How does an Air Source Heat Pump Work?

With the talk about gas boilers being discontinued in coming years, we’re often asked how does a heat pump work? in this short post, we’ll explain all.

Our project in Queensferry, working on the new Pupil Referral Unit at Plas Derwyn & Queensferry CP and the extension and refurbishment of the existing Plas Derwyn & Queensferry CP Primary School has a few very notable renewable technologies being deployed throughout the build.

The first of these is the installation of three Mitsubishi Air Source Heat Pumps (ASHP’s). These are being used to heat the building and the water.

View across the three Mitsubishi CAHV ASHP units at Queensferry PRU by Kimpton


To a layperson, an air sourced heat pump seems to work with “alchemy”. How can it seemingly create heat on the inside, when it’s cold outside?

Perhaps the easiest thing to do is to explain how a heat pump works.

How does a heat pump work?

An air source heat pump does look a little like it’s breaking the laws of thermodynamics, which says that energy cannot be created or destroyed. The way it works however, is quite simple. It’s often described as a refrigerator in reverse. Rather than cooling the air, the system is reversed and the air is warmed. If you feel the warmth at the back of your fridge, it starts to make sense.

But how can it take the heat from temperatures as low as -20 degrees outside?

Heat pumps are not a new technology, in fact, they have been used in Scandinavia for many years. They have much colder winters than us here in the UK and lower average temperatures overall.

The secret is simple. There is an energy input, which is the energy needed to power the compressor. A heat pump system will normally generate 2-4 times more energy output than the energy input to the compressor. This is known as a Coefficient of Performance (COP).

When refrigerant is compressed it becomes a liquid and when it’s allowed to expand, it becomes a gas. This compression of the gas into a liquid creates heat. As the pressure is reduced, it expands and cools. The more it expands, the colder it gets. The downside to this is that when the gas expands more to achieve lower temperatures, to maintain that temperature difference, the harder the system needs to work and the more power will be consumed to compress it again. This is why there is a variation in the COP.

Even if it is below freezing outside, the liquid in the coils will be de-pressurised until it reaches the temperature needed to warm to the space. The lower the temperature outside, the more pressure is required to convert it back to a liquid and the greater the energy input required to achieve the same heat output. Even if the air outside is freezing, when it passes over the coils that are even colder, the external temperature will heat the much colder liquid in the coils.

On the inside of the building, this warmer gaseous liquid is then compressed again and it is superheated. If you use an airline in a garage to pump up your tyres, you’ll notice that the pipe gets warm in the same way. This heat is then transferred to the heating system inside and the cycle is complete.

As it passes to the outside of the building, the liquid passes through an expansion device. As the pressure drops, the temperature cools and it becomes a gas again.

The more heat that is required, the more the gas needs to be pressurised. For an underfloor system, the temperature required can be as low as 35 degrees, for a well-insulated new build with radiators, this would normally be 55 degrees and with hot water too, it needs to be 65 degrees or more. To achieve this latter temperature, the gas needs to be compressed to a max of 40 bar.

Ultimately, using heat exchangers, this heat is drawn from the system and passed into the more conventional ‘wet’ heating and domestic hot water system.

In this installation at Queensferry, the maximum temperature for the heating and to comply with regulations for the hot water system, will be 65 degrees.

Here at Kimpton, we are seeing more and more enquiries for ASHP installations and are on-site with a number of them at present. As a straight replacement for gas, the potential for cost savings may be small or negligible, however, the huge winner is the Carbon reduction.

Is an Air Source Heat Pump suitable for any building?

Any building can be heated using an air source heat pump, whether commercial or residential. The efficiency and subsequent running cost will be the big difference between building types. A new build with triple glazing, ultra-low U values for insulation and underfloor heating will deliver incredibly high COP and low running costs. In an old building, or where underfloor isn’t an option, with conventional radiators and poor insulation, there may need to be additional low surface temperature (LST) radiators to be able to heat the space.

As this installation is progressed, we will share more information and introduce you to the other renewable technologies being installed.

If you are looking to reduce your carbon footprint, future proof your business or just want to explore the concept of Heat Pumps, then you can see more about our range of heat pump services over here, or please call the head office engineering team on 0151 343 1963


Diagram courtesy of EDF Energy, used with thanks.

Creating the
sustainable building
environments of tomorrow

Quick enquiry

If you would like to discuss any of our services, please call 0151 343 1963 or complete the form below and we will be in touch shortly.

    Creating the
    sustainable building
    environments of tomorrow