How does a heat pump work?

A heat pump moves heat from one place to another using a refrigeration cycle. Instead of generating heat by burning fuel or resistive electric heating, it transfers heat from a colder area (source) to a warmer area (sink). That same cycle can be used for heating or cooling a building.

Basic components

• Evaporator (outdoor or indoor coil): absorbs heat from the source by evaporating a refrigerant.
• Compressor: raises the pressure and temperature of the refrigerant vapor.
• Condenser (the opposite coil): releases heat to the sink as the refrigerant condenses to a liquid.
• Expansion device (valve or capillary): drops pressure of the liquid refrigerant so it can evaporate again.

How the cycle works (heating mode)

1. Low-pressure cold liquid refrigerant enters the evaporator. There it absorbs heat from the source (e.g., outdoor air, ground, or water) and evaporates into a low-pressure gas.
2. The compressor compresses that gas, increasing its pressure and temperature.
3. Hot high-pressure gas flows to the condenser inside the house and releases heat to the indoor air or hydronic system as it condenses to a liquid.
4. The liquid refrigerant passes through the expansion device, its pressure and temperature fall, and the cycle repeats.

Cooling mode

• The cycle is the same but reversed with a reversing valve: the indoor coil becomes the evaporator (absorbs heat from inside) and the outdoor coil becomes the condenser (rejects heat outside). In other words, it works like an air conditioner.

Key points

• Coefficient of Performance (COP): efficiency metric equal to heat delivered divided by electrical energy consumed. Typical COPs are greater than 1 (often 2–5), meaning more heat energy moved than the electrical energy used.
• Sources: air-source (air to air), ground-source/geothermal (ground-to-water or ground-to-air), and water-source models. Ground-source tends to be more stable and efficient because ground temperature is steadier.
• Limits: performance drops as the temperature difference between source and sink increases. Air-source units are less efficient in very cold weather; some systems use electric resistance backup or hybrid designs.
• Practical considerations: proper sizing, refrigerant choice, ducting or hydronic distribution, defrosting strategy for cold climates, installation quality, and maintenance affect performance and comfort.

If you want, I can compare air-source vs ground-source systems, show a simple P–h description of the refrigeration cycle in plain words, or explain COP and how to estimate running cost for your climate and electricity price.