How does a heat pump work?

Heat pumps work by taking natural warmth out of the surrounding environment to provide hot water and heating to your home. There are three main types of heat pump: ground source, air source and water source. The name of each one describes where the appliance takes its heat from, and, therefore, each one works in a slightly different way.

• Absorption

Before anything else can happen, the heat must first be collected. Depending on the type of heat pump, the heat will be absorbed by a refrigerant either from the ground, air or water source. This takes place in the evaporator with the refrigerant changing from liquid to gas in the process.

• Compression

The refrigerant is now a gas instead of a liquid and the gas is compressed to increase its temperature even more. It is this heat that can be used to warm up your radiators or underfloor heating.

• Condenser

As the refrigerant transfers the heat that is to be used in your home’s heating system, it gradually cools down and condenses back into a liquid. This occurs in a second heat exchanger. One of the reasons why heat pumps are so efficient is because cooler water from the central heating can continue to absorb the heat from the liquid refrigerant within this second exchanger.

• Repeat

Once the refrigerant has cooled right down, it passes through an expansion valve. In here, the liquid’s pressure is decreased and it returns to the evaporator to begin the whole cycle again.

A ground source heat pump takes its heat from the ground. Pipes are laid under the ground or via a borehole surrounding your home and a solution of water and antifreeze flows through them. It is this solution that transfers the heat from the ground and begins the process above.

In an air source heat pump system, instead of taking the heat from the ground, a fan located outside the property takes in air from outside to heat the refrigerant. The same process is used to heat the solution, turning it into a gas and compressing it.

If you have a substantial amount of water around your property, such as a large pond or lake, you may be able to use a water source heat pump to supply your home with hot water. Just like in an air source system, pipes containing the antifreeze solution can be laid directly in the water source. However, it’s important that the volume of water is large enough. As the solution in the pipes absorbs heat from its environment, the water temperature can drop, causing it to freeze. This shouldn’t happen if the volume of water is big enough for the size of your home.

Even if you live in a cold climate, heat pumps can be used when the ground or air is as
low as -15 °C. 

Before you undertake a heat pump installation, a suitably qualified engineer will come out to the property to check that the site and property is satisfactory. To qualify for government RHI funding in the UK you will need an MCS report (Microgeneration Certification Scheme) They may need to assess the amount of insulation you have, as this could impact the size of your heat pump. To do this, they may require access inside your house and the loft. If you’re having a water source heat pump installed, they will want to check the water on your land too.

An air source heat pump is relatively easy to install, as long as there’s a suitable place to put the outdoor unit. An outdoor unit will be installed and cables and heating pipes will be put through the wall to connect it to the heating system.

A ground source heat pump is usually more expensive to install because of the additional labour. The ground will need to be dug or drilled so that the pipes or bore holes can be inserted. You will need to ensure that you have good access for diggers and other large pieces of equipment, as well as the right amount of space to lay the pipes - the bigger the home, the more pipework will need to be laid. The job would need to be planned, but once the pipes are laid, they can stay there for a long time without needing to be accessed. 

As the UK works towards reducing its carbon footprint and increasing its use of renewable energy, more people are looking to invest in a heat pump. Things to consider when purchasing are :

1. The installation cost
2. Electricity usage
   
   

Both of these things should be looked at realistically so that you can work out the annual cost compared to that of a gas boiler.

It’s worth noting that the installation of the heat pump only needs to be done once - when the pipework is in place, it’s only the heat pump units that need to be replaced in the future. This means that a replacement heat pump could cost considerably less.

Some people may think that heat pumps use a lot of electricity too, however, bear in mind your gas bill would drastically reduce and it is more important to think about total energy usage and costs.

We have previously calculated that the average heat pump will use around 4,000 kWh of electricity per year. At a cost of around 14 pence per kWh, your heat pump can run for £560 per year (based on COP of 4) which is less than a gas boiler. Together with the government grant scheme this can make the cost of heat pump ownership cheaper than that of a standard gas boiler. Unlike a boiler, a heat pump can run on renewable energy, such as solar energy provided by solar panels. This means that you could greatly reduce your electricity consumption from the National Grid - as you would be generating some of your own power on site.

You can also claim money back from the government using their Renewable Heat Incentive (RHI) scheme. If your installation qualifies for the scheme, you could receive yearly payments of up to £1,000 for seven years. This additional payment could contribute towards the installation costs of your heat pump system. To qualify for RHI both your heat pump and installer must be MCS approved.