Ground Source Heat Pump Antifreeze and Glycol

The fluid flowing through your ground source heat pump ground loops is not just water. It is a carefully formulated antifreeze solution that prevents freezing, enables efficient heat transfer, and protects the pipework. Most installers do not explain this clearly, leaving homeowners with unnecessary questions and concerns.

This guide covers what the fluid is, why it matters, how it is maintained, and what you need to know as a homeowner.

What Is in Your Ground Loops?

The fluid circulating through the underground pipes is a mixture of water and antifreeze -- most commonly a glycol-based solution. This fluid is sometimes referred to as "brine" in technical documentation, though it is not salt water.

The antifreeze serves three purposes: it prevents the fluid from freezing in the ground loop (critical in the coldest ground conditions), it optimises heat transfer between the ground and the heat pump, and it protects the system from corrosion.

Types of Glycol Antifreeze

Most UK installers use propylene glycol for domestic installations due to its lower toxicity and environmental profile. Ethylene glycol offers marginally better heat transfer but requires more careful handling. Your installer will specify the correct type and concentration for your system.

Correct Concentration

The antifreeze concentration must be maintained within the manufacturer's specified range -- typically 20-30% glycol by volume. Too little antifreeze risks freezing. Too much reduces heat transfer efficiency (the fluid becomes too thick to circulate efficiently).

The correct concentration depends on the minimum ground temperature at your location and the heat pump manufacturer's specifications. Your installer calculates this during system design.

Maintenance and Checks

Antifreeze maintenance is straightforward:

Annual concentration check: Your service engineer uses a refractometer to measure the glycol concentration. If it has dropped (due to minor leakage and top-up with plain water, or dilution over time), they top up with the correct glycol mix.

Pressure monitoring: A drop in ground loop pressure suggests a slow leak. While rare in HDPE pipe, leaks can occur at joints or connections. Any lost fluid is replaced with the correct glycol mix.

Full fluid change: Rarely needed. In a well-maintained sealed system, the same fluid can circulate for the life of the ground loop. If contamination occurs (unusual), the system can be flushed and refilled.

Safety Considerations

The antifreeze solution is contained in a sealed, pressurised system. Under normal circumstances, you will never come into contact with it. However:

Propylene glycol is classified as low toxicity. It is the same base chemical used in food-grade antifreeze and some pharmaceutical products. Skin contact is not harmful, and small accidental ingestion is not dangerous (though not recommended).

Ethylene glycol is more toxic and can be dangerous if ingested. If your system uses ethylene glycol, the sealed system design means risk is minimal, but you should be aware of the type installed.

Environmental: In the unlikely event of a ground loop leak, propylene glycol is biodegradable and poses minimal environmental risk. Ethylene glycol is less biodegradable but still contained in small quantities. The HDPE pipe used in ground loops has a design life exceeding 50 years, making leaks extremely rare.

Homeowners interested in the full environmental picture of renewable heating, including how heat pumps compare with solar energy systems on sustainability metrics, will find that ground source technology uses minimal consumables over its lifetime.

Frequently Asked Questions

What fluid is in ground source heat pump ground loops?

A mixture of water and antifreeze, typically propylene glycol or ethylene glycol at 20-30% concentration. Sometimes called "brine" in technical documentation.

How often does ground loop antifreeze need replacing?

The fluid does not need full replacement under normal conditions. Concentration should be checked during annual services and topped up if needed.

Is ground loop antifreeze toxic?

Propylene glycol (most common in UK) is low toxicity and biodegradable. Ethylene glycol is more toxic but contained in a sealed system. Both are safe under normal operating conditions.

What happens if the antifreeze concentration drops?

The fluid could freeze in extreme conditions, potentially damaging pipes and the heat pump. Low concentration also reduces efficiency. Annual checks prevent this from becoming an issue.