Compares laboratory and field-generated data, explains the functional results of ozone water treatment use for HVAC cooling water systems. Classifies these effects in the areas of biological growth control, corrosion rate control and scale control or retardation. Discusses limitations on the application of ozone. Examines field results from multiple sites and compares them to laboratory-generated data. Discusses theories of mechanisms based on the accumulated information. Discusses specific situations such as under- ionisation and soft and hard water.KEYWORDS: year 1996, Ozone, water treatment, cooling, laboratory testing, corrosion, antiscale, corrosion protection, biocides