Is it more economical to set the water temperature thermostat on a central heating system high or low?

Typical heating boiler setpoint is around 180*, if you are looking to economize, look into having a reset control installed. The reset control will adjust the boiler setpoint based on outdoor air temperature. On a 70* day you would have 70* boiler water temp with a reset schedule of 1.5, for every degree outdoor temp drops below 70 the boiler temp would rise 1.5* so on a 0* day, boiler temp would be 175*. The reset schedules are adjustable.