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The results are not conclusive
As some of you may remember, I have been keeping very close records of the fuel burn of the system in the museum I keep watch over for several years now, and I have also been trying various experiments -- within admittedly rather narrow limits -- on the effect of varying building temperature and setbacks. So the following is based on ten years' worth of records.
A couple of disclaimers.
First, and most important, you mileage may vary. This is just one system, and one building.
Second, the effect of infiltration, which is very significant, is strongly affected by wind -- and while there are decent wind records for certain weather stations, they are limited. I have extracted and corrected for the effect of wind to the extent that I can with the available data, but that leaves a tremendous amount of variation from that source which obscures the information from the temperature changes.
Whew. That said...
Conclusions to date.
First, higher indoor temperature settings do mean increased fuel burn. Not rocket science, but it is nice to have the theory and the study results confirm each other! the effect is approximately (but not exactly) linear.
Second, a reasonably small setback (3 degrees) may make a small fuel savings, but the effect is not statistically significant.
I have only one data point for a larger setback, and that from some years ago (before Charles!) when the burner failed for a day. The "setback" was about 15 degrees; the fuel burn on that one occasion was statistically significantly greater (about 20%).
I can't study larger setbacks in my position, as the damage to items in the museum would be too great. There are a lot of things which don't take kindly to fluctuating temperatures... such as pianos, paintings, books...
For what it's worth.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch