From Policy to Physics
In the previous article in our Future Homes series, we explored why developers across the UK are increasingly specifying air source heat pumps in new housing developments. Tighter Building Regulations, ambitious carbon reduction targets and the transition towards low-carbon homes are all accelerating their adoption.
Understanding why heat pumps are being installed, however, is only part of the story.
For most homeowners, a much simpler question comes first.
Air source heat pumps continue extracting thermal energy from the outside air even during freezing weather. Cold air contains less heat than warm air, but it still contains enough thermal energy to heat a well-designed home
How do they actually work?
Perhaps more importantly, how can a heating system that uses just one kilowatt-hour (kWh) of electricity apparently deliver three or even four kilowatt-hours of heat without breaking one of the most fundamental laws of physics?
It's a question that has generated countless headlines, lively debates and more than a little confusion.
In this article we'll leave planning policy, regulations and government targets behind and instead look inside the technology itself. We'll follow the remarkable journey of heat through an air source heat pump, explain the refrigeration cycle in straightforward language and uncover why these systems behave so differently from the gas boilers many of us have grown up with.
By the end of the article, you'll understand not only how a heat pump works, but also why it works.
It Sounds Impossible... But It Isn't
If you've been looking at buying a new home recently, there's a good chance you've come across the phrase "air source heat pump." You may also have seen claims that these systems can deliver three or even four kilowatt-hours of heat while using just one kilowatt-hour of electricity.
At first glance, that sounds impossible.
Most of us remember learning at school that energy cannot be created or destroyed. If that's true, how can a heating system apparently produce three or four times more energy than it consumes? It almost sounds as though a heat pump is breaking one of the most fundamental laws of physics.
Fortunately, it isn't.
An air source heat pump obeys exactly the same laws of physics as every other heating system. The difference is that it doesn't create heat in the way a gas boiler does. Instead, it uses electricity to transfer heat that already exists in the outside air into your home.
That distinction is the key to understanding the technology.
A traditional gas boiler releases heat by burning fuel. Every unit of useful heat comes from the chemical energy stored within the gas itself. A heat pump works differently. Think of it less as a heater and more as a transport system. Its job is to collect low-temperature heat from the outside air, raise it to a useful temperature and deliver it into your home's heating system.
Once you understand that heat is being moved rather than made, the headline figures begin to make much more sense.
So where does this heat come from? How can there still be useful thermal energy in the air when the temperature outside is close to freezing? And why does the system need electricity at all?
Those questions take us to the heart of one of the most ingenious pieces of engineering found in today's homes.
WaterMatters Journey
When we first began planning the Future Homes series, I found myself returning to a lesson I'd learnt decades earlier in GCSE Physics: energy cannot be created or destroyed.
That single principle was enough to make me question the claims being made about heat pumps. If a system uses one kilowatt-hour of electricity, how could it possibly deliver three or four kilowatt-hours of heat? Was the industry simplifying the science, or was I misunderstanding what was really happening?
Rather than accepting or dismissing the claims, I decided to go back to the physics.
What I discovered wasn't a loophole in the laws of thermodynamics. Instead, it was a reminder that understanding often begins by asking the right question. A heat pump isn't creating extra energy. It's transferring energy that already exists in the environment. Once I understood that distinction, everything else began to fall into place.
If you've ever looked at the numbers and wondered how they can possibly be true, then you're asking exactly the same question that started this investigation.
Figure 1: Where Does the Extra Heat Come From? A heat pump uses electricity to power the refrigeration cycle while collecting naturally occurring thermal energy from the outside air. The electricity drives the process, while most of the heat delivered into the home comes from the environment.
The Air Around You Is Full of Energy
To understand where that additional heat comes from, we first need to understand something surprising about the air around us.
It might seem strange to talk about extracting heat from the outside air on a cold winter's day. After all, if it's only 2°C outside, or even below freezing, surely there isn't any useful heat left to collect?
In fact, there is.
The key is understanding what temperature really measures.
Temperature isn't a measure of how much energy exists within something. Instead, it tells us how energetically the molecules are moving. Even when the outside air feels cold to us, billions upon billions of molecules are still moving continuously, carrying thermal energy with them.
The only point at which molecular movement stops completely is absolute zero, a temperature of -273.15°C. Anything warmer than that still contains thermal energy.
Since even the coldest winter day in Britain is well above absolute zero, there is always heat available for a heat pump to collect.
