NATE Gas Heating Practice Exam

Selecting a lower speed tap setting on a furnace indoor blower motor will have what effect on temperature rise?

1. The temperature rise through the furnace to increase

2. The temperature rise through the furnace to decrease

3. The temperature rise through the furnace will not be affected

4. The furnace to act as a humidifier

The correct answer is: The temperature rise through the furnace to increase

When selecting a lower speed tap setting on a furnace indoor blower motor, the effect on temperature rise is influenced by the airflow rate through the heat exchanger. A lower blower speed means that the air passes more slowly over the heat exchanger. This slower airflow allows the air to spend more time in contact with the heated surfaces of the furnace, enhancing heat transfer. As a result, the air will absorb more heat, leading to an increase in temperature rise as it exits the furnace. This efficient heat absorption occurs because the longer the air stays in the heat exchanger, the more energy it can capture, resulting in hotter air being delivered to the home. Therefore, selecting a lower speed tap on the blower motor stimulates a greater temperature rise through the furnace. Other options do not accurately reflect the principles of heat transfer in a furnace system. A decrease in temperature rise or no effect at all on the rise would not apply when airflow is reduced, as the relationship between airflow speed and heat absorption is crucial in determining how effectively the furnace can warm the air. The idea that the furnace would act as a humidifier is unrelated to the speed of the blower motor and temperature rise; humidity control involves different processes and is not directly impacted by blower speed.