Active cooling
Normal heat pump cooling, as opposed to ‘passive’ or ‘free’ cooling
Air conditioning
Originates from the system that both cools and controls the humidity of air in a building. It is now loosely used to describe any room cooling application.
Air – Air Heat Pump
A heat pump which uses air as the medium for both heat source and heat sink. It extracts energy from the air outside and distributes it indoors using a fan. It can also be used in reverse to cool the building. Usually called a ‘split’ – having an indoor and outdoor unit
Air – Water Heat Pump
A heat pump which uses air as the medium for heat source and water as the medium for heat sink– its the normal Air Source Heat Pump
Ambient temperature
Outside temperature
Antifreeze
An additive that gives water a lower freezing point. Ethylene or Propylene Glycol is most commonly used in heat pump systems.
Auxiliary heating
Also referred to as supplementary, back-up or top-up heating, this could be an electric element or a boiler.
Back-up heating
See Auxiliary Heating
Bivalent (see Hybrid)
When a heat pump operates alongside a boiler or other heating device it is said to be bivalent, though a somewhat outdated term. There are different control strategies that are usually programmed in the heat pump’s controller. ‘Hybrid’ could be a more familiar term to use here
Borehole
Vertical hole drilled in the ground. A ground source collector pipe can be installed in a borehole. Often 100m deep
Brine
Normally, this is salt water. Curiously it is sometimes used differently for heat pumps (mostly German ones). It’s the fluid used in the source collector and usually consists of a mixture of water with either Ethylene or Propylene glycol.
Arguably the term is actually incorrectly used here.
Buffer cylinder
Water cylinder or tank which acts as a very short-term store of heat. It is generally fitted to minimise short-cycling (meaning frequent stop/start of the compressor).
Cascade .
Used to describe multiple heat pumps, side-by-side in parallel, that operate in sequence to control the output. Note- in refrigeration, ‘cascade’ is used more correctly, as in-line, or one system feeding into another.
Calorific value (cv)
Amount of heat that is potentially available when a fuel is burnt. It is expressed in kJ/kg and assumes 100% conversion efficiency.
CO2 heat pump
System where the refrigerant used is Carbon Dioxide and the ‘Transcritical’ cycle gives rise to different characteristics.
Circulator
Correct term for a ‘water pump’. A central heating pump should, strictly speaking, be referred to as a domestic circulator.
Closed loop
System using sealed plastic ground-source pipes which usually contain a glycol antifreeze. It is the most common trench or borehole ground source system.
Collector loop
The matrix of pipes which is used to extract or dissipate heat energy from or to the external ground, bedrock or water adjacent to the premises.
Compressor
The heart of a heat pump. With its integral electric motor, it compresses the gaseous refrigerant, enables a pressure difference and enables the ‘up hill’ heat-transfer process.
Condenser
Heat exchanger on the ‘hot’ side of the system where refrigerant condenses.
Coefficient of Performance (COP)
Used for measuring the efficiency of a system and found by dividing the useful heat output by the energy input. e.g. a heat pump that produces 4 kWatts of heat for 1 kWatt of input power has a COP of 4. (See also Seasonal Performance Factor)
COP
See Coefficient of Performance
Defrost
Any Air Source unit used in temperatures of around 7°C and below will need an automatic defrost mechanism. This can be reverse-cycle or hot-gas.
De superheater
A device fitted to the outlet pipe of the compressor to capture a small proportion of the heat at an elevated temperature. Used to heat DHW hot water simultaneously whilst in cooling mode
Delta T, ΔT, dt or DT.
Delta is a Greek letter (Δ) that signifies a change in something. Delta T is a difference between two temperatures and is denoted by ‘K’ rather than ‘°C’. For example: flow temperature = 35°C, return = 30°C, dt = 5K.
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.
Efficiency
Loosely used as ‘effectiveness’ but it is more accurately defined as output divided by input. Energy-efficiency of a heating system is specifically the heat output divided by the energy input.
Emitter
Component that ‘emits’ the heat into a building, e.g. radiators or under floor heating.
Enthalpy
Total heat that any solid, liquid or gas holds. It is expressed in kilojoules per kilogram, kJ/kg.
Expansion Valve
A component part that controls the refrigerant flow. Often called an EEV (electronic expansion valve) or LEV (linear expansion valve)
Evaporative cooling
Intake air to a room passes over a wet material. The evaporation causes some cooling. Air must be relatively dry. Not effective for re-circulation.
Energy Efficiency Ratio (EER)
Used to define the energy efficiency of refrigeration and air-conditioning plant. It’s the COP for cooling
Floating condensation
A outdated term that originated from the refrigeration industry and is occasionally used in relation to heat pumps. A better term to use is ‘weather compensation’.
Flow
This confusing term is used to describe the pipe taking heated water out from a boiler. It could also be labelled ‘outlet’ from heat pump. Some confusion can arise when the term is used on the source (cold) pipes. The pipe returning to the boiler is called the ‘return’.
Glide
Characteristic of some refrigerants that are made of mixtures. The ‘glide’ refers to the temperature gradient as the refrigerant changed phase.
Horizontal collector (Ground source)
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.
