The history and future of the heat pump

Posted: April 21, 2026

In the winter of 1938, engineers in Zurich faced a familiar and difficult challenge. The city's historic Town Hall needed heating, but oil was expensive and difficult to secure. Switzerland, landlocked and without its own fossil fuel reserves, had been struggling with fuel shortages since World War I and was growing anxious about its energy dependence as war once again loomed over Europe. Their solution seemed to defy common sense: they would warm the building using the icy cold waters of the Limmat River, which flowed right past the Town Hall.  

How could these Swiss engineers extract warmth from a river barely above freezing? The answer lay in the physics of the heat pump, which relies on principles of thermodynamics that had been understood for nearly a century but rarely put to practical use.

Today, the heat pump has emerged as a critical technology as the world scrambles to decarbonize heat production. Heating living spaces and running industrial processes is responsible for roughly half of global energy consumption and nearly 40% of CO₂ emissions. Here’s how the humble heat pump made its journey from Victorian thought experiment to a modern-day sustainable heating essential.

The origins of the heat pump

The theoretical foundations of the heat pump emerged from the great thermodynamic discoveries of the nineteenth century. In 1824, a young French military engineer and physicist named Sadi Carnot published Reflections on the Motive Power of Fire. Drawing an analogy between heat and falling water, Carnot reasoned that just as a water wheel produces work from water flowing from a higher to a lower elevation, a heat engine could produce work from heat flowing from a higher to a lower temperature. In his short treatise, Carnot also introduced the concept of reversibility: the idea that motive power could be used to produce a temperature difference. Though he later died in obscurity at the young age of 36, Carnot's ideas went on to become foundational to thermodynamics.

In 1852, the British mathematician and physicist Lord Kelvin expanded on Carnot's ideas by proposing that a heat engine could be run in reverse: instead of converting heat into work, a machine could use work to extract heat from the cold outside environment and concentrate it indoors to warm a building. He called the proposed device a “heat multiplier.” A few years later, in 1856, Austrian mining engineer Peter von Rittinger created what is widely considered to be the first working heat pump, which he used, not to heat rooms, but to dry salt in salt marshes.

The physics of the heat pump were sound, but the economics of using it to heat buildings were not. Throughout the late nineteenth and early twentieth centuries, coal and later oil were so cheap to burn (and heat pump technology so slow to improve) that the complexity of heat pump systems made little financial sense. Why invest in developing an elaborate machine to extract heat from cold air or water when you could simply burn cheap fuel?

How energy supply disruptions helped heat pumps grow

The economics of the heat pump began to change in the 1930s, particularly in Switzerland. Switzerland’s lack of domestic fossil fuels made it vulnerable to supply disruptions, while its abundant hydroelectric power offered cheap electricity to run compressors. The 1938 Zurich Town Hall heat pump installation was a major success and marked the first large-scale heat pump installation in Europe. The heat pump system worked so well, in fact, the Town Hall continued to rely on it until 2001. The installation demonstrated that the technology could work at scale, reliably extracting heat from frigid river water to warm a major civic building. Other projects soon followed. Beginning in the late 1930s, Swiss engineers began installing heat pump systems in buildings throughout the country, drawing warmth from lakes, rivers, and groundwater.

But as the war drew to a close, cheap oil began to flood the market. Oil prices fell steadily, and the fully automated oil-fired central heating systems that arrived with them offered homeowners something heat pumps couldn't match: quick, thermostat-controlled warmth. Meanwhile, Switzerland's electricity supply was tightening as hydroelectric expansion hit its technical and financial limits. Experts began recommending that heat pumps only be used alongside backup boilers to handle peak demand.

Postwar building practices compounded the problem. The construction boom that swept Switzerland and the rest of Europe after the war produced poorly insulated buildings that were cheap and fast to build. These buildings demanded heating systems that could compensate for massive heat loss with rapid, high-output warmth—exactly what an oil-fueled boiler provided. From the mid-1930s to 1960, only about sixty heat pump systems were installed across the entire country, almost all of them in large industrial or public buildings. The technology that had seemed so promising during wartime fuel shortages was quietly sidelined by an era of abundant, inexpensive oil.

Improving heat pump technology

For decades, heat pumps remained a niche technology, installed where specific conditions—expensive fuel, cheap electricity—aligned. The 1973 oil crisis changed the calculus again, almost overnight. When OPEC's embargo sent petroleum prices soaring, the idea of extracting cheap heat from the air or ground began, once again, to sound pretty appealing.  

Interest in heat pumps surged around the world throughout the 1970s and early 1980s. Manufacturers rushed products to market, and installations multiplied. But early systems had significant limitations: they worked poorly in cold weather, when heating was needed most, and often proved unreliable. When oil prices eventually stabilized and fell in the mid-1980s, much of the momentum dissipated, and the heat pump fell out of fashion once again.

Nevertheless, over the following decades, the technology continued to improve. Engineers developed variable-speed compressors that could modulate output rather than simply cycling on and off. Refrigerants became more efficient. Most importantly, innovations in cold-climate heat pumps pushed the operating threshold far below freezing, addressing one of the technology's fundamental weaknesses.  

Modern heat pumps and the future of clean heating

Today's systems bear little resemblance to the finicky machines of the 1970s. Modern cold-climate heat pumps operate efficiently even at temperatures of minus 25 degrees Celsius. Smart controls optimize performance, and installations have become increasingly streamlined.

Modern heat pumps can convert clean electricity into heating at remarkable efficiency. A modern heat pump can deliver 3 to 4 units of heat for every unit of electricity consumed—moving heat energy from outside to inside, or to use Lord Kelvin’s term, multiplying the heat out of thin air by harvesting warmth from the environment. When powered by renewable electricity, they enable nearly carbon-free heating.

Still, old ideas about heat pumps’ limitations continue to persist. And despite many countries recognizing the important role heat pumps will play in the global energy transition, they account for only around 10% of global building heating needs. To reach net-zero global emissions by 2050, the International Energy Agency estimates the global heat-pump stock would need to almost triple in the next four years.  

The most recent surge in heat pump popularity came only a few years ago. Growth rates reached double digits in both 2021 and 2022 as global oil and gas prices spiked once again. In Europe, sales grew by nearly 40%. But by 2023, sales across much of the world had slumped again as prices stabilized. The most recent oil shocks may trigger yet another boom in the heat pump boom-bust cycle.  

Good ideas have a way of finding their moment, even if it takes a century and a half of wars, oil shocks, climate concerns, and technological refinement to get there. But only time will tell whether this cycle will finally break the pattern, or whether cheap fossil fuels will sideline heat pumps yet again.

Resources:

^{Heat pumps, International Energy Agency
History of heat pumps, Swiss Federation
The materiality of space heating: heat pumps, Irene Pallua
How heat pumps of the 1800s are becoming the technology of the future, Yale Climate Connections
Technological innovations in heat pump systems, Renato Lazzarin
Heat pumps, EBSCO
How long have heat pumps been around, Engineer Fix}