Air source heat pump

Air source heat pumps are becoming increasingly popular as a low-carbon heating option. They are energy-efficient with relatively low running costs. They are also becoming much more affordable as the technology becomes established, and especially because of funding available from government schemes.

Types of heat pump

There are two basic types of heat pump, the air source heat pump and the ground source heat pump. All heat pumps perform the same function, transferring existing heat from the surrounding environment into your home using simple physics and ingenious engineering.

An air source heat pump is an electrically-powered system for transferring heat from the air outside into your home.

They deliver their heat to your central heating system (this is known as an air-to-water heat pump) and they are best suited to well-insulated properties.

How does an air source heat pump work?

Thermal energy (heat) is everywhere. Even when we think it is cold, there is thermal energy to be captured. Heat energy will naturally flow from a warmer place to a colder one.

The outside is often colder than the inside in the UK and warmth will generally escape your home. The way heat exchange technology works means that even though the air outside might be significantly cooler, you are still supplied with enough energy to heat your home. How does it do it?

The external part of an air source heat pump (the evaporator) drives air towards a colder refrigerant gas. As long as the refrigerant gas is cooler than the outside air, some of the heat flows from the warmer air into the colder gas. The large volume of air driven through the heat pump means that even cold air provides sufficient energy.


Heat exchange in an air source heat pump

Because heat is generated by the collision of molecules in a substance, compressing a gas means there is less space for the molecules to avoid each other. There are more collisions and so more heat is generated. This is why the refrigerant gas in the heat pump, now warmed by the air, is compressed. This heats up the gas even further.

The heated gas gives up some of its captured thermal energy to the water or air inside your home. This is then used to warm your home. The compressed gas is allowed to expand again. It returns to its decompressed and cooler starting point, ready to pull more heat from the air and begin the cycle again.

Energy Efficiency

You can efficiently heat an adequately insulated house with an air source heat pump. A lot more energy is collected from the environment than is needed in the form of electricity to power the heat pump.

Energy efficiency is the ratio between the energy input and the energy output measured in Kilowatt hours (KWh). Below is a diagram of a 14KW heat pump. The electrical input is 3KW, which means the remaining 11KW is gathered from the air outside, adding up to the 14KW output.

Energy efficiency diagram of an air source heat pump

You can find out the energy efficiency of a heat pump by its Seasonal Coefficient of Performance (SCoP). For example, an SCoP of 3 means that over an average year, the heat pump will output three times as much heat energy as the electrical energy required to run it.

At Plumb-Line, we calculate the SCoP for your heat pump in the context of average temperatures in your location and the output requirements for your home. We take into account the design of your home and your specific heating system, considering other factors like the number and size of your radiators, for instance.

Design of your central heating system

You may have a central heating system well suited to the installation of a heat pump.

The aim of a well-designed system is to reduce the heating water temperature as much as possible. The closer the required temperature is to the source temperature (ie the outside air), the more efficient the heat pump will be and therefore the lower the running costs.

The general rule is that the greater the surface area radiating heat into your home, the better. If you have underfloor heating and/or large surface-area radiators then the water used to heat your home can be at a lower temperature.

In the case of an air-to-air heat pump, you will need to provide the energy for hot water from somewhere else. You can opt for a hybrid system, for instance, which still uses a boiler for hot water.

You may also want a hybrid system, combining an air source heat pump with another fuel source, if you need a higher power output at certain times.

Heat pumps are flexible and can be adapted and combined depending on the characteristics of your home. We will return to the subject heat pumps, their advantages and suitability for different homes, in future blogs.

If you’re thinking about getting an air source heat pump for your home then contact us for a quote. We can advise you whether it is an appropriate technology for your circumstances and we will conduct a survey and provide a free no-obligation quotation. We installed around 50 heat pumps under the government’s Green Homes Grant scheme so we can show you plenty of examples of very satisfied customers.