Glossary

An airlock refers to when air bubbles form in heating system pipework (e.g the oil pipeline) and causes a blockage.

Air source heat pumps use the principles of thermodynamics to absorb energy from the surrounding environment (the outdoor air) and convert it into usable energy to provide heating for our homes. Visit our Education Area to learn more about how air source heat pumps work.

This legislation is set by Government to monitor the level of pollutants within inside and outside air with the Air Quality Regulations 2010 containing limit and target values which must be adhered to.

Ambient air refers to the air around ‘us’ that contains thermal energy which can be used as heat energy to power various devices or systems.

The ambient temperature refers to the temperature of any medium, for example, the temperature of air that surrounds an appliance or building.

Baffles are spiral pieces of metal which are used in all of Grant’s oil-fired boilers. They are located in the heat exchanger and cause turbulence and back pressure which assist the extraction of heat when the fuel is burning.

Bioenergy refers to types of renewable energy that are created from organic matter. It can either be used straight as a fuel or processed into a liquid or gas first.

Biofuels are a type of renewable energy. Made from organic matter, biofuels are liquid fuels which can be used in a variety of applications from fuel for vehicles through to heating homes. Read more about biofuels in home heating systems here.

A boiler is an appliance that provides hot-water or central heating. The most common types of boiler include gas, LPG and oil boilers.

Boiler Short Cycling refers to the repetitive operation of the boiler when there is no demand from the heating system for the boiler to run. The boiler fires to heat the water within it in response to the boiler thermostat. As the water in the boiler heats up quickly, the boiler thermostat is soon satisfied and the boiler stops firing. Heat is then lost from the boiler to the surrounding air through the casing and also to the outside via the flue system until the boiler thermostat re-starts the boiler again. This is not only highly inefficient, wasting energy and increasing the fuel bill, but will also damage the boiler shortening its working life. Short cycling can be prevented by using a room thermostat and programmer to control the boiler, providing the so-called ‘boiler interlock’ so that the boiler will only fire when there is a demand for space heating. A boiler may also short cycle if it is oversized for the heating system it is serving. This is again highly inefficient and damaging to the boiler. If you have concerns that your boiler is short cycling, please contact your local heating engineer.

Boiler pressure measures the pressure of the steam or water in your boiler and heating system at any time and can be shown via the pressure gauge on your boiler.

The boiler thermostat is a control, set by the installer user or manufacturer, that limits the temperature of water passing through the boiler by shutting off. The boiler thermostat is different to the thermostats you may have in some of your rooms and it is important that you do not adjust the boiler thermostat – visit our blog to read more about the role of the boiler thermostat and room thermostats in a heating system.

The Boiler Upgrade Scheme is a Government initiative available in England and Wales to encourage householders to swap from fossil fuelled heating systems to renewable heating systems, such as air source heat pumps. BUS provides eligible homeowners with a grant of £7,500 to help offset the installation costs of a new low carbon heating system when replacing a traditional fossil fuel boiler such as a gas boiler. A temporary uplift in BUS funding is available to eligible households using an oil or LPG boiler who can access a grant of up to £9,000 (available for applications submitted on or after 21st July 2026, this temporary uplift is expected to be available until the end of March 2027.) Read more about the Boiler Upgrade Scheme here.

Building Regulations refer to a set of approved documents which detail mandatory minimum standards for the design and construction of UK buildings that must be adhered to when building new properties or making changes to existing ones. Heating systems are included with these regulations so the installation of a new heating system must adhere to current Building Regulations.

A bunded tank is a double skinned tank that has the capacity to hold the full contents (e.g. oil) should the inner tank start leaking.

Carbon dioxide is an odourless and colourless gas that is produced by burning carbon and organic compounds, and by respiration. It is released into the atmosphere and associated with climate change.

A check valve is fitted in the cold water supply to an unvented hot water cylinder and it prevents the backflow of heated water into the cold water mains. The expansion of the water, when heated in the cylinder, is accommodated by a separate expansion vessel connected to the cylinder.

A circulating pump moves liquid around a closed circuit, for example water within a heating system.

