Loop Fluids

Two things are critical to ground-source system operation. The selection of a suitable heat transfer fluid and a pump to circulate it through the ground loop. Ordinary water can be used in warmer climates where the ground temperature stays warm and the heat pump's heat exchanger refrigerant temperature does not drop below 32°F. A corrosion-inhibited antifreeze solution having a freeze point of 10°F or more below the minimum expected loop temperature is suggested.

Ground-Coupled system loops will typically operate from 30 to 95°F in northern climates and from 40 to 95°F in southern climates. Except in the southernmost climates, an antifreeze solution will be required to prevent freezing in the heat pump heat exchanger. Remember, the refrigerant in the heat pump evaporator is about 10°F lower than the exit water temperature. Freeze protection is necessary if the minimum operating evaporator refrigerant temperature approaches 32°F in the middle of winter.

The antifreeze solution should be non-toxic, non-corrosive, and have properties that will reduce pumping power. The antifreeze and water solution choices are:

  1. Glycols Ethylene or Propylene
  2. Alcohols Methyl (methanol), isopropyl or ethyl (ethanol)

Salts may be safe, non-toxic, and have good heat transfer properties, low cost and long life. However, you should avoid using them because they are very corrosive and difficult to clean up. Using salt will also void the warranties on some pumps. Salts require special metal components. The added cost of cupro-nickel fluid-to-refrigerant heat pump heat exchanger can easily be justified in a salt brine system.

Glycols are relatively safe and generally non-corrosive, have fair heat transfer, and medium cost. Their disadvantages are toxicity, viscousness at low temperatures requiring more pump power, and a finite life.

Alcohol and water mixtures are relatively non-corrosive, have fair heat transfer, and a medium cost. Their disadvantages are flammability and toxicity.

Methanol has been used successfully in northern climates where heat pump loop operating temperatures go as low as 25°F.

Water and glycols have been used successfully in southern climates where the operating loop temperatures remain warm. Since alcohols are non-corrosive, their use is popular in both northern and southern climates.