Knowledge Hub



Air bubbles can form in pipework e.g oil pipeline, causing a blockage.

Air Source Heat Pump

Uses the principles of thermodynamics to absorb energy from the surrounding environment and convert into usable energy and provide heating for our homes. Learn more about how air source heat pumps work

Air Quality Standards

This legislation monitors the level of pollutants within inside and outside air to an adequate level.

Ambient Air

The air around ‘us’ that contains thermal energy which can be used as heat energy for power of a product or source.

Ambient Temperature

The temperature of any medium e.g. air that surrounds an appliance or building.


Baffles in water heating boilers are spiral twisted pieces of metal inserted into the heat exchanger causing turbulence, in addition to back pressure for better heat extraction.


A renewable energy that is created from organic matter. It can 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.


An appliance that provides hot-water or central heating.

Boiler Short Cycling

This is 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 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.

Boiler Pressure

This 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.

Boiler Thermostat

A control, set by the user or manufacturer, that limits the temperature of water passing through the boiler by shutting off.

Boiler Upgrade Scheme

The Boiler Upgrade Scheme is a Government initiative to encourage householders to swap from fossil fuelled heating systems to renewable heating systems, such as air source heat pumps . BUS provides 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. Read more about the Boiler Upgrade Scheme here.

Building Regulations

Building regulations are mandatory minimum standards for any design and construction to UK buildings that have to be adhered to when performing such tasks.

Bunded Tank

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

Carbon dioxide

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.

Check Valve

Fitted in the cold water supply to an unvented hot water cylinder, this valve prevents backflow of the heated water into the cold water main. The expansion of the water, when heated in the cylinder, is accommodated by the expansion vessel connected to the cylinder.

Circulating Pump

Moves liquid around a closed circuit e.g. water within a heating system


A Specific amount of space required around a heating appliance for safe and efficient operation.

Cold Water Inlet Manifold

This 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.

Combi Boiler

A combi (combination) boiler produced bother heating and hot water from one compact appliance.  

Combustion Air Vent

A vent that MUST be provided for Conventional flued boilers


A process whereby an engineer checks the settings and controls on the boiler, to ensure it is operating at maximum efficiency and meets current building regulations


Slightly acidic liquid which occurs when flue gas temperatures drop below ‘dew point’ and the steam present condenses.

Condensate Trap

Used to collect the moisture from the condensed gases in the boiler

Condensing Boiler

A condensing boiler has 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 the heat energy that would otherwise be lost to outside through the flue system.

Contents Gauge

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.

Conventional Boiler

Sometimes referred to as a traditional or heat only boiler. It 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.

Conventional Flue

Relies on the natural circulation of air to expel the products of combustion up the chimney. Air for combustion is supplied from the room the oil boiler is situated in


Every heating system needs a controller which enables it 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 installed, it must be correctly set up otherwise the system may be unable to operate correctly.


The active destruction of boiler metal by the pitting action of dissolved oxygen 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 through 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 models are designed to be electrically heated but also incorporate a high performance solar coil to allow easy connection to a solar thermal system. (See also indirect)

Domestic Hot Water

Water that has been heated for use in your home.

Dual Thermostat

This 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 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.

ECO (Energy cut out) – 2-port valve

Used on an unvented hot water cylinder, the energy cut out is a safety control that 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 (i.e. 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.


This is the relationship between energy supplied to the boiler and energy output from the boiler.

Energy efficiency

An approach to reducing energy consumption, while maintaining a certain level of energy output

Energy Saving Trust

An independent organisation that dedicated to promoting energy efficiency to both domestic and commercial audiences.


Energy Related Products Directive. Designed to aid the EU in achieving reductions in carbon, NOx and SO² emissions


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

Expansion Relief Valve

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.

Expansion Vessel

This 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 heat generator 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.


External volumiser

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 sited directly behind the Aerona³ heat pump and it provides a minimum volume of water to allow the heat pump to controls its modulation.

Feed and Expansion (F&E) tank

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

Fire valve

A fire safety shut-off valve used with oil burning equipment. It is installed in the oil supply pipeline outside the building

Filling Loop

A pipe that connects central heating system to the mains water supply, used to re-fill (re pressurise) a central heating system with water.

Flexi foot

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.

Flexi hoses

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.

Flow Rate

Flow rate is the volume of fluid which passes per unit time which is measured in mL/ph.


Connects to a boiler to remove combustion gasses

Flue Gas

This is the gas that is emitted due to the combustion of the fuel.

Gas Oil

Type of fuel oil distilled from petroleum and heavier than paraffin oil. Commonly used as a heating fuel.


The refrigerant glide is the difference between the saturation temperatures at a given pressure, of the different elements (refrigerants) in a blend.

Header Tank

Usually found in the loft, a water storage tank or feed and expansion tank.


Thermal energy produced from the results of combustion, chemical reactions, and movement of electricity or friction.

Heating Calculations

Calculated by our design team heating calculations determine the heat requirements of your room/s.

Heat Exchanger

This part within a 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 Loss

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.

Heat Output

Level of heat produced by a product / appliance.

Heat Transfer

The movement of heat from one area to another.

