{"id":13049,"date":"2026-09-17T08:00:01","date_gmt":"2026-09-17T06:00:01","guid":{"rendered":"https:\/\/felsaris.com\/?p=13049"},"modified":"2026-10-03T15:48:06","modified_gmt":"2026-10-03T13:48:06","slug":"how-does-a-heat-pump-work","status":"publish","type":"lexikon","link":"https:\/\/felsaris.com\/en\/lexikon\/how-does-a-heat-pump-work\/","title":{"rendered":"How does a heat pump work?"},"content":{"rendered":"

Heat pumps are considered a key technology for the energy transition and are increasingly gaining ground in the heating market. But how does a heat pump actually work? At its core, it uses a thermodynamic principle to raise ambient heat from air, soil, or groundwater to a higher temperature level in order to heat buildings or prepare domestic hot water. Unlike conventional heating systems, a heat pump does not burn fuel. Instead, it transports heat from a colder place to a warmer one. This process is based on a closed refrigerant cycle characterized by four key steps: evaporation, compression, condensation, and expansion. In this article, we explain the technical fundamentals, efficiency, different types of heat pumps, and what matters for correct system sizing.<\/span><\/p>\n

Heat pump function explained simply<\/b><\/h2>\n

The way a heat pump works is best compared to a refrigerator in reverse. While a refrigerator removes heat from its interior and releases it to the outside, a heat pump removes heat from the environment and delivers it to the heating system. At first this may sound contradictory, because in winter it seems counterintuitive to extract heat from an already cold medium like outdoor air. This is exactly where the thermodynamic sophistication lies, because even at subzero temperatures, air still contains usable thermal energy.<\/span><\/p>\n

The core of every heat pump is a closed loop in which a special refrigerant circulates. This fluid has the special property of evaporating at very low temperatures. The cycle runs through four phases that repeat continuously:<\/span><\/p>\n

    \n
  1. Evaporation:<\/b> The liquid refrigerant flows through a heat exchanger called the evaporator. There it absorbs heat from the environment, such as outdoor air, the ground, or groundwater. With the added energy, the refrigerant evaporates even though the outdoor temperature is low.<\/span><\/li>\n
  2. Compression:<\/b> The resulting refrigerant vapor is drawn in by an electrically driven compressor and strongly compressed. Compression significantly increases both the pressure and temperature of the gas. At this point, the refrigerant temperature is well above the required supply temperature of the heating system.<\/span><\/li>\n
  3. Condensation:<\/b> The hot, high-pressure gas now enters the condenser, which is another heat exchanger. Here, the refrigerant releases its heat to the heating water that flows through radiators or underfloor heating. As it transfers heat, the refrigerant cools down and becomes liquid again.<\/span><\/li>\n
  4. Expansion:<\/b> The liquid refrigerant passes through an expansion valve, where the pressure is reduced abruptly. This pressure drop also causes the refrigerant temperature to fall sharply. It is then ready to absorb heat again in the evaporator, and the cycle starts over.<\/span><\/li>\n<\/ol>\n

    This continuous process makes it possible to generate several times more heating energy with a relatively small amount of electrical energy for the compressor. The electrical energy is mainly used to drive the compressor, while most of the heat comes from the environment.<\/span><\/p>\n

    The physics behind the heat pump<\/b><\/h2>\n

    \"Hot<\/p>\n

    From a physics perspective, a heat pump operates against the natural direction of heat flow. Normally, heat flows from a warmer place to a colder one until temperatures equalize. A heat pump reverses this process by using mechanical work to lift heat from a lower to a higher temperature level.<\/span><\/p>\n

    This mechanism is based on the second law of thermodynamics and the principle of the Carnot heat pump. The smaller the temperature difference between the heat source and the heating system, the more efficiently the heat pump operates. This is why low-temperature heating systems such as underfloor heating are ideal for heat pump operation.<\/span><\/p>\n

    Refrigerant selection plays a decisive role. Modern heat pumps use refrigerants with low global warming potential that can evaporate and condense efficiently even at low temperatures. The thermodynamic properties of the refrigerant largely determine both the efficiency and the operating limits of the heat pump.<\/span><\/p>\n

