What are heat pumps and how do they work?

An environmentally friendly way to heat your home.

With the climate crisis and political landscape forcing us to consider renewable fuel alternatives to heat our homes, many of us have been turning to heat pumps as an environmentally friendly solution.  

But if you’re new to the technology, there’s a raft of questions that need expert explanation and that’s where Saga Exceptional can help. We’ve spoken to heating specialists to find out what a heat pump is and how it works. 

Air to air heat pump positioned on exterior house wallCredit: Shutterstock/NAPA

Gas boilers are being phased out 

The UK Government’s ambition is that, from 2035,  all boilers are replaced with lowcarbon technology when they reach the end of their life. This is part of its heat and buildings strategy setting a path to net zero by 2050. “If your boiler needs replacing anyway,” says the Energy Saving Trust, “it’s a good time to look at installing a heat pump.” 

Under the Government’s Boiler Upgrade Scheme, property owners in England and Wales can gain £5,000 towards the cost and installation of an air source heat pump, and get £6,000 off the cost and installation of a ground source heat pump. Check your eligibility for the scheme on the Government’s Boiler Upgrade Scheme website. 

What is a heat pump?

Heat pumps capture heat and move it inside or outside the home

A heat pump system is powered by electricity and essentially moves heat from one place to another. They can either heat or cool a home, making them ideal for use throughout the year. In winter, a heat pump will extract warmth from outside and move it inside. During the summer, it works in reverse and cools a home by expelling hot air outside.  

In this article, we’ll be looking at air source and ground source heat pumps. 


How many heat pumps have been installed in the UK? 

Although it’s difficult to get exact figures, according to heating manufacturer Worcester Bosch, there are between 400,000 and 500,000 heat pumps installed in UK homes, around 30,000 of which are ground source heat pumps. 

How do heat pumps work?

They use natural sources of energy to create heat

Heat pumps extract thermal energy (heat) from the air, ground or water and use it to generate heating and hot water for your home.  

“Unlike a traditional boiler, a heat pump does not burn anything to create heat; instead it moves heat from one place to another,” explains home heating expert David Hilton, from Heat and Energy Ltd. “Combustion produces a huge amount of heat in a very short space of time, but the heat pump delivers a small amount of heat over a long period of time.” 

“The technology that the heat pump uses to ‘move’ the heat is nothing new. We all have at least one ‘heat pump’ in our homes, we just call it a fridge.”

“Our fridge simply moves heat from inside the box and dumps it on the grid behind the fridge,” continues Hilton. “In the same way that we would not expect our fridge to cool our homes on a hot day, we need to make sure that the heat pump can heat our homes in winter.” 

In simple terms, a heat pump takes air from outside or heat from the ground and warms it up to a suitable temperature. This heat is then used to create warm air and hot water in our homes.

The heat pump cycle

The National Grid explain the process in six steps outlined below: 

  1. Heat is collected from the outside air or the ground. It passes over the outside of the heat exchange surface. The heat is absorbed by a cold refrigerant liquid below the surface of the heat exchange. 
  2. The heat is enough to allow the refrigerant liquid in the heat pump to evaporate and turn into a gas. 
  3. The gas moves through a compressor, which increases the pressure and causes the temperature of the gas to rise. 
  4. The heated gas passes through the internal heat exchange surface. It can then be blown around the home or transferred to a central heating or hot water system. 
  5. Once the heat is transferred into the home, the gas falls in temperature and returns to a liquid state. It is then pumped back towards the external heat exchange.
  6. The cycle repeats itself until your home reaches the temperature set on your thermostat.  

Vaillant has a useful video on its website illustrating how heat pumps work. 

What is an air source heat pump and how does it work?

Energy is taken from the outside air

“An air source heat pump extracts energy from outside air,” says Martyn Bridges, director of technical services at Worcester Bosch. “It’s simply a case of using the outside air to warm up a refrigerant circuit within the heat pump and, via a compressor, transferring this heat into the heating and hot water system.” 

