AIR SOURCE HEAT PUMP: Is one of the types of Heat Pump available. It takes heat from the outside air and moves it into the building. Two different models are available, Air to Air and Air to Water.

AIR TO AIR HEAT PUMP: Heat is extracted from the outside air and transferred directly to the indoor air.

AIR TO WATER HEAT PUMP: An outdoor unit extracts heat from the outside air and delivers it via water or glycol circulated through fan coils.

ALTERNATIVE ENERGY: Usable energy intended to supplement or replace energy derived from fossil fuels.

ANNUAL EFFICIENCY:The average relationship between supplied and utilised energy during a period of 12 months. Annual efficiency includes both heating and hot water.


BOILER: A boiler is a closed case in which water or other fluid is heated. The heated or vaporised fluid exits the boiler for use in various processes or heating applications.

BOREHOLE: Holes drilled into the earth. In a closed loop system, loop pipe would be laid in a series of boreholes. In an open-loop system, the borehole world be drilled into the underlying aquifer and used to pump water to provide flow to the heat pump.

Btu:  British Thermal Units. A method of measuring a quantity of heat. A BTU is the amount of heat required to raise the heat of one pound of water by 1°F. A BTU can be calculated by taking the number of watts and multiplying it by 3.413. 1 watt = 3.413 BTU’s.

Btuh: British Thermal Units per Hour


CARBON FOOTPRINT: Total amount of greenhouse gas emission caused by a business, activity, home etc.

CFM: Cubic feet per minute of air flow.

CLOSED-LOOP: Closed Loop systems pump an anti-freeze solution through pipes buried in the ground. The loops can be installed either horizontally in trenches dug in the ground, or vertically in a borehole or series of boreholes drilled into the ground. The overall length of these loops depends upon the thermal conductivity of the ground in which they are lain. Heat loss between the antifreeze solution and the loop-pipe; the loop-pipe and the grout surround and the grout surround and the ground reduces the efficiency of these systems in comparison to an Open-Loop system. However a Closed-Loop System can be installed anywhere in the country, whereas an Open-Loop system depends on the prevailing Hydro-Geology. Back up heating is usually recommended for use with Closed-Loop Systems.

COMPRESSOR: The compressor is at the core of a refrigeration or heat pump system. Its purposes are twofold. Firstly it circulates the refrigerant fluid through the circuit like a pump and secondly it compresses and raises the pressure and temperature of the refrigerant vapour so that it can be easily condensed back into a liquid, enabling the heat process transfer to continue.

CONDENSER: In refrigeration systems the condenser is the heat exchanger where hot compressed refrigerant gas is condensed to a liquid and further cooled to continue its journey around the circuit.

CoP: Coefficient of Performance. The ratio of useful output to the amount of energy input. Generally used as a measure of the energy efficiency of heat pumps. COP equals the heat output in BTU per hour divided by the heat equivalent of the electric energy input. The higher the COP, the higher the efficiency of the heat pump. The COP rates a heat pump’s ability to efficiently use electricity in its operation.

CYCLING LOSSES: The actual efficiency of a heating or cooling system is reduced due to


DECC: Department for Energy and Climate Change.

DEFROST CYCLE:  In some circumstances, a coat of ice may form on the evaporator in the outdoor unit of an Air Source Heat Pump. This coating of ice is removed with an automatic defrost cycle, periodically. The frequency of this cycle is controlled automatically by a combination of time and temperature of the external coil.

DIRECT SYSTEM: A direct system operates with the refrigerant in either or both heat exchangers in direct contact with the source or destination medium.


EER: Energy Efficiency Ratio. Another term used for the CoP. The term EER is often used in place of CoP when cooling is being discussed instead of heating.

ENERGY REQUIREMENT:  The amount of kilowatt hours (kWh) required to heat a home and provide hot water for 12 months. This is excluding household electricity for lights, ovens, dishwasher etc.

EST: Energy Saving Trust. Government funded, independent non-profit organisation created to manage and co-ordinate various programmes to improve energy efficiency. It has a particular focus on the domestic sector.

EVAPORATOR: The heat absorbing mechanism in a heat pump.


