Engie Spectrum High Temperature Heat Pump

Relentlessly pumping carbon emissions into the atmosphere is at the root of the climate emergency we are experiencing, and heating and hot water generation are a major cause.

Indeed, they are the UK’s biggest sources of carbon, accounting for more than 35% of total UK emissions, including industrial processes.

Unsurprisingly, this makes the heating and hot water generation one of the main targets of decarbonisation. Heating in the UK is currently dominated by fossil fuels, with 85% (about 24.5 million homes and more than two million businesses) supplied directly by the ‘high carbon’ mains gas grid.

Achieving net zero – a stated aim of the UK government and one of its flagship environmental policies – will require almost all buildings in the UK to transition to low carbon heating in the next 30 or so years. The big question is how?

The government believes that we will need to introduce a decarbonisation policy for the largest buildings in the mid-2020s to ensure organisations are on the pathway to clean up their buildings by 2050.

It has therefore proposed a package of policies to grow low carbon heat pump deployment in the coming years, which it has set out in its much-lauded Heat and Buildings Strategy.

How do heat pumps work?

The concept of heat pumps is by no means new, but the technology has come on leaps and bounds since it was first proposed in the mid-1700s.

Heat pumps operate like fridges in reverse, using a refrigerant to absorb the heat that can be found naturally in the ground, air, or water. This refrigerant is compressed to increase the temperature still further, and this heat can be used to raise the temperature of cold water in the system.

An early champion of heat pump design was William Cullen, a Scottish physician, agriculturalist, and chemist, who was also a pioneer of artificial refrigeration. He is credited with discovering the scientific principle behind heat pumps in 1755. But it was not until the 1940s that the heat pump was first used to heat buildings.

Since then, the popularity of heat pumps has soared, not least because there have been a slew of developments that make the heat pump the most cost-effective source of heating currently on the market.

Energy efficient heat pumps

The exploding approval ratings won by heat pumps is largely due to their energy efficiency. Heat pumps are more efficient than other heating systems because the amount of heat they produce amounts to more than the amount of electricity they use. Because they transfer rather than generate heat, heat pump systems consume far less energy than traditional electric-driven or fossil fuel heating systems.

The amount of heat produced for each unit of electricity used is known as the coefficient of performance (CoP). CoP is based on the relationship between the power put into a system (measured in kW) compared with the amount of power that is returned.

In short, CoP is equal to power output divided by the power input. The higher the resulting number, the more efficient the system. Heat pump systems typically exhibit a CoP of between 2.5 and 3.0, although some can go considerably higher – up to around 5.0. This compares with less than 1.0 for as typical fossil fuel-based heating system.

Benefits of heat pumps

But the advantages of heat pumps don’t end with impressive energy efficiency figures. There are even more compelling reasons to consider them.

For example, they can provide an all-in-one heating and cooling system for both summer and winter comfort, they offer lower running costs than the alternatives, they reduce carbon emissions compared with fossil fuels, they are safer because there is no combustion involved in their use,  they involve less maintenance than a combustion-based heating system, they save space since there are no fuel storage requirements and they have a long lifespan.

Heat pumps’ drawbacks include higher upfront costs (although these are usually offset quickly by the payback advantages). They are also susceptible to power outages (although a robust electricity network can offset this potential problem).

On  balance, therefore, heat pumps clearly represent a smart investment in the long run.

Heat pumps & existing heating infrastructure

A major barrier to the uptake of heat pumps in existing installations has been difficulties in connecting them to a conventional heating infrastructure (including, for example, heat emitters such as radiators) because of their relatively low heat output. Refurbishment costs and the practical problems of modifying these existing heating systems mean that conventional heat pump use is not possible.

High temperature versions of heat pumps, however, extend their range by offering output temperatures of more than 55 deg C with a maximum output flow temperature of 120 deg C. They can be used in a wide range of applications including hot water production and space heating, but also district heating schemes, waste heat recovery, and thermal storage.

Types of high temperature heat pump

There are, essentially, three main types of high temperature (HT) heat pumps.

Carbon dioxide (CO2)

CO2 has traditionally been specified in industrial and major healthcare type applications. A CO2 can deliver higher hot water temperatures at lower ambient temperatures than a conventional heat pump and therefore reduce energy demand. A CO2 air source heat pump can deliver 80°C hot water at -10°C ambient with a coefficient of performance of 2.8.

However, CO2 has the limitation of return water temperature not being above 40 deg C so it’s only really been practical in domestic sanitary hot water applications rather than for heating. CO2 is high pressure, which makes the machines more of an industrial build, so they are not particularly accessible for smaller domestic demands.

Two-stage booster

The system requires heat pumps and water treatment for each stage as well as pipework, interfaces, and controls for each stage.


Air-to-water heat pumps that take the heating directly to 70–75-deg C from -4 deg C. These are particularly applicable in older buildings which currently contain traditional heating systems and require constant domestic hot water.

Higher output temperatures provided by HT heat pumps can negate the need to make changes to the emitter/distribution system which leads to considerable fitting cost savings and faster, easier installation.

They also expand the opportunities for different types of projects and therefore produce carbon and cost savings across a wider range of projects and applications including public buildings and hotels, in district cooling, and for energy suppliers and the waste management industry.

The technology has also proved effective in food manufacturing, among other sectors.

And HT heat pumps with refrigeration are particularly effective in data centres to cool servers while, thanks to its heat recovery function, simultaneously providing heat for the building, for water heating, and for conditioning the recirculated air in the server rooms.

Find out more about Klima-Therm’s range of high temperature heat pumps