As they take up less space than ground source heat pumps and are easier to install, they are the most common type of heat pump to be installed. And according to Greenmatch, while ground source heat pumps can deliver heating and cooling with higher efficiency than air source heat pumps, due to their higher cost and more complicated installation, they only represent a small portion of global sales.  

Air source heat pumps look like air conditioning units and are mounted outside a property. Although, to work effectively, they need clearance from walls and plants.  

“For any heat pump, there also needs to be space within the home for a hot water cylinder,” explains Wilkins, “which could be an issue for many homes that have switched to a combi boiler, as they may have lost this original space.” 

However, if you don’t have a lot of extra space, Wilkins suggests: “Hot water cylinders can also be placed in garages or lofts. Although, as these are normally cold spaces, additional frost protection may be necessary.” 

The size of the outside unit will depend on your heating requirements – the more you need, the larger the unit. It’s also worth knowing that the unit’s fans will create some noise, although modern units are far quieter than their predecessors. Heatable states on its website that air source heat pumps operate at a noise level of 40-60 decibels, which is about the same level as an air conditioning unit, but it’s unlikely you’ll hear the noise when you are inside your home. 

Types of air source heat pumps 

There are two types of air source heat pumps – air to water and air to air. Air to water heat pumps heat radiators, underfloor heating and hot water, while air to air heat pumps transfer heat from the outside air to air inside your home. 

What is a ground source heat pump and how does it work?

Heat is taken from underground

A ground source heat pump relies on natural heat from underground to heat your home and hot water. As the name suggests, the system is embedded in the ground within a series of pipes, which means you’ll need land around your property to install the system. 

Stephen Bielby, operations manager at the Ground Source Heat Pump Association (GSHPA), explains: “The pipework is either buried in the ground horizontally or inserted into boreholes.”  

If buried horizontally, it will be to a depth of around 1-1.5m (3ft 4in-4ft 11in), while boreholes will need to be to a depth of around 80-200m (262-656ft).  

According to the National Grid, a full ground source heat pump system comprises a ground loop – a network of water pipes buried underground – and a heat pump at ground level. The size of the loop will depend on the size of your home and heating needs.  

To generate heat, a mixture of water and anti-freeze (the refrigerant) is pumped around the ground loop, absorbing the heat stored underground. The water mixture is then compressed and passed through a heat exchanger, which in turn extracts the heat and transfers it to the heat pump.   


The benefit of gaining heat from underground is that it remains relatively constant throughout the year, regardless of the weather conditions above ground. 

Illustration of house with horizontal ground source heat pump systemCredit: Shutterstock/vchal

A heat pump heating system needs designing

Why you need to size up the system and get it right

David Hilton explains how installing a heat pump system “is not simply a case of switching from one system to another. It needs careful planning, along with a home survey, to ensure your heating system – including any radiators and underfloor heating – will adequately heat your home.” 

“As a heat pump delivers a small amount of heat over a long period of time, we need to design and manage our heating systems in a very different way,” adds Hilton. 

“We need to know how much heat is required in the home, how much heat there is in the air or ground (depending on where we are getting the heat from) and how big the ‘engine’ needs to be to move the heat. This is the crucial design stage.  

“Once the heat pump has moved the heat from the air (or ground) then the heating distribution system (underfloor heating or radiators) needs to be sized to allow the heat to get into the room. Underfloor heating is ideal due to its large surface area, but radiators can also be sized to work at low temperatures. The larger the surface area, the better. This does not necessarily mean that the radiators will be massively bigger than those that you already have, but these must be considered within the design of the whole system.  

“Finally, it may be necessary to increase the pipe size of your central heating system. There are only two ways to deliver more heat in water.”

Make it hotter or move it faster  

“Making it hotter will make the heat pump less efficient, so we don’t want to do that. Therefore, we must move it faster.  

“If you move water through a pipe faster than 1.5 metres (4ft 11in) per second, it gets noisy, so to get the right heat to each room we may need to install larger pipes, and yes this could be disruptive.”  

“A good survey will determine what needs to be changed. A heat pump is part of a system and the whole system needs to be designed and balanced, or there will be performance issues.”  

