Thermo. Controls

Take your heating system into the 21st Century. Weather Compensating Mixing Controls will adjust the water temperature in the heating zones due to the outdoor temperature. Outdoor reset is method of temperature compensation that can greatly reduce your heating bill by reducing the heat you are pumping into your house when the outdoor temperature is more temperate. It will also improve the thermal comfort of your home or business by eliminating temperature overshoot due to less heat stored in the heating tubing.

These controls are intended to attach to mixing valves but there are also controls to adjust temperature through variable speed injection pumping.

See here for more:

http://extremehowto.com/how-to-save-energy-and-money-with-an-outdoor-reset-control/