Scaling problems are more frequently observed in hot water equipment fed with remineralised reverse osmosis permeate compared to hot water systems fed with conventionally purified water. These problems occur when permeate from reserve osmosis is remineralised to a hardness to meet the regulations. The effects of temperature, initial calcium concentration, and the addition of two types of scaling inhibitors (humic substances and phosphate) on scaling formation were investigated using synthetic water to mimic remineralised RO permeate and tap water. No scaling was observed at room temperature for solutions with an initial concentration of about 40–60 mg/L Ca²⁺. At higher temperature, the saturation index increased, and scaling occurred. In conventional hot tap water, scaling occurred after prolonged exposure time but was about 50% less compared to synthetic water, probably due to the presence of scaling retarding compounds in the tap water such as humic substances. In hot synthetic water, scaling was also retarded when humic substances were dosed. Humic substances at its optimum dosage (3 mg/L C) was more efficient for inhibition of scaling than phosphate at its optimum dosage (1 mg/L P).