This glossary explains all important terms concerning heat pumps. It is being expanded continuously and serves as a library of knowledge for all those that are interested in heat pumps.
A
AI Integration:
Using artificial intelligence in order to optimize the heat pump workings. Self-learning algorithms control operations according to user behavior, weather and electricity tariffs. Can raise efficiency by 10-15%.
B
Bivalence Point:
The outside temperature below which the heat pump alone cannot cover the thermal demand anymore. An additional heating device, for example an electric heating rod, is activated. Well thought out systems very rarely fall below this point, if at all.
Buffer Storage:
Heat storage tank that stores heating water. Decouples heat generation and heat demand over time, reduces cycling, and increases efficiency. Especially useful for on–off heat pumps.
Building Energy Act (GEG):
German law that regulates energetic requirements for buildings, including specifications for heating systems. Amended in 2023, it was the topic of intense public debates on heat pumps.
Brine:
Anti-freeze liquid mixture that circulates in ground collectors or boreholes. Mostly water with Glycol addition. Takes water from the ground and transports it to the heat pump.
C
Carnot Process:
Ideal thermodynamic circulation process that describes the maximum heat pump efficiency that is theoretically possible. Realistically achievable values reach up to 50-60% of the Carnot-efficiency factor.
CO2 Intensity:
Quantity of CO2-emissions per produces unit of energy. For electricity in Germany 2024: circa 363 g CO2/kWh (363 grams of CO2 per Kilowatt-hour). Consistently decreases because of the expansion of renewable energies. Relevant for the eco-balance of heat pumps.
Combination Storage Tank:
The combi-tank stores both domestic hot water (drinking water) and heating water, often in separate compartments or by heat exchangers. Allows for a more flexible mode of operation.
Compressor:
The ‘engine’ of the heat pump. It compresses the gaseous refrigerant, thereby raising its pressure and temperature. Modern compressors are especially optimized for specific refrigerants and work efficiently and quiet.
Condenser:
Heat exchanger in the refrigeration circuit in which the refrigerant condenses, thereby emitting heat to the heating system.
Construction Age Category:
Period in which a building was constructed. The year of construction hints towards the original energy standard but it is not critical for the suitability of a heat pump. More relevant are both the current state of renovation and the possible flow temperatures.
COP (Coefficient of Performance):
Performance ratio of the heat pump in a standardized scenario. It indicates how much heating energy is produces out of one unit of electric energy. Example: A COP of 4.0 means that out of 1 kWh of electricity a heat pump generates 4kWh of heat. The COP is measured at defined operational points, e.g. A2/W35 (air intake at 2°C, water at 35°C flow temperature).
Cycling:
The sport or activity of riding a bicycle. Cycle racing has three main forms: road racing (typically over long distances), pursuit (on an oval track), and cyclo-cross (over rough, open country). Oxford Languages
Cycling (the actual one):
Repeated turning off and on of the heat pump. Should be avoided because it lowers efficiency and lifespan. Modern inverter heat pumps can modulate their power and thereby avoid cycling.
D
Domestic Hot Water (DHW) Share:
Percentage of total energy demand that is used for domestic hot water. Typically 15-25% in single family homes. In well insulated buildings with a low heat demand, this percentage can be significantly higher.
Dynamic Electricity Tariffs:
Electricity Tariff in which prices change according to times of day and power supply. It allows for cost-savings of up to 30% if the heat pump runs primarily during cheaper times (e.g. at noon when there is a lot of solar power in the grid).
E
Efficiency:
Generally: The ratio of use/benefit (produced heat) to effort (consumed electricity). See also COP, SPF and SCOP.
Electronic Expansion Valve:
Most important component of the heat pump’s refrigeration circuit. It precisely regulates the refrigerant flow, thereby optimizing the efficiency factor. Modern electronic valves are far more efficient than older mechanic variants.
EU F-gas Legislation:
European legislation concerning the gradual reduction of climate-damaging fluorinated gases (F-gases) in refrigeration devices and ACs (air-conditioning). Enables the development of natural refrigerants like R290.
Evaporators:
Heat exchanger in the refrigeration circuit in which the refrigerant evaporates (becomes steam) and takes in heat from the ambience (air, soil, water).
F
Field Study:
Scientific study of heat pumps in real life scenarios (in contrast to lab tests). Delivers the most reliable data on actual efficiency and performance.
Flow Temperature:
The temperature of the heating water in the circuit. Distinction between supply temperature (from the heating device to the heat emitter, e.g. radiator, underfloor) and return temperature (back to the heating device). The lower the supply temperature, the more efficient the heat pump.
G
Gas Boiler:
Heating device that produces heat by burning natural gas. Modern gas boilers reach an efficiency of 90% (0.9). Comparison to a heat pump: Lower efficiency, dependent on fossil fuels, higher CO2 emissions.
Geothermal Boreholes:
Pipe systems placed vertically into the ground (typically between 50m and 150m deep) that extract thermal energy from the soil. They require less area than collectors but require drilling.
Geothermal Collectors:
Horizontally placed piping in the ground through which a brine-fluid flows that extracts heat. Geothermal collectors require a larger property area compared to boreholes but are cheaper in installing.
