How Does an Air Source Heat Pump Work?

An air-source heat pump uses electricity to transfer heat from outside air into the home. It does not burn fuel at the property, but running cost and carbon performance depend on the system, tariff and electricity used.

In this article, we'll provide you with a step-by-step guide answering the question: how do air source heat pumps work? We'll also explain a variety of factors to consider before you go ahead with the installation, to help you get the best out of your heat pump.

The popularity of air source heat pumps in homes and commercial properties across Essex is no accident. These mechanisms are changing the way people heat their homes, all while reducing fossil fuel usage and helping the planet.

Instead of burning oil or gas, an air-source heat pump transfers heat from outside air into a water-based heating system. A well-designed system can deliver several units of heat for each unit of electricity used, but performance varies with outdoor and flow temperatures.

While the concept of the air source heat pump may be simple, the technology behind it is a little more complex. Let’s go through it step by step.

The most notable aspect of air source heat pumps is the large fan on the front of the unit. This fan is responsible for drawing in the outside air. Even in the coldest temperatures, the air contains thermal energy (heat).

Outside air passes across a heat exchanger containing refrigerant. The product’s specified operating range, output and efficiency at the project design temperature should be checked in the manufacturer data rather than inferred from one generic temperature.

The incoming air is a lot warmer than the temperature of the refrigerant, which begins to absorb its heat. It then starts to change from a liquid to a low-pressure gas, before entering the next stage of the process: compression.

The evaporated refrigerant is drawn into the compressor, which significantly warms its temperature. It does so by squeezing the gas molecules together, pressurising them and increasing the gas’ internal energy.

The refrigerant has now transformed from a low-pressure gas into a high-pressure gas.

The now-heated gas passes through a condenser. Its heat will then be transferred into the water for the central heating system which runs the other side of the condenser. Once the heat has been transferred, the refrigerant gas will pass through an expansion valve.

This expansion valve will suddenly release the pressure, quickly cooling the refrigerant. This will condense it back into a liquid, before the process restarts again.

As previously mentioned, the heat pump doesn’t use fossil fuels to instigate this process. Instead, the heat pump runs on electricity; but how much does it use?

Electricity consumption cannot be predicted from the heat pump’s nominal output alone. It depends on the building’s annual heat demand, hot-water use, seasonal performance, flow temperature, controls, weather and any supplementary heating.

A heat pump can deliver more heat energy than the electrical energy it consumes because it transfers heat from the environment. This does not mean free heating: Energy Saving Trust advises that current running costs vary and can be slightly higher than a new gas or oil boiler in some homes.

Plus, if you invest in other forms of renewable energy in your home, such as solar panels and solar battery storage, you can save even more.

Before you go ahead with an air source heat pump installation, there are a few points you should consider.

Heat pumps work best in well-insulated homes. While they can still operate if your home lacks high-quality insulation, you’ll run the risk of losing a lot of the heat that the pump produces. To counteract the heat loss, your pump will have to work harder, using up more electricity for a similar/worse result.

Heat pumps operate at lower water temperatures than gas boilers. This means that you are likely to need larger radiators with greater surface areas to effectively heat the room. Unfortunately, this will add extra money onto the installation costs.

Eligible property owners may be able to use the Boiler Upgrade Scheme. The installer applies on the owner’s behalf and the grant is deducted from the quotation and invoice, subject to the current Ofgem rules.

Bear in mind that the scheme is only set to run until March 31st 2028.

Heat pumps are not small and will require your home to have adequate outdoor space for installation. You will also need to make space indoors to store the water cylinder, which takes up a fair bit of space. Heat pumps aren’t silent either, which is another factor to consider when thinking about placement.

• Room-by-room heat loss and annual heat demand

• Design outdoor temperature and proposed flow temperature

• Heat-pump capacity at the stated design conditions

• Radiator or underfloor-heating outputs

• Hot-water cylinder and household demand

• Expected seasonal performance and electricity tariff