

See Timers and Programmers.

Back boiler

A back boiler is a device which is fitted to a residential heating stove or open fireplace to enable it to provide both room heat and domestic hot water or central heating. The device is a water filled heat exchanger enclosed at the rear of the burning chamber with a hot water output at the top of the chamber and a cold water feed at the bottom.


When water flows in the wrong direction, usually as a result of siphoning, or possibly a faulty diverter valve.

Balanced Flue

With this system, the boiler can draw fresh air from outside the building, and eject air too.


Also called a balltap or fill valve. A filling mechanism for water tanks, (e.g. in flush toilets).

Banging Sounds

Caused by overheating due to any of the following: build-up of scale due to hard water, faulty boiler thermostat, blocked chimney, lack of water in the system or the pump not working.

Bleed Valve

This is an air release valve used to release air from a central heating system. A bleed valve can be found on all water radiators and sometimes at high points in the pipework.


Any obstruction in a pipe or airway.

Blocked Chimney

If you have a Solid-Fuel Boiler, a blocked chimney can prevent expulsion of waste products. Mechanical defects and debris can cause blockages.


A device providing a draught on the fire, increasing combustion.


A boiler heats water or another fluid under pressure. The fluid then leaves the boiler for use in heating applications. Only a Gas Safe Register approved expert should deal with faulty boilers.

Boiler-Energy Manager or Optimiser

This device collets data about temperature from internal and external sensors, so that the boiler can operate a maximum fuel efficiency.

Boiler Fume Thermostat

A control feature that measures temperature of gasses entering the flue after the burner.

Boiler Interlock

An arrangement of controls, ensuring that the boiler does not fire without a demand for heat. This is achieved by fitting a room thermostat.

Boiler Scrappage Scheme

The scheme allowed qualifying households’ to receive £400 off the cost of a new boiler. Up to 125,000 households could benefit from the scheme.


A slotted screw in the centre of a water circulating pump. Used to position the pump rota in place and can be used to vent the pump of air.

Capillary Connector

A joint that connects 2 pipes, with solder usually being used to provide the seal.

Carbon Monoxide Detector

A carbon monoxide detector or CO detector (also known as a carbon monoxide alarm) is a device that detects the presence of the carbon monoxide (CO) gas in order to prevent carbon monoxide poisoning. CO is a colorless and odorless compound produced by incomplete combustion. It is often referred to as the “silent killer” because it is virtually undetectable without using detection technology.
CO detectors are designed to measure CO levels over time and sound an alarm before dangerous levels of CO accumulate in an environment, giving people adequate warning to safely ventilate the area or evacuate.
If you have any gas appliances in your house, we would highly recommend you buying a carbon monoxide detector.


A film of moisture and entrained solids, caused by poor water condition within the boiler.

Ceramic Taps

A tap to control the flow of water that only needs to be to be turned a quarter of the way to go on and off.

Check Valve

A type of backflow preventer installed in a pipe run that allows water to flow in only one direction. A check valve is also known as a clack valve, non-return valve or one-way valve.

Circuit Board Electronics

Electrical circuitry, often of low voltage and non-maintainable, found in the majority of modern appliances.

Circuit Pressure

A central heating system’s operating pressure. Combi boilers and refrigeration systems have a critical circuit pressure, at which they operate most efficiently.

Circulation Pipe

This is the main water flow to and from the central heating boiler for the radiator circuit. The normally smaller diameter pipes to the radiators are known as “branches”.

Circulation Pump

A device used to pump water to the central heating system. A non-ferrous type should be used for the hot water because a standard cast steel pump would quickly rust away.


A container holding a small amount of water usually used for flushing toilets.


This is suitable only for traditional or open-fuel boilers, not Combination Boilers.


This is the heating element in a domestic hot water tank.

Cold Mains Supply

The water piped to the premises by the Utilities or Water Company.

Cold Water Storage Tanks

Used for storing cold water, or water that is to be heated using a domestic hot water system.