Geothermal Heat Pump (Brine-Water Heat Pumps):
A heat pump that extracts thermal energy from the ground, mostly through bore holes and ground (geothermal) collectors. Advantage: The soil has relatively consistent temperatures (ca. 8-12°C) throughout the years which means that higher efficiency can be reached. Disadvantage: Higher installation costs because of ground-boring and other earthworks.
Global Warming Potential (GWP):
Greenhouse potential of a substance compared to CO2. Critical for the evaluation of refrigerants. Examples: CO2 has a GWP of 1, R410A’s GWP equal 2088, R32 has GWP=675, and R290 (propane) has GWP=0.1. The lower the more climate-friendly.
(Quality) Grade:
Measurement for the quality of a heat pump: It describes the rate of the actual reached COP to the theoretically possible COP according to the Carnot process. Values of around 0.5 are very good and show that a heat pump runs closely to its physical optimum.
H
Heating Curve:
Setting of the heating controls that defines which flow temperature should be achieved at which outside temperature. Doing this right is critical for efficiency. Rule of thumb: Lowering the heating curve by one degree Celsius improves the SPF by 0.1.
Heating Device:
Device that provides thermal energy, e.g. heat pumps, gas or oil boilers, pellet boilers. In hybrid systems multiple heating systems can be combined.
Heat Distribution:
System that distributes the heat in a room, for example radiators, underfloor heating, wall heating, fan-coils, convectors. The type of heat distributor determines the necessary supply temperatures.
Heat Exchanger:
Component that transfers heat between two media without mixing them. In heat pumps there is the evaporator (takes in heat from the heat source) and the condenser (emits heat to the heating system). Optimized heat exchanger geometry raises the performance factor by about 20%.
Heating Load:
Heating performance that is required at the lowest expected outside temperature (design temperature) to maintain the desired room temperature. Base value for dimensioning the heat pump.
Heat Loss:
Amount of energy that a building loses towards the outside world. Depends on insulation, windows, air-tightness, etc. It determines heat demand and therefor the required heating performance. The lower the heat loss the lower the system temperatures can be.
Heat Pump:
Device that ‘pumps’ heat from a low temperature level (ambient) to a high level (heater). It uses a thermodynamic cycle and requires electrical energy to run out of which it produces multiples of thermal energy. Can also be used for cooling.
Heating Rod:
Electric heating addition in the heat pump system. It serves as a backup in case of drastically low outside temperatures or in an emergency. In correctly designed systems it is activated very rarely (mostly below 2% of operating time) and has a neglectable influence on efficiency and cost.
Heat Source:
The ambience out of which the heat pump extracts energy. Main types: Outside air (most common, easiest installation), soil (most efficient), ground water (very efficient, but requires a permit). The choice of the heat source influences investment costs, efficiency and required space.
Hybrid System:
Heating system which combines a heat pump and another heating device, e.g. a gas boiler or pellet burner. The systems can complement each other or take turns. In practice, the heat pump usually covers the majority of the heating demand.
Hydraulics:
The theory of flowing liquids. In the context of heating systems: The correct design and installation of pipes, pumps, valves, etc. Careful hydraulic planning is critical for high efficiency. Hydraulic errors can reduce system efficiency by 20% or more.
Hydraulic Alignment:
Optimization of flow-quantities in all heating circuits so that every room is supplied with the right amount of water. Major improvement of efficiency and comfort. Especially important for heat pumps.
(Green) Hydrogen:
Hydrogen produced via electrolysis using renewable electricity. Discussed as a potential energy carrier for the energy transition. For building heating, numerous studies show it is neither efficient nor cost-effective compared to heat pumps. Efficiency is roughly 3–4 times lower than direct electricity use in heat pumps.
I
Inverter Technology:
Modern control technology that allows the heat pump to adjust its performance continuously (not in steps). Advantages: Higher efficiency (at least 10% compared to pulsed/cycled systems), more even heating supply, lower noise development, longer lifespan.
L
Leakage Flow:
Unwanted flows in heating systems that stem from faulty hydraulic planning or installation. They can drastically reduce efficiency, for example when water flows by the heat exchanger without absorbing sufficient heat.
Lower Temperature Radiators:
Specially designed radiators that emit enough heat even at lower supply temperatures (35-50°C). They have larger areas or improved heat exchange than standard radiators. Ideal for heat pumps in existing (older) buildings because they can prevent having to renovate the whole heat distribution system.
M
Monoenergetic Operations:
Operating mode in which the entire heat supply relies on a single form of energy (here: electricity). The heat pump covers close to 100% of the heat demand, only in rare peak load situations is it supported by a heating rod.
Monovalent Operations:
Operating mode in which the heat demand is covered only by the heat pump without any support. Requires sufficient dimensioning for the coldest of days.
N
Noise Level:
(Audio-) Volume of a heat pump measured in dB(A). Modern devices reach 35-55 dB(A) which corresponds roughly to the noise levels of a fridge. Within 20 years, it was reduced by 10-15 dB(A) (which means halving the perceived noise).
