It’s critical to install the right size furnace
If you install a furnace that is too large for your home, you will experience a condition called short-cycling. Your furnace constantly cycles on and off, over-heating your home then cooling down below the set temperature. On the other hand, a furnace that is too small for your home will not produce enough heat to keep you comfortable on the coldest days.
over-sized RESIDENTIAL furnace short cycling
PROPERLY SIZED FURNACE WITH SMART THERMOSTAT
A furnace that frequently cycles on and off will require more maintenance and fail sooner than a furnace that is properly sized for your home. The reason has to do with increased wear and tear on the heat exchanger. By analogy, think of a car with only city miles and no highway miles. City miles involve a lot of starting and stopping, which wears on the engine. The over-sized furnace constantly starts and stops, accumulating wear and tear on its "engine," the heat exchanger. The result is that the over-sized furnace works harder and fails sooner.