A fluid with low viscosity and high specific heat is easier to pump, because it is less resistant to flow and transfers more heat. Other properties that help determine the effectiveness of a fluid are its corrosiveness and stability. The following are some of the most commonly used heat transfer fluids and their properties. Air will not freeze or boil, and is non-corrosive. However, it has a very low heat capacity, and tends to leak out of collectors, ducts, and dampers.
Water is non-toxic and inexpensive.
It has a high specific heat, and a very low viscosity, making it easy to pump. Unfortunately, water has a relatively low boiling point and a high freezing point. Make sure that no toxic dyes or inhibitors have been added to it. Most glycols deteriorate at very high temperatures. You must check the pH value, freezing point, and concentration of inhibitors annually to determine whether the mixture needs any adjustments or replacements to maintain its stability and effectiveness.