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In our glossary you find the most important terms regarding heating systems including a detailled explanation.



  • Advance

    Vaillant Advance is our loyalty scheme for installers. Installers get access to a whole network of dedicated service, technical and support teams, as well as a whole host of business support tools.

  • Air source heat pump

    Air source heat pumps use the external temperature to extract heat and are the most common type of heat pumps, suitable for use even in apartment and flat situations off the ground floor. Some electricity energy is required to allow this, though it is only around 25% of the total energy used during the process. Air source heat pumps will operate in low temperatures too.

  • App

    An app is a software application that is downloaded by a user to a mobile device. Vaillant has produced apps for Apple and Android devices (downloadable from the App Store and Google Play) so that its vSMART and VRC 700 heating controls can be controlled from a mobile phone or tablet.


  • Backlit display

    Vaillant uses a blue, backlit display in its digital displays. This proven technology offers outstanding contrast for clear information and low energy consumption.

  • Boiler Plus

    Boiler Plus is legislation introduced by the UK Government in 2018 as part of its ‘Heat in Buildings’ programme. This programme was designed to improve the ways homes in the UK use energy by increasing the efficiency of their heating systems. The introduction of Boiler Plus is the first major piece of legislation since the UK went “all-condensing” in 2005 - becoming the first country to do so in the process.

    All boilers fitted in the UK must now have a minimum ErP efficiency of 92% and all installations must have time and temperature controls fitted, if they aren't already present and working correctly. For boilers that don’t already meet the target 92% efficiency, controls can be added to ensure compliance with this legislation. Further information on which Vaillant boilers already comply with Boiler Plus legislation and which controls should be fitted to ensure compliance, click here.

  • Boiler service

    A annual boiler service is a check and test carried out by a Gas Safe registered engineer. When you book an annual service through Vaillant, our installers will carry out a rigorous 12-point check to ensure your boiler is working efficiently. Find out more about Vaillant boiler service here.

  • BUS

    BUS also known as the Boiler Upgrade Scheme is a Government scheme to encourage more homeowners in the UK to opt for a low carbon heating technology.

    The scheme is designed to help attain the UK’s net zero emissions target by 2050 by making low carbon technologies more affordable to install by reducing the upfront cost .

    Homeowners in England and Wales can get up to £7,500 off the cost of a heat pump.

    The scheme is expected to run until 2028, and has been earmarked a budget of £450 million over the three years as part of an almost £4bn strategy to help cut carbon emissions.

    Read more about the scheme here.


  • Central heating

    A central heating system is a collective heating system that heats all radiators in a building simultaneously. In most cases, it also supplies hot water in a separate heating circuit. Some central heating systems use district heating from an energy provider, others use local heating. When local central heating systems are used, a boiler or condensing boiler is installed in the boiler room. The primary fuels used are heating oil or natural gas.

    The heat produced by combustion is either fed directly to the heat exchanger, which uses it to heat the heating water, or is supplemented by the condensation heat of the gases produced, in the case of condensing boilers. Circulation pumps are used to circulate the heating water in the heating pipe system and supply each radiator with heat, which can then be regulated by thermostats located on the radiators. The cooled down heating water flows back to the boiler and the cycle is then complete.

    Modern central heating systems can also be combined with alternative energy sources, such as solar thermal energy or heat pumps, which the government subsidises. Mini modular thermal power units as a source of energy for central heating can produce electricity in addition to heating and hot water through cogeneration.

  • Combi/combination boiler

    A combi or combination boiler is an all-in one heating appliance that doesn’t need a storage tank. It’s perfect for homes with one bathroom or where there’s limited space. Heating systems run by combination boilers only heat the water they use, so they’re economical too!

    Learn more about combi boilers here.

  • Condensing boiler

    All boilers fitted in the UK have to be condensing boilers. They work by condensing water vapour in the exhaust gases so that they recover the latent heat of vaporisation, which would otherwise have been wasted. The condensed liquid vapour leaves the system via the condensate pipe.

    Learn more about condensing boilers here.

  • Condensate pipe

    The condensate pipe is usually a white overflow pipe, similar to one you may find on a washing machine or sink waste pipe. It allows your boiler to dispose of condensation to an outside drain via this pipe.

    Learn how to defrost your condensate pipe here.


  • eBUS

    eBUS is the proprietary language used by Vaillant products so they can ‘talk’ to each other. It is this intelligent communication that allows Vaillant boilers to carry out more complex decisions rather than just turning on and off.

    All Vaillant products are eBUS compatible from 2006+.

  • Energy efficiency

    Energy efficiency is the measure of the amount of energy used to accomplish a purpose, such as heating a room. Energy efficiency is optimal if the benefit can be achieved with minimal use of expended or renewable energy and without any loss. The more energy efficient a device is, the more it conserves energy, cuts costs and reduces climate-damaging CO2 emissions.

