Sizing a new gas furnace
 

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)?

A 40,000 input furnace @ 90 % efficiency will give an output of 36000.

Thanks, that part I understand. But a furnace is typically slightly over sized to account for worst case conditions - very low temperatures for a few days. My question is how much should a furnace be over sized?

If I know the heat loss has been calculated correctly by using up to date heat loss/gain methods and also having the proper design temperature for the area, I like to have approx 10 % added.
In your case,I suggested 36000 total output, which would be approx 10% more than your calculation.
One of the main reasons to go a bit over the actual heat loss figure by 10% is to compensate for any "short-lived" temperature drops in your local area weather.
Note; oversizing a furnace by too much could create what is known as "short cycling" and hence cause temperature "swings " within the home

Thanks - short cycling is exactly what I'm trying to avoid.