I have never tried to make science out of it but it seems like the lowest temp you can run and still deliver enough BTUs to the house wil burn the least wood. Low temps come at a price though as creosote builds up real fast at lower temps even with bone dry wood. My systems is over kill from the size of the boiler to all of my heat exchangers so the temps I use can be lower than most. We dont run it all summer but start early and burn late in the spring when little to no heat is needed. Temps are the 80s this week and we are set at 145-155. Highest I have even been was 165-175. The controller allows for me to change the spread between on and off but I have never seen reason to foool with it.