Clearances refer to the specific amount of space which is required around a heating appliance to enable its safe and efficient operation. For example, an oil boiler will need to be installed with sufficient clearances to allow for servicing and maintenance access whereas as an air-to-water heat pump will need sufficient space around it to allow for an unobstructed airflow and to allow for servicing and maintenance access.

The cold water inlet manifold is also often referred to as a ‘combination valve’ or ‘Monobloc unit’. Used with an unvented hot water cylinder, this assembly includes a check valve, strainer, pressure reducing valve and expansion relief valve. This unit often has a ‘balanced cold water’ connection, to provide a cold water supply at the same pressure at the hot water supplied from the unvented cylinder, for mixer fittings, e.g. shower valves.

A combi (or combination) boiler is an all in one compact, single unit boiler which produces both heating and hot water. Combi boilers are ideal solutions for when homeowners do not want or need a separate cylinder.

A combustion air vent supplies fresh outdoor air containing oxygen to an appliance burning fuel, such as an oil boiler. A combustion air vent MUST be provided for and installed with conventional flued boilers.

Commissioning is an important part of the installation of any new central heating system. During the commissioning stage, an engineer will check the settings and controls on the boiler or heat pump, to ensure it is operating at maximum efficiency and meets current Building Regulations. Commissioning a boiler or heat pump will also involve the installing engineer carrying out testing on the wider system to ensure the main heat source and complete heating system are working correctly.

Condensate is a slightly acidic liquid which occurs when flue gas temperatures drop below ‘dew point’ and the steam present condenses. Condensate is produced by boilers and heat pumps.

A condensate trap is used to collect the moisture from the condensed gases produced in a boiler or the condensate produced by a heat pump. A condensate trap must be correctly positioned to ensure the safe removal of condensate away from the unit and building.

A condensing boiler has a second (condensing) heat exchanger where the flue gases are cooled to below their dew point by the water returning from the heating system. This has the effect of condensing the water vapour present in the flue gases, and liberating releasing the heat energy that would otherwise be lost to outside through the flue system. A condensing boiler is more efficient than a standard efficiency boiler – all of Grant’s Vortex oil-fired boilers are condensing boilers.

A contents gauge is a device that allows the user to monitor the oil level in an oil storage tank to ensure that it is topped up before the oil runs out. Traditionally these were of the sight gauge type (see ‘Sight gauge’ definition below) but electronic contents gauges are now more commonly used as a sight gauge must be located within the oil tank bund and this is not possible with integrally bunded oil tanks.

A conventional boiler is sometimes also referred to as a traditional or heat only boiler. A conventional boiler is designed for properties with a heating and hot water system that is connected to a separate hot water cylinder. A cold water storage tank would also need to be located in the loft.

A conventional flue is a boiler flue system that uses the natural flow of air to remove combustion gases safely through a chimney or flue. The boiler takes the air it needs for combustion from the room where it is installed, so the room must have suitable ventilation to allow the boiler to operate safely. Ventilation requirements depend on the type of boiler and where it is installed and should always be checked by a qualified heating professional.

Controls are an integral part of any central heating system and every heating system, whether using an oil boiler or air source heat pump, needs a controller which enables its settings to be set, viewed and adjusted. Controls can vary from simple thermostats through to more sophisticated options which give homeowners and their heating engineers greater control of a heating system. It is vital that whatever type of control is installed, it must be correctly set up otherwise the system may be unable to operate correctly. Learn more about oil boiler controls and discover more about heat pump controls here.

Corrosion refers to the active destruction of boiler metal by the pitting action of dissolved oxygen which is contained in the boiler water.

The cylinder in a heating system stores the hot water which is then fed to the hot water taps in the kitchen, bathroom and utility room etc. A hot water cylinder has one or more inner coils through which the system water passes and heats up the water contents inside the cylinder – this system water passes through the coil once it has been warmed up by the heat source(s) such as a boiler or heat pump. Please click here to read more about a hot water cylinder works. If a combination boiler is installed in a property, a separate hot water cylinder is not required because the combi boiler incorporates its own expansion vessel.

