--- title: "What is Monopropylene Glycol %?" description: "Monopropylene Glycol is an antifreeze ingredient used in many heating and heat-pump systems. Testing its concentration helps confirm that the system has sufficient frost protection and can identify dilution, incorrect dosing or loss of heat-transfer fluid. The correct percentage depends on the product being used and the required frost-protection temperature." url: https://help.fernox.app/en/articles/177-what-is-monopropylene-glycol-percent collection: "Chemical Parameters" updated: 2026-10-02 language: en alternates: [es, fr, nl-be, fr-be, de, nl, sv, pl, cs, tr, ro, sk] --- # What is Monopropylene Glycol %? Monopropylene Glycol is an antifreeze ingredient used in many heating and heat-pump systems. Testing its concentration helps confirm that the system has sufficient frost protection and can identify dilution, incorrect dosing or loss of heat-transfer fluid. The correct percentage depends on the product being used and the required frost-protection temperature. ### Monopropylene Glycol — % **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. ### Why is Monopropylene Glycol important to test? 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. ### Important interpretation note 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.