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Last updated: 24 March 2022

Ground Source Heat Pump Antifreeze

Why use antifreezes for a ground source heat pump?

Even though the ground source heat pump loops (heat collectors) are laid underground, they still have to be protected from low temperatures. Burying collector pipes deeper in the ground won’t prevent them from freezing, even when they are installed above or below the frontline. This is because the freezing of a geothermal pump loop is caused by the pump itself, precisely, by the heat transfer principle employed in a heat pump.

As the ground source heat pump constantly removes the accumulated heat, the smaller and shorter a loop, the longer time it requires to absorb new quantities of heat, hence its susceptibility to freezing. This drawback can be overcome by installing bigger tubes, that should prevent the fluid’s temperature dropping below the freezing point. However, bigger tubes mean more pipelines (in some cases, up to 3 times more), which can strain your budget and subsequently, lengthen the payback period.

Thus, an antifreeze liquid is used in most of the ground source heat pump loops, so as to make sure that the coolant can easily flow even below 0 °C. In the following lines we present five antifreeze types that can be used for underground heat collectors.

Ground source heat pump antifreeze types

  • Methanol (methyl alcohol, wood alcohol). Since methanol bears high toxicity levels, its application in ground source heat pumps is strictly regulated. In certain countries it is against the law to use methanol for underground heat collectors, if these are buried deeper than 5 metres. Such a precaution is due to the risks associated with the methanol leakage, that under certain conditions could lead to groundwater contamination. In addition to this, methanol is a highly flammable substance that is prone to explosion, if misused. Thus, due to its somewhat ‘unstable nature’ we would advise to refrain from using methanol in favor of more chemically stable and environmentally friendly options like the ones we cover in the next lines.
  • Ethanol (ethyl alcohol, denatured alcohol). Although not as toxic as methanol (if it is pure ethanol), it is still highly flammable and can lead to explosions and asphyxiation. Nevertheless, as an antifreeze it is similar to the methanol heat transfer properties. It flows properly, it is a good heat exchanger, and can stay in a liquid state as long as the temperatures don’t drop below the -9,4 °C mark. However. since it would be expensive to use pure ethanol as antifreeze, the denatured ethanol is used instead. Many of the denaturing agents are toxic and harmful to the underground pipes. The ethanol that is denatured using petroleum based products for instance, can dissolve most of the piping. Still, there are antifreeze manufacturers that are developing ethanol antifreeze liquids that are safe for earth loops, but nevertheless, harmful to people.

Heat Pump Pipes

  • Ethylene Glycol, otherwise known as the ‘car antifreeze’, is a very poisonous substance. On top of being a poor heat transfer agent, it flows badly at temperatures below 0 degrees Centigrade (Celsius). In some countries this type of antifreeze is banned due to the risks of contaminating groundwater sources. Therefore, we strongly recommend avoiding the use of ethylene glycol.
  • Propylene Glycol. In contrast to the already mentioned antifreeze fluids, the propylene glycol has low toxicity levels and is generally considered safe. Nonetheless, its viscosity at certain temperatures might limit its application in some of the ground source heat pump loops. Thus, it is important to undertake a careful planning before installing underground heat collectors, especially if you will be using propylene glycol as antifreeze. It is also important to remember that propylene glycol is toxic for animals, specifically cats - hence, make sure to keep your antifreeze sealed and away from kids and pets.
  • Calcium Chloride is a decent antifreeze that bears good heat transfer properties, flows well and is not as toxic as the other antifreezes. However, it is very corrosive, which makes its use quite problematic if certain measures are not considered. If calcium chloride is used as antifreeze, then one has to make sure that all metal fittings and pipes are made of brass, the ground source heat pump’s heat exchanger is made of cupro-nickel and other pump’s components that can be susceptible to corrosion are covered with protective layers.

Before deciding on the antifreeze type that would be appropriate for your heat pump, it is important to get in touch with local and/or regional regulatory agencies that oversee the approvement of groundworks in your area. In so doing, you’ll make sure that you are authorized to carry out the planned installation works and that the antifreeze you will use meets the safety and environment protection requirements.

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