A tempering valve takes the hot water from a water heater (60-65 Celsius) and the cold water supply and mixes them to create a tempered hot water supply limited to 50 Celsius.
The tempering valve is installed to protect against severe scalding. When the temperature of the water is below 50 degrees it gives you enough time to react and either:
1. Remove your hand or your self from the flow of water
2. Regulate the temperature using the cold water
This is most important when either children or the elderly are using the hot water fixture. This is now law in NSW that a tempering valve be fitted as per the new government regulations on the Fair Trading website.