jetsam
Minister of Fire
Low won't even get the downstairs to 70 if it's much below 50. And won't do anything for upstairs. I do 12 hour burns down to about 35 or so unless the wind is howling. Below that it is 8 hours sometimes less.
You DO have a window open. Did you check the guest bedroom?
(I am poking some fun at bholler here, but the underlying truth is that everyone's envelope is really different, and one 2000sf house may have very different BTU requirements from the next 2000sf house.)