Of course, colder air contains less thermal energy than warmer air. That's one reason why heat pumps become slightly less efficient during very cold weather. But less heat doesn't mean no heat.
The challenge isn't finding heat.
The challenge is collecting it efficiently.
This is where an air source heat pump begins to demonstrate some remarkably clever engineering. Rather than trying to heat the outside air directly, it uses a specially engineered fluid called a refrigerant that can absorb heat even at very low temperatures.
That refrigerant is the real secret behind the system.
WaterMatters Insight
Cold doesn't mean there isn't heat. It simply means there is less of it.
Even air at 0°C contains significant thermal energy. A heat pump doesn't need warm air to operate. It simply needs air that is warmer than absolute zero (-273.15°C) so that heat can naturally flow into the refrigerant.
Can a Heat Pump Really Work Below Freezing?
One of the most common questions about heat pumps is also one of the simplest.
If the temperature outside is below freezing, where does the heat come from?
The answer is that freezing is simply another point on the temperature scale.
While water freezes at 0°C, the molecules in the air continue moving and continue carrying thermal energy. The air may feel bitterly cold to us, but from a scientific perspective there is still energy available to transfer.
What changes as the temperature falls isn't whether heat exists, but how much is available and how easily it can be collected.
As the outside air becomes colder, the heat pump has to work harder to extract the same amount of thermal energy. This is why its Coefficient of Performance (COP) generally falls during very cold weather. The system remains perfectly capable of heating the home, but it uses slightly more electricity to move each unit of heat.
Modern air source heat pumps are specifically designed to operate in cold climates. Many manufacturers rate their systems to continue working at temperatures well below -15°C, with some models capable of operating at -20°C or lower. Countries such as Norway, Sweden and Finland, where winter temperatures regularly fall far below those experienced across most of the UK, have relied on heat pumps for many years.
This often surprises people.
The common misconception is that a heat pump somehow "runs out of heat" once temperatures fall below freezing. In reality, the limiting factor isn't whether heat exists. It's how efficiently that heat can be collected and concentrated into a useful temperature.
So if thermal energy is available in the outside air, how does the heat pump actually capture it and bring it into your home?
The answer lies in a beautifully engineered process known as the refrigeration cycle.
Figure 2:Can a Heat Pump Really Work Below Freezing? Freezing temperatures do not mean heat disappears. Air at 0°C, or even below, still contains thermal energy that can be absorbed by a refrigerant and transferred into the home. As temperatures fall, the heat pump works harder, reducing efficiency but continuing to operate.
WaterMatters Insight
Freezing doesn't mean heat disappears.
A heat pump doesn't stop working because the outside temperature falls below 0°C. As long as the air remains warmer than absolute zero, it still contains thermal energy. The colder it becomes, the harder the heat pump must work to collect that energy, but the underlying physics never changes.
Following the Journey of Heat
We've established that even cold winter air contains useful thermal energy. The next question is how a heat pump captures that energy and transfers it into your home.
The answer lies in a continuous process known as the refrigeration cycle.
Although the name may sound unfamiliar, you've almost certainly encountered the same scientific principles before. Refrigerators remove heat from inside your fridge and release it into your kitchen. Air conditioning systems remove heat from inside a building and release it outdoors.
An air source heat pump simply reverses the process.
Instead of moving unwanted heat out of your home, it captures naturally occurring heat from the outside air and moves it indoors.
At the heart of the system is the refrigerant introduced in the previous section. Unlike water, which boils at 100°C, refrigerants are specifically designed to boil at extremely low temperatures. This allows them to absorb heat from the outside air, even during the middle of winter.
The refrigerant circulates continuously through a sealed system, changing between a liquid and a gas as it passes through four distinct stages.
Step 1: Collecting Heat
A fan draws outside air across a heat exchanger known as the evaporator. Inside the evaporator is cold liquid refrigerant. Because the outside air is warmer than the refrigerant, heat naturally flows into the fluid.
As the refrigerant absorbs this energy, it boils and changes from a liquid into a gas.
Step 2: Concentrating the Heat
The refrigerant gas now enters the compressor, the only major component powered directly by electricity.
Compressing the gas increases its pressure, and as the pressure rises, so does its temperature.
You may have experienced the same effect when inflating a bicycle tyre. After pumping for a while, the pump becomes noticeably warm. Compressing a gas forces its molecules closer together, increasing the number of collisions between them and raising the temperature.
This is where the electricity supplied to the heat pump performs its most important task. It powers the compressor, enabling the low-temperature heat collected from the outside air to become warm enough to heat your home.
