The old adage that bigger is better does not apply to HVAC equipment. Unfortunately, too many homes have over-sized furnaces. A furnace that is too big will short cycle. It only runs for short periods and then shuts down. This creates an On/Off cycle that becomes the norm. The problem with this is that the comfort level is severely compromised. The thermostat reaches the desired setting too fast and shuts off the furnace. But the rooms that are the furthest away from the thermostat are always colder. A properly sized furnace will stay on much longer and heat the entire house evenly while also conserving energy.
Over-sized furnaces will also have premature failure of the heat exchanger.
Continued over-firing results in cracks or seam failure in the heat exchanger. The fix for that is to replace the heat exchanger or the furnace. These problems can easily be solved by making sure that a proper heat load calculation be performed on your house when purchasing a new furnace.
The equipment manufacturers are aware of this problem and are offering multi stage burners to minimize the short cycling with new furnaces. The ultimate furnace for comfort and maximum efficiency is the modulating furnace. This unit is rate at 98% efficiency and regulates the gas output based on need. We believe int his technology so much that we installed it in our own building.