Combination Boiler (also know as Combi boiler)

A combi boiler is a highly efficient water heater and central heating boiler combined in a single unit.

Water is heated on demand from within the unit itself, so a combi boiler (unlike a conventional boiler, which typically requires a copper cylinder to store hot water) can be a practical solution if space within your home is at a premium. Combi boilers are typically compact; they can be hung conveniently on a kitchen wall and their installation is less disruptive and less expensive than that of conventional boilers.

The absence of a storage cylinder means that significant savings on water heating can be made. Although the speed at which hot water is produced may be slightly slower than with a conventional boiler, hot water is nevertheless delivered through your taps or shower head at mains pressure. This may mean that you can enjoy a high pressure “power” shower without the need for an additional pump.

Combi boilers are the most popular form of boiler in the United Kingdom, accounting for more than 50% of new condensing boiler sales.

Combustion Chamber

The area within the boiler where the fuel is burnt.

Combustion Gases

These are the products given off when fuel is burnt. They consist mainly of CO2 and water vapour.

Compression Connector

Similar to a Capillary Connector, but instead of solder to hold it together, nuts are used instead.

Condensing Boiler

A condensing boiler is a central heating boiler that has improved efficiency over a standard boiler. Condensing boilers achieve their extra efficiency by extracting extra energy from the hot exhaust gases produced by the boiler. A condensing boiler captures much more usable heat from its fuel than a non-condensing boiler.
Condensing boilers get their name because they enter what is called “condensing mode” periodically. In other words, they start to extract heat from the exhaust gases that would otherwise escape through the flue, in the process turning water vapour from the gas back into liquid water or condensate.
Condensing boilers are available in heat only, system and combi variations.

Conventional Boiler (also known as regular boiler)

If you are replacing an older model of boiler, the chances are that you will have a regular (conventional) boiler. A typical conventional system incorporates a boiler and extended controls, a feed and expansion cistern, and a hot water cylinder (usually in the airing cupboard) which is often fed by a cold water storage cistern located in the loft.

Conventional Open Flue

A heating appliance that uses internal air for combustion.
The advantages of regular boilers are that the hot water flow rate will tend to be higher than that with combination boilers as the hot water is stored within the hot water cylinder ready for use rather than being heated on demand. If you find that your family often have several hot water taps on at the same time, say a shower running in the bathroom and hot water running in the kitchen, then this stored hot water can be an advantage.

Copper Cylinder (Storage Cylinder)

A part of a conventional boiler and central heating system, used for storing heated water for use in the home. Typically found in airing cupboards and may be fitted with an immersion heater.


CORGI is the UK ‘s Confederation of Registered Gas Installers – Registration with CORGI is a legal requirement for a business or self-employed person working on gas fittings or appliances. CORGI oversees the safety of gas installations.
Gas Safe Register replaced the CORGI gas register in Great Britain and the Isle of Man on 1 April 2009.


When hard-water deposits or oxygenated water reacts with the system’s metal components, corrosion occurs. This can cause deterioration of the boiler components.

Corrosion Inhibitor

A corrosion inhibitor is a chemical compound that can be added in a small concentration stops or slows down corrosion in your boiler.

Delayed Start

A feature that saves energy and therefore money. It works by monitoring the ambient temperature at the time that the heating is due to come on. If the room temperature is high, the heating start time can be delayed, reducing fuel wastage and increasing energy efficiency.


A process that is used to remove limescale (calcium carbonate) from within a boiler.


A pressure comparison device. Common applications in a boiler are the diverter valve and air pressure switch.

Digital Programmer

These have an electronic display, but do the same job as the Time Clock. They have separate controls for the heating and hot water.

Double Radiators

A radiator with two heating panels.

Double-Skinned Pressed-Metal Panel

Manufactured from two metal sheets for extra strength.

“Drain the System”

A term used to describe the process of intentionally emptying the water from a central heating system.


A valved device used to drain a system.

Duct Warm-Air System (Gas)

A system where the gas is combusted to give hot air, which is then distributed through the property.