FOSSIL FUELS: Fuels that are derived from organic matter sediments and formed underground. Common examples include natural gas, coal and petroleum.


GEOTHERMAL ENERGY: Heat from the Earth. It is a clean, renewable resource that provides energy in a variety of applications and resources. The heat of the Earth is available everywhere and is considered a renewable energy as the heat emanating from the centre of the earth is essentially limitless. Geothermal energy is obtained by deep drilling in suitable geological areas where volcanic activity can provide heat from the Earth’s crust. This is a separate and distinct technology and should not be confused with ground source heat pumps.

GREEN DEAL: The Green Deal aims to improve the energy efficiency of more than 14million homes by 2020. It will allow energy companies to provide both domestic and non-domestic customers with a range of property improvements which will increase energy efficiency and reduce heating costs. Green Deal Providers, such as B&Q, E.ON and British Gas will provide robust, quality work to peoples homes and businesses with no upfront capital cost. The costs incured by making these improvements will be recouped through installments on a property owners energy bill over a number of years. The Green Deal is not an advance payment or personal loan therefore no obligation moves with the property owner should they move.

GREEN DEAL ASSESSMENT (GDA): A Green Deal Assessment is required if you wish to apply for the domestic RHI. It only applies for retrofit installations – self builds will only require an Energy Performance Certificate (EPC). A GDA incorporates an EPC and an occupancy assessment. Any RHI payment made for an installation will be calulated using a GDA report.

GROUND SOURCE HEAT PUMP: (GSHP) The earth is continually absorbing heat from the sun, which is stored in the ground. The ground temperature is constantly between 8 and 14°C all year round at depths below 1 metre. A Ground Source Heat Pump is designed to use this constantly renewed energy to supply your domestic heating and hot water requirements. It is a central heating and /or cooling system that pumps heat to or from the ground. The design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems.


HEAT EXCHANGER: a device for the transfer of heat energy from one medium to another. It can take a variety of different forms, but the most recognisable would be a radiator in which hot water is circulated through pipes or plates and gives up its heat to the surrounding air.

HEATING ONLY HEAT PUMP: Ground source heat pump systems are often optimised specially to deliver heating only. These units are often capable of delivering greater overall energy efficiencies than reversible units because the system is designed for a constant unvarying load above the freezing point of water at the evaporator section and can dispense with the defrost cycle.

HEAT PUMP: EN 14511 – part 1 defines a heat pump as an “encased assembly or assemblies designed as a unit to provide delivery of heat. It includes an electrically operated refrigeration system for heating. It can have means of cooling, circulating, cleaning, and dehumidifying the air. The cooling is by means of reversing the refrigeration cycle.

HEAT SINK: the medium (air, water or earth) which receives the heat rejected from a heat pump.

HEAT SOURCE: the medium (air, water or earth) from which heat is extracted by a heat pump.


INDIRECT SYSTEM: has a secondary medium, water or glycol, circulated through the heat exchangers. By this means, the refrigerant is contained in one remote place and refrigerant quantities can be minimised.



kW: Kilowatt unit of power. Used to specify the thermal performance of a Heat Pump as well the power energy it consumes.

kWh: Kilowatt hours. The standard unit of sale of electricity. It is the equivalent power consumed by a purely resistive load of 1000 Watts (1kW) for 1 hour.



MCS: Micro-generation certification scheme. MCS is owned by the DECC. It is a scheme devised by the Government to regulate both products and installers operating within the UK market.

MCS accredited: The Micro-generation Certification Scheme recognises quality installers of renewable technologies. Suppliers must be MCS accredited in order for your system to be eligible for grants or subsidies.

MICRO-GENERATION: Any power generation equipment that produces less than 50kW of electricity. This is achievable in homes and small businesses with the use of products such as heat pumps and solar panels.



OFGEM: The Office of Gas and Electricity Markets. The industry regulator in the UK.

OPEN-LOOP: Open-Loop systems are the most effective Ground Source Heat Pump option. They take heat out of the ground water, usually abstracted from an aquifer (a body of saturated rock through which water can easily move). As the water is in direct contact with the ground there is no heat loss between surfaces. The water is extracted via a borehole and pumped through a heat pump, where heat from the water is extracted. The water is cooled by approximately 6˚before being returned to the ground, where it immediately begins to regain heat from the earth. Open-Loop systems can also provide cooling at a fraction of the cost of conventional air conditioning.

