In the majority of the USA and Canada, residential geothermal heat pump earth loops must be protected from freezing. Many people that have contacted us have asked, "If I bury my earth loop deeper, will I still need to use antifreeze to protect my earth loop from freezing?" This question tells us that there is a misconception of why the earth loop is subject to freezing. If your earth loop is installed above the frost line, yes it will freeze, but even if you install your earth loop below the frost line, the fluid may still freeze. Freezing in an earth loop is caused by the geothermal heat pump taking heat from the loop fluid, not the winter air temperatures.
The more undersized a loop, the longer it takes for the earth to recharge the heat in it, as the geothermal heat pump removes the heat. We have been asked, "Can't we just design bigger earth loops, so the earth loop fluid doesn't drop below the freezing temperature?" Sure, this can keep the earth loop above freezing. However, earth loops must be designed for a reasonable payback period. If we design to keep earth loop fluid above freezing, it can require twice as much pipe in the earth loop in many geographical locations, and in some places it requires 3 or 4 times as much pipe. Since it is not cost-effective to use this much pipe, we use antifreeze to keep the fluid flowing below 32°F instead.
What types of antifreeze can be used in geothermal earth loops?
For years methanol was the choice for many geothermal heat pump installers. Methanol works very well because it is less expensive to buy, and flows better (has low viscosity) down to 15°F than other types of antifreeze. It also has good heat transfer ability (it holds acceptable amounts of heat, compared to water). However, 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. Methanol is not our preferred antifreeze, because of the danger of fire and explosion when working with it, and because of the restrictions that have been imposed on it by many local and state health codes.
*However, research has shown that methanol does biodegrade rapidly in the natural environment; even deep below the ground ("In soil or groundwater, rapid biodegradation is expected with the half-life ranging from 1 to 7 days.").
Ethanol is another antifreeze that has been used for geothermal earth loops. It has similar characteristics to methanol as an antifreeze: 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 (the type of alcohol that people drink) is not as toxic as methanol, but pure ethanol is too expensive to be used as an antifreeze, so denatured ethanol is used instead. These denaturing agents are generally very toxic. Ethanol can be denatured with methanol, pine-based solvent, gasoline, rubbing alcohol, or other such 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. We have used a brand called "Geosafe" (safe for pipes, but still dangerous for people), but there are others available.
Ethylene Glycol is a very poisonous antifreeze. It also becomes very thick and flows badly (has high viscosity) at temperatures below 35°F, and has fairly low heat transfer ability. Also, most states have prohibited its use, because of the dangers of contaminating and poisoning groundwater. We therefore do not recommend ethylene glycol be used in an earth loop.
Propylene glycol is a non-toxic antifreeze. It is used in the food preparation industry and is considered safe*. However, it has some viscosity problems that limit its use as an antifreeze for flowing fluids. Special care must be taken when designing and calculating earth loops that will use propylene glycol, or the fluid flow will be too fast in the summer, and too slow in the winter.
Propylene glycol's low toxicity makes it the only earth loop antifreeze allowed in many states. We recommend this antifreeze, but only for earth loops that have been properly sized and designed by a professional earth loop designer.
*Propylene glycol is toxic to cats, however, so don't let them drink it.
Calcium Chloride performs well as an 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 cupro-nickle. Any metal fittings, pipes, or pumps must be made from brass, or better material, for corrosion protection. We also recommend this antifreeze.