Safest lowest temp when its warm outside? I think it would vary from place to place. As it gets warmer outside draft can fail. A few days ago when it got up over 60 degrees, I opened the manual damper, gave it lots of over the fire air (which just gets heated and goes up the chimney) and cut the primary air feed to a sliver to hold the furnace at about 180 with 130 on the pipe and a .01 - .02" WC on the manometer. I also have vents on each side of the basement to keep air pressure neutral with outside. So for a safest lowest temp, thats about the best I can achieve.