If a furnace nameplate specifies a temperature rise of 40 to 70 degrees and measurements indicate a rise of 80 degrees, what is the most appropriate action?

The most appropriate action to take when the measured temperature rise exceeds the specified range from the furnace nameplate is to increase the fan speed. A temperature rise that is higher than the specified range indicates that the furnace is producing more heat than the system is designed to handle at its current airflow rate. By increasing the fan speed, you improve the airflow through the heat exchanger, which helps to distribute the heat more effectively throughout the home while bringing the temperature rise back into the manufacturer's specified range.

Increased fan speed enhances the ability of the system to remove heat from the heat exchanger, promoting better heat transfer to the air passing over it. This adjustment can help prevent issues such as overheating, which could lead to system inefficiencies or damage.

The other options would not address the issue effectively. For example, adjusting the thermostat or decreasing fan speed would likely exacerbate the problem by reducing airflow or system responsiveness, potentially resulting in even higher temperature rises. Inspecting for blockages might be a relevant step if there were no increase in fan speed available, but in the context of managing airflow and temperature, adjusting the fan speed is the immediate and most effective response.