A purpose made passageway to allow the installation of water pipes, electrical cables, etc.

Electrolytic Action

If acidic water is present in a central heating system, some metals like copper and aluminium can act like a battery, resulting in corrosion.

Electronic Ignition

Most modern boilers have an electronic ignition that lights the burner automatically when the heating system controls call for heat.

Expansion Tank

A small tank used in closed water heating systems and domestic hot water systems to absorb excess water pressure, which can be caused by thermal expansion as water is heated, or by water hammer. It is also known as an expansion vessel.

Fan Assisted Convector Heater

This appliance uses an electric fan to distribute the warmed air.

Feed-and-Expansion Tank

This provides water for the heating system and the domestic hot water cylinder and acts as a reservoir for any overflow as a result of expansion.

Finned Pipe

Gives a greater heat output.

Finned Radiators

The fins increase the heat output of the radiator by increasing the area that head dissipates from.

Float Operated Valve

This is the ballcock within the toilet or tank used to control the flow of water.

Floor Standing Boiler

These boilers can be traditional or combination and are seated on the floor. Floor Standing boilers can fit in spaces between kitchen units or, in larger homes, in a pantry or a garage.


A flue is a duct, pipe, or chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. They usually operate by buoyancy, also known as the stack effect, or the combustion products may be ‘induced’ via a blower.
As combustion products contain carbon monoxide and other dangerous compounds, proper ‘draft’, and admission of replacement air is imperative. Building codes, and other standards, regulate their materials, design, and installation.

Flue Liner

An insulated liner within an existing chimney that ensures expelled gases are at the right temperature and ensures there is a sufficient draw of outside air to maintain correct combustion.

Flue Outlet

This is where the boiler flue discharges to the exterior of the building.


A closed space where the fuel is burned


A device protecting an electrical appliance, or circuit, against excessive current flow.

Gas Safe Register

Gas Safe Register exists to protect you, your family, and your property from dangerous gas work.
To carry out work on gas installations and appliances safely and legally, engineers in Great Britain and the Isle of Man must be on the Gas Safe Register. We make sure the 120,000 engineers on the register are safe by inspecting the gas work they have carried out.
Gas Safe Register replaced the CORGI gas register in Great Britain and the Isle of Man on 1 April 2009.

Gas Shut Off Valve

This is usually located next to the gas meter. If the meter box is located outside, then you would normally need a gas key to open the box.

Gauge glass

An indicator of the water level in the boiler, enclosed in glass.

Gravity Circulation

In an um-pumped water circuit, hot water rises and cold water sinks, therefore, where there is a heating source, circulation occurs.


A small, elliptical manhole. Used for maintenance of repair

Handwheel Valve

A hand-operated isolation valve with a circular grip.

Hard Water

Water that contains concentrates of soluble calcium carbonate greater than 20ppm.

Header Tank

Another name for a cold water storage tank.

Heat Emission

Any heat energy radiated from source.

Heat Exchanger Coil

A device used to transfer heat energy to another medium, i.e. hot gasses in a boiler transferred to the central heating water. The primary heat exchanger usually serves the radiator coil.

Heat Output

Is the rated output of an appliance, usually given in kW, BTU, or Therms.

Heating Controls

Heating controls allow you to choose when the heating is on, how warm it is, and where you want to be heated. It will also make sure that the boiler is only turned on when it needs to be.
Once you have installed the controls, it is important to ensure it is used correctly. This is the easiest way to keep your rooms at a comfortable temperature and by doing so, help to reduce your household’s fuel bills and CO2 emissions, too.

Heating Flow

The hot water pipe that runs from the boiler to the radiator system.

Heating Return

The pipe returning cooler water from the radiator system to the boiler for re-heating.

Hissing Sounds

See Banging Sounds.

Horizontal Duct

Service channel that runs parallel to the ground.

Hot Water Circuit

The water circuit from the boiler to the hot water storage tank.

Hot Water Cylinder

This is a storage vessel for hot water, that enables it to maintain heat.

