ThermometerFreezing or ‘going cold’ in an earth loop is not caused by the winter air temperatures. It happens when the geothermal heat pump is taking heat from the loop field faster that the loop field can regenerate from the earth around it. This typically happens when the loop field is under sized for the Btu requirement of the home or building.

When a loop field takes heat out of the ground it must “recharge” by drawing in heat farther and farther away from the loop. If a loop field is undersized it cannot recharge fast enough and the temperature in the loop can drop to below 32 degrees and freeze up. The ground does not freeze, but the water in the loops can.

It is highly unlikely that a loop field that is properly designed to carry the BTU load of the building will ever freeze of go cold. But to be on the safe side, antifreeze is still added to provide an additional layer of protection.


Propylene Glycol

Propylene glycol is highly recommended because it is a non-toxic, earth and people friendly antifreeze. It is used in the food preparation industry and is considered safe. Propylene glycol’s low toxicity makes it the only earth loop antifreeze allowed in many states. It has some viscosity problems that limit its use as antifreeze for flowing fluids.

Special care must be taken when designing and calculating earth loops size when propylene glycol is used to avoid the fluid flow being too fast in the summer, and too slow in the winter.

UMR GeothermalUMR Best Practice


UMR uses only Food Grade Propylene Glycol in all of its loop fields.


There are several other types of antifreeze that can be used in loop fields.  Some are quite toxic or very expensive and are only used in specific situations.

Ethanol (Denatured Alcohol, Ethyl Alcohol)

When used as antifreeze, ethanol has similar characteristics to methanol. It flows well, has good heat transfer, and good freeze protection at 15° F. Ethanol is also very flammable and can cause explosions and asphyxiation.

Pure ethanol is not as toxic as methanol, but pure ethanol is too expensive to be used as antifreeze, so denatured ethanol is used instead. The denaturing agents are generally very toxic. Ethanol can be denatured with methanol, pine-based solvent, gasoline, rubbing alcohol, or other similar chemicals.

Ethanol that is denatured with petroleum-based products will dissolve earth loop piping, and can’t be used for earth loop antifreeze. Some brands of ethanol-antifreeze are available that have been specifically designed for use in geothermal earth loops. They contain denaturing agents that are safe for geothermal pipe.

Methanol (Wood Alcohol, Methyl Alcohol)

This was the choice for many geothermal heat pump installers for many years. Methanol is less expensive to buy and flows better because it has lower viscosity (down to 15° F) than other types of antifreeze. It also has good heat transfer ability which improves the performance of the loop field.

On the down side, methanol is extremely poisonous to humans and other animals. It evaporates quickly, and can asphyxiate a person if all of the safety precautions are not followed. Methanol is also highly flammable, and can cause an explosion.

Because of methanol’s toxicity, some states in the USA have outlawed its use in earth loops buried deeper than 20 feet, and other states have outlawed its use in all earth loops, to protect the groundwater if the loop should leak. Therefore Methanol is not recommended antifreeze for geothermal loop fields.

Ethylene Glycol (Car Antifreeze)

Ethylene Glycol is very poisonous. It becomes very thick and flows badly (has high viscosity) at temperatures below 35° F, and has fairly low heat transfer ability. Most states have prohibited its use because of the dangers of contaminating and poisoning groundwater.

Calcium Chloride

Calcium Chloride performs well as antifreeze in earth loops. It is non-toxic but extremely corrosive. If this type of antifreeze is used, the geothermal heat pump’s water coil heat exchanger must be cupronickel. Any metal fittings, pipes, or pumps must be made from brass, or better material, for corrosion protection.