    Heat pump efficiency and performance metrics<\/b><\/h2>\n

    The efficiency of a heat pump is described by two key metrics: the Coefficient of Performance (COP) and the Seasonal Performance Factor (SPF). These values indicate how much heating energy can be produced from one unit of electrical energy.<\/span><\/p>\n

    Coefficient of Performance (COP)<\/b><\/h3>\n

    The COP describes the ratio of delivered heating output to electrical input under defined conditions. A COP of 4 means, for example, that the heat pump produces 4 kWh of heat from 1 kWh of electricity. The remaining 3 kWh come from the environment at no additional fuel cost.<\/span><\/p>\n

    However, the COP is not a constant value. It depends strongly on operating conditions. In particular, the temperature difference between the heat source and the heating system has a major influence. The higher this difference, the more work the compressor must do, and the lower the COP becomes. For an air-to-water heat pump, the COP drops significantly on cold winter days, while higher values are reached in spring or autumn.<\/span><\/p>\n

    Seasonal Performance Factor (SPF)<\/b><\/h3>\n

    The SPF is the more practical metric because it reflects actual operation over an entire year. It accounts for fluctuating outdoor temperatures, varying heating loads, and real electricity consumption including auxiliary drives such as circulation pumps. An SPF of 3.5 means that over the year the heat pump provides an average of 3.5 kWh of heat from 1 kWh of electricity.<\/span><\/p>\n

    Modern heat pumps achieve SPF values between 3 and 5, depending on system type, design, and building insulation. Brine-to-water heat pumps that use geothermal energy generally achieve higher SPF values than air-to-water heat pumps because ground temperatures remain more stable throughout the year.<\/span><\/p>\n

    Factors influencing efficiency<\/b><\/h3>\n

    Several factors have a significant impact on heat pump efficiency:<\/span><\/p>\n

    Temperature level of the heat source:<\/b> The warmer the source, the higher the efficiency. Soil and groundwater offer more stable and higher temperatures than outdoor air.<\/span><\/p>\n

    Supply temperature of the heating system:<\/b> Low supply temperatures, as typical for underfloor heating (30 to 35 \u00b0C), enable much better efficiency than radiators requiring 50 to 60 \u00b0C.<\/span><\/p>\n

    Control strategy:<\/b> Intelligent control systems that consider weather data and building behavior optimize operation and increase efficiency.<\/span><\/p>\n

    Hydraulic balancing:<\/b> A correctly balanced heating system ensures that all rooms are supplied evenly without forcing the heat pump to produce unnecessarily high supply temperatures.<\/span><\/p>\n

    Types of heat pumps<\/b><\/h2>\n

    Heat pumps can be classified according to the heat source they use. Each design has specific advantages and disadvantages that should be considered during selection.<\/span><\/p>\n

    Air-to-water heat pump<\/b><\/h3>\n

    The air-to-water heat pump is the most commonly installed variant. It extracts heat from outdoor air and transfers it to the heating water. Its popularity comes from comparatively simple and cost-effective installation, since no excavation work or well drilling is required.<\/span><\/p>\n

    However, the efficiency of an air-to-water heat pump fluctuates more than other types because outdoor air temperature varies significantly over the year. On cold winter days, when heat demand is highest, the source temperature is simultaneously lower, which reduces the COP. Modern units are designed to operate reliably at outdoor temperatures down to minus 20 \u00b0C and are becoming increasingly efficient through optimized refrigerants and intelligent defrost strategies.<\/span><\/p>\n

    Air-to-water heat pumps can be built as monobloc units, where all technology is housed in a single outdoor unit, or as split systems with an outdoor and an indoor unit. Both variants have advantages and disadvantages regarding space requirements, refrigerant lines, and installation effort.<\/span><\/p>\n

    Brine-to-water heat pump (ground source heat pump)<\/b><\/h3>\n

    Brine-to-water heat pumps use heat stored in the ground. A frost-protected fluid known as brine circulates through ground collectors or borehole heat exchangers. This brine absorbs geothermal heat and transports it to the heat pump.<\/span><\/p>\n

    Ground collectors are installed horizontally at about 1.5 meters depth and require a relatively large plot area. Boreholes, by contrast, are drilled vertically down to up to 100 meters and save space, but they require permits and involve higher investment costs.<\/span><\/p>\n

