Technical definition
Monoethylene Glycol (MEG), also known as ethylene glycol, is a measurement of the concentration of monoethylene glycol present in the circulating heating-system fluid, expressed as a percentage (%).
Monoethylene glycol is commonly used as the base fluid in antifreeze and heat-transfer formulations because, when mixed with water, it lowers the freezing point of the solution. This makes it particularly useful in systems exposed to low temperatures, including air-source and ground-source heat pump circuits and other heating or cooling applications requiring frost protection.
Fernox Heat Transfer Fluid HP-EG, for example, is an inhibited heat-transfer fluid based on ethylene glycol. Fernox specifies different product concentrations according to the required frost protection, ranging from 25% product concentration for approximately -10°C protection to 50% for approximately -33°C.
Simplified definition
Monoethylene Glycol is an antifreeze ingredient used in some heating and heat-pump systems.
Testing the percentage tells you how much glycol is present in the system fluid. This helps determine whether there is enough antifreeze to protect the system from freezing and whether the fluid has become diluted or incorrectly mixed.
Monoethylene Glycol is important because the concentration of glycol directly influences the system's frost protection and heat-transfer characteristics.
An abnormal or unexpected Monoethylene Glycol result may indicate:
Insufficient glycol concentration, resulting in inadequate frost protection.
Dilution caused by topping-up the system with water.
Loss of glycol-containing fluid through a leak followed by replacement with plain water.
Incorrect initial mixing or dosing.
A system that has been drained and refilled without restoring the correct glycol concentration.
Excessive glycol concentration compared with the intended system specification.
Introduction of a different fluid or heat-transfer product.
Deviation from the concentration recorded during a previous water test.
The most immediate concern with insufficient MEG is freezing. Water expands when it freezes, meaning inadequate frost protection can result in serious damage to exposed parts of the system.
This may contribute to:
Frozen pipework.
Damage to heat exchangers.
Cracked components.
Leakage following thawing.
Loss of circulation.
Pump or valve problems.
System shutdown during cold weather.
Potentially expensive component replacement.
This is particularly important for heat-pump systems, where external pipework and equipment may be exposed to sub-zero ambient temperatures.
Fernox HP-EG demonstrates the relationship between concentration and frost protection: its specified in-use concentrations range from 25% to 50%, corresponding to increasing frost protection from approximately -10°C to -33°C. Fernox also specifies a minimum 25% product concentration to provide adequate corrosion protection from the inhibited formulation.
However, the concentration should not simply be made as high as possible. Increasing the amount of glycol changes important fluid properties, including viscosity and heat-transfer performance. Fernox notes that its ethylene-glycol-based HP-EG has lower viscosity than comparable propylene-glycol products, reducing the energy required to circulate the fluid.
Testing MEG concentration can therefore help:
Confirm the required level of frost protection.
Detect dilution through water topping-up.
Confirm that the system was correctly filled.
Identify loss or replacement of heat-transfer fluid.
Check that the concentration remains within the product manufacturer's specified range.
Support investigation of unusual system performance.
Confirm that the system fluid remains appropriate for its operating environment.
Monoethylene Glycol should therefore be considered alongside Refractive Index, pH, inhibitor-related measurements and the specification of the particular heat-transfer fluid being used.
Monoethylene Glycol % should not normally be assessed against a single universal pass/fail value. The correct concentration depends on the heat-transfer fluid being used, the required frost-protection temperature and the manufacturer's specification.
For example, the current Fernox HP-EG specification provides the following relationship between product concentration and frost protection:
25% — approximately -10°C
30% — approximately -14°C
35% — approximately -18°C
40% — approximately -22°C
45% — approximately -28°C
50% — approximately -33°C
It is also important to distinguish between Monoethylene Glycol %, heat-transfer product concentration and Refractive Index.
Monoethylene Glycol % describes the amount of MEG detected in the fluid. Product concentration describes how much of a formulated heat-transfer product has been mixed with water; that product may also contain corrosion inhibitors, buffers and other additives. Refractive Index is a physical measurement that can be used to estimate glycol or product concentration when the correct calibration is known. Fernox HP-EG, for example, contains monoethylene glycol together with specifically formulated inhibitors and an antiscalant.
For your Fernox App documentation, it would therefore be worth confirming whether the laboratory's “Monoethylene Glycol %” is reported as % by volume, % by mass, or as a calculated product-equivalent concentration. These are not necessarily numerically interchangeable and the app should reflect the laboratory method used.