Direct cylinders are classified as direct because the stored water is heated directly by one or more electric immersion heaters installed within the cylinder. Some direct cylinder models are designed exclusively for electric heating, while others also incorporate an internal solar heat exchanger coil, allowing solar thermal energy to contribute to the heating up of the domestic hot water. The addition of a solar coil does not change the cylinder’s classification as a direct cylinder. (See also indirect cylinder).

Domestic Hot Water, which is often abbreviated to DHW, refers to the water that has been heated up (by a boiler, heat pump or solar thermal system) for use in your home with the hot water that runs from your taps.

A dual thermostat consists of two separate thermostats – a control thermostat and overheat thermostat – in a single housing. Dual thermostats are often used on unvented hot water cylinders and some oil boilers, e.g. the Vortex Grant Boiler House boiler range.

The control thermostat allows the operating temperature to be set (i.e. the required water temperature when used on an unvented cylinder) whilst the overheat thermostat is a safety device that will operate if the temperature exceeds the pre-set limit. The overheat thermostat can be manually reset by pressing in a button once water temperature has cooled down.

The energy cut out is a safety control used on an unvented hot water cylinder. The energy cut out consists of a 2-port (or zone) valve fitted on the boiler primary flow to the cylinder coil. To heat the water in the cylinder, this valve is opened by an electrical supply from the heating/hot water system controls (eg. the programmer and the cylinder control thermostat) allowing the hot primary water from the boiler to flow through the cylinder coil and heat the hot water. In the event of the water temperature in the cylinder exceeding the pre-set temperature limit, the overheat thermostat will operate, cutting the electrical supply to the 2-port valve. As a result, the valve closes, shutting off the primary flow to the cylinder coil stopping any further heating of the water in the cylinder.

Efficiency, when referring to a central heating system, refers to the relationship between the energy supplied to a heat source (such as a boiler or heat pump) and the energy output from the heat source.

Energy efficiency refers to an approach to reducing the energy consumption of an appliance, such as an oil boiler or air source heat pump, while maintaining a certain level of energy output (ie. desired room temperatures for space heating).

The Energy Saving Trust is an independent organisation that is dedicated to promoting energy efficiency to both domestic and commercial audiences to achieve their mission to address the climate emergency and provide guidance abut energy efficiency and clean energy solutions.

The Energy related Products Directive, often abbreviated to the ErP Directive, is an EU initiative which the United Kingdom adopts. It is designed to aid countries in achieving reductions in carbon, nitric oxides (NOx) and sulfur dioxide (SO²) emissions.

The EvoLink Smart is a hybrid heating product supplied by Grant UK. The Grant EvoLink Smart Hybrid System Hub enables the addition of an Aerona³ or Aerona 290 air source heat pump with most types of existing domestic heating systems, such as a gas or oil boiler. Read more about the EvoLink Smart Hybrid System Hub.

An expansion relief valve is often part of the cold water inlet manifold used with an unvented hot water cylinder. This valve is located upstream of both the check valve and pressure reducing valve and will automatically discharge water if the pressure of the heated water exceeds the required level.

An expansion vessel is the one of main components found in a sealed heating system. Its purpose is to accommodate the increased water volume as the system heats up and to maintain a positive pressure in the system when it cools down. Expansion vessels are usually located in or close to the primary heat source within the property. It is important that the expansion vessel is sized correctly and able to accommodate the volume of water that is in the system when it is heated. 

An external volumiser can be installed alongside an air source heat pump when there is no room inside the property to locate a volumiser or low loss header. With Grant’s 30 litre external volumiser , it is designed to be sited directly behind the Aerona³ heat pump and it provides a minimum volume of water to allow the heat pump to control its modulation.

A Feed and Expansion Tank is a cistern that supplies a central heating circuit with water and accommodates the expansion of the water when it is heated by a boiler.

A fire valve is used with oil burning equipment, such as an oil-fired boiler. It is a safety shut-off valve which is installed in the oil supply pipeline located outside a building.

The Filling Loop is a pipe that connects a central heating system to the mains water supply and it is used to re-fill (re-pressurise) a central heating system with water.

Flexi feet are an accessory used with air source heat pump installations. When an air source heat pump is installed, it should be raised off the ground which can be achieved using a flexi foot kit . Grant’s flexi feet are anti-vibration mounts which not only raise a heat pump off the ground but they also minimise vibration (and sound) produced by the heat pump.

