What is an air source heat pump? A complete guide for UK homeowners

An air source heat pump (ASHP) is an energy-efficient and cost-effective heating and cooling system that runs on electricity controlling the temperature of your home and providing a reliable supply of hot and cold water (depending on type, model and make), keeping your home comfortable all year round. With support from UK government grants, ASHPs are becoming a popular choice for sustainable, low carbon home heating. This guide covers what an air source heat pump is, how it works, its benefits and drawbacks, and the types available on the market today.

The working principle

An air source heat pump operates similarly to a refrigerator or a car air conditioner, but with enhanced efficiency for heating and cooling your home. It consists of four main parts: a compressor, condenser, evaporator, and an expansion valve.

The basic explanation

In simple terms, an ASHP absorbs heat from outside air to warm a special refrigerant fluid within the system. As this fluid warms, it changes to gas, which is then cooled in the condenser and turns back to liquid. The compressor and expansion valve adjust the gas and liquid pressures, optimizing the system’s efficiency. If you want cold air connect a fan to the evaporator, for hot air connect a fan to the condenser it is as simple as that.

The detailed explanation

My PhD was in the development of advanced adsorption heat pumps to speed up the decarbonisation of homes in the UK so I will try to explain it the way I studied it from journals.

The principle behind heat pumps and other cooling/heating technologies had been developed around the 1800s when Faraday displayed a new concept for refrigeration that utilises the adsorption effect in which a compressor or a condenser is not required (it is a must have in a typical Vapour Compression Refrigeration System that dates back to 1805 by Oliver Evans see figure below).

An air source heat pump basically uses the best in both worlds combining the strengths behind the adsorption technology with the strengths of the vapour compression technology. That is utilising a low grade heat source to increase the efficiency of the cooling or heating system without needing an adsorbent (which has limitations) since the compressor and condenser do its job.

In very simple terms heat is taken from the outside air to heat up a special fluid in liquid form inside the evaporator, when this fluid heats up it changes to gas which then cools down in the condenser turning it into liquid, the expansion valve and the compressor helps regulate the pressure of the liquid and gas respectively; i.e Compressor ensures the gas pressure is in the optimum range and the expansion valve ensures the liquid pressure is in the optimum range. You may be asking how does the system work in winter when it is super cold like at -3 degree celsius and the answer is simple the fluid is specifically chosen depending on the working conditions that are specific to the country in which the heat pump will be operating. For example the refrigerant R134 (I am sorry to use it as an example I know its a greenhouse gas ) has a boiling temperature of -36 degree celsius the pressure in the compressor/expansion valve will affect its liquid phase/ gas phase temperature but you get the point.

What are the benefits and disadvantages of an Air source heat pump?

Benefits

Here are the key benefits of using an air source heat pump:

• Government Support: The UK government offer grants of up to £7,500 for homeowners in England, Wales, and Scotland to encourage switching from traditional boilers to sustainable heating solutions.
• Energy Efficiency: ASHPs are highly efficient, often achieving 500% efficiency (SCOP), which means you can get 5 watts of heat for every watt of electricity used.
• Renewable and Sustainable: Powered by electricity, ASHPs work well with renewable sources like solar panels, making it a fantastic choice for those aiming for a sustainable, zero-emission lifestyle.

Disadvantages

• High Initial Cost: Installing an ASHP can cost between £7,000 and £9,000, depending on the size and specific requirements of your home.
• Noise Levels: ASHPs can produce noise (similar to an older fridge), ranging between 40 to 60 dB, due to the compressor and condenser. However, newer models feature acoustic designs to reduce sound.
• Home Insulation Requirements: Homes may need upgraded insulation to maximize the efficiency and cost savings of an ASHP, which may add to upfront costs.

What types of air source heat pumps are there?

There are so many types of air source heat pumps and new products from top manufacturers like Mitsubishi and Viessmann are constantly introduced to the market. We have partnered with major manufacturers, distributors and MCS certified installers to help you make your home more sustainable at an affordable cost.

FAQs About Air Source Heat Pumps

1. How much does it cost to run an air source heat pump?
Running costs vary depending on your home’s insulation and energy needs, but ASHPs are generally cheaper to run than traditional heating systems, especially when paired with renewable energy sources.

2. Do air source heat pumps work in cold weather?
Yes, air source heat pumps work effectively in cold temperatures. Modern ASHPs use refrigerants with very low boiling points, allowing them to extract heat even in sub-zero conditions.

3. Are there grants available for installing air source heat pumps?
Yes, the UK government currently offers grants of up to £7,500 through the Boiler Upgrade Scheme to help homeowners switch to sustainable heating systems like ASHPs.