Heat pumps convert low-temperature heat into higher-temperature heat - even in winter when it is well below freezing.
This process takes place in a closed circuit and involves constantly changing the state of the working fluid (evaporation, compression, condensation, expansion). Refrigerators operate on the same principle to extract heat from inside and discharge it to the outside.
By contrast, heat pumps take stored solar heat from the environment around the house - soil, water or air - and release it together with the input energy in the form of useable heat to the heating and hot water circuit.
The heat pump cycle
The heat pump operates on the basis of a working fluid (coolant) which changes state (liquid/gas) in a continuous cycle and absorbs and releases heat.
The heat pump cycle is essentially based on the 
Carnot cycle.
The diagram illustrates the operating cycle of a heat pump:
1. Evaporation
In a heat exchanger the liquid refrigerant absorbs energy from the heat source (water, soil or air) and evaporates as the temperature rises.
2. Compression
In a compressor the introduction of electrical energy causes the vaporised but still cold coolant to compress and heat up. The coolant leaves the compressor as a hot gas.
3. Liquefaction/condensation
The hot gas flows into the liquefier, releases energy to the heating system, condenses and leaves the condenser as hot, liquid refrigerant. This energy heats the hot water for heating or domestic use to the desired temperature.
4. Expansion
The hot, liquid refrigerant is transferred to the expansion valve. In the expansion valve the pressure is reduced very rapidly. The temperature of the coolant also drops very quickly without releasing energy. The cold, liquid refrigerant is transferred to the evaporator and the cycle begins again.
The heat pump heating system
The heat pump heating system consists of a heat source system (WQA), the heat pump (WP) and the heat utilisation system (WNA).
Figure: Heat pump heating system
All system components must be perfectly matched to one another in order for the system to operate correctly and to ensure high energy efficiency ratios (seasonal performance factor).
Key data must be recorded on the system data sheet, and the system must be documented in accordance with the OCHSNER quality assurance standard.
The following operating modes are possible:
- Monovalent
The heat pump is the only heat generator. The heat pump covers 100% of the heat demand at all times. Suitable for flow temperatures of max. 55°C (65°C for the "plus" range).
Systems using the ground or water as a heat source are operated monovalently. - Bivalent parallel (heat pump + auxiliary heating system)
[If a heat rod is used as the auxiliary heating system the term used is monoenergetic.]
The heat pump operates on its own up to the cut-in point. After the cut-in point the heat pump operates together with the boiler or heat rod. The maximum flow temperature is 65°C.
This mode is mainly used in new developments using air as the heat source or for conversions in building renovations. - Bivalent alternative (heat pump or auxiliary heating system)
The heat pump heats in isolation up to the switchover point. After the switchover point the boiler heats in isolation. Suitable for flow temperatures up to 90°C. Mainly used for retrofit systems.
Example: At a cut-in point of 0°C the air/water heat pump still delivers a good 88% of the annual heat output even at a really low outdoor temperature of -16°C.
The lower the cut-in point of the second heat generator, the higher the contribution made by the heat pump to the annual heat output. The annual heat output also depends on the climatic zone and operating mode.
System variants:
System variants
Energy efficiency ratio
e = heat output/input power = (environmental energy + input power) / input power
The energy efficiency ratio e indicates the heat output as a ratio of the input power used. The term coefficient of performance (COP) is sometimes used as an alternative to the energy efficiency ratio.
An energy efficiency ratio of 4 means that four times the electrical energy used is converted into useful heat energy. The energy efficiency ratio is an instantaneous value.
The potential energy supplied over an entire heating period in relation to the electrical input power used gives the seasonal performance factor. A distinction can also be made between the coefficient of performance of the heat pump and that of the the system as a whole.
Temperature difference determines energy efficiency ratio
The energy efficiency ratio is dependent on the temperature difference between the heat source and the heat distribution: the lower this temperature difference, the more efficient the heat pump. That is why getting the overall system design right is so important.
Substances which evaporate at low temperature and also have a high internal heat are used as working fluids (coolants). Only chlorine-free working fluids are permitted. These have no ozone-depleting potential (ODP = 0). R 134 a, R 407 C and propane all satisfy these conditions.
OCHSNER uses R 134 a and R 407 C as standard. Both of these safety refrigerants are non-flammable and non-toxic. The ester oil used with the working fluid is biodegradable. This means that the system can be installed in any room. With heat pumps which use flammable working fluids (propane), on the other hand, there are restrictions and rules governing installation.
Benefits