Nominal Power:
The heating performance of a heat pump in a standardized scenario (e.g. A7/W35 für air-to-water heat pump). Relevant for choosing the right size of the device.
O
Outside-Air Heat Pump (Air-to-Water Heat Pump):
A heat pump that extracts thermal energy from the ambient (outside) air and uses it for heating. It also works in temperatures below freezing, because even cold air holds usable thermal energy. Advantages: Easy installation, no earthworks necessary. Disadvantage: Slightly lower efficiency than geothermal heat pumps, especially at very low outside temperatures.
Over-Dimensioning:
The heat pump’s design is too large for the actual heating load. Can lead to frequent cycling. Less problematic for inverter heat pumps than for on-off ones.
P
Panel Heating:
Heating system with large heat emission surface, e.g. underfloor, wall, or ceiling heating. Advantage: Works on low flow temperatures (typically 30-35°C) which raises heat pump efficiency. Not necessary for heat pumps but advantageous.
Pellet Boiler:
Boiler which is fueled by wooden pellets. Sometimes, it is used in hybrid systems with a heat pump. Real life often shows that the heat pump covers the majority of the heating load and the pellet boiler becomes largely redundant.
Performance Value:
See COP. The performance value describes the instantaneous (current) efficiency of a heat pump under specific operating conditions.
Power-to-X:
Collective term for technologies that transform electric energy into other forms of energy, for example Hydrogen (Power-to-Gas) or artificial fuels. Less useful for heat pumps and more expensive than the direct use of electric power in heat pumps.
R
R290 (Propane):
Natural refrigerant with very low GWP (0.1). Sustainable and climate-friendly alternative to synthetic refrigerants. Allows for higher supply temperatures which makes heat pumps more suitable for existing buildings. Market share continuously on the rise.
R32:
Synthetic refrigerant with a medium-ish GWp (675). Is being used as an interim solution and steadily replaced by R290.
R410A:
Older synthetic refrigerants with very high GWP (2088). Due to the EU F-gas legislations it is disappearing from the market.
Radiators:
Traditional heating emission system that runs on higher flow temperatures that panel heating (typically 40-70°C). Modern heat pumps can work efficiently with radiators, especially with lower-temperature ones.
Refrigerant:
Special fluid in the refrigeration circuit that evaporates at low temperatures and emits heat when condensing. Important refrigerants: R410A (outdated, high GWP), R32 (interim solution), R290/propane (fit for the future, very low GWP). Natural Refrigerants are on the rise.
Refrigeration Cycle (/Circuit):
Every heat pump’s heart. A closed system in which a refrigerant circulates and, through evaporation and condensation, “pumps” heat from a lower to a higher temperature level. It consists of an evaporator, compressor, condenser, and expansion valve. Cycle and circuit can both be used but cycle usually describes the process and circuit the hardware.
Remote Maintenance:
Monitoring and maintenance of the heat pump via an internet connection. Functions as an early warning system, optimization of the settings, and quick problem solving without on-site attendance.
Renovation Status:
Describes the energy-related condition of a building after modernization measures (insulation, window replacement, etc.). More important for heat pump suitability than the building’s age, but not a strict requirement.
Return Temperature:
The temperature of the heating water that flows back from the radiators/underfloor heaters/etc. to the heat pump. The lower the return temperature the more efficiently the heat pump can work.
S
Seasonal Coefficient of Performance (SCOP):
Calculated seasonal performance value according to European norm EN 14825. Signifies the expected average efficiency during a specified heating period, based on lab measurements and model calculations. Lies between COP (instant) and SPF (real year-long).
Seasonal Performance Factor (SPF):
The most important value for the real efficiency of an operating heat pump. It signifies how much thermal energy the heat pump generated out of one unit of electric energy over the course of one year. Example: An SPF of 3.5 means that out of 1 kWh power the heat pump produced 3.5 kWh heat on average. Based on actual field studies, not laboratory measurements.
Self-Consumption Rate:
Share of the self-produced electricity (e.g. from photovoltaic) that is used directly at home. With intelligent controlling heat pumps can reach up to 70% of self-consumption and therefore drastically lower power costs.
Share of Coverage:
The percental share of the heat supply that is covered by the heat pump. In monovalent operations it lies at nearly 100%, in bivalent systems the heat pump shares the load with a second heating device.
Smart Grid Ready:
Labelling for heat pumps that can communicate with intelligent power grids. Enables grid-friendly operation and participation in flexibility markets.
Supply Temperature:
The temperature of the heating water, that flows from the heat pump towards the radiators/underfloor heating/etc. Most important parameter for efficiency: The lower the supply temperature the higher the SPF. Typical values: 30-35°C for underfloor heating, 40-55°C for radiators, and up to 75°C for specially designed high-temperature heat pumps in unrenovated buildings.
System Temperature:
Collective Term for the operating temperatures in the heating system, especially supply and return temperature. Lower system temperatures raise the efficiency of the heat pump drastically.
U
Underfloor Heating:
The most common type of panel heating. Water circulates through pipes in the floor and evenly emits heat. Ideal for heat pumps due to low operating temperatures.