How do heat pumps work?

In a nutshell, a heat pump is a device that transfers thermal energy from one point to another.

We use them all the time, even if we don’t know we’re doing it – our fridge freezers use this technology through evaporation and cooling of a liquid to reduce the storage temperature.

But recently, heat pumps have become more of a talking point because the technology is being increasingly deployed in the reverse direction – to provide domestic and commercial heating, transferring energy from an exterior source to heat radiators, underfloor heating systems and hot water.

Heat pumps – either air or ground – are used to extract thermal energy in the outside air or from the ground, and moving it inside where it is compressed and the heat transferred using a series of coils.

Efficient and clean

As an energy technology, heat pumps are very efficient and clean. While they do use electricity to power a part of the operation, they don’t burn fossil fuels directly to generate heat. What’s more, they work effectively in temperate climates like the UK and deliver heating even when external  temperatures are as low as -20 degrees Celsius.

Heat pumps generally are more efficient and so cheaper to operate than electricity night storage heaters, oil boilers or LPG boilers while they can be around the same lifetime cost to run as gas-fired central heating. An added incentive is that a heat pump should run for 20 years and more, compared with the 10 to 15-year average working life of a gas boiler.

The renewable technology is being installed in a wide range of buildings, notably new construction, as the government announced plans to ban installation of gas boilers in new builds from 2025.

Air source heat pumps

We can dive into more detail around air source heat pumps and outline the advantages and disadvantages of what might become one of the main ways we heat our homes as we move to the national Net Zero target, reducing carbon emissions to the equivalent of zero within a 30 years.

An air source heat pump is fitted outside a building and has a system of coils filled with refrigerant liquid. It works by drawing in air via a fan or impeller and the liquid in the coils absorbs heat from the air and evaporates. This gas passes through a compressor which greatly increases the temperature.

This heat is transferred through inner coils in the building and released with the refrigerant flowing back outside to absorb more air heat.

You can choose between two types of air source heat pump. Air-to-water heats water which is then circulated around the home through low-flow radiators or underfloor heating systems, and can also heat water in a storage tank for kitchen and bathroom use. Air-to-air uses fans to circulate warmed air but are not used for heating water.

Low running costs

A big advantage of air source heat pumps is their relatively low running costs but they are not suitable for every home or commercial space. They work well with underfloor heating but the design and construction of the system needs to be carefully planned together with excellent insulation and airtightness in the property.

Running costs will also vary but there could be substantial savings, particularly in properties off the gas grid. To take one example, for every 1 kWh of electricity, an air source heat pump can produce 3kWh of heat so if we take the mean average annual demand for most homes as 12,000 kWh, that will need 4,000 kWh of electricity. If we price this at £0.13 a unit, the annual heating cost is around £520.

While cheaper and simpler to install than ground source heat pumps their efficiency is affected by external air temperatures. Another consideration is that the fans generate noise and so need to be sited correctly.

Heat pumps can run at 60 degrees and as most current central heating systems are over-specified homeowners should not need to install bigger radiators in most cases, or have to install underfloor heating, bearing in mind at the same time of the need for a highly insulated property.

The system would need to operate continuously during winter months and there is the noise factor from the fans. In an older home that cannot be insulated, a back-up heating system might be necessary, such as solar thermal panels for hot water and pellet-burning stove.

Costs of an air source heat pump

Installation costs of an air source heat pump depend on the size of property but on average can be between £6,000–£8,000 with additional expense incurred with any modifications to the heat distribution system.

Ground source heat pumps

Ground source heat pumps work by taking heat from the ground outside or from a body of water, using either closed loop for the ground or open loop piping for water.

Closed loop piping is used in the ground, with a continuous supply of refrigerant that works in much the same way as the air source heat pump, while open loop piping uses the water from a well or lake to do a similar job.

For the ground-based system, a water and anti-freeze mix is pumped around the buried piping array, and absorbs heat from the sun that is captured in the earth. This is extracted using compression and expansion technology.

How much heat is harvested is determined by the amount of piping as well the type of soil. For example, clay holds more heat that sand.

There are two types of installation, depending on the space available and the particular type of ground condition.

A horizontal array has the piping laid in a closed loop serpentine pattern, buried in a trench that is usually 1.2m deep with either straight or slinky pipe construction. A large garden space is required for this set-up. For example,  500m² to provide energy for a 10kW solution in clay soil, with double the space if the soil is sandy.

With a vertical array boreholes are drilled into the ground and each connected at the top in a closed loop. Again, the amount of energy needed and the soil conditions will determine how many boreholes are needed as well as their depth. You would need at least three boreholes drilled a depth of 70-100 metres for an 8kW pear pump

Heat can also be extracted from lakes or ponds, as long as the body of water is large.

The pump is installed in the property and these range in size from a large filing cabinet to a model that can fit under a kitchen sink.

The cost of installing a typical ground source heat pump system in a 3 to 4-bedroom house would usually be over £10,000 – with a general guide of £1,200 per kW capacity, and bearing in mind the effect of different soil conditions as well as house structure.

Ground source heat pump running costs

A 4-bedroom house that needs 11,000kWh of heat for space and 4,000kWh for hot water would consume around £500 of electricity per year (assuming total cost of 15p/kWh), which is actually considerably cheaper than gas.

So ground source heat pumps can save money on energy bills when compared to other heat sources from gas to direct electric heating and oil, with lower maintenance costs.

Renewable Heat Incentive

One of the biggest draws for heat pumps is the government support for them in the form of higher rates in the Renewable Heat Incentive (RHI) programme. How much you get depends on size of property and the type of heat pump chosen.

Installing air source heat pump technology in a 3-bedroom detached house, you could be in line for an annual RHI payment of £1,297, while ground source heat pumps would give you £3,339 a year, according to figures from Which? magazine.