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As part of our maintenance work we carry out inspections of the glycol transfer fluid in systems, which can be the cause of low heat pump efficiency and even breakdowns. Here, we look at what glycol actually is and how some simple precautions can be taken to ensure that your heat pump is not suffering due to poor glycol quality.

The glycol, otherwise known as brine, that circulates through the ground collector loops in heat pump systems comes in two main varieties. Propylene glycol is an organic-based fluid, meaning that leakages do not adversely affect the surrounding ground. Ethanol-based glycols are cheaper, but would cause damage were they to leak and are notifiable to the Environment Agency.

In basic form all glycols are clear, almost odourless liquids which normally have the consistency of syrup. Dye is added to provide colour. They have high boiling points and low vapour pressure, hence are ideal for transferring energy from one area to another.

Propylene glycol

Due to containing biological organisms, propylene glycols will decompose over time. When they do, the fluid becomes thicker and darkens, thereby reducing the efficiency of the heat pump system and shortening the fluid’s effective lifetime. To counteract this, additives that minimise biological growth are added. Alongside these, inhibitors are also added that protect against corrosion, scaling and deposits in order to help protect the metals that are found in common heat collector circuits.

Propylene glycols tend to be more expensive to produce. They have very good thermal transfer properties and their low environmental risk means that they are commonly used in heat pump systems throughout the country.

Ethylene glycol

An alcohol-based glycol, ethylene has slightly better heat transfer characteristics than propylene glycol and is cheaper. It is also a little less viscous which has the corresponding effect of marginally improving pump efficiency. Unfortunately, it is also toxic to humans, animals and plants meaning that any leakage can potentially be very dangerous. It is in use in many heat pump systems, hence the need for a careful ongoing check to be kept on the state of the collector array components. Some parts, such as connectors, are more prone to leakage and need regular checking to make sure that their integrity is not compromised. As the concentration of ethylene glycol in a brine mix increases, so bacterial growth is inhibited. Therefore, high concentrations of ethylene are often chosen in order to minimise bacterial growth, so also making the solution more dangerous in the event of a leak.

As part of our heat pump maintenance contract, we will carry out tests on your brine system to ensure that glycol quality is maintained to a high standard. We can top up glycol levels where we are able – mixing two types of glycol is not recommended and does not provide a long term solution. If the quality has degraded to a level where replacement is needed we will only use high quality propylene glycols such as those manufactured by Sentinel.

If you think that degraded glycol is affecting the performance of your system, and would like to see efficiency restored to higher levels, contact us online, by phone or This email address is being protected from spambots. You need JavaScript enabled to view it. and speak to one of our helpful team.


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