Step 3: Heating Your Home
The hot, high-pressure refrigerant then flows through another heat exchanger called the condenser.
Here it transfers its heat into the water circulating around your home's heating system.
As the refrigerant gives up its thermal energy, it cools and changes back into a liquid while the warmed water carries that heat to your radiators or underfloor heating.
Step 4: Ready to Begin Again
Finally, the liquid refrigerant passes through an expansion valve.
Its pressure drops rapidly, causing its temperature to fall once again.
The refrigerant is now ready to absorb more heat from the outside air, and the cycle repeats continuously for as long as the heat pump is operating.
WaterMatters Insight
The compressor doesn't create heat. It upgrades it.
One of the biggest misconceptions about heat pumps is that the compressor is where the heat is "made". In reality, its role is to increase the pressure and temperature of refrigerant that has already absorbed thermal energy from the outside air, allowing that heat to be transferred efficiently into your home's heating system.
Why 1 kWh of Electricity Can Deliver 4 kWh of Heat
We've now followed the complete journey of heat from the outside air into your home.
That leaves one final scientific question.
If the fan, compressor and circulation pumps are all powered by electricity, how can a heat pump deliver three or four times more heat than the electrical energy it consumes?
The answer lies in recognising that electricity isn't supplying all of the heat.
Instead, it powers the refrigeration cycle, allowing the heat pump to collect additional thermal energy that already exists in the outside air. The electricity drives the fan, compressor, circulation pumps and control systems. Together, these components enable heat to be transferred from the environment into your home.
An everyday analogy helps illustrate the point.
Imagine an electrically powered conveyor belt moving boxes from one warehouse to another.
The electricity powers the conveyor belt.
It doesn't create the boxes.
A heat pump works in much the same way. Electricity provides the energy needed to move heat, while most of the heat itself already exists in the surrounding air.
For example, a heat pump might use 1 kWh of electricity to operate the refrigeration cycle while collecting approximately 3 kWh of thermal energy from outside.
The result is around 4 kWh of useful heat being delivered into your home's heating system.
Nothing has been created.
Nothing has appeared from nowhere.
The laws of physics remain completely intact.
The energy has simply been transferred from one place to another.
Figure 3: Inside an Air Source Heat Pump An exploded view of a typical air source heat pump showing the main components that make the refrigeration cycle possible. Together, these components continuously absorb, concentrate and transfer heat from the outside air into the building.
Understanding COP
Engineers describe the performance of a heat pump using a measure called the Coefficient of Performance, usually shortened to COP.
Unlike the efficiency rating traditionally associated with boilers, COP isn't expressed as a percentage. Instead, it compares the amount of useful heat delivered with the amount of electricity consumed.
If a heat pump uses 1 kWh of electricity and delivers 4 kWh of heat, it has a COP of 4.
If it delivers 3 kWh of heat, its COP is 3.
The higher the COP, the more effectively the heat pump is transferring naturally occurring thermal energy into the home.
However, COP isn't a fixed number.
It changes continually depending on operating conditions.
Outside air temperature, the temperature of the heating water, the insulation of the building, the design of the heating system and even how the homeowner uses the controls all influence performance.
On a mild spring day, when plenty of thermal energy is available in the outside air, a heat pump may achieve a relatively high COP.
During colder weather, the system must work harder to collect the same amount of heat, so the COP naturally falls.
This doesn't mean the heat pump has become faulty or inefficient.
It simply reflects the fact that collecting heat becomes progressively more difficult as less thermal energy is available in the outside air.
Perhaps the easiest way to visualise this is to imagine cycling.
Cycling along a flat road requires relatively little effort.
Cycling uphill still gets you to your destination, but you have to work much harder to achieve the same result.
A heat pump behaves in much the same way.
As outdoor temperatures fall, it continues moving heat into your home, but the effort required to collect each unit of thermal energy increases.
For this reason, manufacturers normally publish a range of typical COP values rather than quoting a single figure.
WaterMatters Insight
COP is a measure of performance, not perfection.
A heat pump doesn't operate at one fixed level of efficiency throughout the year. Just as a car's fuel consumption varies depending on driving conditions, a heat pump's COP changes with the weather, the home it serves and how the heating system is operated.
Figure 4: Understanding COP The Coefficient of Performance (COP) compares the amount of heat delivered by a heat pump with the electrical energy it consumes. A higher COP means more useful heat is transferred into the home for every unit of electricity used.
