Antifreeze for heat pumps: frost protection and when it is needed

Short answer: If a heat pump has water-filled pipes outdoors, frost protection must be part of the system design. Antifreeze fluid may be one solution, but its type and concentration must not be chosen by a rule of thumb: follow the equipment manufacturer’s requirements and the fluid’s technical data sheet.

Why is freezing a concern for heat pumps?

Water expands when it freezes. In a closed circuit, this can put severe stress on pipes, heat exchangers, pumps and fittings.

The risk is especially relevant when part of the hydraulic circuit is outdoors and ambient temperatures can fall below freezing.

Do monobloc systems require extra care?

Usually yes, because many monobloc systems carry water between the outdoor unit and the indoor installation. This section may be exposed to sub-zero temperatures.

In a split system, the connection between the outdoor unit and the indoor module mainly contains refrigerant. This changes how hydraulic frost protection must be assessed.

For a full comparison, see monobloc vs split.

Is the controller’s anti-freeze function enough?

Active protection functions are useful, but they generally depend on electricity, functioning controls and the ability to circulate or heat the fluid.

Good system design must also consider abnormal situations such as power cuts, shutdowns or equipment faults.

When should antifreeze be used?

Antifreeze can be considered when the hydraulic circuit faces a freezing risk and the equipment manufacturer permits the fluid in question.

We do not recommend selecting the concentration using a universal rule. The protection temperature depends on the product, concentration and fluid properties. Use the antifreeze manufacturer’s tables and documentation.

Does antifreeze affect the installation?

Yes. A water-glycol mixture does not have exactly the same properties as water. Viscosity and heat transfer change, potentially affecting flow rate, pressure losses, the circulation pump and transferred heat output.

The more hydraulically sensitive the system is, the more important it is to design around the actual fluid being used.

Does more antifreeze always mean better protection?

Not necessarily. Choose the concentration for the required protection level and within the manufacturer’s limits. An excessive concentration can impair hydraulic properties and heat transfer.

What other protection methods are available?

Depending on the equipment and project, protection may involve several measures: automatic circulation, anti-freeze functions, auxiliary heating, dedicated mechanical components, suitable pipe routing or protected fluid.

Not every measure suits every unit; check compatibility in the manufacturer’s documentation.

Does pipe insulation solve the problem?

Thermal insulation slows heat loss, but without heat input it cannot keep a pipe above freezing indefinitely. Insulation is necessary, but it is not absolute frost protection.

What if the system is shut down for several days?

An extended shutdown differs from normal operation. If protection relies on circulation, the compressor or an electric heater, consider what happens when the system is deliberately switched off or electrical power is unavailable.

For holiday homes or installations that may be left without electricity, address this before installation.

Should antifreeze be checked after commissioning?

Yes. Fluid can be lost during repairs, topping up or servicing. If only water is added, the original concentration may change.

Regular fluid checks and adherence to manufacturer recommendations are part of proper system operation.

What should be checked during design?

• the lowest possible temperature at the site;
• any sections of pipework outdoors;
• the volume of exposed water;
• the unit’s automatic functions;
• how the system behaves during a power failure;
• material compatibility with the antifreeze fluid;
• flow rate and pressure losses with the actual fluid;
• the heat pump manufacturer’s requirements.

Is antifreeze also necessary in a split system?

The word “split” alone is not enough to answer. Check where the water circulates and which hydraulic components are exposed to sub-zero temperatures.

Frequently asked questions

Can I use automotive antifreeze in my heat pump?

Choose a fluid suitable for HVAC use and compatible with the installation. We do not recommend a product simply because it is generally described as “antifreeze”.

How much antifreeze should be added?

Set the concentration according to the fluid product and required frost-protection temperature, following the fluid’s technical documentation and the equipment manufacturer’s instructions.

If I have antifreeze, can I skip insulation?

No. Outdoor pipes must be properly insulated regardless of the frost-protection strategy.

See also: the ECO-TEC heat pump guide and the heat pump catalogue.