Heat pumps work by moving existing heat from one place to another using a refrigeration cycle, instead of creating heat by burning fuel.

Core idea: moving heat, not making it

Even in cold air, ground, or water, there is usable heat energy above absolute zero. A heat pump uses a special fluid called a refrigerant to pick up that heat at a low temperature, “upgrade” it to a higher temperature with a compressor, and release it inside your home. Because it moves energy rather than generating it, a heat pump can deliver 2–4 units of heat for every unit of electricity it uses.

The main components

Most modern heat pumps (air‑source, ground‑source, or water‑source) share the same basic parts.

  • Evaporator coil : Where the cold liquid refrigerant absorbs heat from outside air, ground, or water and boils into a gas.
  • Compressor : An electric pump that squeezes the refrigerant gas, raising its pressure and temperature.
  • Condenser coil : Inside the building, where the now‑hot gas gives up its heat to indoor air or water and condenses back to liquid.
  • Expansion device (valve) : Drops the pressure and temperature of the liquid refrigerant so it becomes cold again, ready to pick up more heat.
  • Reversing valve : Lets the system switch direction so it can either heat or cool.

Step‑by‑step: heating mode

In heating mode, the “source” is outside (air, ground, or water) and the “sink” is indoors.

  1. Cold liquid refrigerant passes through the expansion valve and becomes an even colder low‑pressure mixture.
  1. In the evaporator (outside), this cold refrigerant absorbs low‑grade heat from the air, ground, or water and boils into a low‑temperature gas.
  1. The compressor squeezes this gas, raising its pressure and making it much hotter than indoor temperature.
  1. The hot gas flows through the indoor condenser coil; a fan or water circuit takes this heat into the home’s radiators, underfloor heating, or air ducts, while the refrigerant condenses back into a warm liquid.
  1. The liquid passes through the expansion valve, its pressure and temperature drop, and the cycle repeats.

Because each loop just moves heat, the system can be highly efficient compared with electric resistance heating or older fossil boilers.

Step‑by‑step: cooling mode (like AC)

In cooling mode, everything reverses: your home becomes the “heat source,” and outdoors is the “heat sink.”

  1. The reversing valve flips the flow, so the indoor coil becomes the evaporator and the outdoor coil becomes the condenser.
  1. Indoors, cold refrigerant absorbs heat from room air, cooling the house and evaporating into gas.
  1. The compressor raises the temperature of this gas.
  1. Outside, the hot gas releases its heat to the outdoor air and condenses back to liquid.
  1. After expansion and cooling of the liquid, the cycle repeats. During this process, moisture from indoor air condenses on the cold indoor coil, so the system also dehumidifies.

Types: air, ground, and water source

Different heat pumps grab heat from different places but use the same basic physics.

  • Air‑source heat pump (ASHP) : Takes heat from outside air using an outdoor unit that looks like an AC box. It is the most common and easier to retrofit.
  • Ground‑source (geothermal) heat pump : Uses buried loops in soil or boreholes; the ground stays relatively stable in temperature, giving higher efficiency but requiring more installation work.
  • Water‑source heat pump : Uses a pond, lake, river, or groundwater as the heat source/sink.

All can provide space heating, cooling, and often domestic hot water when paired with the right indoor equipment.

Why heat pumps are such a big topic now

Heat pumps have become a trending topic because they cut emissions and energy use compared with gas or oil boilers, especially as electricity grids add more renewables. Governments and utilities in many countries now offer incentives to install them, and articles published in the last few years emphasize their role in decarbonizing home heating.

From a forum‑style perspective, the recurring discussion threads usually revolve around:

  • Upfront cost vs. long‑term savings.
  • Performance in very cold climates (often depends on model and good installation).
  • Comfort differences: lower‑temperature but more continuous heating compared with hot, on‑off radiators.

A simple way to picture it: a heat pump is like a fridge in reverse. Your fridge moves heat from inside the box to your kitchen; a heat pump moves heat from outside (air/ground/water) into your home in winter, and then flips the direction to act like an air conditioner in summer.

TL;DR: A heat pump uses a refrigeration cycle (evaporator, compressor, condenser, expansion valve plus a reversing valve) to move heat between indoors and outdoors, efficiently providing both heating and cooling by transferring heat instead of burning fuel.

Information gathered from public forums or data available on the internet and portrayed here.