Heat pumps are revolutionising home heating by providing an energy-efficient alternative to traditional gas or electric systems. But how exactly do they work? In this guide, we’ll break down the science behind heat pumps in a way that’s easy to understand, helping you make an informed decision about whether a heat pump is right for your home.
The Basics: What Is a Heat Pump?
A heat pump is a device that transfers heat from one place to another using a refrigerant and a process called the vapour compression cycle. Unlike conventional heaters that generate heat, a heat pump moves existing heat from the outside air (or ground) into your home, even in cold weather. This makes it incredibly efficient, often delivering 3-4 times more energy than the electricity it consumes.
Why Heat Pumps Are So Efficient
The secret lies in thermodynamics:
- Heat naturally flows from warm to cold areas. A heat pump reverses this process, extracting heat from cold outdoor air and transferring it inside.
- Refrigerants play a key role. These special fluids absorb and release heat as they change between liquid and gas states.
- Pressure manipulation boosts efficiency. By compressing and expanding the refrigerant, the heat pump maximizes heat transfer with minimal energy input.
The Science Behind Heat Pumps: Key Principles
Before diving into how a heat pump operates, let’s cover some foundational concepts.
1. Heat Moves from Hot to Cold
Heat energy always travels from warmer areas to cooler ones. The greater the temperature difference, the faster the transfer. A heat pump exploits this by using a refrigerant colder than the outside air to absorb heat, even on chilly days.
2. Available Heat in Cold Air
You might think freezing temperatures mean no available heat, but that’s not true! Even at 3C, the air contains thermal energy (measured in Kelvin). Absolute zero (-273C) is the only point with no heat, so there’s always energy to extract.
3. Refrigerants: The Magic Fluid
- Evaporation (Liquid → Gas): The refrigerant absorbs heat from the outside air, vaporising in the process.
- Condensation (Gas → Liquid): The refrigerant releases heat when it condenses back into a liquid, warming your home.
4. Pressure and Temperature Relationship
- Compressing a gas heats it up (think of a bike pump getting warm).
- Reducing pressure cools it down (like releasing air from a tire).
- Boiling points change with pressure. For example, water boils at 68C on Mount Everest due to lower air pressure.
These principles are the backbone of the vapour compression cycle, which we’ll explore next.
How a Heat Pump Works: The 4-Step Cycle
A heat pump operates in a continuous loop with four main stages:
1. Evaporation (Absorbing Heat)
- Location: Evaporator (outdoor unit)
- Process: The refrigerant, at low pressure and temperature, absorbs heat from the outside air, causing it to evaporate into a gas.
- Key Point: Even cold air contains enough heat to boil the refrigerant.
2. Compression (Boosting Heat)
- Location: Compressor
- Process: The gas is compressed, drastically increasing its temperature and pressure. This is the only stage that uses electricity.
- Result: The refrigerant becomes a high-temperature, high-pressure gas, ready to release heat indoors.
3. Condensation (Releasing Heat)
- Location: Condenser (indoor unit)
- Process: The hot gas passes through a heat exchanger, transferring its heat to your home’s central heating system (radiators or underfloor heating). As it cools, it condenses back into a liquid.
- Efficiency Win: One unit of electricity generates 3-4 units of heat!
4. Expansion (Cooling the Refrigerant)
- Location: Expansion valve
- Process: The high-pressure liquid passes through a valve, rapidly dropping in pressure and temperature.
- Result: The refrigerant returns to a cold, low-pressure liquid, ready to restart the cycle.
This loop repeats continuously, efficiently heating your home even in winter.
Why Heat Pumps Are Perfect for Homeowners
Energy Efficiency
Heat pumps can achieve 300-400% efficiency, meaning they produce more heat than the energy they consume. Compare that to gas boilers, which max out at 90-95%.
Lower Carbon Footprint
By using renewable heat from the air or ground, heat pumps slash CO₂ emissions, especially when paired with green electricity.
Cost Savings
Though installation costs are higher, government incentives (like the UK’s Boiler Upgrade Scheme) and lower running costs make heat pumps a smart long-term investment.
Common Questions About Heat Pumps
Do heat pumps work in cold climates?
Yes! Most models today operate efficiently at -15-20C.
Are heat pumps noisy?
Modern units are quieter than a dishwasher (around 40-60 decibels).
How long do heat pumps last?
With proper maintenance, 15-20 years.