Hot Water Thermostat

This will be fitted to the side of the hot water tank. It controls the highest temperature of the hot water. Some boiler systems don’t need them.

Immersion Heater

An electrical device for heating water. Usually acts as a back up in case of boiler failure, but can be used on their own to provide hot water without the need for a boiler.


Chemicals introduced into your central heating system that will slow down the rusting process of your radiators. We recommend that good quality brands (Kamco, Fernox, Sentinal), are used and handled by a professional, and introduced following a power flush and topped up every three years.

Instantaneous Gas Water Heater

Often in the form of a wall mounted or floor boiler that heats as water flows through it.

Landlords Gas Safety Certificate

If you rent out property then as landlord, you are responsible for ensuring that all gas fittings and flues are maintained in good order, and that gas appliances and flues are checked for safety every 12 months. You must keep a record of all safety checks for at least two years and issue it to existing tenants and to any new tenants before they move in. Tenants are responsible for the maintenance and safety of appliances they own.
In all cases, you should consult a Gas Safe registered installer who will be able to give you more detailed advice on what checks need to be carried out.
You have a duty under the Gas Safety (Installation and Use) Regulations 1998 to arrange maintenance by a Gas Safe Registered engineer for all pipe work, appliances and flues, which you own and have provided for your tenants use. You must also arrange for an annual gas safety check to be carried out every 12 months by a Gas Safe Registered engineer. You must keep a record of the safety check for 2 years and issue a copy to each existing tenant within 28 days of the check being completed and issue a copy to any new tenants before they move in.

LCD Display

Liquid Crystal Display. It’s a small readable screen fitted on most boilers and programmers, showing information about the appliance.

Lime Scale

An off-white, chalky deposit found inside a poorly maintained central heating system. Lime scale can reduce a boiler’s Heat Exchanger’s efficiency and can cause Banging Sounds in the system.

Lockshield Valve

This valve is found on the side of the radiator, and can’t be turned after it has been set on installation.

Mains Water

Water supplied by the utilities or water companies.

Mixer Shower

A shower which combines the cold and domestic hot water meaning temperature and flow can be adjusted.

Mixer Taps

Taps combining domestic hot and cold water through one output. Can be adjusted to be hotter and colder.

One-Pipe System

A system where the radiators are run in series, with the flow in and return out on the same pipe.

Open Flue Boilers

Also known as conventional flue boilers, these can be a back boiler, wall mounted or free standing. Open flue boilers are less efficient and in certain situations dangerous. An open flue boiler must sacrifice some efficiency in order to vent unwanted products of combustion from the house.
If for any reason the flue gets damaged or the ventilation grill gets blocked, or even a fault develops within the boiler itself, poisonous fumes could enter the living space. So open flue boilers should never be considered when fitting or replacing a central heating boiler, although new boilers do have extra safety devices.
All new boilers now are fanned flue and room sealed , which is the best and safest option.


See Boiler-Energy Manager.


Black circular rubber ring. Most commonly found in mixer tap.

Outlet Valve

An isolation device that controls substances leaving an appliance.

Overflow Pipe

A device fitted to a tank to safely discharge extra water.

Panel Heating

Heating system consisting of wall or floor or baseboard or ceiling panels containing electric conductors or heating pipes. Panel radiators are heating units placed on, or flush with, a flat wall surface and intended to function as a radiator.

Perforated Panel

This is a panel that is usually fitted to the front of a cabinet to dissipate the heat.

Pilot Light

A small, continuous flame (in a hot water heater, boiler, or furnace) that ignites gas or oil burners when needed. In natural gas furnaces, water heaters, and room heating systems, a safety cut-off switch is usually included so that the gas supply to the pilot and heating system is shut off by an electrically operated valve if the pilot light goes out.

Pipe Joints

The connection point between two pipes, or a pipe and a component.

Pitched Roof

A pitched roof is a roof with at least two slopes at more than 10 degrees to the meeting point.