Why Heat Pumps Work Best in Well-Designed Homes
Understanding how a heat pump works is only part of the story.
How well it performs also depends on the home it is heating.
This is one of the biggest differences between a heat pump and a traditional gas boiler.
Gas boilers are designed to produce very hot water, often at temperatures of 70°C or more. That allows them to heat a room quickly, even if the building loses heat relatively easily through its walls, windows or roof.
Heat pumps take a different approach.
Rather than producing short bursts of very high temperatures, they are designed to deliver a steady supply of warmth over longer periods. This makes them particularly well suited to homes that are well insulated and carefully designed to retain heat.
Think of it like filling a bath.
A gas boiler is rather like turning the hot tap fully on for a few minutes.
A heat pump is more like running the tap steadily, maintaining a comfortable level without repeated bursts of heat.
Because heat pumps usually operate at lower water temperatures, they often work best with larger radiators or underfloor heating. These systems provide a greater surface area, allowing the same amount of heat to be transferred into a room without requiring extremely hot water.
This doesn't mean heat pumps cannot be installed in older properties.
Many existing homes are successfully heated using heat pumps. However, improvements such as better insulation, upgraded radiators or reduced draughts may help them perform more efficiently.
For new-build homes, the picture is rather different.
Modern Building Regulations require significantly higher standards of insulation and airtightness than were common just a few decades ago. As a result, many new homes naturally complement the way heat pumps operate, requiring less energy to maintain comfortable indoor temperatures throughout the day.
In other words, the heat pump is only one part of a much bigger system.
The performance of the walls, roof, windows, ventilation and heating controls all contribute to the comfort and efficiency of the home.
WaterMatters Insight
The most efficient heating system is the one that doesn't need to replace lost heat.
Good insulation, high-quality windows, effective ventilation and careful design all reduce the amount of heat escaping from a building. The less heat a home loses, the less work any heating system has to do to keep it comfortable.
Figure 5: A Modern Home Works as One System A heat pump performs best when it forms part of a well-designed, energy-efficient home. Good insulation, low-temperature heating systems, hot water storage and smart controls all contribute to lower energy use and improved comfort.
Understanding the Technology Behind Tomorrow's Homes
For many homeowners, an air source heat pump is unlike any heating system they've used before. It doesn't behave like a traditional gas boiler, it operates at different temperatures, and it relies on moving heat rather than creating it.
That can make the technology seem unfamiliar.
Yet once you understand the science, it becomes surprisingly logical.
Every day, millions of refrigerators quietly move heat from inside a cabinet into the room around them. An air source heat pump simply applies the same scientific principles in reverse, moving naturally occurring thermal energy from outside your home to the inside.
Whether heat pumps become the dominant heating technology of the future remains to be seen. Technology will continue to evolve, government policy will change, and different solutions may prove more appropriate for different homes and locations.
What seems much more certain is that the homes we build over the coming decades will become increasingly integrated systems.
Heating, insulation, ventilation, electricity generation, battery storage, rainwater harvesting, wastewater treatment and smart controls will all work together to improve comfort, reduce energy consumption and make better use of our natural resources.
Understanding how each of these technologies works is becoming just as important as understanding how a boiler or a fuse box worked for previous generations of homeowners.
The purpose of this series isn't to tell readers which technologies they should choose.
Instead, it's to provide the knowledge needed to ask informed questions, understand the options available and make better decisions as our homes continue to evolve.
Understanding how a heat pump works doesn't tell us whether it is the right solution for every home. It does, however, allow homeowners, developers and policy makers to have better informed conversations about the role it may play in the homes of the future.
WaterMatters Insight
Good decisions begin with good understanding.
Whether you're buying a new home, renovating an older property or simply trying to understand the technologies shaping Britain's housing future, knowledge doesn't tell us which choices to make, but it does help us ask better questions.
Looking Ahead
Heating is just one of the hidden systems that make a modern home work.
In the next article in our Future Homes series, we'll turn our attention to something every one of us relies on every single day, yet rarely stops to think about: drinking water.
Every time you turn on a tap, clean, safe water appears almost instantly. But where has it come from? How has it been collected, treated, tested and transported before reaching your kitchen? And how will climate change, population growth and the demand for thousands of new homes affect one of Britain's most critical pieces of infrastructure?
Join us next time as we follow the remarkable journey of a single drop of water from source to tap, revealing the hidden engineering that keeps our communities supplied every day.
