What Is a Heat Pump and How Does It Work?

Heat pumps are easier to use than you may think

As the UK and many other countries transition toward Net Zero goals, heat pumps are emerging as a transformative solution for home heating. With their potential to drastically reduce carbon emissions and increase energy efficiency, understanding how heat pumps work is essential for homeowners, developers, and anyone interested in sustainable living.

This guide breaks down the physics behind heat pumps in simple terms, explains how they operate in cold weather, explores the different types of systems, and evaluates their efficiency compared to traditional gas boilers.

What Is a Heat Pump?

A heat pump is an energy-efficient heating and cooling system that transfers heat from one place to another using electricity. It doesn’t create heat by burning fuel like a gas boiler does—instead, it harvests naturally occurring thermal energy from the air, ground, or water outside your home and uses it to warm the inside.

At its core, a heat pump works like a refrigerator in reverse. Just as the back of a fridge feels warm while the inside remains cold, a heat pump extracts heat from the outdoors—even in cold temperatures—and transfers it indoors.

How Do Heat Pumps Work?

Step-by-Step Breakdown of Heat Pump Operation

1. Evaporation: The process begins by exposing a special refrigerant to outside air, ground, or water. Even at temperatures below freezing, this refrigerant absorbs thermal energy and evaporates into a gas.

2. Compression: A compressor then compresses the gas, significantly raising its temperature. This is where most of the system’s electricity is used.

3. Heat Transfer: The hot gas passes through a heat exchanger, transferring heat to the home’s heating system—radiators, underfloor heating, or hot water storage.

4. Condensation: As the refrigerant releases its heat, it cools down and condenses back into a liquid.

5. Expansion and Cooling: The liquid refrigerant is then passed through an expansion valve, reducing its pressure and temperature before the cycle begins again.

This cycle repeats continuously, efficiently using a small amount of electricity to move a large amount of heat.

Why Heat Pumps Work in Cold Weather

Many people ask: “Can heat pumps work when it’s freezing outside?” The answer is yes.

Thermal energy exists in the air down to -273°C (absolute zero). Most refrigerants used in heat pumps boil at around -15°C to -20°C, meaning that they can still evaporate and absorb heat even in freezing conditions. This makes heat pumps suitable for colder climates—including places like Scandinavia where they are widely used.

Types of Heat Pumps

Heat pumps are categorized based on where they extract heat from and how they deliver it inside. Here are the most common configurations:

By Heat Source (Outside):

• Air Source Heat Pumps: Extract heat from the air.

• Ground Source Heat Pumps: Pull heat from the ground.

• Water Source Heat Pumps: Use lakes, rivers, or aquifers as a heat source.

  
  

By Heat Delivery (Inside):

• Air-to-Air: Transfers heat from outside air and blows warm air inside.

• Air-to-Water: Transfers heat from outside air and uses it to warm water in radiators or underfloor heating.

• Water-to-Air: Transfers heat from water sources and delivers it via air.

• Water-to-Water: Transfers heat from water and delivers it through water-based heating systems.

Most UK homes use hydronic (water-based) systems like radiators and underfloor heating, making air-to-water or ground-to-water systems the most popular choices.

Heat Pump Efficiency: Understanding the Coefficient of Performance (COP)

A key advantage of heat pumps is their high efficiency, measured by the Coefficient of Performance (COP).

COP = Heat Output (kWh) / Electricity Input (kWh)

For example, a COP of 3 means the system produces 3 kilowatt-hours of heat for every 1 kilowatt-hour of electricity consumed.

Typical COP Values:

• 3 to 4 is considered average.

• Above 4 is excellent performance.

By comparison, modern gas boilers typically have an efficiency of around 92–94%, meaning they output 0.92 to 0.94 kWh of heat per 1 kWh of gas consumed. This clearly shows that heat pumps outperform gas boilers in primary energy efficiency, particularly when powered by renewable electricity.

Why Heat Pumps Are Considered ‘Magic’

The real "magic" of heat pumps lies in their ability to use the sun’s stored energy in the environment to do most of the work. The electricity only powers the compressor—nature provides the heat. That’s why people often say: “Buy one, get three”—you invest one unit of electricity and get up to three (or more) units of heat.

Heat pumps: A Smart Step Toward Net Zero

Heat pumps are not just a new type of heating system—they represent a paradigm shift in home energy. They are clean, efficient, and increasingly necessary in our journey to decarbonize residential heating.

Whether you're retrofitting an existing property or building a new one, understanding the basics of how heat pumps work helps you make informed choices for a more sustainable future.