What condition might cause a furnace to short cycle?

A furnace short cycling refers to the phenomenon where the heating system frequently turns on and off in short intervals rather than running for a sufficient duration to adequately heat the space. This can be detrimental to both the furnace and the heating system’s efficiency.

Improper thermostat placement can lead to inaccurate temperature readings. If the thermostat is located near a draft or in direct sunlight, it may detect a higher or lower temperature than the actual ambient conditions, causing the furnace to shut off prematurely. Similarly, if the air filters are clogged, this can restrict airflow, leading to overheating. When a furnace overheats, it activates a safety feature that turns it off to prevent damage. This combination of inaccurate temperature readings and airflow issues results in the furnace short cycling.

In contrast, low refrigerant levels typically relate to air conditioning systems rather than heating units, and dirty condenser coils primarily affect the cooling process as well. Inadequate duct size can lead to airflow problems but does not directly cause a furnace to shut down prematurely. Therefore, the conditions mentioned in the correct answer correctly identify common issues that can disrupt the proper operation of a furnace and lead to short cycling.