What could cause a high stack temperature in a furnace?

A high stack temperature in a furnace typically indicates that the combustion process is not functioning as efficiently as it should be. When there is too much air supplied to the burner, it can lead to a condition known as "overfiring." In this scenario, the excess air can result in higher temperatures because it may not mix well with the fuel, leading to incomplete combustion. This incomplete combustion can cause the heat generated to exit the stack at higher temperatures.

Using the correct amount of air is crucial for optimal combustion. Ideally, a balanced air-to-fuel ratio ensures that the fuel burns completely, producing efficient heat and maintaining lower stack temperatures. Therefore, too much air disrupts this balance, causing the stack temperature to rise.

Other factors like insufficient air supply, dirty filters, or low fuel pressure can also influence furnace efficiency, but they generally lead to different conditions. Insufficient air can cause incomplete combustion, potentially lowering stack temperatures, while dirty filters can obstruct airflow, leading to similar effects. Low fuel pressure typically results in less fuel being burned, which would not result in high stack temperatures either.