The flexi hoses which are supplied with Grant Aerona air source heat pumps are flexible hoses which connect the heat pump to the wider heating system.

The flow rate in an air source heat pump system refers to the volume of fluid which passes per unit time which is measured in mL/ph. The flow rate is an important factor when designing a heat pump system and achieving desired heat pump performance.

A flue system connects to a boiler to remove combustion gasses away from the boiler and safely release them outside of a property. A flue system will typically comprise of stainless steel tubing which can be assembled inside, outside or through a chimney depending on a property’s requirements.

The flue gas refers to the gas that is emitted due to the combustion of the fuel, such as kerosene oil used in Grant Vortex oil boilers.

Gas oil is a type of fuel oil distilled from petroleum and is heavier than paraffin oil. While gas oil is commonly used as a heating fuel, it does not comply with ErP requirements and is not to be used with Grant’s oil-fired boiler range.

Global Warming Potential, which is typically abbreviated to GWP, represents the amount of time a gas remains active in the atmosphere and its impact on warming the atmosphere during this time. It is a term which is commonly referenced when comparing refrigerant gases used in air source heat pumps. The lower the GWP value a refrigerant gas has, the less damaging it is to the atmosphere.

A header tank is a water storage tank which is usually found in the loft and is a requirement for central heating systems which are not sealed systems. A header tank, which is sometimes referred to as a feed or expansion tank, supplies a boiler and the wider system with cold water. Please note, a header tank is separate to a domestic hot water cylinder.

Heat is thermal energy that is produced as a result of chemical reactions, such as combustion, mechanical processes producing heat due to friction, the flow of electric current through a resistive material, or solar radiation heating objects on the surface of the earth.

Heating calculations are an integral part of planning the installation of a new heating system, whether a traditional boiler, renewable heat pump or hybrid heating system. They include a series of assessments of your home to calculate the space heating requirements for each of your rooms and the amount of heat escaping from your property (through the walls, windows and roof) and are used to determine the size of boiler or heat pump your home will need. Grant UK’s in-house Design Team can assist homeowners and their installers by completing room-by-room heat loss calculations for heat pump installations.

A heat exchanger is where heat energy is extracted and transferred from one part of a heating system to another. The heat exchanger within an oil boiler is usually sat behind the burner and transfers heat from the burner flame to the system water without them coming into direct contact.

Heat output refers to the amount of heat produced by a product or appliance, such as oil-fired boiler or heat pump. The heating output of an appliance is measured in kilowatts (kW).

Heat transfer is the movement of heat from one area to another. In a central heating system, heat transfer can take place within the heat source (as it extracts heat energy from the fuel or renewable energy source) and elsewhere on the system, such as when heat is emitted through radiators into rooms.

The Heat Pump Association UK (HPA UK) is a trade organisation and is the UK’s leading authority on the use and benefits of heat pump technology. Formerly known as the Heat Pump Association (HPA), the HPA UK was established following the merge of the HPA, Ground Source Heat Pump Association and Heat Pump Federation. It includes many of the UK's leading manufacturers of heat pumps, components and associated equipment. Grant UK is a member of the HPA UK.

HETAS is a national organisation working for consumer safety and the wider public interest in safe, efficient and environmentally responsible use of biomass and solid fuels.

The Heating and Hotwater Industry Council (HHIC) is a member organisation that is committed to effectively driving, supporting and promoting the sustained growth of the domestic heating and hot water industry in the UK, with a focus on decarbonising heating and hot water solutions.

The Home Energy Scotland (HES) Scheme is a funding initiative provided by the Scottish Government which gives eligible households access to a grant and/or loan to support the costs of installing renewable technology in their home. The HES funding includes a grant of up to £7,500 for eligible homes when they install a new air source heat pump. Read more about the Home Energy Scotland Scheme here.

HVO stands for Hydrotreated Vegetable Oil. It is a second generation biofuel, made from organic matter and waste materials, which can be used as a ‘drop-in fuel’ to replace kerosene oil in domestic heating systems. All of Grant’s Vortex floor stand oil boilers are HVO compatible. Learn more about HVO here.

