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A quick guide to the essentials

The ABC of heat pumps

Air heat pump

See ‘Air-to-water heat pump’

Air-to-water heat pumps

An air-to-water heat pump is an electrically powered machine that makes the solar energy stored in the ambient air usable. The Ecodan air-to-water heat pump systems always consist of two parts: an outdoor unit that removes energy from the outdoor air and an indoor module that is required for transporting the energy into the heating system.

Annual performance factor

When determining the energy efficiency, the annual performance factor is applied to more than just one single outdoor temperature condition. Instead, this is based on three outdoor temperature conditions in line with VDI 4650 that are then evaluated according to the time frequency.

However, the purely mathematical value that is calculated usually deviates from the actual value. The actual value of the annual performance factor is strongly influenced by user behaviour and other factors such as the hot water temperature in the drinking water cylinder or the duration and intensity of the winter.

Bivalence temperature/point

The bivalence temperature is the outdoor temperature at which there is a change in bivalent mode between a heat pump and a second heat generator.


See ‘Mode’

Blocking periods

The following blocking periods apply for interrupting the energy supply in the case of heat pumps: the German Federal Electricity Tariff Regulation (BTOElt) permits energy providers to interrupt the electricity supply for the operation of heat pumps for up to 2 consecutive hours, but for no longer than 6 total hours within a 24-hour period.

Buffer storage tank

In heating systems with a heat pump, the buffer storage tank fulfils a range of important functions. In addition to increasing the average run times of the heat pump by heating and storing heating water for a longer period of time, the buffer storage tank also serves as system separation, provides the required energy in defrosting mode and acts as a hydraulic shunt.



See ‘Liquefier’

Carbon dioxide (CO2)

Carbon dioxide is generated through the burning of fossil fuels. As a heat pump operates without combustion technology, it differs from a boiler for fossil fuels in that it does not pollute the environment with CO2 during operation.

CE marking

The CE marking is a mark of conformity that certifies compliance with the relevant European guidelines and thus enables the movement of goods within the EU. Since 26 September 2015, CE marking has only been awarded to heat generators that meet the minimum efficiency standards and minimum emission standards of the Ecodesign (ErP) Directive.

Circulating pump

The electrically operated circulating pump enables the transport of a fluid medium (e.g. water) in closed piping systems.

Commissioning of heat pumps

Mitsubishi Electric provides a commissioning service for heat pumps in the case of new systems that are to be installed. This provides the benefit of having certified partner companies perform a series of predefined tasks in advance. In addition to the inspection of the installation, for example, the heat pump system is properly connected for cooling purposes, the control unit of the system is optimally configured and this is all explained to the operator.


The compressor is a key component of a heat pump. There are various compressor types and these take different approaches to performing two tasks, namely transporting the captured heat into the building and heating the refrigerant by means of compression. For the purpose of compressor control, Ecodan systems use inverter boards that enable modular, demand-dependent output adjustment (and thus demand-dependent power consumption).

Condensate tray

The condensate tray acts as a collecting container for the water that is collected on the evaporator, which can then be centrally drained. This therefore prevents any dripping onto the floor.

Condensation temperature

When the condensation temperature is reached, the refrigerant changes from a gaseous state to a liquid state. This change of state makes it possible to transfer the energy to the heating system.

COP (Coefficient of Performance)

The COP (Coefficient of Performance) value states the ratio of the heating output to the electric power used. The COP values specified by Mitsubishi Electric have been determined under standardised test conditions according to EN 14511 and are always stated together with the underlying heating output, outdoor temperature and water flow temperature of the measurement.

Cylinder unit

A cylinder unit is part of the Ecodan heat pump system. The module is installed in the building and connected to the outdoor unit via water-carrying or refrigerant-carrying pipes, depending on the design in question. A cylinder unit contains an integrated DHW cylinder that is made of stainless steel and holds up to 200 litres.


In the case of outdoor temperatures approaching freezing, the humidity that is present in the air settles on the evaporator of the heat pump in the form of ice. In order to maintain the efficiency of the air-to-water heat pump, the evaporator must be defrosted as necessary. This procedure takes place automatically using small quantities of heat, e.g. from the buffer storage tank.

