The hidden history of the heat pump

01Lord Kelvin saw it first

In 1852, the physicist William Thomson, later Lord Kelvin, published a paper describing what he called a heat multiplier. His insight was that a machine could use mechanical work to move heat from a colder body to a warmer one, delivering more thermal energy than the work it consumed. He recognised that this was not a violation of energy conservation, because the extra heat was drawn from the environment.

Kelvin was building on the earlier work of Sadi Carnot, who had analysed the theoretical limits of heat engines in 1824. Carnot showed that a reversible engine could run backwards as a heat pump. Kelvin turned the abstraction into a concrete proposal, though he did not build a working device.

02The first machines

The Austrian engineer Peter von Rittinger built the first practical heat pump around 1855. His system used vapour recompression to evaporate salt from brine at a saltworks in Upper Austria. Rittinger recognised that the heat rejected during compression could be reused to evaporate more brine, an early example of industrial heat recovery.

Through the late nineteenth and early twentieth centuries, heat pumps appeared in niche industrial applications: brewing, paper drying, and food processing. The technology worked but competed against cheap coal and gas. Without an economic incentive, there was little pressure to improve efficiency or reduce costs.

03Zoelly and the residential vision

In 1919, the Swiss engineer Heinrich Zoelly filed a patent for a ground-source heat pump intended for residential heating. Switzerland lacked domestic fossil fuels, and Zoelly saw that a heat pump drawing energy from soil or groundwater could reduce dependence on imported coal. His patent described a system remarkably similar to modern ground-source units.

The patent was granted in 1920, but commercial deployment took decades. The Great Depression, two world wars, and the abundance of cheap heating oil delayed the technology. Zoelly's design was sound, but the economic and industrial conditions did not yet support it.

04Cheap fuel buried the idea

From the 1920s through the 1960s, oil and gas were inexpensive, widely available, and supported by growing distribution infrastructure. A furnace that burned cheap fuel was simpler, cheaper, and more familiar than a heat pump that required electricity, refrigerant, and specialised installation. The economic case for heat pumps was weak.

This period illustrates a recurring pattern in technology adoption: a superior idea can remain dormant when the economic environment does not reward its advantages. The heat pump was not wrong; it was premature.

05The oil crisis changed the math

The oil crises of 1973 and 1979 dramatically increased the cost of fossil fuels and exposed the vulnerability of import-dependent economies. Governments in Scandinavia, Switzerland, and North America funded research into heat pumps as an alternative. Sales surged briefly, but early units were often unreliable, and the market collapsed when oil prices fell again in the 1980s.

The bust left a legacy of mistrust in some markets. Installers went out of business, and consumers who had bought early units faced expensive repairs. The technology needed another two decades of compressor, refrigerant, and controls development before it could deliver the reliability that households require.

06Climate policy as a second driver

By the 2000s, climate policy provided a new and durable rationale for heat pumps. Buildings account for a large share of energy-related carbon emissions, and most of that comes from heating. Electrifying heating with heat pumps, especially where electricity is generated from low-carbon sources, offers a pathway to decarbonisation that fossil-furnace replacements cannot match.

Governments began offering subsidies, mandating efficiency standards, and setting phase-out dates for fossil heating. The economic environment that had suppressed the technology for a century began to reverse. By the 2020s, heat pumps were outselling gas furnaces in several European countries.