This is a great question that I get routinely asked. And the answer all depends on your house. Some contractors will solely base it on the number of square feet you have in your house, some will base it on the size of your air conditioner and some will simply replace your furnace with the exact size you currently have in your house.
The correct way to determine the proper size furnace for your house is through the use of a Manual J load calculation. This calculation will take into consideration the square footage of your home, the type of windows you have installed along with the number and size, the amount of insulation, the direction of the house, the number and type of doors, the use of blinds and/or curtains, the landscape, the number of fireplaces, the duct work, and the general construction of the home. This information is then computed with a formula to determine the heat loss of your house at a specific temperature.
In the Chicagoland area, the generally accepted design standards for a furnace are -10 degrees and 72 degrees. That means when heating your home, your contractor will design the system so that on a -10 degree day you can achieve a 72 degree temperature inside. Oak Brook is the odd municipality that requires a design of -4 degrees and 72 degrees. Many municipalities now require this load calculation when applying for a permit to replace your furnace.
While you now know how many BTUs are needed to heat your home on a -10 degree day, this will not provide you the proper size furnace. This will all depend on the efficiency of the system your choose. For example if your load calculation came back with 68,000 BTUs you would need to install at a maximum of a 90,000 BTU 80% efficient furnace or at a maximum of a 75,000 BTU 95% efficient furnace. Since all furnace manufactures have some slight variance in size, I recommend working with a contractor that carries and has multiple lines of equipment to ensure the best fit for your home
Once a load calculation has been completed you and your contractor will be able to select the proper size furnace for your home to achieve the maximum comfort for your home and optimal performance of your system.