Introduction: The Superstar of Heating Technology

Heat pumps have been appearing in the media repeatedly in recent years and are of particular interest to homeowners. Modern heat pumps are said to be efficient, quiet and, above all, sustainable.

Impressive Figures

• Over 70% of all new building projects in Germany (2023) plan a heat pump as the primary heat source
• Global turnover is expected to reach 70 billion US dollars in 2024
• The operating principle was discovered as early as the 17th century
• The first underfloor heating with heat pump was installed in 1968

Why the Boom?

Driving factors for heat pumps are:

• High energy prices for fossil fuels
• Growing environmental awareness
• Technological progress
• Combination with solar energy enables CO2-neutral heating

Heat Pump and Refrigerator: Close Relatives

Heat pumps and refrigerators are actually close relatives – almost like brothers. Why are these devices so similar, even though they were developed for completely opposite tasks?

At first glance, both devices seem to perform opposite tasks – but a look under the bonnet reveals astonishing similarities:

Device	Takes heat from	Releases heat to
Refrigerator	Interior	Environment (back)
Heat Pump	Environment	Interior (heating)

The operating principle is identical – only the goal is reversed!

Physical Fundamentals

To understand heat pumps, we must first clarify two concepts:

1. States of matter
2. Heat transfer

The Physics of States of Matter

Solid, liquid and gaseous – these three states of matter are ubiquitous. But what exactly is a state of matter?

Definition: The current status or appearance of matter, determined by the movement of particles.

Fundamental law of nature: As temperature rises, particles move faster and more vigorously.

The properties of the three states of matter differ fundamentally:

State	Particle Movement	Structure
Solid	Vibrate in place	Ordered structure
Liquid	Move, stay connected	Partially ordered
Gaseous	Move freely	No structure

Important: The transition from one state to another requires energy absorption or release. This is precisely what the heat pump exploits!

The Two Laws of Thermodynamics

The science of heat – thermodynamics – has two fundamental rules:

First Law: Conservation of Energy

Energy cannot be created from nothing or destroyed. It can only be converted.

Examples:

• Electrical energy → Heat (electric heater)
• Chemical energy → Heat (fire)

Second Law: Direction of Heat Flow

Heat always moves from hot to cold.

Nature always tries to achieve an energetic equilibrium.

Example fireplace: Heat leaves the hot fireplace and warms the cold room – never the other way round.

The Three Types of Heat Transfer

Heat can travel from one place to another in various ways:

Type	Description	Example
Conduction	Direct contact of two materials	Hand on radiator
Convection	Heat transport by moving gases/liquids	Warm air rises
Radiation	Electromagnetic waves	Solar warmth

Conduction

The fast-moving particles of the warmer material collide with the slower particles of the colder one. This is how heat transfers through direct contact.

Convection

Warm air has lower density and rises. It takes the heat with it and transports it to another location.

Cycle:

1. Air is heated → density decreases → rises
2. At the top, the air cools → density increases → sinks
3. At the bottom, it is heated again → cycle closes

Thermal Radiation

Electromagnetic waves in the infrared range. Needs no transfer medium – that is why solar warmth reaches us through the vacuum of space.

The Operating Principle of the Heat Pump

The heat pump "pumps" heat from one place to another – just like a water pump transports water.

The Basic Idea

The heat pump:

• Extracts heat from the environment (even from cold air!)
• Compresses this heat to a higher temperature level
• Releases the heat to the heating system

But How Does One Extract Heat from Cold Air?

The secret lies in the refrigerant – a special liquid that evaporates even at very low temperatures and absorbs heat in the process.

The Cycle in Four Phases

Phase 1: Evaporation (Heat Absorption)

1. The liquid refrigerant flows through the evaporator (heat exchanger)
2. Ambient air is drawn in by a fan
3. Even cold air contains heat energy
4. The refrigerant absorbs this heat and evaporates (becomes gaseous)

Phase 2: Compression (Temperature Increase)

1. The gaseous refrigerant reaches the compressor
2. The compressor mechanically compresses the gas
3. Through compression, pressure rises and thus temperature
4. The refrigerant now has a usable, high temperature

Phase 3: Condensation (Heat Release)

1. The hot, compressed gas flows to the condenser (second heat exchanger)
2. The heat is transferred to the heating water
3. The refrigerant condenses (becomes liquid again)
4. The heated water flows to underfloor heating or radiators

Phase 4: Expansion (Pressure Reduction)

1. The liquid refrigerant still has elevated pressure
2. The expansion valve releases the pressure
3. Pressure drops → temperature drops
4. The refrigerant is back to its initial state

Then the cycle starts again!

Summary of the Phases

The four phases of the heat pump cycle can be summarised clearly:

Phase	Component	Process	State
1	Evaporator	Heat absorption	Liquid → Gaseous
2	Compressor	Pressure increase	Gaseous (hot)
3	Condenser	Heat release	Gaseous → Liquid
4	Expansion valve	Pressure reduction	Liquid (cold)

No Contradiction to Physics!

At first glance, the heat pump seems to violate the second law of thermodynamics: heat flows from cold (outside air) to warm (heating).

The solution: Energy (electricity for the compressor) is used to reverse the natural heat flow. The system as a whole follows the laws of nature!

The Trick

1. The refrigerant is colder than the outside air → heat flows in (physically correct)
2. Through compression, the refrigerant becomes warmer than the heating water → heat flows out (physically correct)

The heat pump does not create energy from nothing – it only transports and transforms it very cleverly!

Advantages and Disadvantages of Heat Pumps

Advantages

Heat pumps offer numerous advantages over conventional heating systems:

Advantage	Explanation
High efficiency	From 1 kWh electricity you get 3–5 kWh heat
Environmentally friendly	No direct CO2 emissions
Low operating costs	Cheaper than oil or gas
Long lifespan	15–25 years
Low maintenance	No combustion = little wear
No fuel storage	No oil tank, no gas connection needed
Subsidies	Government grants available

Disadvantages

Despite the many positive features, there are also aspects to consider:

Disadvantage	Explanation
High purchase costs	€10,000–25,000 depending on type
Electricity dependent	Requires electrical power
Efficiency drops in cold	Less efficient at very low temperatures
Noise	Outdoor unit can be audible
Low flow temperatures	Not suitable for all heating systems
Space requirement	Outdoor unit or ground works needed

Conclusion

Which components work together exactly, you will learn in the article The Components: Heat Exchanger, Compressor and Expansion Valve.

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The Complete Article Series "Heat Pumps"

1. The Anti-Refrigerator: How Does a Heat Pump Work? – You are here
2. The Components: Heat Exchanger, Compressor and Expansion Valve – Components
3. Heat Pump Key Figures and Sizing – COP, SPF and more
4. Operating Modes: Monovalent, Bivalent and Hybrid – Operating modes
5. Heat Pump Types and the Dream Team with Solar Systems – Air-water, ground-source & solar

Sources

• DESTATIS: Heat Pumps in New Buildings 2023
• Mordor Intelligence: Heat Pumps Market
• Heizung.de: History of the Heat Pump
• LEIFIphysik: Heat Transport
• Chemie.de: Thermodynamics

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With our free Heat Pump Calculator, you can calculate the seasonal performance factor of your heat pump according to VDI 4650 – including operating costs and CO₂ balance.

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