Do bin data analysis where temperature occurance is given in 93) 8 hour blocks of daily data. See NAVFAC MILITARY Weather Data Tables. Do (2) sets, one with reset, one without. Total up Btu for each temperature bin = Design Btuh load x count of hrs x (ti-to actual)/(ti-to design).
Take the total Btu difference between w/ & w/o rest.
Say average efficiency = 80% so Btu fuel input saved = Btu saved /0.8
