Hi, there! Our ancient furnace is in death throes, but I don't know how many btus I should have for the top floor of a 100-yr.-old graystone (with new vinyl windows, but not much insulation in the walls). Our flat's about 1100 sq ft. The furnace will be located in the basement and come up to our second story. We're thinking of 90% AFUE or up. We live in Chicago.