A hybrid heating system combines two types of heating technologies to fulfil the heating and hot water demand of a property. Typically, a hybrid heating system will comprise of a traditional heat source combined with a renewable heat source, such as oil or gas boiler working alongside an air source heat pump.

Ignite is a term used to describe the beginning of the combustion process. It occurs when a mixture of gaseous substances containing enough heat energy to trigger a spark start the process of combustion.

An immersion heater is an electric heating element that is positioned in liquid which is to be heated, typically in a domestic hot-water cylinder. The role of an immersion heater is to provide back-up heating for the cylinder if the original heat source is unable to do so.

Indirect cylinders are classified as indirect because the stored water is heated indirectly through one or more internal heat exchanger coils connected to one or more heat sources, such as a boiler, heat pump or solar thermal system. Some models feature a single coil for connection to one heat source, while others incorporate twin coils to connect two independent heat sources, such as a boiler and solar thermal system or a heat pump and solar thermal system. (See also direct cylinders.)

An installer is a qualified individual who can carry out fitting, commissioning, maintenance and servicing of a heating system. Sometimes also referred to as a heating engineer.

An isolation valve is used to close off water to a specific area or appliance allowing selective distribution as and where necessary. Also referred to as an isolating valve.

Kerosene is a light fuel oil obtained by distilling petroleum and it is used especially in domestic heating boilers and jet engines. Grant Vortex oil boilers operate using kerosene oil as standard.

Legislation refers to statutory law(s) set by a governing body or similar. For example, the UK and Scottish Governments set laws which are detailed in legislation which must be abided to. Such laws and legislation are in place which heating installations must adhere to.

A low loss header creates hydraulic separation within a heat pump system. The Grant combined low loss header/volumiser creates hydraulic separation to maintain the correct flow rate through an Aerona³ heat pump while also providing a suitable volume of system water. Learn more about the Grant low loss header/volumiser here.

Manual valves are found on most central heating systems and they directly control the flow of a substance, such as the system water.

The Grant MagOne DUO is a central heating filter that protects new system components from damage by preventing the build of debris in the pipework. A MagOne DUO filter can increase the effectiveness and lifespan of new and existing heating systems.

The Mean Water Temperature, sometimes abbreviated to MWT, refers to the average (mean) temperature of the flow and return water in the heating system. Mean Water Temperature is commonly used with heat pump heating systems when monitoring the temperature of water flowing from the heat pump (flow temperature) and the temperature of water returning from the wider system back to the heat pump (return temperature) as it can indicate system performance.

A merchant is a stockist of supplies including the products and accessories needed to install and maintain a domestic heating system. Grant heating products, including Vortex oil boilers, Aerona air source heat pumps and spares, are available to order from merchants located throughout Great Britain. To find a local merchant who sells Grant products, please visit our online Find a Merchant search.

Modern heating systems are often split into separate zones - for example a downstairs zone and an upstairs zone - allowing homeowners to set specific time and temperature settings for each of these zones to suit their own lifestyle and temperature preferences. Each heating zone is wired back to a central wiring centre and in turn to the system controls, a solution is needed to house all of the zone wiring in one single enclosure. The Grant EP001 wiring centre, which is compatible with Aerona³ heat pumps only, is an example of such a product providing a simple and convenient means of making all system control connections for up to four heating zones plus one domestic hot water zone in one wiring centre – read more in our Home Heating Blog.

Net Zero Carbon is the goal of achieving an overall balance between the production of carbon dioxide and the amount of carbon dioxide removed from the atmosphere. Targets have been set by Government to reduce carbon emissions with a current goal of achieving Net Zero Carbon emissions by 2050 which will mean that the amount of carbon dioxide being produced is equal to or less than the amount of carbon dioxide being removed from the atmosphere. Read more about net zero carbon targets here.

Non-return valves are located within a heating system to allow water to flow through them in only one direction. These automatic two-port valves have two openings, one for fluid to enter and the other for fluid to leave.

Off-peak refers to a set period of time when demand for energy, such as electricity, is low. Some energy providers reduce the cost of the electricity they supply during off-peak hours, offering cheaper tariffs to those using their electricity during this low-demand period rather than during peak times of the day.