OPERATING PERFORMANCE: A way of assessing how the heat pump is performing at any moment in time where all parameters are known.

OUTPUT REQUIREMENT:  The amount of kilowatts required to heat a home (and produce its hot water) on the coldest day of the year.




REA: The Renewable Energy Association represents renewable energy producers and promotes the use of all forms of renewable energy in the UK.

RECC: Renewable Energy Consumer Code (previously the REAL consumer code). By adhering to the RECC, installers provide a high standard of consumer service when selling renewable sources to domestic customers.

REFRIGERANT: The heat transfer fluid contained in a heat pump refrigeration circuit. Normally this is a chemical contained in a hermetically sealed circuit that has a low temperature boiling point. Refrigerants can be one of a number of man-made Fluorocarbons or a Hydrocarbon compound.

RENEWABLE ENERGY: Energy which comes from natural sources such as water, wind and sunlight. Renewable energy is derived from natural processes that are constantly replenished. In its various forms, it derived directly from the sun, or from heat generated deep within the earth.

RENEWABLE HEAT INCENTIVE (RHI): The government scheme used to reward consumers for generating their own energy through systems. The scheme is currently available for commercial installations. The domestic RHI is due to be launched in Spring 2014.

RENEWABLE HEAT PREMIUM PAYMENT (RHPP): a Government scheme that pays a one off lump sum in order to help fund the installation of systems such as solar thermal or heat pumps.

RENEWABLE ENERGY: Energy that is generated from unlimited resources such as the sun, the wind, flowing water etc.

RENEWABLE RESOURCE: A resource that can be renewed or replenished over a humas time scale.

REVERSE CYCLE HEAT PUMP: A reverse cycle system is a refrigeration system that can, by means of a valve that reverses the flow of the refrigerant fluid, change the operation of the system from heating to cooling.

RETURN ON INVESTMENT (ROI): the term used to assess the return on your initial capital. E.G If you paid 12,000 for an Open-Loop Ground Source Heat Pump System, the ROI would consider your fuel savings and generation tariff payments in order to give a percentage rate of how quickly your initial investment would be paid back to you.


SCoP: Seasonal Coefficient of Performance. Average CoP over the entire heating season.

SEASONAL PERFORMANCE FACTOR (SPF): A measure of the heat energy extracted from the ground over the heating season divided by the amount of electricity used to do so. It measures the efficiency of a GSHP installation. The overall SPF will be dependent on the ground conditions, the heat delivery system used and the efficiency of the heat pump itself. The SPF will improve if the ground is charged with heat in the summer in order to reduce the amount of work needed by the heat pump in winter.



TEMPERATURE SET POINT FOR HEATING: The temperature at which heating systems are triggered to stop the temperature falling below a set level.


UK LOW CARBON TRANSITION PLAN:  A government plan plotting how the UK will meet the 34% emissions cut on 1990 levels by 2020.

UK RENEWABLE ENEGRY STRATEGY: Explains how and why the government intends to increase the UK’s use of renewable electricity, heat and transport.

UNDERFLOOR HEATING: An efficient method of distributing heat into a building to provide comfortable space heating. Lower temperatures (around 40°c) can be distributed over a larger area than the higher temperatures (around 70°c). It makes a good match for GSHP’s as a GSHP will deliver a higher CoP if a lower delivery temperature can be used.



WATER SOURCE HEAT PUMP: Central heating and /or cooling system that pumps heat to or from surface or underground water such as a lake, river or well.

WATER TO WATER: A heat pump where the collecting medium (source) is either ground water or a glycol solution and the destination medium is also water or glycol. These systems are invariably indirect.

WATER TO AIR: Water source systems are used where a supply of ground water, ponds or lakes are available. These systems are invariable indirect and require filtration to remove particles from the water source before it enters the heat exchanger. In the UK, permission from the Environment Agency is required if abstracting more than 20,000 litres of ground water per day. The majority of domestic installations would not require a licence.




ZONING: A method of partitioning your home into independently controlled comfort zones for enhanced comfort and efficiency.