DHW cylinder

A DHW (domestic hot water) cylinder is a container for storing hot drinking water with a maximum temperature of 90°C. The cylinder is normally thermally insulated and is heated indirectly via the heat pump, meaning that the drinking water is heated using a heat exchanger. Depending on the nominal volume, the Ecodan heat pump cylinders in the WPS series feature smooth pipe heat exchangers with large dimensions and surfaces of between 3.2 and 6.2 m2 to help ensure quick heating.


The dimensioning of a heat pump describes the heating output adapted in line with the heat demand, which forms the basis for efficient operation of the system. If the dimensioning of a system is too large or too small, the system will generate unnecessary costs in terms of procurement and/or operation and it will not run in a particularly economic manner.



The efficiency of a heat pump characterises its degree of effectiveness. Efficiency is specified using figures such as the annual performance factor, the COP value or the SCOP value.

EHPA Quality Label

The Quality Label (formerly known as the D-A-CH Quality Label) issued by the European Heat Pump Association (EHPA) is intended to ensure a high level of quality among heat pumps available on the market. The Quality Label is only awarded to products that meet the technical, design-related and service-specific quality guidelines for heat pumps as compiled by the EHPA. These include a spare parts supply for at least 10 years, a nationwide service network in Germany and more besides. Compliance with these guidelines guarantees a high level of energy efficiency and reliability for heat pump systems.

Electric heating rod

The electric heating rod supports the operation of a heat pump where required.

Electrical connection

The electrical connection of a heat pump may only be implemented by a specialist. The heat pump must first be registered with the responsible energy provider. A dedicated electricity meter is required for the heat pump in the event that its electricity consumption is to be billed using a heat pump tariff. In the case of an electrical power connection, it must be noted that heat pumps with a higher capacity require a three-phase power connection featuring 400 volts (see ‘Operating voltage’).

Energy label

Since 26 September 2015, all heat generators have been identified with an energy label. The appearance of the labels and the criteria for attaining specific energy efficiency grades are governed by the energy labelling regulations of the EU.

Energy provider blocking periods

Periods during which the energy provider can prevent the supply of heat pump electricity; see ‘Blocking periods’.

Evaporation temperature

The evaporation temperature is the temperature at which the refrigerant changes from a liquid state to a gaseous state, absorbing ambient heat in the process.


The term ‘evaporator’ refers to the heat exchanger of a heat pump, in which heat is drawn from the outdoor air at a low temperature and low pressure by means of the evaporation of the refrigerant.

Expansion tank

The expansion tank is used so that even the water that has expanded due to heating can be held without any problems. The steel container is filled with nitrogen and fitted with a membrane in its interior. It is important to ensure that the expansion tank features sufficient dimensions to accommodate the additional water volume that is generated.

Expansion valve

An expansion valve plays an important role in a heat pump. Attached between the condenser and the evaporator, it serves to lower the refrigerant pressure and to regulate the injection of the refrigerant.

Flow temperature

The flow temperature is the temperature at which the heating water flows from the heat generator into the heat distribution system. In systems featuring pure panel heaters (underfloor heating, wall heating), this is between 25 and 40°C.

Heat demand/heat load

The term ‘heat load’ refers to the heat demand of buildings that is to be determined according to DIN EN 12831.

Heat exchanger

A heat exchanger is a component that is able to transfer the heat of a particular medium to another medium or the same medium without the media being mixed in the process. Heat exchangers are deployed e.g. in hot water cylinders as coiled pipes or pipe bundles for transferring the heating water heat to the drinking water.

Heating capacity

The heating capacity of a heat pump refers to the usable amount of heating energy that is output within a particular period under specific conditions and is measured in kilowatts (kW). In the case of heat pumps, the operating parameters – comprising the outdoor temperature and the water flow temperature – are always stated. For example, A2/W35 refers to an outdoor temperature of 2°C and a water flow temperature of 35°C.

Heating current

Various energy providers offer additional special tariffs for the use of electrical space heating systems and hot water supply systems. This makes it possible for night storage heaters, heat pump heating systems and hot water supply systems to be operated using cheaper heating current.

Heating energy demand

The heating energy demand is the amount of energy that the heating system must provide over the course of the year for the heating of rooms and drinking water. The following factors are considered when calculating the heating energy demand: the transmission heat demand (i.e. heat that is lost via external walls, windows and the roof) and the ventilation heat demand (i.e. heat that is lost via active and passive ventilation). This demand is reduced via internal heat gains (such as body heat and device heat) and via passive solar heat gains (such as radiation through south-facing windows). In Germany, the procedure for calculating the annual heating energy demand is defined by the German Energy Saving Ordinance (EnEV).