    The major advantage of brine-to-water heat pumps is the stable ground temperature, which typically remains between 8 and 12 \u00b0C throughout the year. As a result, these systems achieve high SPF values of 4 to 5 and operate very efficiently.<\/span><\/p>\n

    Water-to-water heat pump<\/b><\/h3>\n

    Water-to-water heat pumps use groundwater as the heat source. Two wells are drilled for this purpose: a pumping well to extract groundwater and a reinjection well to return it after heat has been extracted.<\/span><\/p>\n

    Groundwater has a relatively constant temperature of about 8 to 12 \u00b0C throughout the year, providing ideal conditions for highly efficient operation. Water-to-water heat pumps achieve the highest SPF values of all heat pump types.<\/span><\/p>\n

    However, permits are required, and the groundwater quality must be suitable. High iron or manganese content can lead to deposits and impair operation. In addition, installation costs are comparatively high due to the well drilling work.<\/span><\/p>\n

    Air-to-air heat pump<\/b><\/h3>\n

    Air-to-air heat pumps transfer the captured heat directly to indoor air, often via mechanical ventilation with heat distribution. They are frequently used in passive houses that have very low heating demand due to excellent insulation.<\/span><\/p>\n

    These systems can provide both heating and cooling while improving indoor air quality through continuous air exchange. However, they are less suitable for domestic hot water production and are therefore often combined with other systems.<\/span><\/p>\n

    Air-to-water heat pump function in detail<\/b><\/h2>\n

    Due to its widespread use, the air-to-water heat pump deserves a closer look. Its operating principle follows the four steps described above, but it has several specific characteristics.<\/span><\/p>\n

    Design and components<\/b><\/h3>\n

    An air-to-water heat pump consists of several main components:<\/span><\/p>\n

    Evaporator:<\/b> A heat exchanger with a large surface area through which outdoor air is guided by a fan. The refrigerant absorbs heat from the air and evaporates.<\/span><\/p>\n

    Compressor:<\/b> The compressor is the heart of the heat pump. It raises the pressure and temperature of the gaseous refrigerant. Modern inverter compressors adjust output continuously to match current heat demand, which significantly improves efficiency.<\/span><\/p>\n

    Condenser:<\/b> Here, the hot refrigerant gas releases its heat to the heating water. The refrigerant condenses and becomes liquid again.<\/span><\/p>\n

    Expansion valve:<\/b> This component reduces the pressure of the liquid refrigerant abruptly, which lowers its temperature so it can absorb heat again.<\/span><\/p>\n

    Controls:<\/b> Modern heat pumps include intelligent controllers that consider outdoor temperature, indoor temperature, and the heating curve to operate the system optimally.<\/span><\/p>\n

    Operating behavior and defrosting<\/b><\/h3>\n

    A distinctive feature of air-to-water heat pumps is the need for defrosting. At outdoor temperatures just above freezing and with high humidity, ice can form on the evaporator. This ice reduces heat transfer and must be removed regularly.<\/span><\/p>\n

    For defrosting, the refrigerant cycle is briefly reversed so that warm refrigerant flows through the evaporator and melts the ice. This process takes only a few minutes and occurs automatically. During defrosting, heating output temporarily decreases, which can be compensated by a sufficiently sized buffer tank.<\/span><\/p>\n

    Noise emissions<\/b><\/h3>\n

    Noise emissions are a relevant topic for air-to-water heat pumps. The fan that draws in outdoor air and the compressor generate operating noise. Modern units are increasingly designed for quiet operation and can achieve sound power levels below 60 dB(A).<\/span><\/p>\n

    During installation, adequate distance to neighboring properties and bedrooms should be ensured. Noise mitigation measures such as acoustic enclosures or vibration-damping foundations can further reduce noise impact.<\/span><\/p>\n

    Heat pump sizing and system design<\/b><\/h2>\n

    Correct sizing of a heat pump is crucial for efficiency, service life, and economic viability. A system that is too small may not cover the required heating load, while an oversized system often operates inefficiently and increases investment costs.<\/span><\/p>\n

    Heating load calculation<\/b><\/h3>\n

    Every design starts with a detailed heating load calculation in accordance with DIN EN 12831. This takes into account:<\/span><\/p>\n