As a general rule of thumb, there are two types of glycols commonly used as heat-exchange fluids: Ethylene and Propylene, but what are the differences?
Glycol, commonly called “antifreeze”, is a heat exchange fluid utilized in HVAC systems to protect hydronic heating and cooling systems from freezing. Typically, glycol is diluted with distilled water and decreases the freezing temperature of the mixture, allowing systems to operate in cold climates. Glycol is a critical component to these systems as without it most systems would experience pipe bursts and costly equipment repairs due to water’s unique characteristic to expand upon freezing. As glycol has a higher viscosity and belongs in the alcohol family (containing an -OH group), the organic compounds reduce the freezing point when combined with water. In this industry there are two types of glycols; Ethylene and Propylene.
The main difference between the two glycols is that ethylene is categorized as toxic, whereas propylene is not, or at least less toxic. Ethylene glycol is a syrupy alcoholic liquid compound with the chemical formula of C2H6O2 (Figure 1). Toxicity aside, ethylene glycol is still the preferred heat transfer fluid as has a superior efficiency when compared to propylene. When applied in a safe manner protected from leaks and seepage into waterways, this fluid can help reduce operating costs as it is more efficient due to being less viscous. This means that pumps can operate better, and overall heat transfer efficiency is greater. In addition, ethylene glycol is typically less expensive than its counterpart.
Figure 1: 3D model of Ethylene Glycol (C2H6O2)
Propylene glycol is an organic compound with the chemical formula of C3H8O2 (Figure 2). Compared to ethylene, it has a higher freezing point at similar concentrations, making it less efficient and more viscous. An important characteristic of propylene glycol is its approval for use by the US Food and Drug Administration, making it available for food and pharmaceutical applications. Where there is the risk of human or animal exposure, propylene is the preferred heat exchange fluid. Although large amounts can be toxic, is it even used for pharmaceutical applications.
Figure 2: 3D model of Propylene Glycol (C3H8O2)
Glycol itself is not an effective corrosion inhibitor, and because of this chemical additives are preferred. As glycol decays over time, it produces organic acids that can decrease system pH, increasing corrosive tendencies and without corrosion inhibitor(s), will accelerate the deterioration of pipe, pumps and HVAC equipment. To avoid such issues, it is recommended that a concentration of glycol no less than 25% should be obtained. Water purity is also important for the longevity of a system. At elevated temperatures, oxygen, metal contaminants, and other impurities will increase the rate of glycol breaking down. Bacteria present can utilize glycol as a food source, leaving acidic byproducts that reduce concentrations. For these reasons manufacturers will typically dilute glycol with distilled water prior to shipment.
For HVAC applications in a well cleaned, concealed environment, glycol can last 15-20 years. With that being said these systems should be tested on a routine basis for concentration, glycolic acids, pH, etc. for preventative measures as if left unchecked, can lead to catastrophic failures. Glycol refractometers are handy tools utilized to determine the freeze points/concentration of glycol fluid solutions and can be used in the field as a quick verification method. Sending samples to the lab periodically will allow for a more thorough investigation. As water treaters, understanding the chemical and physical properties of glycol will allow us to better support these systems and provide useful testing, both in the field and in laboratory settings.
List of Resources:
 Madhusha, B. (2017, November 15). Difference Between Ethylene Glycol and Propylene Glycol | Definition, Properties, Uses. Pediaa.Com. Retrieved December 5, 2021, from https://pediaa.com/difference-between-ethylene-glycol-and-propylene-glycol/
 Go Glycol Pros. (n.d.). Propylene Glycol Vs. Ethylene Glycol. Retrieved December 5, 2021, from https://goglycolpros.com/pages/ethylene-glycol-vs-propylene-glycol
 4G, A., & 4G, A. (2019, June 26). Ethylene Glycol vs. Propylene Glycol. HVAC Engineering Academy. Retrieved December 5, 2021, from https://www.hvacradar.com/2019/06/ethylene-glycol-vs-propylene-glycol.html
 Ponschok, D. (2017, July 26). Using Glycol in Hydronic Heating Systems. EP Sales Inc. Retrieved December 5, 2021, from https://www.epsalesinc.com/using-glycol-hydronic-heating-systems
 D. (2021, March 30). Propylene Glycol vs. Ethylene Glycol. Dynalene, Inc. Retrieved December 5, 2021, from https://www.dynalene.com/news/2021/01/propolene-glycol-vs-ethylene-glycol/
 Hieronymus, S. (2014, April 24). Everything you wanted to know about glycol. ProBrewer.Com. Retrieved December 5, 2021, from https://www.probrewer.com/library/refrigeration/everything-you-wanted-to-know-about-glycol/