Water

Water is the least expensive circulating loop fluid. Its main disadvantages are its relatively high freezing point of 32 degrees, and that it expands upon freezing, rupturing pipes.

It has a density of 8.334 pounds per gallon at 60 degrees and a specific heat, for practical purposes, of 1.000 Btu per pound per degree. At 40 degrees its viscosity is 1.550 centipoise.

The refrigerant temperature in the evaporator runs about 10 degrees lower than the exit water temperature. Antifreeze protection is required when there is a possibility that the minimum operating evaporator refrigerant temperature will drop to or below 32 degrees.

The coldest expected evaporator exit water temperature during heating operation determines the required fluid freezing point. The earth coil fluid freezing point should be about 10°F below that exit temperature. This freezing point then determines the solute concentration required to protect the heat pump evaporator from freezing.

Glycols and alcohols

Glycols and alcohols are available as concentrated liquids, salts as granular or powdered solids, and potassium acetate as a 50 percent solution. The brine is mixed with water, on- or off-site, prior to being charged into the piping system.

The solutions of alcohols in water - methanol and ethanol waters - have both good and bad properties for circulating fluid use. Methanol water has a low frictional pressure drop and a relatively high heat transfer coefficient. However, it is highly volatile, highly flammable, and highly toxic.

Ethanol waters are less toxic and less flammable than methanol waters, but are still highly flammable. They also are more viscous, and have a larger pressure drop and lower heat transfer. Corrosion inhibitors can be added to make them non-corrosive. Ethanol is more expensive than methanol but less expensive than the ethylene glycol solutions.

Ethylene glycol in water is a common and often favorable alternative. It offers low corrosivity, volatility and flammability. Properly prepared and inhibited, it is relatively non-corrosive. It is the most common antifreeze for automotive use.

The main disadvantages of ethylene glycols are:

• They are somewhat toxic and must be stored and handled with care.

• The viscosity becomes high at low temperatures.

• They have a definite life and the pH should be checked yearly; and spent solution replaced.

Propylene glycol solution is an alternative with the only true advantage of being less toxic than ethylene glycol. Below about a 20°F freezing point, the viscosity gets excessively high, resulting in high pumping costs.

The quality of the water used with the glycols is important. It must be "soft" and have a low concentration of chloride and sulfate ions. Factory-inhibited ethylene and propylene glycol solutions are sold by manufacturers.