How Do Air Source Heat Pumps Work?

When it comes to low-carbon home heating, air source heat pumps are leading the way. If you’re wondering exactly what they are and how they work, you’re not alone. At Cinergi, we’re passionate about demystifying green technology so you can make an informed choice for your home or business.

Unlike gas boilers, air source heat pumps don’t generate heat; they transfer heat. Think of it as working like a fridge in reverse. A fridge pulls heat from inside the unit to keep food cool and releases that heat into your kitchen. An air source heat pump does the opposite: it extracts heat energy from outdoor air, concentrates it using refrigerant and electricity, and transfers heat into your central heating system and hot water cylinder.

Even when the outside air feels cold, it still contains ambient temperature energy. That’s what makes air source heat pumps ideal for both heating your home and reducing your home’s carbon emissions, not to mention reducing your energy bills.

Read on for Cinergi’s straightforward guide on how air source heat pumps work. For more information, visit our dedicated ASHP pages or drop us a line to ask us a question.

What are the main components of air source heat pumps?

Modern air source heat pump systems are made up of several components that work together to transfer heat from the outside air into your property:

Outdoor unit

This is the part of the system that sits outside your home. It looks similar to an air conditioning unit and contains:

• A fan to draw in outside air
• An evaporator coil
• A refrigerant liquid that absorbs heat energy

External heat pump units are robust, and designed to be weather resistant, even in cold climates. The dimensions of a typical domestic unit are between 80cm to 150cm high, 80cm to 120cm wide and 30cm to 50cm deep. Most homeowners choose to place them on an external wall. Read about the new rules on planning permission from May 2025.

Hot water cylinder

Unlike a combi boiler, which heats water instantly on demand, a heat pump works more slowly and efficiently at lower temperatures. So it needs a hot water cylinder to preheat and store water, ensuring there is hot water ready when you need it.

Refrigerant

This special heat transfer fluid cycles through different states – liquid to gas and back again – as it absorbs and releases energy. It’s crucial to how heat pumps work.

Compressor

The compressor increases the pressure of the refrigerant, raising its temperature significantly so it can release enough heat to warm your home.

Expansion valve

This part cools the refrigerant by reducing its pressure, preparing it to absorb more heat from the next batch of outdoor air.

Heating system

Air source heat pumps work best when paired with underfloor heating systems or larger radiators, which are designed to run at lower flow temperatures. While the radiators may not feel hot to the touch, they will still heat your room to the same comfortable temperature — just in a slower, more efficient way. By gently warming the fabric of the building (including walls, floors, and furniture), the system helps retain heat for longer, keeping your home warm and cosy with less energy. The only difference is that it takes a bit more time to reach the desired temperature.

Hybrid heat pumps

Homes with high heat demand — such as poorly insulated or very old properties — may benefit from a hybrid heat pump system. This combines a heat pump with a traditional boiler, ensuring the home stays warm even when the heat pump alone can’t meet peak demand, especially on the coldest days. A hybrid system offers flexibility, using the heat pump for most of the heating (efficient and lower-cost) and relying on the boiler only when extra power is needed.

Is the heat pump technology different to ground source heat pumps?

While the external components are different, air and ground source heat pumps share the same core principle: extracting heat from the environment and moving it into your home.

While air source heat pumps extract heat from the air, ground source heat pumps draw energy from the soil via underground pipes. Ground systems are generally more efficient year-round due to the stable temperature of the earth, but they’re more expensive and disruptive to install and require significant outdoor space for the pipework.

How an air source heat pump system works

Let’s take a closer look at how this clever technology really works.

1. Air is drawn in from outside by the fan in the external unit and passed over the evaporator coil.
2. The refrigerant liquid inside the coil absorbs heat from the air and evaporates into a gas.
3. The compressor boosts the pressure and temperature of the gas.
4. The hot gas travels to the internal heat exchanger, where it releases its energy into your heating system and hot water supply.
5. The refrigerant cools, condenses back to a liquid, and flows through the expansion valve.
6. Its pressure and temperature drop again, and it returns to the evaporator to start the cycle over.

This continuous process allows the system to produce more heat energy than it consumes in electricity, offering lower running costs and impressive energy efficiency compared to traditional gas boilers or other fossil fuel boilers.

Are air source heat pumps efficient?

If designed, installed and maintained correctly, heat pumps are extremely efficient. Many can deliver up to four times more heat energy than the electrical energy they consume. That’s a coefficient of performance (CoP) of around 3:1.

Factors that influence an air source heat pump’s efficiency:

• Outside temperature – Performance is strongest in milder weather, though modern systems are designed to work in cold air and colder climates.
• Insulation – A well-insulated home retains heat more effectively, allowing the heating system to run at lower flow temperatures. Good insulation can also reduce or remove the need for a more powerful (and more expensive) heat pump, lowering upfront installation costs.
• System design – When combined with larger radiators or underfloor heating, heat pumps are at their most efficient.
• Installation quality – A poorly sized or badly fitted system will not perform well. That’s why professional air source heat pump installation matters.

Heat pump compared to traditional boilers

If you’re unsure whether to replace your gas boiler, here’s a simple comparison:

Looking for a reliable heat pump installer on the South Coast? Speak to Cinergi

At Cinergi, we make air source heat pump installation simple, reliable and stress-free. If you’re exploring options, we’ll guide you through it with clear, impartial advice tailored to your home and energy needs.

Our customers rate us as a 5-star service for our excellent workmanship and great customer service. Read our reviews.

If you live or work on the South Coast, get in touch today to speak to one of our friendly, expert team or book a discovery call.

Frequently asked questions – how do air source heat pumps work?