The first and most important step in sizing a furnace is to do the heat loss calculation. When you are finished you will have a number that is the Design Heat Loss of the house. Let's say that number is 52,234 BTUH. This means that the house loses 52,234 BTU's each hour when the outdoor temperature is the Winter Outdoor Design temperature for your area.
Obviously, you want a furnace with an output of at least 52,234 BTUH. Most contractors, would add a safety factor to the requirement. After all, weather data is averaged to come up with the Winter Design Temperature and you want to be warm even on colder than average winters. Our own personal recommendation is a 15% to 25% safety factor.
So, 52,234 plus 20% = 62,680 BTUH, this means a furnace with a 60,000 to 70,000 output would do nicely. Why not add a big safety factor, like 100% to be really safe? There are a number of reasons why not. Operating costs go up. A furnace that runs only for short bursts uses more fuel, much like city driving compared to highway driving.
Life of furnace may be shortened due to overheating heat exchanger. Initial costs of the furnace and the larger ductwork go up. Comfort level may go down due to short bursts of hot air and long off cycles. Furnace life may be shortened due to condensation on the heat exchanger.
