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Jargon buster

There can be some interesting but often confusing names and terms associated with boilers and central heating systems. We’ve tried to list the most common ones here to make it a little easier for you.

However, if you are still struggling to understand something, feel free to get in touch with our experts at Arktek, we are here to help!

Annual service

Just like your car needs to have a service to make sure the brakes are safe and to change the oil, a boiler needs to be serviced annually by a Gas Safe registered engineer, to make sure it is running safely and efficiently. It is also a requirement of the warranty to have an annual service.

Back Boiler

A back-boiler unit (BBU) is a boiler that is built in behind an electric or gas fireplace and provides hot water and central heating in homes.

Balanced flue

A flue system designed by the boiler manufacturer. Typically supplied in kit form, balanced flue systems provide flexibility in flue termination, such as low/high level horizontal or vertical. Balanced flues also provide fresh air directly to the boiler to support combustion.


Benchmark is a nationally recognised scheme that makes boiler manufacturers and installers responsible for best practice during installation, commissioning and servicing of domestic heating and hot water products in line with Building Regulations in England and Wales. It is managed and promoted by the Heating and Hot Water Industry Council (HHIC). Benchmark requires installers to complete a Benchmark checklist when commissioning a new boiler. Failure to install and commission according to the manufacturer’s instructions and complete the Benchmark commissioning checklist will invalidate the warranty for gas boiler installations.

Bleeding the radiators

If there is air trapped in your central heating system, and the tops of your radiators are cooler than the rest, you may have to ‘bleed’ the radiator. This is easily done by using a radiator key to open the small valve at the top of the radiator to let the air out. When water starts to come out, the air is gone and you need to tighten the valve again. Check the pressure gauge on the boiler when you have bled the radiators, to see if you need to re-pressurise the system.

Boiler SEDBUK efficiency

SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) is the average annual efficiency achieved in typical domestic conditions.

Carbon monoxide (CO)

A colourless, odourless, poisonous flammable gas formed when gas or LPG does not burn properly. If your boiler is not working correctly or your flue is blocked or badly fitted, carbon monoxide can accumulate in your home and it can poison or kill.

Combination Boiler

A boiler designed to provide instantaneous hot water on demand, and heating.

Condensate pipe

Condensing boilers remove so much of the heat from the flue gases that sometimes the water left in the gas condenses. This ‘condensate’ is discharged through a narrow plastic pipe called the condensate pipe. Sometimes, if it has been fitted outside and is not insulated, the water can freeze in the pipe in very cold weather. This makes the boiler shut down to protect itself, and it can’t be restarted until the condensate pipe has been thawed. We have made videos to show you how to thaw the condensate pipe and also what you can do to ensure it doesn’t freeze again.

Condensing boiler

Available as a regular boiler or combination boiler, condensing boilers are the most efficient type available. Such boilers help reduce heating bills and carbon emissions. In most regions, condensing boilers should be installed to meet with building regulations.

Consumer Unit

A type of distribution board comprising a type-tested co-ordinated assembly for the control and distribution of electrical energy, principally in domestic premises, incorporating manual means of double-pole isolation on the incoming circuit(s) and an assembly of one or more fuses, circuit breakers, residual current operated devices or signalling and other devices proven during the type-test of the assembly as suitable for use.


The hot water storage cylinder supplies hot water that has been heated by a system or heat only boiler to the taps. It can also have an immersion heater, so it can be heated by electricity if the boiler is not switched on.

Cylinder thermostat

A switching device fitted to the domestic hot water cylinder that controls the temperature of stored water. The thermostat should be set to at least 60°C to prevent the formation of Legionella bacteria.

Diverter Valve

The diverter valve is a part inside the boiler that switches between hot water at your taps and the central heating.

Energy-related Products (ERP)

The ErP Directive, which is a new regulation set by the European Union, is designed to drive improvements in the efficiency and performance of heating and hot water products. Its purpose is to ensure that end users are aware of the level of energy efficiency inherent within their appliances.

Expansion Vessel

Expansion vessels are found in most boilers, especially combi boilers, and are needed to cope with the extra water created by expansion during the heating process. Expansion vessels work at pressure. If this pressure is lost, the expansion vessel cannot do its job and the boiler will fail to start or will automatically shut down.

Flow Rate

Flow rate is the term used to describe the rate at which water comes out of taps or a shower. A poor flow rate means that the flow of water from some or all of your taps or shower (or both) will be low – your sink and bath will take a long time to fill, or the flow of water from your shower will be weak.

Flue termination

The point at which boiler exhaust gases are expelled from the boiler into open air.

Fused Spurs

A Fused Spur is an electric point with its own removable fuse and an isolation switch. Current regulations require your new boiler to have an electricity supply from a fused spur, as this ensures that when an engineer carry’s out routine maintenance on your boiler, there is no danger of electrocution. The engineer is able to turn off the switch and remove the fuse, and carry out the service without the danger of someone accidentally switching on the appliance. If your existing boiler does not have a fused spur, then we will need to fit a new one close to the new boiler position. This will be carried out by a fully qualified and approved electrician, which if it’s required we have arranged and included in this quotation.

Gas oil

Higher in viscosity (thicker) than Kerosene, a burning oil more suited to larger or non-domestic boiler installations. Also known as 35 second, Class D oil to BS 2869.

Gas Safe Accreditation

Gas Safe Accreditation is the hallmark for gas safety in the UK. An installer must be on the Gas Safe Register to be legally allowed to work on your boiler or any other gas appliances in your home.

Heat Exchanger

A Device that allows heat from a fluid (a liquid or a gas) to pass to a second fluid (another liquid or gas) without the two fluids having to mix together or come into direct contact.

