I recognize the importance of correctly sizing a gas furnace. I used a spreadsheet program to do my own Manual J calculation and came up with 31,400 btu / hour heat loss at 5 degrees, for SE Michigan for my 2100 sq ft. Colonial house. To cross check this I looked at my gas consumption for 2008 and compared to the heating degree days for my area in the same time period. This calculation gave me a heat loss of 32,400 btu / hour.
My question is this : What kind of a safety factor should I use to size the furnace (beyond the efficiency rating correction)?