Surfaces that have tiny openings which allow liquid to be absorbed, or to pass through.

Power Flush

Power Flushing is the process by which heating systems are cleaned using water at high velocity, but low pressure, so no physical damage is caused to the system. The process can be made even more effective with the addition of powerful cleansing and mobilising agents.
It is prudent to power flush a heating system immediately before fitting a new boiler to an existing system to prevent possible future problems. Many systems are found to contain corrosion and sludge, even if no flow problems have yet shown themselves.
The build up of sludge in hot water central heating systems is inevitable, no matter what fuel source is used. Over a period of time this sludge can block pipes and prevent the radiators from distributing heat around your house.
Sludge can also seriously affect the efficiency of your central heating system; it will lead to higher fuel bills and can bring forward the time when you need to replace some or all of the system.
A power flush is an expensive process, so please make sure your central heating system actually needs it by consulting several heating engineers.

Pressure Gauge

A gadget that indicates the pressure of water in the system pipe work.

Pressure Relief Valve

A safety device that resets itself once excess pressure in a boiler has been relieved.

Pressure Vessel

A hot water storage container, usually at water mains pressure.

Primary air

Air that’s introduced at the burners, along with fuel.

Primary Heat Exchanger

See Heat Exchanger Coil.


The presence of boiler water caused by steam flowing into the steam line.


See Timers and Programmers.


A device used to circulate water around the central heating system. The only pumps that are really necessary are primary pumps, as gravity will allow the water to circulate.

Pump Impeller

The part of a circulating pump that drives the water.

Pump Outlet

The water port on the circulation pump where the water flows out from.

Push Fit Connector

This is a plumbing connector suitable for both hot and cold water. The Copper pipe or plastic is simply pushed in.


Transfers heat from the hot water to the rooms.

Radiator Cabinet

A decorative or protective surround for a radiator. Can also be a clothes airing cupboard.

Radiator Valves

Used to control and isolate the water flow and return on a Radiator


An electrical component, often found on the printed circuit board used to switch secondary equipment such as water pumps, gas valves, and flue fans.

Rising Main

The Pipe that supplies appliances, tanks and taps with pressurized mains water.

Room Sealed Flues

By far the most common flues to find on a modern boiler. Boilers with a room sealed fanned flue can only be fitted to an outside wall. A double skinned pipe draws air in from outside and feeds waste gases back out again. In this situation flue gases cannot escape directly back into the room, and is therefore a safer option. Room sealed flues can operate through the natural draught of the air, or may be fan assisted to allow for longer flue runs.

Room Thermostat

See Thermostat.


A common term for corrosion of iron and its alloys, such as steel. It’s formed by the reaction of the metal with oxygen in the presence of water.

Safety valve

An automatic valve which releases excess pressure within the boiler.


Scale builds-up due to hard water. You can treat the system with a descaler. Drain, flush and refill the system afterwards.


SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) is the method for calculating the seasonal efficiency of central heating boilers, The system was developed under the UK Government’s energy efficiency best practice programme with the co-operation of boiler manufacturers and provides a basis for fair comparison of different models of boilers.
The SEDBUK rating is the average annual efficiency achieved in typical domestic situations. It takes into account sensible assumptions about climate, control, pattern of usage and other similar factors.
SEDBUK ratings provide a fair comparison of average boiler efficiency and this clear rating system is now applied to all boilers. The rating goes from A-G, with A being the best and most efficient and G being the least efficient.
The SEDBUK rating when used within an average domestic household, is an excellent benchmark for comparing new boilers and also assessing whether your current boiler would be better off being replaced due to its low efficiency rating.

Skirting Heaters or Radiator

Designed for installation in place of wooden skirting board. These tend to have relatively low output per unit length but they do input heat more evenly around the room.


Black sludge (magnetite) clogs the pump and builds up in the bottom of Radiators, reducing their heat output.

Small-Bore Copper or Plastic Pipes

General term to identify 15mm and 22mm pipes.