OFGEM is the Office of Gas and Electricity Markets. OFGEM are a non-ministerial government department and they are the energy regulator for Great Britain. They work with energy companies, government and National Energy System Operator with a key focus on protecting the interests of existing and future electricity and gas consumers.

OFTEC (the Oil Firing Technical Association) is a competent persons scheme that recognises installers who are approved to fit and maintain oil, solid fuel and renewable heating equipment. Operating in the UK and Northern Ireland, OFTEC is a trade organisation for the heating and cooking industries and they strive to promote high quality professional and technical standards through the development and provision of assessment and training courses.

An oil filter is a device used in an oil-fired boiler heating system to remove any solid matter from the oil supply to a boiler. A typical oil supply system will have a coarser filter (or strainer) of 70mm in the oil supply pipe at or near the tank to remove any larger particles from the oil. A second finer filter (15mm) is located closed to the boiler, to protect the burner oil pump and nozzle. Oil filters should be serviced annually with the boiler.

An open vented cylinder is a low pressure hot water storage solution. Low pressure hot water storage. Water is supplied to the cylinder from a cold water cistern/tank (usually situated in the loft) and not directly from the mains, like unvented cylinders.

The Open Safety Vent pipe or OSV pipe is a requirement for an open vented heating system as it provides a means of pressure relief and the release of steam in the event of a boiler thermostat failure. It also allows air to be released from the system. It should be installed so that it discharges into the feed and expansion cistern, below the cistern cover but above the water level.

An O-ring is a seal in the form of a ring, typically made from rubber. It a common component used on all types of central heating systems.

A photocell is a sensor within the burner on a boiler that reacts based on the presence of light. It is a safety component that monitors the boiler combustion chamber for a flame to ensure it has fired correctly.

Plume refers to the water vapour which can often be seen at the flue terminal and it is similar to your breath on a cold day. Condensing oil boilers operate at extremely high efficiency levels, producing cool flue gas temperatures which result in a plume of vapour. This plume (steam) is a normal condition of condensing boiler operation and indicates that the appliance is working efficiently. Grant UK supply low level balanced, high level and vertical flue kits which can move this plume to a higher level outside of a property.

A pressure relief valve is a safety device designed to protect equipment and system pipework from excessive pressure. It automatically opens when the system pressure exceeds a predetermined set point, releasing fluid or gas to reduce the pressure. Once the pressure returns to a safe level, the valve closes automatically, helping to prevent damage to equipment, maintain system integrity, and improve operational safety.

A pressure jet oil burner is a burner that atomises fuel oil, blowing it at pressure into the combustion chamber of a boiler in the form of a fine mist or vapour. Grant Vortex oil-fired boilers use this type of burner.

The Pressure Reducing/Regulating Valve controls the pressure of the cold water entering an unvented hot water cylinder. It reduces the incoming mains water pressure to a safe consistent level, helping to protect the cylinder and its components while ensuring the system operates safely and efficiently. The valve is factory set to the correct operating pressure for a specific cylinder and should not normally require adjustment. In most installations it is built into the cold-water inlet assembly, commonly known as a combination valve, which may also incorporate other safety features such as pressure relief valve, strainer and non-return valve.

Pumping over occurs in a central heating system which has been incorrectly configured, whereby water is discharged into the feed and expansion tank through the expansion pipe, while the pump is running.

A quotation is a price for carrying out work and/or supplying equipment. For example, for a new heating system, a quotation should itemise all the products and equipment required as well as the labour charges for installation.

R32 refrigerant is a type of refrigerant which is used in air source heat pumps, such as the Aerona³ R32 heat pump. Learn more about R32 refrigerant in heat pumps.

R290 refrigerant is a type of refrigerant used in R290 heat pumps, including the Aerona 290 air source heat pump range. R290 refrigerant has a very low Global Warming Potential value of 3 which means it is more environmentally friendly compared to other traditional heat pump refrigerants. Learn more about R290 refrigerant in heat pumps.

