Technical definition
Monopropylene Glycol (MPG), also known as propylene glycol, is a measurement of the concentration of monopropylene glycol present in the circulating heating-system fluid, expressed as a percentage (%).
Monopropylene glycol is widely used as the base fluid in antifreeze and heat-transfer formulations because mixing it with water lowers the freezing point of the solution. It is commonly used in air-source and ground-source heat pump circuits, solar thermal systems and other closed heating or cooling systems where frost protection is required. Fernox currently uses monopropylene glycol in products including Heat Transfer Fluid HP-15c and Antifreeze Protector Alphi-11.
Fernox HP-15c, for example, is an aqueous monopropylene-glycol solution containing specifically formulated inhibitors and stabilisers. Its recommended in-use concentration ranges from 33% to 50%, providing increasing frost protection as concentration rises.
Simplified definition
Monopropylene Glycol is an antifreeze ingredient used in some heating and heat-pump systems.
Testing the percentage shows how much glycol is present in the circulating fluid. This helps determine whether the system has enough antifreeze protection and whether the fluid has become diluted, lost or incorrectly mixed.
Monopropylene Glycol is important because its concentration directly influences the frost protection provided by the system fluid.
An abnormal or unexpected Monopropylene Glycol result may indicate:
Insufficient glycol concentration, resulting in inadequate frost protection.
Dilution caused by topping up the system with plain water.
Loss of glycol-containing fluid through a leak followed by replacement with 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 heat-transfer fluid.
A significant change from previous laboratory results.
Insufficient MPG can leave exposed parts of the system vulnerable to freezing during low ambient temperatures. This is particularly important for air-source heat pumps, where external pipework and components may be exposed directly to sub-zero conditions.
Inadequate frost protection can potentially contribute to:
Frozen pipework.
Cracking or damage caused by freezing fluid.
Damage to heat exchangers.
Leakage after thawing.
Loss of circulation.
Pump and valve problems.
System shutdown.
Expensive component failure.
Fernox products demonstrate the relationship between MPG-containing product concentration and frost protection. For example, current Alphi-11 guidance gives approximately -11°C protection at 25% concentration, -15°C at 30%, -18°C at 35% and -22°C at 40%.
For HP-15c, the specified concentrations are 33%, 40% and 50%, corresponding to approximately -14°C, -22°C and -34°C frost protection respectively.
However, more glycol is not automatically better. The concentration needs to match the particular product and system design. Glycol affects physical properties such as viscosity and heat-transfer performance, and Fernox specifically notes that its ethylene-glycol HP-EG product has lower viscosity than comparable propylene-glycol products and therefore requires less pumping energy.
Testing Monopropylene Glycol can therefore help:
Confirm the required level of frost protection.
Detect dilution following 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 manufacturer's specified range.
Support investigation of unexpected heat-transfer or circulation performance.
Verify that the system fluid remains suitable for its operating environment.
Monopropylene Glycol should therefore be considered alongside Refractive Index, pH, inhibitor-related measurements and the specification of the particular heat-transfer fluid being used. Fernox also provides refractometer scales specifically for propylene glycol concentration and corresponding frost protection.
Monopropylene Glycol % should not normally be assessed against a single universal pass/fail concentration. The correct percentage depends on the specific heat-transfer product, required frost-protection temperature and manufacturer's specification.
Different Fernox MPG-based products illustrate this clearly. HP-5c is specified for use between 10% and 30% product concentration, whereas HP-15c is specified between 33% and 50%, because the products are formulated for different levels of frost, corrosion and microbiological protection.
It is also important to distinguish between:
Monopropylene Glycol % — the amount of MPG detected in the system fluid.
Heat-transfer product concentration % — the proportion of the complete formulated product in the system.
Refractive Index — a physical measurement that can be used to estimate glycol concentration when the appropriate calibration is known.
Frost Protection Temperature — the approximate minimum temperature protection corresponding to the particular product and concentration.
These values are related but are not necessarily numerically interchangeable. A formulated heat-transfer product contains not only glycol but may also contain corrosion inhibitors, buffers, stabilisers and biocidal components. Fernox HP-15c, for example, is described as an aqueous monopropylene-glycol solution containing specifically formulated inhibitors and stabilisers.
For the Fernox App, it would therefore be important to confirm whether the laboratory's “Monopropylene Glycol %” represents percentage by mass, percentage by volume, or a calculated product-equivalent concentration. The displayed interpretation and thresholds should match the analytical method being used.