Hydraulic balancing

Hydraulic balancing is a procedure performed at a predefined flow temperature that involves each individual heater within a heating system or each heating circuit within a panel heater being supplied with precisely the amount of heat required in order to achieve the desired room temperature. Hydraulic balancing is a prerequisite for obtaining funding for heating modernisation through the KfW development bank. Hydraulic balancing is also required by the German Federal Office for Economic Affairs and Export Control (BAFA), which is responsible for the grants issued as part of the market incentive programme for the use of renewable energies.


A hydrobox is part of the Ecodan heat pump system. The module is installed in the building and connected to the outdoor unit via water-carrying or refrigerant-carrying pipes, depending on the design in question. Important heating system components such as the buffer storage tank and the drinking water cylinder are also connected to the hydrobox.

Installation instructions

Installation instructions are provided for the outdoor module and the indoor module alike and it is essential that these are observed. The correct installation of the units is particularly important in ensuring optimum functionality.

Inverter technology

In cutting-edge air-to-water heat pumps, inverter technology ensures precise adjustment of the output. In essence, the compressor is continuously adjustable, which helps influence the power consumption of the compressor and monitor the heat output of the overall system.


A liquefier or capacitor is a heat exchanger on the heat pump. The refrigerant is liquefied in this heat exchanger, causing the stored heat to be output to the consumers.

Membrane expansion tank

See ‘Expansion tank’


A basic distinction is drawn between three modes: monovalent, monoenergetic and bivalent. In monovalent mode, the layout of the heat pump is such that it can cover the heat demand of the building by itself. In monoenergetic mode, the electric heating rod can additionally be used. Bivalent mode enables the use of an extra heat generator in the system,  such as a gas boiler. 

Modulation, continuous

See ‘Inverter technology’

Monoblock heat pump

A monoblock heat pump consists of two modules: an outdoor unit and a hydrobox or cylinder unit inside the building. The monoblock system significantly simplifies installation from the refrigeration point of view: here, the plate heat exchanger is located in a closed cooling circuit in the outdoor unit. This means that energy is transferred from the outdoor unit to the indoor unit by well-insulated water pipes (flow and return).


See ‘Mode’


See ‘Mode’

Operating voltage

The operating voltage is the voltage stated in volts that is required to operate the heat pump. Depending on the nominal output, Mitsubishi Electric provides Ecodan heat pumps that require an operating voltage of 230 volts (1 phase) or 400 volts (3 phases).

Outdoor installation

If an air-to-water heat pump is installed outdoors, the evaporator (and potentially also the compressor) of the unit is installed outside of the building. This eliminates a considerable space requirement in the interior of the building. It also means that no air ducts or large wall breakthroughs need to be created.

Panel heater

Thanks to the large heating surfaces, a panel heater can cope with relatively low flow temperatures of the heating water. In the process, the water flows through the pipelines routed in the floor  (underfloor heating), on the wall (wall heating) or beneath the ceiling (overhead heating). The low water temperatures promote efficient operation of the heat pump. The panel heaters also ensure mild, even heat across the entire surface, thereby achieving a greater level of comfort.

Performance factor

The performance factor describes the ratio of generated thermal energy to the applied electrical energy. As this value varies significantly over the course of the year, the annual performance factor is often applied when considering the system efficiency as this considers the ratio on an annual basis. 

Performance factor = Wuse / Wel
Wuse = heating energy in kWh;
Wel = used electrical energy in kWh.

Plate heat exchanger

A plate heat exchanger is used to transfer energy from the refrigerant to the water. This consists of a number of stainless steel plates that are arranged in parallel with one another. The refrigerant and the water alternately flow through the gaps between these plates. The large heat exchanger surface that is generated as a result helps ensure an efficient exchange together with small outer dimensions.

Power consumption

The power consumption is the generated power in kilowatt hours (kWh) that is required in the form of electric current for the operation of a heat pump.

Power Inverter

Power Inverters are a form of Mitsubishi Electric inverter technology used in air-to-water heat pumps. Operation is guaranteed at temperatures as low as -20°C with these compressor types. A special Power Receiver, which is used for supercooling of the refrigerant, is combined with two individually controlled expansion valves to achieve an optimum heat output in outstanding energy-saving operation. Typical areas in which the Power Inverter is used include new buildings and existing buildings with good insulation and large heat transfer surfaces such as underfloor heating.