A radiator is a type of heat emitter which is used in central heating systems. Radiators are typically made from steel, aluminium or iron and they transfer heat from their water contents into rooms. Read more about the different types of radiators available.

Recycling refers to the process of converting old waste materials into new products.

A refrigerant is a chemical substance which can absorb heat and transfer it from one place to another as it changes state between a liquid and gas. Air source heat pumps use refrigerant, usually R32 or R290 refrigerant, to absorb the heat energy from the outside and transfer this to be used to heat the home inside. Read more about how heat pumps use refrigerant.

Renewables, such as renewable heating systems, use a natural source or resource of energy such as water, wind or solar power.

The Renewable Heat Incentive was a Government financial initiative which encouraged the use of renewable fuels to heat properties. This Scheme closed to new applications in March 2022.

Scale refers to the damage caused by the build-up of limescale in a central heating system. It can potentially destroy components so must be kept in check.

SEDBUK stands for the Seasonal Efficiency of Domestic Boilers in the UK which has mostly been replaced by the ErP Directive now. With regard to heating, it is a list of boiler efficiency ratings.

Setback control is a term often referred to with air source heat pump heating systems. Setback controls can lower the overall energy consumption of a heating system by giving the user the ability to change temperature settings automatically at night and during the day when no one is at home. During a setback period, when the home is either unoccupied or at nighttime, the temperature settings are reduced but not allowed to create too large a temperature difference compared to the warmer temperatures set during occupied periods. Learn more about setback controls.

A sight gauge is a component used in oil-fired heating systems, and it consists of a vertical tube (usually plastic) connected to the oil outlet of the oil tank. The level of oil in the tank is indicated by the height of the oil in the sight tube. Many are fitted with a small valve between the tank and the sight tube. This valve must be opened to connect the sight tube to the tank to check the oil level. This is often overlooked by the user, who can mistakenly think that the height of the oil in the tube indicates that they have plenty of oil in the tank only to find that the tank is actually empty and all they have is the oil in the sight tube.

A smart controller is one that can be monitored remotely using WiFi connectivity to link to apps and dedicated browser-based platforms which give homeowners greater flexibility and control of their heating system, whether at home or away from their property.

Soakaway is a pit typically filled with hard core into which waste water or condensate is piped so that it drains slowly out into the surrounding soil.

Solar PV stands for Solar Photovoltaics and this type of systems captures energy from the sun using photovoltaic panels to generate electricity. These panels convert sunlight into DC electricity which is then fed into an inverter. The inverter changes the DC current into AC which can be used to power homes and businesses.

Solar Thermal systems are different to Solar PV systems. Solar thermal technology utilises the heat from the sun to generate a domestic hot water supply. Discover more about solar thermal hot water systems.

A solenoid valve is an electromechanically operated valve controlled by an electric current.

Solid Fuels are supplied in a solid state and used to heat some types of home. Examples of solid fuels would be wood or coal.

A strainer is fitted in the inlet cold water supply to an unvented hot water cylinder and it prevents the other controls fitted downstream from being contaminated by any solid matter in the water supply. It is often incorporated into the pressure reducing valve and part of the cold water inlet manifold.

A standard boiler is a conventional heating system which comprises of a boiler, a separate hot water cylinder and a cold water storage tank which is typically located in the loft.

Standing heat loss figures are a measure of the amount of stored heat that escapes from an appliance, such as a water heater or a hot water cylinder, over a period of time. The heat loss figures are a measure of the kilowatt hours lost over a 24 hour period.

System balances refers to the process of adjusting the radiator valves to achieve the same mean water temperature in each radiator on the system so that they all deliver the required heat to the room in which they are installed. This is done by adjusting the ‘lockshield’ valve on each radiator to obtain the required temperature difference across the radiator i.e. between the flow and return pipes. For a condensing boiler this temperature difference across each radiator should be 20K (or 20°C). If the temperature difference is too high, the water is flowing too slowly through that radiator and the lockshield needs to be opened to allow more flow through the radiator. Alternatively, if the temperature difference is to low the water is passing through the radiator too quickly, so the lockshield need to be closed down to restrict the flow to achieve the required temperature difference.

