The history of the heat pump begins in 1824, with the French engineer Sadi Carnot's book on heat engines. The idea of heating buildings this way was put forward in 1852 by William Thomson (later Lord Kelvin), and the first working system that reused heat by compressing steam was built in the mid-1850s at a saltworks in Austria by Peter von Rittinger. One of the first large heat pumps to heat a building went into service in Zürich in 1938. Heat pumps spread after the oil crises of the 1970s, and today's units are shaped by inverter compressors and refrigerants with a low global warming potential.
Below we follow that journey through the milestones that explain why a heat pump today is designed the way it is.
Theory: Carnot and Thomson
In his "Reflections on the Motive Power of Fire", published in 1824, Sadi Carnot showed that a heat engine obtains work from heat flowing from a hot source to a cold one, and that the upper limit of the work it can obtain depends only on those two temperatures. Run in reverse, this ideal cycle, which we now call the Carnot cycle, moves heat from cold to hot by spending work from outside. That is exactly what a heat pump is.
In 1852 William Thomson made a proposal that took the principle to the scale of a building. He calculated that a machine compressing and expanding air could heat a building with less energy than burning the same fuel directly, and noted that the same arrangement could be used to cool buildings. This proposal was the first clear statement that a heat pump could be used to heat a building.
The theory still has a direct meaning today: the highest efficiency a heat pump can reach depends on the difference between the temperatures at which it takes in and gives off heat. With outdoor air at 0 °C, the theoretical upper limit is about 8.8 for producing 35 °C water and about 6.0 for 55 °C water. Real units typically reach around half of that limit. This is also why underfloor heating, with its low water temperature, suits heat pumps so well.
Refrigerating machines pave the way
The hardware that made the heat pump possible was in fact developed for refrigeration. In 1834 in London, Jacob Perkins patented the first vapour-compression refrigerating machine, which compressed an evaporating fluid, condensed it and let it evaporate again. This cycle of evaporation, compression, condensation and expansion is still the basis of today's heat pumps.
In the late 1850s the French engineer Ferdinand Carré developed an absorption refrigerating machine that ran on heat instead of a compressor and used ammonia and water. We describe the present-day form of that approach in our article on the absorption heat pump. In the 1870s Carl von Linde's ammonia compression machines brought refrigeration to industrial scale, breweries first among its users. You can read the history of refrigeration itself in our article on the history of the chiller.
The first application: Rittinger and salt
The first working system that took heat from one place and reused it at a higher temperature was built not to heat a building but for production. In the mid-1850s, at the saltworks in Ebensee, the Austrian mining engineer Peter von Rittinger developed an arrangement that compressed the steam rising from boiling brine and used it again as the heating medium. Compressing the steam raises its temperature, so the same steam can be used again to boil the brine and less fuel is needed.
Known today as mechanical vapour recompression (MVR), this method is still used in evaporators, drying lines and distillation plants. Rittinger's solution also showed the real strength of the heat pump: spending a little energy to lift heat that would otherwise be thrown away at a low temperature to a temperature where it is useful.
Heating buildings: Zürich, 1938
The heat pump entered building heating in the first half of the 20th century, and one of the pioneers was Switzerland. In a country that imported its coal but had plenty of hydropower, producing heat with electricity reduced its dependence on imported fuel. In 1938 Zürich's city hall began to be heated by a heat pump that used the water of the river Limmat as its heat source.
The arrangement in Zürich still holds today: a low-temperature source that is plentiful and continuous (a river, a lake, the ground or the air), an evaporator that takes heat from that source, and a condenser that feeds the building's heating circuit. An air-source heat pump today does the same job with the outdoor air.
After the war and the oil crises
After the Second World War, air-to-air heat pumps for homes began to be produced, particularly in the United States. In those years of cheap oil and gas, however, heat pumps struggled to compete with boilers that cost less to buy, and for a long time they remained a small market.
The turning point came with the oil crises of 1973 and 1979. As fuel prices climbed quickly, energy efficiency reached government agendas for the first time. Heat pump installations grew fast in Northern Europe in this period, and ground-source systems became widespread, especially in Sweden. The technology moved from a laboratory idea to a mass-produced heating appliance.
The story of refrigerants
The first refrigerating machines ran on fluids such as ether, ammonia, sulphur dioxide and CO₂, some of them toxic or flammable. CFCs (such as R12), which spread in the 1930s, looked safe and quickly became the standard, followed by HCFCs (R22).
In 1974 Mario Molina and Sherwood Rowland showed that CFCs deplete the ozone layer. The Montreal Protocol, signed in 1987, set out the phase-out of ozone-depleting refrigerants, and HFCs with no effect on ozone (R410A, R134a) took their place. HFCs had a problem of their own: a high global warming potential (GWP). The 2016 Kigali Amendment provided for cutting HFCs as well.
In air conditioners and heat pumps, R22 was gradually replaced by R410A. R410A did not harm the ozone layer, and because it works at a higher pressure, the same capacity could be reached with smaller pipes and compact heat exchangers. Its GWP, however, was above 2,000, so a new generation of refrigerants was developed that has started to replace it in turn.
Today the industry is moving to R32, with its lower GWP, and to a natural refrigerant, propane (R290). We compare the climate impact of these refrigerants in figures in our article on the environmental impact of heat pumps.
Today: inverters, vapour injection and R290
There was a shift on the source side too. The ground- and water-source systems that came to the fore during the oil crises needed infrastructure such as boreholes, wells or a lake. Air-source heat pumps need only an outdoor unit, which makes them far easier to install; as their problems with efficiency in cold weather were solved, they became the most common type in Europe.
In recent decades the biggest change has been in control. Older units ran by switching the compressor on and off at full power. An inverter-driven compressor adjusts its speed continuously to demand; it raises efficiency at part load and reduces noise and wear. Vapour-injection (EVI) compressors and cascade systems, which run two circuits one after the other, have also taken the heat pump into cold climates where the boiler used to be the only option.
The AD series are built on this knowledge. The Boran series produces 65 °C water at an outdoor temperature of −25 °C, using R290 and a vapour-injection compressor. The Ayaz series reaches 75 °C water at −30 °C by running an R290 circuit and an R134a circuit in cascade. The control board and software that run the compressor, fan and expansion valve are developed by our research and development team.
The limit Carnot described in 1824 still applies; what has changed is how close we get to it. A correctly sized inverter heat pump running at a low water temperature in a well-insulated building is today's answer to two centuries of that knowledge.
Choosing today's heat pump
We explain step by step how a heat pump works in our article on what a heat pump is and how it works. You can find the series that suits your climate zone, with its technical tables, among our heat pump models.
