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Heating terms explained simply

Helping you see clearly

If 'unvented', 'sealed system' or 'heat only' sound like gobbledegook to you, our jargon buster is here to help you understand the language your heating engineer uses.


Annual service 

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.

ASHP/air source

ASHP/air source - An air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside, and uses the heat for radiators, underfloor heating or warm air convectors in the home.


BAR - A unit of pressure relating to water, gas or air.

BBU/Back Boiler Unit

BBU/Back Boiler Unit - A back boiler is a small, compact boiler fitted at the back of an open fireplace hearth. It sits behind an electric fire (in the chimney breast) and provides hot water and central heating to the property.


Benchmark - 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 Hotwater Industry Council (HHIC). Benchmark requires installers to complete a Benchmark checklist when commissioning a new s instructions and complete the Benchmark commissioning checklist will invalidate the warranty for gas boiler installations.


Biomass - The name given to fuel that comes from biological matter, such as wood pellets. It is often known as carbon neutral, because as much energy is absorbed when the plant was growing as is released when it is burned. Biomass boilers use biomass fuels rather than natural gas or LPG.

Bleeding the radiators

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 repressurise the system.

BP/burner pressure

BP/burner pressure - In a boiler, gas is supplied through the gas valve then mixed with air. The mixture in the burner is at a specific pressure and air/gas ratio to make sure that it burns consistently, and at the best possible efficiency. When your installer commissions the boiler, he must record the burner operating pressure in the Benchmark checklist, as part of the general combustion measurements he has to do.

CHP/combined heat and power

CHP/combined heat and power - Combined heat and power (CHP) is the simultaneous production of heat and electricity from a single source, close to where they are to be used. This means that the heat and electricity generated can be used in the building extremely efficiently because there is no energy loss during transportation to another site.

CO/carbon monoxide

CO/carbon monoxide - 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.

CO2/carbon dioxide

CO2/carbon dioxide - A colourless, odourless gas produced by burning carbon and organic compounds and by respiration. It is naturally present in air (about 0.03 per cent) and is absorbed by plants during photosynthesis. It is a major component of greenhouse gas, responsible for global warming and climate change.

Combi/combination boiler

Combi/combination boiler - A boiler that heats water instantly when you turn on a hot tap. It does not need a water storage cylinder or water tanks in the loft. The water it heats comes straight from the mains, so the flow rate at the tap is usually very good. See diagram to the right.

Condensate pipe

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.


Controls - Controls for your boiler or heating system include room thermostats, room programmers and digital timer or mechanical timers. Controls help you get the best out of your heating system and make it work efficiently and cost effectively.

CWM/cold water mains

CWM/cold water mains - The water that comes from the water mains, directly into your home, under mains pressure.


Cylinder - 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.

DHW/domestic hot water

DHW/domestic hot water - The water that comes from your hot tap or is stored in a hot water cylinder.

Expansion vessel

Expansion vessel - As the water in your system heats up, it expands and the expansion vessel is a small, pressurised container that accommodates the extra volume.

F&E tank/feed and expansion tank

F&E tank/feed and expansion tank - Usually two tanks found in the loft; the larger one feeds fresh water to the hot water storage cylinder and the other, smaller tank maintains the correct level of water in the heating system. The tanks also allow for the expansion of water when the system gets hot.


Flue -The flue is a bit like an exhaust pipe, taking away the waste gases from the boiler's burner. It also supplies air into the burner so the boiler can burn the best mix of gas and air, making it as efficient as possible. Flues are either concentric - they have one pipe inside the other - or twin pipe - one pipe takes the exhaust away and the other supplies air to the boiler.

FR/flow rate

FR/flow rate - The rate at which water comes out of your tap - usually measured in litres per minute (LPM). If you have a poor flow rate, it will take a long time for your bath or sink to fill. You heating engineer can recommend ways to increase your flow rate. If you have a combi boiler, the flow rate refers to how much hot water can be heated as it flows through the boiler, at a given rate, usually litres per minute. If you need large quantities of hot water, a system or heat only boiler may be better for you than a combi.

Fuel poverty

Fuel poverty - A household is said to be in fuel poverty when it spends more than 10% of its income on energy to heat the home.

