Hydronic fluids are heat transfer fluids composed of three key ingredients:

  • Water, to provide heat transfer.
  • Ethylene or propylene glycol, to depress the freeze point of the fluid.
  • A balanced inhibitor system,  to protect common metals of  construction.

Water is nature’s heat transfer fluid, a liquid used since the dawn of time to heat and cool. It freezes at 32°F (0°C) and boils at 212°F (100°C). To extend the liquid range, other chemicals are added such as ethylene and propylene glycol.

Ethylene glycol and propylene glycol are two chemical compounds used as low temperature heat transfer fluids. They share many similar qualities; however, they also have important differences. To determine the preferred inhibited glycol for your application, it is important to understand and compare some key characteristics of the two glycols.

When focusing solely on physical performance, inhibited ethylene glycol solutions often are preferred over propylene glycol solutions because ethylene glycol solutions possess more desirable physical and heat transfer properties. It is important to consider a propylene glycol-based hydronic fluid with an HT1 rating from NSF International, however, for applications involving possible contact of the coolant with foods, beverages or food-contact surfaces.

The most important physical properties of both ethylene and propylene glycol are:

  • Relatively low vapor pressure.
  • High boiling points.
  • Ability to lower the freeze point of  water.

Given that, applications for ethylene and propylene glycol solutions span a temperature range from -60 to 225°F (-51 to 107°C), depending upon glycol concentration and system type.

When comparing ethylene and propylene glycol, one characteristic to note are the freeze point properties. Pure ethylene glycol has a freeze point of about 9°F (-13°C). By contrast, pure propylene glycol has a freeze point of about -4°F (-20°C). Table 1 shows the various concentrations of glycol and water mixtures and the corresponding freeze points.

The freeze points shown in table 1 are the initial (equilibrium) temperatures at which the first crystals appear in the solution. As the table shows, ethylene glycol is a more efficient freeze-point depressant for hydronic fluids. This is due to the fact that an aqueous solution requires less ethylene glycol to achieve the desired freeze point. Therefore, the solution contains a higher concentration of water, and water provides the most efficient heat transfer capacities.

Viscosity is another important characteristic of coolants. Glycols are more viscous than water alone. Solution viscosity increases with an increase in glycol concentration or a decrease in temperature. At low temperatures, propylene glycol solutions become considerably more viscous than ethylene glycol solutions. Therefore, if utilizing a propylene glycol solution, it is important to evaluate the pumping requirements of industrial systems which operate near or below 0°F (-18°C). In the typical range of operating temperatures, the viscosity of ethylene glycol does not impede pump efficiency. However, when anticipating continuous low temperature operation, an increase in pump horsepower is generally allowed. Table 2 compares the viscosity of both ethylene glycol and propylene glycol.

Specific gravity is a referenced physical property when working with glycol solutions. With a direct relationship to density, it is important to consider data on specific gravity in the calculation of a number of engineering expressions. Specific gravity measurements can be a useful tool to quickly approximate the glycol concentration of a given solution.

While it is relatively easy to use specific gravity to determine the concentration of an ethylene glycol solution, it can be challenging to use specific gravity to determine the percent concentration of a propylene glycol solution. Small changes in specific gravity measurement represent rather large changes in propylene glycol content. Therefore, in order to prevent a significant error, it is important to take extreme care when using specific gravity as a tool to measure the propylene glycol content in a hydronic fluid. Furthermore, specific gravity is affected by the temperature of the fluid. Most tables or charts showing glycol concentration versus specific gravity will be accurate for one specific temperature. When the fluid is measured at a different temperature and compared to this data, the comparison will not be a true comparison and could mislead the user.

An optical refractometer is a common, simple and reliable tool to determine glycol concentrations. Measuring the refractive index is a preferred laboratory method for both ethylene and propylene glycol solutions. Most instruments are temperature corrected automatically, and the concentration of the glycol solution — and the corresponding freeze point — can be read directly from the calibrated gauge.

When determining the concentration of a glycol solution, an inaccurate reading can occur when the hydronic fluid is heavily contaminated. In these instances, a laboratory can use the Karl Fischer method to determine the water content of a sample of hydronic fluid.

Unlike aqueous solutions of methanol or other low-boiling alcohols, when typical solutions of glycol (25 to 60 percent) boil, the vapor contains very low glycol content. Because hydronic fluids are designed for closed-loop systems, it is important to avoid vented or open applications due to the appreciable loss of water during evaporation. When it is necessary to add makeup water to restore the solution to the proper concentration, the use of high quality — distilled, deionized or condensate — water is recommended. This will help avoid adding undesirable chemical species such as calcium, magnesium, chlorides and sulfates to the hydronic fluid.

Consideration for the environment and the potential toxicity of industrial products has become a commonly referenced topic. The Environmental Protection Agency has listed ethylene glycol as a hazardous chemical. It has a reportable quantity (RQ) for spills of 5,000 pounds or more. The EPA has not listed propylene glycol as a hazardous chemical.

Ethylene glycol is toxic to humans and other mammals. A lethal dose of ethylene glycol for humans by ingestion is about three ounces (85 grams). Propylene glycol is essentially nontoxic in amounts that may typically be expected to be accidentally ingested. Additionally, propylene glycol that meets the specifications of the Food Chemicals Codex, 3rd edition, and the United States Pharmacopoeia, is considered to be generally recognized as safe (GRAS) by the Food and Drug Administration.

In summary, both products have pros and cons pertaining to quality, capacity and usage. Propylene glycol often is the product of choice when toxicity is an issue or if there is an immediate environmental concern. However, if these concerns are not an issue, ethylene glycol can be considered due to its superior physical attributes. In either scenario, consider partnering with a reputable inhibited glycol manufacturer for optimal product support.