How does a heat pump work?

5 min read

A heat pump extracts free thermal energy from the environment (air, ground, or groundwater) and uses a small amount of electricity to raise this heat to a higher temperature level to heat a building [1]. Basically, it works like a reverse refrigerator: while a refrigerator transports heat from its interior to the outside, a heat pump pumps environmental heat into the house [2].

The refrigeration cycle (also known as the heat pump cycle) is the heart of the system. A special refrigerant circulates within it, continuously changing its state of matter, pressure, and temperature to transport heat from a low to a high level. This happens in four steps:

1. Evaporation (heat absorption): In the evaporator, the liquid refrigerant absorbs heat from the environmental source. Due to its very low boiling point, it evaporates even at low temperatures and becomes gaseous. The absorbed energy is mostly free from the environment [1].

2. Compression (temperature increase): The compressor sucks in the gaseous refrigerant and compresses it under high pressure. This causes the temperature to rise sharply. This step requires electrical energy and is crucial for bringing the heat up to heating levels [2].

3. Condensation (heat release): In the condenser, the hot, high-pressure refrigerant releases its heat to the heating system (e.g., radiators or underfloor heating). It cools down, condenses, and becomes liquid again. The released heat heats the building [1].

4. Expansion (pressure reduction): The expansion valve abruptly lowers the pressure, causing the temperature of the refrigerant to drop sharply. It is now cold and liquid again, ready for the next cycle in the evaporator [2].

Special case with cold outdoor air: Even at freezing temperatures (e.g., -5 °C), the air still contains usable thermal energy, as absolute cold is only reached at -273.15 °C. The refrigerant is then even colder (e.g., -15 °C) and can absorb heat because heat always flows from warmer to colder. The low boiling point of the refrigerant (often below -20 °C) enables this. However, efficiency decreases during severe frost because the compressor must work harder [2].

The Seasonal Performance Factor (SPF/JAZ) measures the real efficiency over an entire year: it indicates how much heat (in kWh) is generated per kWh of electricity consumed. Unlike the COP (Coefficient of Performance) under test conditions, the SPF considers all real influences such as temperature fluctuations and defrost cycles. Good values for air-to-water heat pumps are 3.5–4.0, for brine-to-water 4.0–5.0, and for water-to-water 4.5–5.5+. An SPF of 3.0 or higher is recommended for economic viability. Low flow temperatures (e.g., with underfloor heating) and stable heat sources significantly improve the SPF [2].

Differences between types: All use the same refrigeration cycle but differ in the heat source. Air-to-water heat pumps are inexpensive to purchase (approx. 25,000–45,000 €), easy to install, and flexible, but have fluctuating efficiency and an audible outdoor unit. Brine-to-water heat pumps (geothermal) are more efficient and stable (SPF 4.0–5.0) but require earthworks or drilling (costs 35,000–60,000 €) and permits. Water-to-water heat pumps (groundwater) offer the highest efficiency (SPF over 5.0) but are the most expensive (40,000–65,000 €) and are strictly subject to permits due to the wells [3].

Suitability for unrenovated old buildings: Yes, often possible, but challenging due to high heat demand and the need for high flow temperatures (up to 60–70 °C). High-temperature heat pumps or hybrid systems (combination with an existing gas or oil boiler) are suitable here. Underfloor heating is ideal (low flow temperatures 30–35 °C), but often expensive to retrofit in old buildings. For radiators, larger models, low-temperature radiators, hydraulic balancing, or minor insulation measures (e.g., roof, windows) help to lower the flow temperature. A professional heating load calculation is essential [4].

Costs and Funding: Acquisition costs range from €25,000–65,000 depending on the type (incl. installation). Operating costs depend on the SCOP (Seasonal Coefficient of Performance) and electricity price – with an SCOP of 4 and a heat demand of 15,000 kWh, it is often under €1,300/year with heat pump electricity. In Germany, the Federal Funding for Efficient Buildings (BEG) subsidizes up to 70% of the costs (basic funding 30%, plus bonuses for replacing old heating systems, efficiency, or income; max. €21,000). Application must be submitted to BAFA/KfW before the measures begin; an energy consultant is recommended. A tax bonus is alternatively possible [3].

Noise Level and Installation: Brine-to-water and water-to-water models are very quiet (comparable to a refrigerator). Air-to-water units reach 35–50 dB(A) at a distance of 1 m. Important: Maintain distance from neighbors and windows (observe TA Lärm), avoid corners and sound-reflecting surfaces, and use vibration dampers and a stable foundation. Night mode reduces noise [1].

CO₂ Savings: Compared to a gas heating system (approx. 200 g CO₂/kWh), a heat pump with an SCOP of 4 and the German electricity mix (approx. 380 g CO₂/kWh) saves over 50% CO₂. With green electricity or a PV system, operational emissions are nearly 100% eliminated. The savings increase as the share of green electricity in the grid rises [3].

Conclusion for Homeowners: Heat pumps are efficient, environmentally friendly, and future-proof. The choice depends on the plot of land, the building, and the budget. Individual consultation by a specialist company and an energy consultant, including a heating load calculation, is indispensable for optimal planning, funding, and efficiency.