Heat transfer fluid: glycol water or pure water?

Understanding the role of the heat transfer fluid in your installations

What a heat transfer fluid does: heat transport and overall performance

The heat transfer fluid is the "messenger" that picks up calories in a heat exchanger, circulates through the circuit, then releases them to the emitters. Its ability to carry heat, its stability and its freezing point all affect overall performance and circulator power consumption.

Circuits involved on site: heating, solar, heat pump (depending on configuration)

In hydraulic heating, it's often water, sometimes a water-glycol mix when part of the network can drop below 0°C. In thermal solar, the primary circuit almost always uses an antifreeze mix. On a heat pump, the main topic is refrigerant, but some setups have a water or water-glycol circuit on the emitter side, or even a "brine" geothermal loop depending on the setup.

Points to watch on site: viscosity, purging, corrosion and material compatibility

On site, the right fluid depends as much on the technical datasheet as on the actual handling. Keep these key points in mind:

• Viscosity. The "thicker" it is when cold, the more flow drops.
• Purging. Trapped air blocks heat exchange and speeds up oxidation.
• Corrosion. Check pH, inhibitors and concentration, avoid mixing products.
• Compatibility. Watch out for seals, aluminium, copper, and maximum temperatures.

Pure water: when this fluid is the right choice

Advantages: simplicity, cost, ease of filling and maintenance

Water remains the simplest fluid to manage. It requires no dosing, is available everywhere, and its heat-carrying capacity is excellent. On budget, you avoid buying additives and periodically replacing a glycol-water mix. Filling and purging are done with standard tools, which limits repeat trips to the job site.

Limits: freeze risk, scale and oxidation depending on water quality

Points to watch. At 0°C, water freezes. In an exposed circuit, this can crack the exchanger, pipes or circulators. Depending on hardness, scale can foul components and reduce heat exchange. Water that's overly oxygenated or poorly treated also promotes oxidation and sludge.

Common use cases: circuits protected from frost and heated technical rooms

This choice works well in a closed circuit installed in a heated volume, or when sensitive parts are protected from frost. Also a good fit for technical rooms kept frost-free and short networks that are easy to drain or flush during maintenance.

Glycol water: managing glycol to secure the fluid in difficult conditions

Why add glycol: frost protection and operating safety

In an outdoor circuit or an unheated zone, a temperature drop can block the heat transfer fluid. Adding glycol lowers the crystallisation point, limits freeze risk and protects exchangers, piping and collectors. This is common on geothermal-loop heat pumps, rooftop networks or long runs.

The trade-offs: performance loss, higher circulator consumption and fluid ageing

Glycol makes the mix more viscous and less effective at carrying heat. The result is a drop in efficiency and the need for a more powerful circulator, meaning more electricity. Over time, chemical ageing can acidify the mix and speed up corrosion and deposits. Annual monitoring and checking pH and inhibitor levels avoid unpleasant surprises.

Choosing the right glycol ratio: a simple method based on the expected minimum temperature

Start from the actual minimum temperature possible on site, add a safety margin, then choose the percentage using the manufacturer's chart (propylene glycol recommended in residential settings). Common benchmarks: around 30% for protection down to about -15°C, 35% toward -20°C. Avoid overdosing — it's like insulating with too thick a jacket: you're protected, but you lose energy comfort.

How to choose between glycol water and pure water for your job site in 2026

The right choice based on frost exposure: outdoors, lofts, unheated network sections

When part of the network runs outdoors, through a cold loft or an unheated technical room, freeze risk becomes the number one criterion. On these sections, a properly dosed glycol water mix protects the installation. In a heated volume with a short network, pure water often remains simpler and more efficient, provided there is genuine frost protection (insulation, pipe lagging, temperature maintenance).

Accounting for 2026 obligations and practices: maintenance, traceability, manufacturer recommendations

In 2026, what makes the difference is maintenance tracking. Record the fluid type, concentration, fill date and brand in the job file. Regularly check pH and corrosion inhibitor, and follow the manufacturer's instructions. Some warranties require a specific antifreeze, or advise against glycol on certain exchangers.

Weighing the overall cost: fluid purchase, maintenance, breakdown risk and callbacks

On overall cost, pure water wins on purchase price, but a freeze mistake can quickly cost a circulator, an exchanger or a mid-winter callout. Glycol costs more and requires checks and top-ups. On exposed job sites, it mainly reduces breakdown risk and callbacks.

Implementation: best practices for a reliable fluid over time

Filling, purging and pressurising: avoiding air and concentration errors

Fill slowly, with the circuit cold, using a fluid prepared according to the instructions. If you're using glycol water, measure the concentration with a refractometer before and after filling. Purge at the high points, run the circulator in sequences, then purge again. Finish by pressurising to the planned value to avoid cavitation and corrosion.

Regular checks: pH, freezing point, appearance and replacing at the right time

An annual check is enough in many cases. Check the pH test, freezing point, colour and presence of sludge. Drift in the freezing point or a cloudy fluid often signals lost inhibitors or contamination. Replace when values fall outside the manufacturer's ranges, after overheating, or after several top-ups.

Managing incompatibilities: aluminium, seals, exchangers and top-up products

Before adding anything, validate material compatibility. Some aluminium alloys, seals and exchangers don't tolerate mixing brands, nor "miracle" additives. Avoid topping up at random. Flush if you change fluid type, and only use top-up products approved by the manufacturer.