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Steam/HW system setback - Energy Savings
My understanding is a 1* set-back for 8-hours will save 3% for those 8-hours. So if you look at 24-hours, that 3% savings is really 1% of the day's fuel consumption.
So a 9* set-back would save 9%. But you need to be sure that the system can bring the building up to normal temp in time for business hours.
Steve from Newton, MA