Is a heat pump better than central heating?

It powers your heating, it doesn’t replace it

“The central heating system distributes the heat and comprises radiators or underfloor heating or both,” says Bielby. “Central heating, like the house itself, has no idea what technology is generating the heat,” he explains. 

“Provided that the radiators or underfloor heating are correctly sized, any heat technology – including boilers and heat pumps – will be more efficient in buildings that are better insulated. But improved insulation is not a required precursor to an effective and successful heat-pump insulation.” 

In fact, he says that there are plenty of examples of Grade I- and Grade II-listed buildings running solely on heat pump technologies. 

Octopus Energy told Saga Exceptional that a heat pump also has a longer lifespan: “There is less maintenance involved: a heat pump can last 20 years, about twice as long as a boiler, and needs less pricey maintenance.”  

Are heat pumps environmentally friendly?

Yes, much more so than a gas boiler

“Heat pumps are an extremely low-carbon heating option,” says the Energy Saving Trust. “They use electricity to produce heat in a highly efficient way. A heat pump captures heat already present in the environment, and the system itself does not burn any fuel and therefore emits no carbon dioxide.” It will use a little electricity to power the compressor, but this isn’t emitted by the system.  

In fact, the heat generated by the heat pump is much greater than the amount of energy used to power the system. “For every 1kW of electricity used by the pump,” says Mark Wilkins, technologies and training director at Vaillant, “you can get up to 5kW of heat in return. 

“Gas boilers, on the other hand, can only make 0.9kW units of heat out of 1kWh units of electricity,” says a spokesperson at Octopus Energy. “This means that with a heat pump you get more energy whilst using less electricity.” 

With a total 17% of the UK’s carbon emissions coming from heating homes, according to the Government’s report Decarbonising heat in homes, published in 2021,it seems heat pumps may soon need to become a standard home heating solution if we want to reduce this figure. The Government is encouraging the transition with its Boiler Upgrade Scheme, offering £5K towards the switch.  

 Can a heat pump heat my whole house?

Absolutely – just make sure it’s the right one

When asked if heat pumps can heat a whole house, it’s a resounding yes from Bridges: “An air to water or ground towater heat pump is designed to heat an entire house and the hot water cylinder, he says. “It’s only air to air heat pumps that can’t do this.” 

“An air to air heat pump is literally an outdoor unit with an indoor fan that blows warm or cold air into the area where it’s positioned. Unless the property is open-plan then they can’t heat a whole house,” he adds. 

Octopus Energy also explains that a heat pump will provide a consistent heat throughout your house, unlike a gas boiler where the temperature can “yo-yo” throughout the day. “A heat pump is designed to stay on for longer without costing you more – keeping your home at a cosy, consistent temperature,” the energy provider says.  

Camilla Sharman

Written by Camilla Sharman she/her


Camilla Sharman is a Staff Writer at Saga Exceptional. Camilla has worked in publishing and marketing for over 30 years and has covered a wide range of sectors within the business and consumer industries both as a feature, content, and freelance writer.  

As a business journalist, Camilla has researched articles for many different sectors from the jewellery industry to finance and tech, charities, and the arts. Whatever she’s covered, she enjoys delving deep and learning the ins and out of different topics, then conveying her research within engaging content that informs the reader. 

It was when she started her family that her freelance career evolved. Having moved into a period house two days before her first son was born, she had the perfect opportunity to combine working from home with writing about her own house renovation projects. Apart from appearing on the cover of Your Home magazine, Camilla’s written for Ideal Homes, Real Homes, House Beautiful, and kitchen and bathroom business magazines.  

It was inevitable that her interest in all things homes would lead her to writing home interest features. As a young girl she had the earliest version of Pinterest – a scrap book full of home inspiration images cut from magazines.  

In her spare time, when she’s not in her kitchen experimenting with a new recipe, you’ll find her keeping fit at the gym. In the pool, stretching at a yoga class, or on a spin bike, exercise is her escape time. She also loves the great outdoors and if she’s not pottering about in her garden, she’ll be jumping on her bike for a gentle cycle ride.  

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