Soil Pipe

A drainpipe that carries wastes from a plumbing fixture, especially from a toilet.

Solid-Fuel Boiler

Any fuel that is not a liquid or gas, i.e. wood, coal, anthracite, etc.

Stack Pipe

This pipe, often found on the outside of the house, is where the toilet waste goes.

Standard Universal Backplate

A mounting plate fixing for a component.


A valve that regulates the flow of fluid through a pipe. The stopcock controls the flow of water and shuts off the entire supply in an emergency. The stopcock can be found under the kitchen sink, bathroom, cloakroom, cellar or basement.

Storage Vessel & Heating Vessel

These water storage tanks are usually located in the loft.

Swarf and Flux

Over time, sludge, debris, copper swarf, flux, etc will collect in your central heating system. If you find that your radiators have cold spots or poor circulation, this may be down to excessive sludge within the system.

System boiler

A system boiler, just like a regular (conventional) boiler works on the principle of stored hot water. However, a system boiler differs from a regular boiler in some important respects.
Firstly, many of the major individual components of the heating and hot water system are built into a system boiler, which means that installation is quicker, neater, easier and more efficient.
Secondly, the hot water is pumped from the system boiler through the heating system to the radiators and hot water cylinder, resulting in a fast response and more economical running costs. The system boiler removes the need for a feed and expansion cistern.


Those copper pipes found under the sink that connect to the water pipe.

Temperature Pressure Relief Valve (TPV)

A horizontal lever-like valve located at the water outlet joint near the top of a boiler.


A room thermostat constantly measures the air temperature of a space and can be set to whatever temperature suits you best. They are usually in halls, stairs or landing areas to sense the temperature of a home’s main living spaces. When the temperature falls below the setting, the thermostat switches on the central heating; once the room reaches the set temperature, the thermostat switches the heating off.

Thermostatic radiator valves

A Thermostatic Radiator Valve, (TRV), is a self regulating valve fitted to hot water heating system radiators. The TRV controls the temperature of a room by regulating the flow of hot water to the radiator.
Thermostatic Radiator Valves (TRVs) are often fitted to radiators to give a more responsive control of the radiator heat output according to the air temperature in the room. The head of the TRV has a sensor which responds to the air temperature around it. When the sensor expands on warming it pushes against a pin which closes the valve.

Time Clock

A time clock has a dial and around the edge of the dial, matching the hours of the day, are ‘pins’ that you can push in or out. When pushed in, these pins tell the system to turn on during the times you have selected.

Timers and Programmers

Timers switch the system on and off at set times during the day. Programmers offer a larger number of on-off programmes to suit your lifestyle, ensuring that the heating is not on while you are out or asleep.

Trapped Air

If your radiators or heating system seem to be making a noise then it may mean that there is air trapped somewhere. It’s usually trapped in the radiators, and can be fixed by bleeding them.

Two-Pipe (Vented) System

A heating system that transmits heat directly to all radiators in the circuit, quickly raising them all to the same temperature.


A mechanical device that regulates the flow of fluids by opening, closing or partially obstructing a passageway.

Vent Pipe

This safety pipe allows expansion of water within a hot water system or Air to escape.

Warm-Air System

See Duct Warm Air System.

Warm Front Scheme

The Warm Front Scheme is a government funded scheme which provides a package of insulation and heating improvements up to the value of £3,500 (or £6,000 where oil, low carbon or renewable technologies are recommended).
Warm Front Grants are available in England only; other schemes operate in Scotland, Wales and Northern Ireland.
More information on Warm Front Scheme and eligibility.


A thin plate (typically disc-shaped) with a hole in the middle that is used to seal or shut off the flow of liquid or gas.

Water tube

A tube in the boiler containing water and steam. Heat is applied from the outside.

Weather compensator

A device that adjusts the temperature of the water in the heating system, depending on the outdoor temperature. As the weather gets warmer, the water flow temperature is reduced to maintain required room temperature.

Wet Central Heating

A heating system that uses water to carry the heat energy.

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