I have had one of these programmers installed today and what i can make of it is you programme time against temperature.
So i have mine set at 21deg from 3pm then at 11pm i have it set for 17deg.
So during the day and night when we dont need a lot of heat the boiler will not fire unless it drops below the set temp.Between 3 & 11 the boiler fires unless its warm anyway.
ChrisK
January 2011