Furnaces that are too small lead to insufficient heating during cold times, while furnaces that are too large waste energy. Choosing a furnace of the right size provides better comfort and savings.
Relying on a furnace that is too small leads to periods where it runs constantly, but is unable to provide sufficient heat. However, there are ways to potentially remedy this problem without having a new furnace installed. Adding better insulation and sealing any potential air leaks can lead to significantly better performance.
Furnaces that are too large operate less effectively than ones that are the right size. Furnaces are least effective when they are first turned on, and furnaces that burn too hot will quickly meet the intended temperature and shut off. Since they cycle more frequently, they spend more time in less efficient modes. Large furnaces also create more heat near vents, leading to areas of the house that are significantly warmer than others. Furnaces sized correctly provide better consistency.
Existing furnaces can sometimes be modified. By adjusting the airflow into the combustion area, large furnaces can be modified to produce less heat. Similarly, small furnaces can sometimes be upgraded to add more power. However, furnaces systems are designed to meet high levels of efficiency, and modified units may operate less efficiently.