What are heat pumps and how do they work?

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 sourc​​e heat pumps. 

​​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 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 exchang​​e. 
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 ​i​nternal 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. 

“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. 

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.   

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.”

“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.”  

“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.”  

“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.” 

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.