Mixing valves alow for control of water temperature to heating zones. Radiant floor heating systems are very susceptible to heaitng problems due to water temperature. Overheating - temperature swings, Underheating, wood flooring shrikage, carpet fiber break dow, etc. Therefore, addition of a mixing contol to a radiant floor system will signigicantly protect your investment.

Mixing valves come in three packages.

1. Manual Set - rotary valves
• Rotate the valve till you get the right temperature and remove the handle to prevent tampering.
• Does not compensate for changes in supply water temperature of increase in load.
2. Thermostatic Mixing Valves
• Adjusts the valve to maintain output water temperature due to changing supply water temperature. (Anti-scale valve)
• Does not compensate for increase in load.
3. Motorized - rotary valves.
• Motorized valves are connected to a control with sensors. The control will sense supply water temperature, load temperature, boiler return water temperature.
• The control can adjust the valve to protect the boiler from low return water temperatures or heating zone from increased boiler temperatures.
• The control can allow hotter or cooler water to the zones for load changes due to weather (outdoor reset).
• See here for more: http://extremehowto.com/how-to-save-energy-and-money-with-an-outdoor-reset-control/