A system boiler directly heats your central heating system and hot water as part of a sealed heating system. Sealed heating systems feature a boiler and separate hot water cylinder but there is no cold water storage tank in the loft. System boilers are heat only and work like a regular (conventional) boiler on the principle of stored hot water however there is no need for a tank in the loft, so it can be an option in a property with little or no loft space or where the space is earmarked for a conversion.

Temperature glide is a term relating to the refrigerants used in air source heat pumps. An air source heat pump uses refrigerant to absorb heat from the outside air and transfer it into the home. Some refrigerants are made from a blend of different refrigerant substances. Each substance in the blend changes from a liquid to a vapour at a slightly different temperature so the refrigerant changes state gradually over a small temperature range - this is known as temperature glide.

Other refrigerants, such as R32 and R290, are made from a single refrigerant substance so do not experience temperature glide. This means they change between a liquid and a vapour at one specific temperature for a given pressure, making their performance more predictable. The use of single component refrigerants helps air source heat pumps deliver consistent operation, supporting efficient performance and simpler system design, installation and servicing.

The terminal guard is fitted to the end of the flue to stop small animals or debris like leaves entering and blocking the flue system.

A Tiger Loop is a de-aeration device that removes air from heating oil. It is fitted on the outside wall of the property.

A timer is a device that can be programmed to switch appliances on and off at specific times.

A thermal store is commonly used with domestic renewable heating technologies as a way of storing excess heat generated, for example a thermal store for hot water.

A Thermostatic Radiator Valve (TRV) is fitted to a radiator in a heating system that is self-regulating (sets the temperature of the radiator automatically) based on the temperature of the room. It controls room temperature by adjusting the flow of hot water to the radiator.

A tundish is a fitting installed on an overflow pipe that provides an air-gap and visual window. It prevents the unsanitary cross connection between an overflow and drain.

An underfloor heating system comprises of concealed underfloor pipework positioned in the floor which can warm a property or dwelling. Underfloor heating makes the entire floor surface area an emitter of heat, warming a room evenly from the floor upwards. Learn more about the different types of underfloor heating systems.

An unvented cylinder is a pressurised water storage unit fed directly from the cold mains water feed. The water contents are heated up by the heat source (a heat pump or boiler for example) and then hot water is delivered at mains pressure, ideal for improving hot water performance from taps. Visit our Knowledge Hub to learn more about hot water cylinders.

A vented cylinder requires a cold water storage tank or cistern (usually located in a loft). The cold water is heated either directly from an immersion heater within the hot water cylinder or indirectly via the central heating boiler, heat pump or solid fuel appliance. Vented systems tend to have lower water pressure. Read more about the different types of cylinders available.

Ventilation refers to the movement of air from an interior space by natural or mechanical methods and replacing indoor air with a supply of fresh air from outside.

A Volumiser is a vessel that is used to increase the volume of a heating system connected to a heat pump. Its primary role is to maintain the minimum volume of fluid required within the heating system even when large parts of the system are not in use, this in turn will help ensure that the heat pump is operating at its maximum efficiency. Various volumiser options are available to suit your system design, both internal and external options as well as combined volumisers/low loss headers for more complex system requirements. 

A water jacket in an oil-fired boiler, also referred to as the water jacket, is a casing that holds the system water which surrounds the combustion chamber. Heat is transferred from the combustion chamber to the water which is then distributed around the wider heating system.

A watt is a unit of power and 1,000 watts is known as a kilowatt (kW). Boilers and heat pumps are rated using kilowatts measuring the amount of heat delivered.

A wattmeter is a device that measures power consumption.

Weather compensation is a type of control which maintains internal temperature by varying the flow temperature from a heat generator relative to the outside temperature. Weather compensation controls are often used with air source heat pumps to improve system efficiency, reducing the flow temperature when it is warmer outside and increasing the flow temperature when it is colder outdoors. You can learn more about weather compensation here.

Zero carbon is the result of no net release of carbon dioxide into the atmosphere.

The zone in a heating system is an internal area that is heated and, depending on the controls set up, its temperature can be independently monitored and controlled. Also referred to as a heating zone, a zone in a heating system will typically be a collection of rooms (such as downstairs or upstairs) or it can be individual rooms.