If you buy a furnace that’s too big, the furnace will turn on and off too frequently (short-cycling) sacrificing comfort, losing efficiency and potentially damaging the unit. If it’s too small, your furnace will run continuously, and your home will not feel heated sufficiently. A properly sized furnace should run for about 40 minutes every hour on the coldest day of the year. Many furnaces today have multi or variable stage burners, which result in longer run times but increased comfort and efficiency. A heat load calculation is detrimental in determining the proper size furnace for any home.