    Electrical appliances are divided into energy efficiency classes ranging from green for very efficient to red for inefficient. For houses, energy efficiency refers to the relationship between system technology, thermal insulation and building sealing. Similarly, there are energy efficiency classes for power generators and heaters, which provide information on how efficiently these utilise their respective fuels. Renewable energies are CO2-neutral, but are not the most efficient form of energy. Combined with conventional heating systems, they significantly reduce the use of fossil fuels and lower pollutant emissions. Energy-saving laws and government subsidies seek to promote the replacement of energy-inefficient appliances with energy-efficient ones.

  • EPC (Energy Performance Certificates)

    An EPC (Energy Performance Certificate) gives your property an energy efficiency rating from A (most efficient) to G (least efficient). The certificate is valid for 10 years. You need one if you plan on selling your home.

  • ErP (Energy Related Products)

    Products that use energy, such as boilers, are rated according to the energy they consume. The ratings usually range from A+++ to G, depending on the product. The ErP rating has to be displayed on the product.

  • Expansion vessel

    An expansion vessel is a small tank fitted to a sealed heating system that provides room for expansion of the water as it is heated. This is to prevent the system pressure getting too high. Read more about how expansion vessels work here.


  • Flow rate

    The rate at which water comes out of the taps or shower.

  • Flow switch

    Flow switches are a major part of a combi boiler as it senses when you turn on a hot water tap and the boiler.

  • Flue

    During combustion, all boilers produce gasses that need to be emitted outside of your home. The pipework for this is known as a flue.


  • Geothermal energy

    Geothermal energy is thermal energy stored in the accessible part of the earth's crust. It is an emission-free regenerative energy source that is available all year round at any location and is generously subsidised by the government.

    A distinction is made between direct and indirect use of geothermal energy. While geothermal heat pumps (brine-water heat pumps) are used for heating, cooling and hot water preparation and use the near-surface heat of the earth directly to evaporate its refrigerant and thus initiate the heat pump process, indirect use involves harnessing the heat of the earth in a geothermal power plant and converting it into electricity. In the case of combined heat and power (CHP), the geothermal heat is simultaneously converted into electricity and used for heating.

    Geothermal heat pumps (brine-water heat pumps) designed for household heating systems normally use geothermal probes requiring a permit or space-intensive geothermal heat collectors to harness geothermal energy. Less energy-efficient geothermal exchange baskets and geothermal trenches (trench heaters) are used less frequently.

    Read more about geothermal heating systems.

  • Ground source heat pump

    Ground source heat pumps use a system embedded into the ground to extract heat. For this reason, your home does need the necessary land to allow installation of either a ground array or borehole system. Installed systems are not noticeable once completed. Some electricity energy is required to allow this, though it is only around 25% of the total energy used during the process.

    Read more about ground source heat pumps here.


  • Heating control

    Heating Controls (also known as a thermostat) are controls that allow you to easily regulate the temperate of your home as well as managing your hot water demands. You can choose a simple analogue heating timers or control it through an app on your smart device.

  • Heat exchanger

    A heat exchanger is used inside a boiler to transfer heat from the gas burner to the water in the system/radiators. They are also used in a combi boiler to heat the hot water for your hot taps and showers. Heat exchangers can be made from either aluminium, or stainless steel. Stainless steel heat exchangers offer superior heat transfer and highly efficient performance.

  • Heat pump

    Heat pumps use environmental energy from the air, earth or ground water to produce heat. They extract heat from the outside and increase it by compressing a refrigerant. In doing so, the refrigerant in the heat pump absorbs the environmental energy from outside air in the air-air or air-water heat pump, soil in the brine-water heat pump or groundwater in the water-water heat pump. Conversely, heat pumps can also be used to cool rooms.

    Heat pumps utilise three-quarters of the heat available from the natural environment. The most efficient are brine-water heat pumps and water-water heat pumps. However, since deep boreholes or groundwater wells and appropriate permits are required, air-water heat pumps, also known as air-to-water heat pumps, are the most commonly used ones. The high purchase price and installation costs of this environmentally friendly energy source are offset by generous government subsidies and ongoing savings in heating costs. Heat pumps combined with underfloor heating are particularly efficient. Used in conjunction with a photovoltaic system, a large part of the electricity required for operation can be generated in an environmentally friendly manner, making you almost completely independent of energy providers.

    Discover more about heat pumps here.

  • Hot water tank

    In heating technology, a distinction is made between heating water and service water. Heating water circulates only within the heating system and transports thermal energy. Service water is drinking water for kitchen and bathroom use. While instantaneous water heaters are the simplest choice when small quantities of hot service water are required, hot water storage tanks are the ideal solution when households require larger quantities of hot service water.

    A hot water storage tank is comparable to a closed container in which there is a pipe coil acting as a heating coil. Drinking water flows around the pipe coil, which contains heating water. The pipe coil now functions as a heat exchanger, transferring the heat to the service water. Inflowing cold water is likewise heated up whenever hot water is drawn. It is important for the hot water tank to have very good insulation to minimise heat loss and maintain a temperature of at least 60 °C to prevent the formation of legionella bacteria.