Integrally bunded tank

A factory-produced oil storage tank comprising a single skin tank enclosed in an outer bund. Integrally bunded tanks also have fitted lids to stop debris and rain water entering the bund area.


A light, low-sulphur and very clean burning oil suitable for use in most types of domestic oil heating and cooking appliances. Also known as 28 second, Class C2 oil to BS 2869.

Magnetic Filters

A central heating system is made up of a number of different metals, some are ferrous (can rust) and some are non-ferrous (can’t rust). These metals can be magnetic (Magnetite) and non-magnetic (non-magnetite). Sometimes these metals break down and little particles break away from your radiators or other components. A magnetic filter is installed on the pipework before the boiler, and prevents any particles that would usually cause blockages from entering it. The debris caught by the magnetic filters do not necessarily have to be magnetic like the name suggests, a good magnetic filter will be able to separate non-magnetic particles as well.

Manufacturer’s Guarantee

Products guarantee against material or manufacturing faults for the duration of the guarantee period. Please read your boiler manufacturer’s guarantee terms and conditions for further details.

Motorised valve

An electrically powered valve used to control where water leaving the boiler goes, that is, to provide heating or hot water. Valves are available which can do both.


Oil is the generic term for kerosene, gas oil and bio-liquids used as fuel for oil-fired appliances. Oil Storage Tanks.

Open Flue

A pipe that carries exhaust gases from a boiler. Open flues commonly use an existing chimney which should be lined to prevent brickwork absorbing exhaust gases. Unlike a balanced flue, an appliance connected to an open flue must be provided with a fresh air supply from outside of the building.

Pilot Light

A pilot light is a small constant gas flame that ignites your boiler when heat or hot water is needed. Pilot lights are only found in older boilers. If the pilot light is not working properly, the boiler will not work.

Plume management kit

An extension to a balanced flue system, these kits provide even more flexibility when having to avoid features such as doors, windows, etc.

Powerflush / System flush

Powerflush is recommended practice prior to the installation of a new boiler as it thoroughly cleans your heating system, ensuring that your new boiler works as effectively as possible. Chemical cleaner will be added to your system, then connected to a portable pump which produce a high-pressure flow of water. Each of your radiators will then be turned on and off, and the pressure is directed all over the system. This process ensures your new boiler works as efficiently as possible, helping to reduce your gas bills.

Pressure Release Valve (PRV)

This is a safety device built into a boiler that activates if the water pressure rises to a level the boiler cannot cope with. The PRV switches the flow of water to a pipe that usually discharges outside of the house.

Pressure Top-up or Repressuring

Every boiler and central heating system needs a bit of attention occasionally, in addition to its normal annual service. This includes repressurising the system and bleeding the radiators. Your boiler’s user guide has details on how to carry out these simple procedures.

Printed Circuit Board (PCB)

Modern boilers are controlled electronically and the PCB is the ‘brain’ of the boiler. If there is a problem with the PCB it may lead to the boiler working incorrectly or not at all.


A programmer is a time-activated switching device that regulates when the boiler comes on for heating and/or hot water. They may be a simple time switch or have multiple functions to give greater control of the heating system.

Radio Frequency Controls (RF)

Wireless controls for your heating system that don’t need to be wired into the boiler.

Regular boiler

A boiler designed to heat water only. The heated water is then circulated around the heating system by external pump, valves, and controls on the system.

Room thermostat

A device that switches heating control equipment on and off according to the temperature set by the user. SEDBUK efficiency (see: boiler SEDBUK efficiency).


Sludge is the material that has settled to the bottom of a heating system (usually your radiators). It will make your heating system run less efficiently and can lead to boiler parts failing.

Standard Efficiency

Old, non-condensing, or standard efficiency boilers can be as little as 60% efficient. Modern condensing boilers are over 90% efficient. Replacing an old boiler can help you make substantial savings on your gas bills.

System Boiler

A system boiler pumps hot water to the radiators and a sealed hot water storage cylinder. Most of the major components of the heating and hot water system, including the pump and expansion vessel, are built into the boiler, making installation easier, tidier and more efficient, and also simplifies servicing.

System Filter

A filter can be fitted to your central heating system to protect it from sludge and rust that builds up over time. This debris can be harmful to your boiler and make your heating system less efficient. Filters come in different designs, including magnetic filters. Your installer will recommend the right filter for your system.

Thermostatic Radiator Valves (TRVs)

These are valves which can be connected to individual radiators to provide temperature control local to the radiator. The fitting of TRVs in a dwelling allows areas such as bedrooms to be maintained at a lower temperature than living areas or the bathroom. The higher the number selected on the TRV, the hotter the radiator and warmer the room.


A thermostat allows you to stipulate the temperature that your central heating system works to. It works by switching the central heating system on and off in order to meet your specific temperature requirements. For example, if the thermostat is set to 20 degrees then the thermostat will monitor the temperature in the air and if it drops to 19 degrees or lower, then your central heating will come on. Once the desired temperature is reached your central heating is then switched off.

Thermostats (Programmable)

A programmable room thermostats are popular as it allows you to stipulate different temperature settings at different times (this depends on the chosen device). For example, you could set your central heating to come on every morning at 7am for 2 hours and operating at 20 degrees, which then switched off throughout the day until 6pm when it will be switched back on ready when you come home.


Your warranty will give you total peace of mind and protection. It means all parts and labour costs are covered if your appliance ever needs repairing while the warranty is valid, subject to our terms and conditions.

Weather Compensation Control

Weather compensation control is an optional control available for modern boilers. It works by monitoring the temperature outside and using this information to help the boiler work as efficiently as possible.


Dividing the building into separate heating zones, each with independent time and temperature control.

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