HP thermal fluid: engineering, safety, environmental and cost considerations


13 April 2023

By Marcus Boskey, Head of Communications – Hydratech

As well as performing its primary role of temporarily absorbing captured heat and transferring it to points of usage, the heat transfer fluid (heat pump antifreeze, glycol, working fluid, brine etc) used in a heat pump system must also be a reliable antifreeze and offer effective corrosion protection.
The type of fluid selected and installed in GSHP, ASHP and geothermal heat recovery systems can have a direct and long-lasting impact on performance, efficiency, output, energy use, and down-time.
In addition to the thermal and pumping characteristics of any heat transfer fluid (HTF), when making the appropriate selection for any heat recovery system it is also important to consider:

  • Corrosivity and Corrosion
  • Toxicity
  • Biodegradability
  • Biological susceptibility and durability (aka lifespan)
  • Flammability
  • Energy efficiency and cost

Corrosivity and Corrosion

Unsuitable and uninhibited HTF can cause corrosion of metal components, as mainly found in the manifolds, pump rooms and energy centres of geothermal and GSHP installations. The corroding part of a component is called the anode, which will tend to dissolve into the HTF and/or coat
other metals of higher nobility. E.g., carbon steel will ‘sacrifice’ itself to copper.

Oxidation Corrosion, as caused by air and/or dissolved oxygen, will tend to dissolve metals evenly across their surface, and most often affects metals which do not form passive protective layers or are naturally resistant to oxidation corrosion.

The most susceptible metals to oxidation corrosion include magnesium, zinc (galvanised pipe), cast iron, ductile iron and carbon steel. Whilst iron and steel can be protected using the appropriate corrosion inhibitors, it is virtually impossible to prevent magnesium and zinc from corroding in a submerged aqueous environment, such as a flooded pipework system.

Galvanic Corrosion can take place when two different metals of varying nobility are ‘connected’ by a fluid, which acts as an lectrolyte. E.g., HTF inside a geothermal or GSHP system. An electric potential difference is generated between the metals and the less noble (less precious) metal acts as an anode and dissolves,while the more precious metal acts as the cathode.

Pitting and Crevice Corrosion are typical in installations where metals protected by a passive coating, such as galvanised or some stainless-steel pipes. An increased local corrosion rate can occur where there is a flaw in the passive coating, when the potential difference is concentrated.

Erosion Corrosion is where metal and synthetic components are worn down over time, due to one or more of the following causes:

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  • High levels of entrained sediment in the HTF, acting as an abrasive inside pumps and bends.
  • Cavitation is most often observed inside centrifugal pumps. E.g., worn impellers and volutes, but can also be found where there are severe changes in HTF direction, or poorly designed pipework etc.

Corrosion Inhibitors
To minimise corrosion, it is very important for HTF to be formulated with effective and proven inhibitors. Specifically, the inhibitor formulation should take into consideration the metals of construction, antifreeze type, susceptibility to air ingress and subsequent bacterial contamination, make-up water quality and effectiveness of pre-commission pipework flushing and conditioning (passivation).


The toxicity of HTF formulations becomes very relevant and important where that HTF might accidentally come into contact with humans, pets, mammals and aquatic life. Emphasis here on the word ‘accidental’ as we all recognise that no business or person would deliberately contaminate a water-source or consume a chemical.
Accidental exposure to HTF can occur for a number of non-deliberate reasons, including spillage, ground-loop failure (split pipe etc.), heat-pump connections weeping etc. As such it is important that
the HTF be classified as non-toxic on the Safety Data Sheet (SDS). As an example; Hydratech's Thermox DTX heat pump antifreeze is classified as being non-toxic (neat). When diluted with water all chemicals become less toxic on a pro-rata basis, so when neat Thermox DTX is diluted with 72% volume water to achieve -15°C freeze-protection, it becomes even less toxic.

To long-term prevent soil or ground-water contamination – in the event of accidental spillage or leak – it is important that HTF formulations completely biodegrade over time. I.E., they should not bio-accumulate.
Our neat and diluted mixtures are classified as being 100% biodegradable and will not bio-accumulate. The five-day biochemical oxygen demand (BOD5) of a 28:72 mixture of Thermox DTX and water is 0.3 grams of oxygen for each gram of mixture. Water has a zero BOD5.

Biological susceptibility and durability (aka lifespan)
Our natural environment contains a myriad of different microbes, including bacteria, algae, fungi and protozoa, most of which are too small to be seen by the naked eye. Microbes live in air, water and soil and can multiply at exponential rates inside process cooling and heating systems if not prevented from doing so.

Hence, it is crucially important for geothermal and GSHP systems to be carefully sterilised before filling with HTF, and for HTF formulations to include chemicals which neutralise and/or supress all form of microbes.

Ethanol is the most often used flammable HTF found in geothermal and GSHP systems, and whilst not often selected for UK installations, Ethanol is still regularly used in Scandinavia. The fire risks of using 20% to 30% Ethanol in solution with water are obvious and as such rarely acceptable to customers or their insurers.

Energy efficiency and cost

The combined HTF characteristics detailed above will directly impact the short, medium, and long-term energy efficiency of associated geothermal and GSHP systems. This will, in turn, effect the Coefficients of Performance (CoP), Running Costs, Return on Investment (ROI-Payback) and reduction in CO2.
Selecting poor quality or unproven HTF may save costs initially, but the long-term
ramifications can be significant.
Our team of sales engineers, chemists and analysts can assist with all aspects of
heat transfer fluid performance, running cost, environmental impact, compatibility, and energy efficiency. By understanding your exact application requirements.