GSHP/ground source heat pump

GSHP/ground source heat pump - A ground source heat pump uses pipes buried under the ground to extract stored thermal energy. This energy is converted to provide clean, efficient and renewable heating and hot water for the home.

Heat only boiler

Heat only boiler - Also known as a 'conventional' or 'regular' boiler. This type of boiler is often found in older homes and needs a feed and expansion tank in the loft to store cold water. It also needs a hot water storage cylinder, and a pump to pump hot water around the system.

High efficiency or condensing

High efficiency or condensing - High efficiency or condensing boilers are much more energy efficient than standard efficiency boilers. Condensing boilers have bigger heat exchangers and recover more heat out of the flue gases before they escape to the outside. Boilers can be made even more energy efficient by adding a passive flue gas heat recovery device like a GasSaver.

LPM/litres per minute

LPM/litres per minute - The rate at which water comes out of your tap, also known as the flow rate (FR).

Micro-combined heat and power

Micro-combined heat and power - Micro-combined heat and power (CHP) is the simultaneous production of heat and electricity from a single source, close to where they are to be used, on a small scale, usually a single home.

Open-vented system

Open-vented system - A heating system that has a feed and expansion tank in the loft.


Output - The output of boilers is usually measured in kilowatts (kW). Your installer can determine the right output for your heating system by carrying out a survey of your home and your personal requirements. If you are having a combi boiler and require a lot of hot water, you will need a high output, like the Baxi Duo-tec Combi 40 HE A. However, if you have a system or heat only boiler, you will need the lowest output required to heat your radiators, because hot water will be stored in the cylinder.

PCB/printed circuit board

PCB/printed circuit board - The PCB controls the boiler electronically. Sometimes, it will develop a fault or stop working properly, or sometimes it may shut down the boiler because another component is not working properly. Your heating engineer should check all the components before changing the PCB.

Pilot light

Pilot light - Old, standard efficiency boilers had a small flame, burning constantly, to ignite the burner when heating or hot water was required. Modern boilers don't have pilot lights because they are lit by electronic ignition.

Power flush

Power flush - A cleaning process that removes all the sludge and debris from your heating system. If you have cold spots on your radiators, they could be filled with sludge that has built up inside the system over many years. If you put a brand new boiler onto a dirty system, it could damage or block the heat exchanger. Your installer should always perform a power flush before installing a new boiler, and add a suitable inhibitor to stop the sludge building up again. This will make your heating system much more energy efficient and reliable.

PP/pounds per square inch

PPI/pounds per square inch - A unit of pressure. On boilers there is usually a pressure gauge to tell you your boiler is working at the correct pressure. If the pressure drops, it could be due to a leak in the system. We have a useful video to show you how to top up the pressure in your boiler. However, you should ask your Gas Safe registered engineer to check for leaks in the system.

PV/photovoltaic solar panels

PV/photovoltaic solar panels - Solar panels that generate electricity.

RF/radio frequency controls

RF/radio frequency controls - Wireless controls for your heating system that don't need to be wired into the boiler.


SEDBUK - Stands for Seasonal Efficiency of Domestic Boilers in the UK, the rating given to a boiler's efficiency.

Smart meter

Smart meter - An electricity or gas meter that records how much electricity is used, and if you have micro-CHP or Solar PV, how much electricity you generate. By 2020, all UK households will have their standard meters replaced with smart meters.

Solar thermal

Solar thermal - Solar panels that use energy from the sun to generate hot water (rather than electricity).

Standard efficiency

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

System boiler

System boiler - A boiler that works with a hot water storage cylinder but does not generally need a tank in the loft to fill the system.

System filter

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.

TRVs/thermostatc radiator valves

TRVs/thermostatic radiator valves - Thermostatic radiator valves can be fitted to individual radiators so you can vary the temperature in each room, to suit your needs.

Unvented/sealed system

Unvented/sealed system - A sealed heating system does not need feed and expansion tanks in the loft. Instead, the system is filled with mains water through a filling loop and there is an expansion vessel in the boiler to accommodate the hot water as it expands in the system. The flow rate on a sealed system is usually very good.

Weather compensation

Weather compensation - An outdoor mounted weather compensation sensor works with the boiler to give greater fuel savings and improved comfort levels in the home.


Download our Jargon Buster

Download our Jargon Buster document, here to help you understand the terms used by heating engineers.

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