Thermal Fluids Frequently Asked Questions

Propylene Glycol and Ethylene Glycol Heat Transfer Fluids

Thermal Fluids, Inc.

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For your convenience, we have answered many of the basic questions you may have regarding glycol based heat transfer fluids below.


​Q:   What is Propylene Glycol?

A:   Propylene Glycol (PG) in chemical terms is known as 1, 2-propanediol and is a relatively non-toxic liquid that is colorless, odorless, and tasteless. Because of its low level of toxicity, it is used in many cooling and heating applications where there maybe incidental contact with humans, animals, foods, and beverages. It is also used in many cosmetic and food products (check your shampoo bottle and food coloring bottles at home).


Q:   What is Ethylene Glycol?

A:   Ethylene Glycol (EG) in chemical terms is known as 1, 2-ethanediol and is a toxic liquid that is colorless, with a glycol odor and sweet taste. Because of its toxicity, ethylene glycol should only be used in applications where there is no chance of human contact. Ethylene glycol is also used in automotive antifreezes and hydraulic brake fluids and is blended in minor amounts into inks and paint solvents.


Q:   Why use an inhibited glycol?

A:   Both ethylene glycol and propylene glycol in their pure forms are aggressively corrosive to the types of materials (metals, etc) found in a common heat transfer system. It is for this reason that both these glycols must be blended with a high quality anti-corrosion (inhibitor) package of chemicals to create a glycol heat transfer fluid (HTF).


Q:   Are there any materials that should be avoided in a heat transfer system that contains glycol?

A:   Yes. Galvanized steel should not be used with either propylene glycol or ethylene glycol. Also, propylene glycol in almost all cases should not be used in systems with PVC components. However, ethylene glycol works fine in systems with PVC.


Q:   Why use purified water when diluting a glycol heat transfer fluid?

A:   Municipal water varies greatly from one town to the next and in the case of larger cities, from one city block to the next. That is why a purified water such as distilled water or de-ionized (DI) water should be used when diluting a glycol heat transfer fluid (HTF). There are two main reasons to use purified water when diluting a glycol HTF. First, the contaminants and/or impurities in water may degrade both the glycol and the inhibitor package reducing the effectiveness of the product. It will also reduce the useful lifespan of the glycol HTF. Second, water hardness has a negative effect on the glycol HTF causing premature inhibitor depletion and reducing the overall effectiveness of the glycol HTF which can lead to corrosion, scale, clogging and, system failure.


Q:   What are the standards for determining water hardness?

A:   The following are the standards for water used for glycol HTF dilution: 

  • Chlorides – less than 25 ppm (parts per million)
  • Sulfates – less than 25 ppm
  • Total hardness (as CaCO3) – less than 100 ppm


Q:   How can the water be analyzed for hardness?

A:   Most local laboratories can test for water hardness, however, most states require local municipalities to test their water on a yearly basis. The results can be obtained by calling your local water department and asking for their current water quality analysis test.


Q:   What is the range for glycol heat transfer fluid (HTF) dilution?

A:   Thermal Fluids recommends a dilution range of 25% to 65% glycol with the balance being purified water. A concentration of less than 25% glycol will not only increase the freeze point (a warmer freeze point), but will also further dilute the inhibitor package which protects the heat transfer system from corrosion. Thermal Fluids highly recommends a minimum concentration of 30% due to possible future water dilution of the heat transfer fluid in the system by untrained maintenance personnel. On the other side, a concentration of greater than 65% reduces the heat transfer efficiency of the mixture.


Q:   Can glycol heat transfer fluids from different companies be mixed together?

A:   In most cases, glycol heat transfer fluids (HTF) from competitors can be mixed together as long as both fluids are of high quality and the same chemical composition (EG or PG). Always check with the manufacturer of the HTF before mixing.


Q:   Why can’t a propylene glycol heat transfer fluid (PGHTF) and an ethylene glycol heat transfer fluid (EGHTF) be mixed together?

A:   Although they are closely related, propylene glycol (PG) and ethylene glycol (EG) are different chemical substances. They both have their own distinct physical properties (freeze point, specific gravity, etc) but by blending the two together, the mixture will have its own physical properties different from either the PG or the EG. This will cause problems in calculating the freeze point, heat transfer characteristics, etc. Also, the inhibitors designed for one fluid are different than the inhibitors for the other and may not be compatible when mixed. 


Please note that if the two fluids are accidentally mixed together, in most cases the system will still operate, but the mixed fluid must be drained and replaced as soon as possible.


Q:   Why can’t automotive antifreeze be used in a heat transfer system?

A:   The vast majority of automotive antifreezes are made from ethylene glycol as are some types of heat transfer fluids (HTF). However, automotive antifreeze contains chemicals specially designed for use in automotive engines. Many of these chemicals are detrimental to the components and materials found in a stationary heat transfer system. Vice versa, the inhibitor chemicals in a HTF will not protect, and may damage, an automotive engine. 


Q:   How long will a heat transfer fluid last?

A:   A high quality heat transfer fluid blended with purified water such as distilled water or de-ionized (DI) water and maintained properly should last a minimum of 7-8 years.


Q:   How can a heat transfer fluid be tested?

A:   Thermal Fluids can test your heat transfer fluid for a minor fee. This service is free if it is a Thermal Fluids glycol product. The main areas to check in a heat transfer fluid are: Reserve Alkalinity, pH, Glycol Concentration, and Specific Gravity. There are also outside independent laboratories that can also perform this service (see our “Links” section).


Q:   Does dye color represent any particular type of product?

A:   No. Different color dyes are used by different companies. There is no standard dye color that signifies whether the glycol is propylene glycol or ethylene glycol. Propylene glycol may be clear or dyed, however, ethylene glycol is almost always dyed. Dyes are inert and have no impact on the functionality of the fluid.