comfort temperature - what temp you have in your rooms when heating on. Most people comfortable with around 21 - 22°.
Setback - what it sits at rest of time. It's generally more efficient to heat a house from "slightly warm already" rather than stone cold, so set your setback temp to about 15°, and around 10° overnight.
This isn't hard n fast, for example my house = naturally cold and draughty and won't hold heat, so no point having setback as high as 15, may as well just have htg on higher when I come in so I just set it to 10 during the day so htg doesn't come on, then 23° in the evenings.
Bunslinger, April 2009
