Glycol for a heat pump – when and which one should be used?

Is it always necessary to use glycol in a heat pump?

Not every heat pump requires the use of glycol. In internal installations where all piping is located within a heated zone, only water is often used. However, in systems exposed to low temperatures – such as a ground-source brine-to-water heat pump or an outdoor monoblock unit – using a glycol solution significantly increases operational safety.

The decision should be made individually, taking into account the unit location, pipe routing and the heat pump manufacturer’s requirements. If you have a split-type heat pump, glycol will almost certainly not be an issue. If you have a monoblock unit – it often will be, as explained below.

Monoblock – glycol or water in an outdoor installation?

In the case of air-to-water monoblock heat pumps, the question often arises: monoblock – glycol or water? The outdoor monoblock unit contains the entire refrigeration circuit, and a water-based heating circuit enters the building. If pipes are routed outside and there is a risk of temperatures dropping below zero, using glycol in the heating circuit increases safety in case of power failure and the inability to perform defrost.

Pure water provides better thermal performance (higher specific heat capacity and lower viscosity), but it does not protect against freezing of the heat pump. Therefore, in many cases glycol in a monoblock heat pump system is used as frost protection, especially when there are no additional electrical safeguards or anti-freeze systems in place.

If you live in a region with severe winters and occasional power outages, it may be worth considering glycol. Unexpected harsh winters do occur, and so do power failures. Many regions, even in highly developed countries, face grid overloads and long-term underinvestment in energy infrastructure – blackouts will continue to happen.

✅ But if glycol is used – the question becomes: which glycol?

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Ethylene glycol vs propylene glycol – key differences

When comparing ethylene glycol vs propylene glycol, three main aspects should be considered: toxicity, physical properties and price. Ethylene glycol is cheaper and has slightly better thermal parameters, but it is toxic. Propylene glycol is safer for both people and the environment, which is why it is more often used in installations where there is potential contact with domestic water.

The differences in thermal conductivity and viscosity are small, but in larger systems they may influence the selection of circulation pumps and pipe diameters.

☑️✅ In practice, in small single-family home systems, the type of glycol will have a marginal and usually unnoticeable impact on performance. So which one is better?

Ethylene or propylene glycol – which is better for a heat pump?

The question ethylene or propylene glycol often arises when designing new systems. In closed systems where there is no risk of contact with drinking water, both solutions are technically acceptable. However, in practice propylene glycol for heat pumps is chosen more frequently due to operational safety.

It is important to remember that any glycol solution should contain a corrosion inhibitor package and be intended for use in heating installations. Random fluids not designed for HVAC systems should not be used.

Propylene glycol for heat pumps – why is it popular?

Propylene glycol for heat pumps is popular due to its low toxicity and good chemical stability. In ground-source installations and outdoor monoblock systems, it provides effective freeze protection even at temperatures below -15 or -20°C (depending on concentration).

Additionally, modern formulations contain anti-corrosion additives and pH stabilizers, which extend the lifespan of heat exchangers, circulation pumps and fittings.

• ☑️✅ Check: Nordic Tec heat exchangers for monoblock heat pumps

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Why do so many heat pump manufacturers require ethylene glycol?

Ethylene glycol is often recommended mainly for technical and economic reasons, not because propylene glycol is unsuitable. Here are the main reasons:

Better thermal properties

Ethylene glycol has lower viscosity (it is “thinner”), better thermal conductivity and slightly better heat capacity at the same concentration.

In practice, this means:

• lower flow resistance, which matters in larger systems,
• lower load on the circulation pump,
• slightly better heat exchange efficiency.

For heat pumps, where every percent of efficiency (COP) counts, this can be important.

Better freeze protection at the same concentration

At the same freezing temperature, an ethylene glycol solution typically has lower viscosity than propylene glycol and more stable characteristics at low temperatures. Therefore, in installations exposed to very low temperatures (e.g. outdoor monoblock units), manufacturers often prefer ethylene glycol. However, this advantage is relatively marginal and practically unnoticeable in smaller systems up to 10 or 15 kW.

Lower cost

Ethylene glycol is often cheaper and more widely used in heating and cooling technology, making it more readily available.

That is why many manufacturer recommendations are based on it.

Manufacturer testing standards

Many heat pump manufacturers test their units primarily with ethylene glycol – it has simply been more common historically. Therefore, recommendations in technical documentation are often based more on standardization than strict technical necessity. In practice, noticeable design differences would require relatively large systems.

• ✅ Also check: Nordic Tec Ba-115 heat exchangers for large industrial heat pump systems

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Summary – monoblock: glycol or water?

The answer to the question monoblock – glycol or water depends on installation conditions and the risk of freezing. In outdoor and ground-source systems, the use of glycol – most often propylene glycol – is a safe and commonly recommended solution.

When choosing between ethylene and propylene glycol, it is worth considering not only technical parameters but also safety and environmental aspects. A properly selected and prepared glycol solution enables many years of reliable heat pump operation.

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