Pressure gauge

A pressure gauge is used to display the water pressure present in the heating system, which is stated in bar.

‘Qualitätssiegel Raumklimageräte’ quality label for air conditioning units

The ‘Qualitätssiegel Raumklimageräte’ quality label confirms that the air conditioning units in question meet the current valid legal provisions and that all important technical data featured in the corresponding data sheets, operation manuals and similar documents is correct. The issuing authority (Fachverband Gebäude-Klima e. V., known as FGK) checks to ensure that this data relates to standardised framework conditions so as to guarantee straightforward comparability between the units of different providers.


Refrigerant is the working material of a heat pump and is used in a closed circuit. It draws energy from the air while simultaneously heating the water. Rather than being consumed in the process, it simply changes its state by evaporating and liquefying.

Remote controller

In addition to the control system contained in the indoor unit, a wireless remote controller can be installed in a location such as the living room. This means that the most important heat pump functions and settings can be conveniently adjusted via the wireless remote controller without the need for complex cabling.

Return temperature

The return temperature is the temperature of the heating water that is recorded when the water flows back out of the heat distribution system towards the heat generator.

Room remote controller

See ‘Remote controller’

SCOP (Seasonal Coefficient of Performance)

The performance data for the SCOP value is recorded at four different measuring points in line with DIN EN 14825. The measuring points are weighted differently according to the temperature profile of the reference climate in Strasbourg. This ensures that the energy efficiency is identified under conditions that are as realistic as possible and that the SCOP value is closer to reality than the COP value.

Screed heating program

Newly applied screed must dry out in accordance with a particular time program in order to prevent cracks or similar issues. The Ecodan control system features an automatic program that can be individually adjusted.

Secondary circuit

The secondary circuit is the heat distribution circuit between the buffer storage tank that is deployed and the consumers.

Solar energy

Solar energy is always used indirectly by heat pumps. They use the solar energy stored in the air, in the ground or in water to heat buildings and to provide domestic hot water (DHW).

Sound power level

The sound power level is a physical measurand relating to the volume that is measured using the unit dB(A) regardless of the distance from the sound source.

Sound pressure level

The sound pressure level is stated using the unit dB(A). It is a physical measurand relating to the volume that is determined depending on the distance from the sound source. The sound pressure level that is stated  is usually recorded at a distance of one metre from the heat pump.

Split heat pump

In the split heat pump, the energy is transported into the building via refrigerant. The plate heat exchanger is located in the indoor unit and the outdoor unit is connected via a refrigerant pipe. Overall system efficiency is enhanced by the split principle, and this is also the preferred solution when large distances have to be bridged between the indoor and outdoor units. Depending on the capacity of the heat pump, pipe lengths of up to 80 metres are possible.

Starting current

When starting a conventional heat pump, a high level of current (starting current) is still required for a brief period despite the ‘soft start’. This high current is not present in an inverter-controlled heat pump, as the compressor starts with minimal speeds and then continuously works its way up to the required output. A soft starter, which prevents the sudden start-up of the compressor, is not required in the case of an inverter-controlled compressor.

Surface-area-to-volume ratio

The surface-area-to-volume ratio is an important value that is used to determine the energy requirement of the building. In principle, compact buildings with a small surface-area-to-volume ratio require less energy than buildings with a large surface-area-to-volume ratio – even if the volume itself remains constant.

System content

The system content describes the total volume of the water in the heating system, including the content of all tanks, pipes, connectors, etc.

Temperature spread

The temperature spread is the temperature difference between the inlet and outlet temperature of a heat carrier medium on the heat pump. It therefore denotes the difference between the flow temperature and the return temperature.

Wall heating

See ‘Panel heater’

Working materials

See ‘Refrigerants’

Zubadan Inverter

The patented Zubadan Inverter technology represents the very best that the field of air-to-water heat pump technology has to offer. The Zubadan cooling circuit delivers stable operation even in the case of low outdoor temperatures. In this way, the system provides full heating performance at temperatures as low as -15 °C. Even at -25 °C, the Zubadan heat pump still enables reliable and efficient operation. This means that, thanks to Zubadan technology, the process of overdimensioning the system as a safety buffer for heating mode is rendered entirely superfluous.