The purpose of the mixing valve, also known as a tempering valve, is to bring down the water temperature from the solar geyser to a maximum of 60 degrees Celsius before the water comes out of your taps. This is a safety precaution, especially when there are kids around, to prevent accidental burns or scalding since solar water heaters can reach temperatures of up to 90 degrees Celsius. It is also specifically stipulated in SANS and is a compulsory requirement for plumbing COC.