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Selecting a heat transfer fluid system

Corrosion can lead to many problems.
Corrosion can lead to many problems.

With greater demands on systems and for improved efficiencies, companies must look at ways to continue to improve fluid performance. Ultimately, this can lead to time and cost savings and to improvements in the life span of cooling and heating systems. Heat transfer fluids are a key component in many heating systems using ground or air source heat pumps. Andrew Murray, senior manager at Kilfrost’s Speciality Fluids Division, examines the impact of heat transfer fluids on the longevity of systems.

While heat transfer fluids are a key component in many heating systems using ground or air source heat pumps, not all companies are paying sufficient attention to the composition or quality of the heat transfer fluids used. A key consideration is the impact that they can have on the condition of a system as a result of corrosion. Corrosion can result in a reduction in system thermal performance due to scaling on the heat transfer surface, decreased flow due to reduced pipe diameters from corrosion deposits, and potentially the need for system component replacement due to damage.

If sourced or used incorrectly, heat transfer fluid can potentially become the most expensive aspect of a heat pump, despite the low initial cost to the customer. The life span of any heating system depends on the quality of the heat transfer fluid used, while it is also important to abide by an efficient and rigorous maintenance regime.

Potential corrosion

Ground source heat pumps can be used to efficiently heat any building by drawing heat from the ground, condensing it and delivering it to the building. Heat transfer fluids are a vital part of this process.

A geothermal heat pump harnesses heat from the ground using a network of pipes buried in either vertical boreholes or ground loops in trenches. A suitably diluted heat transfer fluid is then pumped around the pipes. The fluid extracts heat from the earth surrounding the pipework. This heat is then condensed to step up the temperature to provide space and water heating. Depending on the type of heat pump, it also has the capacity to work in reverse and provide a cooling system by expelling heat to the ground.

Corrosion of these pumps can lead to many problems; the most significant being damage that may result in fluid leakage. Other problems may include reduced heat transfer caused by surface scaling. Additionally, concerns include the clogging of particulate filters and damage to mechanical seals.


Many heat transfer fluids are glycol-based. Glycols themselves are not particularly corrosive in concentrate form. However, when they are diluted with water to achieve the required frost protection, system corrosion becomes a real problem for the installer and ultimately the end user. Bacterial growth and scaling within the system can also become an issue as a result of hard and/or inferior water. So, what can be done to prevent rust within systems and to increase their longevity?

The quality of water needs to be considered when trying to prevent corrosion. Using a high-quality product and mixing it with poor quality water makes little sense. The corrosive effect of natural water can vary considerably depending on its chemical composition. Hardness of water also needs to be considered, as it may introduce calcium and magnesium, which form scale on metal surfaces within the system.

Mixing an inhibited glycol-based heat transfer fluid, with water of a sufficient quality, enables the system to last longer by reducing corrosion, bacteria and scaling. This saves both time and money for the customer. For example, less time needs to be spent on maintaining the system and on repairs. The need to replace expensive capital equipment is also reduced.

Suppliers and installers of systems therefore face significant reputational and financial risk if the fluid they are installing falls short. An efficient inhibited heat transfer fluid gives the installer and the end user greater peace of mind.

Kilfrost’s ThermatransSustainTTS is a bio-derived propylene glycol based concentrate fluid for use in ground and air source heat pumps and industrial thermal applications. All of Kilfrost’s Thermatrans inhibited heat transfer fluids are designed to harness and transfer naturally occurring heat. They have been specifically designed to deliver higher resistance to degradation, scale, bacterial growth and corrosion. They all offer excellent freeze protection, cleaner circuits, efficient heat transfer and long term cost benefits to system maintenance.

Lack of standards

As the industry recognises the challenges of system corrosion, an international standard, known as the ASTM 1384 corrosion test standard is playing an increasingly important role.

The ASTM standard is carried out in laboratory conditions where specified pieces of commonly used metals, including aluminium, iron, steel, brass, solder and copper are immersed in an inhibited glycol/water mixture which is aerated to simulate in situ conditions at 88°C. The pieces of metal are specified at set weights and sizes and are weighed before the test begins. The test is carried out over a period of four weeks.

At the end of the test, the pieces of metal are weighed again and the before and after weights are compared for signs of corrosion. In order to pass the test, the mass change in the metals before and after the test must not exceed those levels stipulated in the standard. The test is important as it demonstrates the ability of the fluid to protect sufficiently against corrosion and thus gives the customer greater confidence. Any fluid can claim to contain corrosion inhibitors but without the presence of a test standard, the extent of protection is difficult to gauge.

Despite its existence, there is still a lack of understanding of this standard in the industry. It does remain voluntary, however some heat transfer fluid manufacturers, such as Kilfrost, have chosen to embrace the standard and adhere to it across the range of products.

Maintenance regimes

While the quality of heat transfer fluids is paramount, maintenance regimes are also key to keeping systems in good condition. Regimes can differ depending on the equipment used. During servicing, fluids are generally checked periodically for pressure, glycol content, and the presence of any foreign bodies which may indicate the breaking down of the fluid.

The various conditions under which cooling fluids are transferred, stored, heated and applied can all have an impact on their performance. Regular quality checks should be performed in order to ensure that fluids are in a satisfactory condition for use all year round. It is also important to combine regular maintenance checks with an examination of the glycol content within the system, in order to maintain sufficient frost protection.

Looking to the future

In a world where supply chains are under increasing pressure to demonstrate their environmental credentials, as well as selecting high quality fluids to reduce system corrosion, many companies are keen to select products that help to reduce their carbon footprint, while maintaining high levels of performance.

Corn-derived inhibited glycol is sustainable and biodegradable with low toxicity levels. For example, Kilfrost’s ThermatranssustainTTS is not only sustainable, but also recyclable. Such products continue to offer greater innovation and improved environmental options in the heating fluids market.

In an increasingly competitive market with greater demands on systems and a drive for improved system efficiencies, it is imperative for companies to look at ways to continue to improve fluid performance by lowering viscosity and increasing the thermal transfer of systems. As well as leading to time and cost savings for the customer, this work may also lead to further improvements in the life span of cooling and heating systems.

In summary, there needs to be a change in the way that heat transfer fluids are selected and used. Kilfrost knows that selecting the right fluid for the system can reduce maintenance costs and downtime, while extending the life of a system and ultimately maximising the investment that has been made.