The COP or ‘Coefficient of performance’ is a measure if energy-efficiency, and found by dividing the useful heat output by the electrical energy input. e.g. a heat pump that produces 4 Kwatts of heat for 1 kWatt of electrical input power has a COP of 4.  The COP will vary depending on the operating temperatures of the system.  Look out for optimistic figures quoted in unfairly favourable operating conditions


Seasonal Performance Factor is similar to COP, but is an average figure taken over the year. It is the total energy input (including pumps and any supplementary electric boost heaters) compared with the total useful heat output delivered to the building.


The word “Efficiency” can be used rather loosely, but on this site, it is the ratio of useful output to energy input. e.g. if an open fireplace loses half its energy up the chimney it is said to be 50% efficient. A bright light bulb might be efficient at lighting a space, but we generally talk of the energy-efficiency. e.g. an old 100W lightbulb might emit only 5W of light. It is therefore only 5% efficient.

Source, or heat source

This is wherever the heat is being extracted from. e.g. the outside air, river or ground. Sometimes referred to as an ambient source.     It could also be recovered heat from extract air or effluent water etc.


This is defined as ‘heat from the ground’. Proper geothermal is heat from the earth’s core extracted from very deep in the ground, as in the steam that powers the whole of Iceland. The term is sometimes used (wrongly to my mind) to describe Ground Source Heat Pumps.

Horizontal collector

This can be either coiled ‘Slinky’ or straight pipes that are buried up to 2m deep in open ground (your garden). The pipe is usually plastic and contains a Glycol antifreeze solution.


The name sometimes used to describe the type of ground collector pipes which are coiled before burying in a trench.     


This is simply a vertical hole drilled in the ground. A ground source collector pipe can be installed into this.

Open Loop

This is the type of source where river or ground water is pumped through a heat pump then expelled to the environment a few degrees colder.

Closed loop

This is where a sealed plastic ground pipes are used which usually contains a glycol antifreeze. This is the most common trench or borehole ground source heat pump system.

DX system

Abbreviation for ‘direct expansion’. This is where the refrigerant flows directly within the ground pipes. This system is less common, and may have some disadvantages.


An additive that lowers the freezing point of water. Ethylene or Propylene Glycol are most commonly used in heat pump systems.


Brine is normally defined as ‘salt water’. However, this term seems to be have adopted to describe any antifreeze mixture. A brine-water heat pump usually means one having glycol antifreeze on the ‘cold’ side and water on the ‘hot’ side.

Emitters or ‘heat emitters’

Another term used to describe radiators or underfloor heating. This is the component that emits the heat into the building.


Abbreviation of ‘heat sink’. This is the name sometimes given to the part where the heat is usefully dissipated, such as radiators in the room, underfloor heating etc. ‘Heat load’ or ‘heat emitter’ are other terms

DHW    Domestic Hot Water

This the stuff that comes out of your taps. With heat pumps it is always stored in a DHW cylinder. Combi-boilers are direct gas water heaters with no storage cylinder.

Buffer tank

This is simply a large water cylinder that may be used to improve the efficiency by reduces the number of stop/starts that the compressor makes, and ensures a high water flow-rate through the heat pump. However, buffer tanks can introduce inefficiencies too. Modern variable-speed heat pumps tend not to need a buffer cylinder. A small buffer tank may also be needed to help with the defrost cycle of an air source heat pump.


This is the working fluid within the heat pump. It evaporates in the ‘cold’ part and condenses in the ‘hot’ part. By doing so, heat is transferred from cold to hot. This fluid is sealed in, and should not leak or degrade within the life of the heat pump.

Heat Exchanger

This is a simple component that transfers heat from one fluid to another. It could be liquid to liquid, liquid to air, air to air. Two heat exchangers are housed within the heat pump, one for the ‘hot’ side (the condenser), and the other for the ‘cold’ side (the evaporator).


This is an extra small heat exchanger fitted to the compressor discharge pipe that can produce a small amount of heat at a higher temperature. If fitted, it can heat domestic hot water to a high temperature whilst also room heating or cooling

Passive Cooling

Passive cooling is where the ground water is simply pumped around underfloor heating. This gives limited amount of ‘free cooling’. It will only be effective with boreholes or large trenches in wet ground. Whilst it may have a limited effect, it can be almost free!

Passive heat recovery ventilation

This is where the out-going exhaust air passes its heat to the incoming fresh air with only the use of a simple heat-exchanger. It uses no heat pump.

Heat Pump Capacity Rating

A heat pump is given a kW heat output rating. This value will vary depending on the working temperatures. The electrical power input will very roughly be between a 1/2 and a 1/5 of the heat output . E.g. a 5kW heat pump will produce nominally 5kW of heat, and require typically between 1 and 2.5kW electrical input, depending on operating temperatures.


Power is usually the electrical rating (Watts). The intensity of a device  e.g. a 3 kW (3,000 watt) electric heater,  or 100w bulb (using 100w or electricity).
I personally don’t like the use of ‘power’ to describe heat from a radiator.  I would use ‘heat output’ (kW) in that case.


This is a quantity of ‘power’ over time, and usually expressed in kWh (kilo-watt hours).  e.g. a vacuum cleaner of 1kW power  would consume 1kWh of energy if used for 1 hour.   A 100w power bulb uses 2.4kWh of energy per day.


This is a sophisticated electrical device that can vary the output capacity of a heat pump by changing the speed of the compressor motor. It therefore can vary the heat output to match the heat demand. (An inverter also acts as a low-current soft-start)


Night set-back is the amount you reduce the heating at night (or in the daytime when you are out). We usually reduce the setting only a few degrees at night when a heat pump is used.

Heating Curve

Almost all heat pumps, and many condensing boilers control the heated water temperature (to the radiators etc) using a Heating Curve.  To explain –  It makes sense to have the radiators warmer when its colder outside.  The ‘curve’ setting dictates the water temperature relative to outside conditions. Each manufacturer has a different number setting.  Read your user manual for a description relating to your product.  As an example, the circulating floor water might be 35°C when its 0°C out, but the water may be only 30°C when its +5°C outside

Soft Start

This is an electrical device that reduces the start surge that is taken by a conventional compressor. It does not save energy, but stops lights flickering, and may reduce wear & tear.


This term is used in RHI payment calculations. It is the amount of energy a building is estimated to use over a year. It is calculated by an Energy Assessor for use in and EPC (Energy Performance Certificate)