  • Load compensation

    Load compensation is the ability to modulate the flow temperature from the boiler based on the actual room temperature, this requires the control and boiler to ‘speak’ the same language

  • Low carbon

    The reduction of CO2 emissions. Low carbon heating includes heat pumps as they don't rely on oil and gas to help heat your home.

  • LPG (Liquid Petroleum Gas)

    Some boilers can be converted to run on LPG where a natural gas supply is not available (an off-gas area). LPG gas for central heating is stored in large cylinders adjacent to the property. Some boilers require an LPG conversion kit (purchased separately) and some are compliant out of the box.

    Read more about LPG here.


  • myVAILLANT app

    The myVAILLANT app allows you to control your heating and hot water from your mobile phone.

  • myVAILLANT Web

    myVAILLANT Web is our free online service portal which gives you control over your boiler’s guarantee, repair and service information.

    The portal allows you to book and manage your boiler repair appointments online, register your Vaillant appliance, and activate your smart home subscription.

    Visit myVAILLANT Web here.


  • Open vent boiler

    Also known as a conventional or regular boiler, an open vent boiler is part of an open vent heating system that includes feed and expansion tanks in the loft (that are open to atmospheric pressure) as well as a storage cylinder. The hot water is heated via the storage cylinder.

    Read more about open vent boilers here.

  • One off repair

    A fixed price repair for your boiler when you need it.


  • Pilot light

    A small constant gas flame that ignites your boiler when heat or hot water is needed. Pilot lights are usually only found in older boilers.

  • Plug-in timer/programmer

    A Plug-in Timer/Programmer is a device that switches your heating on and off at specific times set by the user. These can be mechanical or digital and may only allow times for a 24hr period to be set or each day programmed individually.

  • PRV (Pressure Release Valve)

    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.


  • Refrigerants

    Refrigerants are natural or synthetic fluids with a low boiling point, which are used in cooling technology and heat pumps and which transport thermal energy.

    The main properties of refrigerants are that they absorb heat in liquid form and evaporate even at low temperatures. The heated refrigerant in gaseous form is then compressed by a compressor, thus increasing the temperature. The refrigerant then releases its heat into the heating system at the desired heat generation rate. The cooled down, once again liquid refrigerant is decompressed and cooled down even further by a valve, which closes the loop and allows it to absorb heat again. In cooling technology, heat is extracted from the room to be cooled by evaporation and then released to the outside.

    Today, partially halogenated organic fluorocarbons (PFC) or hydrofluorocarbons (HFC) are primarily used because of their high output. Other biologically non-toxic refrigerants include carbon dioxide, ammonia, ammonia dimethyl ether or water. The now banned refrigerant made of chlorofluorocarbons (CFCs) damaged the ozone layer because of the chlorine in it.

  • Radiators

    Radiators can be divided into radiators, convector heaters and panel heating systems.

    Radiators are usually steel water-conveying hollow structures that act as heat exchangers and release the generated and stored energy as heat to the ambient air Sectional radiators are the original form and are now only found in old buildings. The tubular radiator is the successor model, which is also used as a towel dryer. Currently the most popular radiator is the space-saving and efficient panel radiator, which also emits more radiant heat.

    Convectors are typical supplementary heating systems, since they can quickly heat up the air in the room, but cannot store it. Electric radiators draw in cooler air, heat it through fins and expel it upwards through an air outlet, with or without a fan.

    Panel radiators are the most efficient kind of radiators. Low flow temperatures, little convection and more radiant heat will improve the indoor climate. These heaters are ideal when used in combination with regenerative energy systems. They are available as floor, wall and ceiling heaters.

  • Room thermostat

    A traditional Room Thermostat senses the temperature of the room and sends a command to the boiler to turn on. It instructs the boiler to work at 100% until the required set temperature is achieved.


  • Smart home

    Also known as ‘eHomes’ or ‘Smart Living’. Homes that are considered ‘smart’ when equipped with a range of interconnected devices, all of which can be monitored and controlled by a computer, smartphone or tablet.

    Read more on smart homes here.

  • System boilers

    A system boiler heats water directly from the mains for your central heating system and hot water, which it stores in a cylinder (rather than a tank in the loft). They are perfect for larger homes that may have more than one bathroom.

    Read more on system boilers here.


  • Thermostat

    A thermostat controls the temperature of your heating system, ensuring the system heats to maintain a set temperature. Thermostats can be standalone or form part of a control.

    See our range of thermostats here.

  • TRV (Thermostatic Radiator Valve)

    A TRV is a self-regulating valve that fits directly onto the radiator. It controls the temperature in the radiator by changing the flow of hot water to the radiator.


  • Water source heat pump

    Water source heat pumps use a system embedded in a water source to extract heat. Water source heat pumps often provide the most reliable source of hot water and heating in homes, thanks to the consistent water temperatures outside.

    Find out more about water source heat pumps.

  • Weather compensation

    Weather compensation is technology that uses environmental data to understand the thermal behaviour in and around a property. The weather compensation feature on Vaillant’s smart controls uses weather forecast data to understand the outside temperature. It then adjusts the heat going to the radiators or underfloor heating system for optimum efficiency.