Hot-gas defrost
Less common method of periodically defrosting an air source heat exchanger
Often used on swimming pool heat pumps
Heat Exchanger
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), one for the cold side (the evaporator).
Heat Capacity
Similar to ‘specific heat’, but based on the volume of a material rather than its weight. Can be called ‘Volume based heat capacity’
Heat Meter
An installed device fitted to the pipework that measures heat quantity produced. It measures liquid flow-rate and temperature rise (or fall).
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 is more energy-efficient to reduce the radiator temperature on milder days. The setting is usually displayed as a graph, and the ‘curve’ dictates the water temperature relative to outside conditions. Each manufacturer has a different method. Read your user manual for a description relating to your product.
Hysteresis
Difference between switch-on and switch-off temperature. It can also be called ‘differential’.
Inverter
Electronic device that varies the frequency, hence rotational speed, of a compressor. It is used to drive and vary the capacity of a heat pump.
Joule
A unit of energy. More commonly-used, than Calorie or British Thermal Unit.
Latent Heat
Energy involved when a substance changes phase, e.g. when a liquid changes to a vapour, or a liquid changes to a solid (e.g. ice)
Mean temperature
Average. Generally, ‘mean’ and ‘average’ are interchangeable.
Mono-energetic
Sometimes used to describe a heat pump that also has an integral electrical top-up heater. The heat is thus mostly heat pump but partly direct electric heating.
Open Loop (ground source)
System where river or ground water is pumped through a heat pump then expelled to the environment a few degrees colder.
Open Loop (heating)
A fairly new term used to describe a radiator circuit where all or most of the radiators are always open and in circulation. e.g. where no (or few) radiator valves are fitted.
Passive Cooling
Cooling without a heat pump. This is achieved by simply circulating the water from the ground directly through the under floor heating system. It is sometimes called ‘free cooling’
Phase change
Changing state between solid, liquid or vapour.
Recovery time
The time taken for a hot water cylinder to be restored to its required temperature, e.g. after a bath.
Refrigerant
The working fluid within the heat pump. It evaporates in one part and condenses in another. By doing so, heat is transferred from cold to hot. This fluid is sealed in and should not degrade within the life of the heat pump.
Return
This term is used to describe the pipe coming back to a boiler or heat pump. This could also be labelled ‘inlet’. It is the opposite pipe to the ‘flow’.
Reverse-cycle defrost
An air source heat pump usually ‘reverses’ to melt ice build-up on the heat exchanger fins. This may happen hourly in winter, and lasts only a few minutes
Rotary compressor
Any compressor without a piston is rotary. Term commonly used to describe small ‘offset-roller’ compressors as used in small air conditioning units
Scroll Compressor.
Compressor which was developed around 1990 and involves two ‘scroll’ shaped components that oscillate relative to each other.
Secondary return hot water loop
Hot water is pumped around a loop to ensure taps run-hot immediately. Commonly used in hotels and large houses. can be energy-wasteful.
Sensible heat
The energy change when a material changes in temperature but does not change phase. e.g. stays a liquid
Set-back
A reduction in temperature-setting for unoccupied or night periods. It is often programmed with daily time settings.
Set point
Temperature setting of a thermostat.
Short-cycling
Term given when a compressor switches on and off fairly quickly. Normally you would not expect a compressor to start more than once every 15 minutes. If it runs for less than 5 minutes, it would be described as ‘short cycling’. Ideally, heat pumps are designed for long run-times at steady-state
Slinky
Sometimes used to describe the type of ground collector pipes which are coiled before burying in a trench. Slinky is a trade mark.
Seasonal Performance Factor (SPF)
Similar to COP, but is the total heat over a year divided by the total electrical input. This also takes into account any necessary electric top-up heating load
Seasonal Coefficient of Performance Factor (SCOP)
This is a calculated prediction of the average COP over the year.
Source
Where the heat is taken from – e.g. the outside air or the ground.
Specific heat
The property of a material relating to its thermal capacity.
Expressed in kJ/kg k, i.e. the amount of energy required to raise the temperature of 1kg of material by 1°C. We have used the ‘heat capacity’ on a volume basis (kJ/m³ k) in this book since it is more relevant. (Note: ‘k’ is the same as ‘°C’ but refers to a change in temperature rather than a specific temperature)
Standing Column
Type of borehole that is used as both a water supply and as a source for a heat pump by re-circulating the water around the borehole.
Steady-state
When a system reaches equilibrium over time. A relatively small heat pump may spend a lot of time at steady-state
Thermal mass
Usually refers to a building or item’s ability to store heat, i.e. a high thermal mass item will take a long time to heat up and also a long time to cool down. In essence, it’s the same as specific heat. A ‘light-weight’ building has low thermal mass.
Transcritical
Vapour compression cycle where the pressure rises above the ‘critical’ point. This transcends the point where ‘liquid’ or ‘vapour’ states cannot be defined. Only applies to CO2 refrigerant
Weather Compensation
Automatic adjustment of the heat pump’s output temperature dependent on the outside temperature. The term ‘curve’ is often used — referring to the curved graph that is used for the adjustment. (samsung call it ‘Water Law’!)
Wet heating system
Heating system that circulates water as the heat transfer medium. e.g. radiators or underfloor pipes