Propylene Glycol Mixture: Geothermal Freeze Protection

Our installers often ask why we recommend a 25% mixture of propylene glycol to water for geothermal loopfields.

Propylene glycol at a 25% mixture by mass yields a freezing point of about 12.5 degrees F. Since propylene glycol is slightly more dense, 1.036 g/cm3 to 1.0 g/cm3 for water, this equates to a mixture by volume of 25% freezing at 12.95 degrees F. 20% by mass freezes at 18 degrees F. This is sufficient for interior hydronic lines.

Our pump sizing is based on the viscosity of a 25% glycol mixture by volume. We use this mixture because it uses the minimum amount of glycol for pumping viscosity losses while providing freeze protection.

The reynolds number also plays a part in the ratio. The greater the volume of the thicker glycol, the faster you have to pump to reach turbulent flow. Use 25% glycol to 75% water with our product in a correctly designed geothermal loop field for optimum performance.

Since our product is inhibited, it has a glycol ratio of 96%.  Four percent is the additives for corrosion resistance and the anti-foaming agent. This is the highest ratio available in this type of glycol. Some products labeled -100 can be as low as 65%. If you mix that at 25% you would actually be only 16.25%.

Download the Freeze Safe 100 PDF Spec Sheet for more information.

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