If temperature measurements on a furnace indicate a temperature rise of 80 degrees while the nameplate specifies 40 to 70 degrees, what should be done?

When temperature measurements on a furnace show a rise of 80 degrees, while the nameplate specifies a range of 40 to 70 degrees, it indicates that the furnace is overheating. In this scenario, increasing the fan speed is an appropriate solution because it enhances airflow over the heat exchanger. This boost in circulation helps to dissipate heat more effectively, thus maintaining the temperature within the manufacturer's specified limits. Proper airflow is crucial for both efficiency and safety; higher fan speeds can prevent potential damage to the furnace components and reduce the risk of over-temperature conditions.

Decreasing the fan speed would worsen the situation by allowing even less airflow, potentially leading to further overheating. Replacing the thermostat or cleaning the filters would not directly address the root cause of the overheating; while filters should always be clean for optimal performance, the immediate concern lies in managing the temperature rise through airflow adjustments.