How can you determine if a furnace is stopped up with soot in the heat exchanger?

To determine if a furnace is stopped up with soot in the heat exchanger, measuring the draft gauge at the inspection door is an effective method. The draft gauge measures the pressure difference within the combustion system. When soot accumulates in the heat exchanger, it can obstruct airflow and affect the draft created by the furnace. A significant drop in the draft measurement would indicate that there is an obstruction, such as soot buildup, hindering the proper flow of exhaust gases.

By using this method, a technician can assess whether the airflow is being compromised, which is crucial for safe and efficient furnace operation. If the draft is below the manufacturer's specified levels, it is a clear indicator that further inspection and cleaning of the heat exchanger may be necessary to ensure optimal performance and safety.