Heat Pump Association

The Heat Pump Association (HPA) is the UK’s leading authority on the use and benefits of heat pump technology. It includes many of the UK's leading manufacturers of heat pumps, components and associated equipment.


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.


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. Learn more about HVO here.

Hybrid Heating

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.


A mixture of gaseous substances containing enough heat energy to trigger a spark starting the process of combustion.

Immersion Heater

An electric heating element that is positioned in the liquid to be heated, typically in a domestic hot-water cylinder


Indirect cylinders have two coils for connection to primary heat sources.


A qualified individual to carry out fitting, commissioning, maintenance and servicing of a heating system.

Isolating/Isolation Valve

A valve used to close off water to a specific area/appliance allowing selective distribution as and where necessary.


Casing around a product / appliance.


Light fuel oil obtained by distilling petroleum, used especially in domestic heating boilers and jet engines


Statutory law(s) set by a governing body or similar.

Low loss header

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

Manual valves directly control the flow of a substance.


A filter that protects new system components from damage and increases the effectiveness and lifespan of existing ones.

Mean Water Temperature

The average (mean) temperature of the flow and return water into the radiator or across the heating system.


A merchant is a stockist of supplies including the products and accessories needed to install and maintain a domestic heating system. To find a local merchant who sells Grant products, please visit

Multi-zone wiring

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 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

The Government target of net zero for UK greenhouse gas (GHG) emissions by 2050

Non-Return Valve (NRV)

In a heating system, non-return valves 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.


A set period of time where demand for energy like electricity is low. An example would be Economy 7.


The Office of Gas and Electricity Markets. OFGEM are a non-ministerial government department and an independent National Regulatory Authority with a key focus on protecting the interests of existing and future electricity and gas consumers.


Oil Firing Technical Association which is a competent persons scheme that recognises installers who are approved to fit and maintain oil, solid fuel and renewable heating equipment.

Oil Filter

This is a device 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.

Open Vented Cylinders

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

Open Safety Vent (OSV)

The OSV pipe (or Open Safety Vent pipe) on an open vented heating system 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 (F & E) cistern, below the cistern cover but above the water level.


A seal in the form of a ring, typically made from rubber.


In the case of an oil boiler, a photocell is a sensor in the burner that reacts based on the presence of light.


Flue gas temperatures are very low on condensing boilers so a plume of water vapour can often be seen at the terminal (similar to your breath on a cold day)

Pressure Relief Valve

Type of safety valve used to control or limit the pressure in a system

Pressure Jet Oil Burner

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

PRV (pressure regulating or reducing valve)

Fitted in the cold water supply to an unvented hot water cylinder, this valve reduces the mains water pressure to the required working pressure of the cylinder. These are usually pre-set at a pressure to suit the cylinder they are to be used with. This valve is now usually incorporated into a ‘cold water inlet manifold’ (see definition below) supplied with an unvented hot water cylinder.

Pumping Over

Occurs in an improperly configured central heating system whereby water is discharged into the feed and expansion tank through the expansion pipe, while the pump is running


A price for carrying out work and/or supplying equipment.



R32 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.



A type of heat emitter.


The process of converting old waste materials into new products.


refrigerant is a chemical substance used in a heat pump to heat the home


A natural source or resource of energy such as water, wind or solar power.


Renewable Heat Incentive. Government initiative encouraging and rewarding the use of renewable fuels to heat properties. This Scheme closed to new applications in March 2022.


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


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 basically a list of boiler efficiency ratings.

Setback Control

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.

Sight Gauge

Once the most commonly used type of contents gauge, this consists of a vertical tube (usually plastic) connected to the oil outlet of the 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 mistakenly thinks 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!

Smart controller

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.


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

Solar PV (photovoltaics) captures energy from the sun using photovoltaic panels. 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

Technology that utilises the heat from the sun to generate a domestic hot water supply.

Solenoid valve

An electromechanically operated valve controlled by an electric current

Solid Fuel

Fuels that are supplied in a solid state. An example would be wood or coal.


Fitted in the inlet cold water supply to an unvented hot water cylinder, this 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’.

System Balancing

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.

System boiler

Directly heats your central heating system as well as hot water for a cylinder. They 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.

Terminal Guard

Fitted to the end of the flue to stop small animals or debris like leaves entering and blocking it.

Tiger Loop

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 device that can be programmed to switch appliances on and off at specific times

Thermal Store

Common in domestic renewable heating technologies as a way of storing excess heat generated

Thermostatic Radiator Valve (TRV)

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 adjucting 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.

Underfloor Heating

Concealed underfloor pipework set to warm a property / dwelling.

Unvented Cylinder

A pressurised water storage unit fed directly from the cold mains water feed and therefore can deliver hot water at mains pressure. Ideal for improving hot water performance from the tap

Vented Cylinder

A vented cylinder requires a cold water storage tank (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 or solid fuel appliance.


The movement of air from an interior space by natural or mechanical methods


A Volumiser is simply 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.


Water jacket

The heat exchanger within a boiler.


A unit of power.


A device that measures power consumption.

Weather Compensation

A control which maintains internal temperature by varying the flow temperature from a heat generator relative to the outside temperature

Zero Carbon

The result of no net release of carbon dioxide into our atmosphere.


An internal area that would normally be cooled, heated or ventilated.

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