Summary of key words and definitions

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.


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

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.

Air Quality Standards

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


An appliance that provides hot-water or central heating.


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.

Bunded Tank

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

Balanced flue (Room sealed)

The boiler is sealed within the room it is fitted. The flue has a twin-wall, where fresh air for combustion is drawn in through the outer pipe and the combustion gases from the boiler are vented outside via the inner pipe.

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.

Biomass Boiler

Biomass boilers are fully automatic appliances, behaving more like an oil or gas boiler, and use advanced controls which cleverly regulate the amount of fuel being delivered to the burner to match the heat demand on the boiler.

Biomass Fuel

Biomass refers to a type of fuel sourced from living/non living plant matter, which could be in the form of either wooden logs, wood pellets or chips – wood pellets being the most energy efficient.

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.

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.


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

Combination valve

PRV, NRV, Expansion relief valve and strainer combined.

Combustion Air Vent

A vent that MUST be provided for Conventional flued boilers


The active destruction of boiler metal by the pitting action of dissolved oxygen in the boiler water

Combi Boiler

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

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.

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.


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

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

Circulating Pump

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

Condensate Trap

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

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.

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.

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.

Central Heating Flow

This is the heated water leaving the boiler and entering the radiator system.


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

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.


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.


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

Expansion Vessel

Accommodates the expansion of the heating system water. Must be correctly sized

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.

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.


Connects to a boiler to remove combustion gasses

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.

Flow Rate

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

Flue Gas

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

Gss 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.

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.

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 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.

Immersion Heater

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


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


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

Lockshield Valve

A valve that is shielded with a plastic cap that prevents it being accidentally changed once adjusted


Boiler lockout is a term used when the boiler has shut down, due to not having the correct parameters to run e.g. it may happen when the boiler runs out of fuel


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

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.


Any combination of nitrogen and oxygen in a compound form.

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 device fitted in the condensate discharge pipe from a condensing boiler to neutralise the acidic condensate before it is discharged into drain or soakaway.

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.

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.


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

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


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

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


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)

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.


Renewable Heat Incentive. Government initiative encouraging and rewarding the use of renewable fuels to heat properties


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.

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.


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

Solenoid valve

An electromechanically operated valve controlled by an electric current

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.

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.


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’.

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!

Solar Thermal

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

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.

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

Terminal Guard

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


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.

T&P (temperature and pressure) Valve

Fitted to an unvented hot water cylinder, the temperature and pressure relief (T & P) valve prevents the water temperature exceeding 100°C in the event of the control and overheat thermostats failing, as required by the Building Regulations. The valve is factory-fitted by the cylinder manufacturer and is pre-set to operate at 90°C and 7bar.

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

Underfloor Heating

Concealed underfloor pipework set to warm a property / dwelling.

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.

Weather Compensation

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


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