What is an indication of a clogged air filter in a gas furnace?

A clogged air filter in a gas furnace primarily restricts airflow, which is why low airflow from vents is a key indication of this issue. When the filter becomes dirty or blocked, it prevents air from properly circulating through the system. This reduced airflow can lead to various operational inefficiencies, potentially causing the furnace to work harder to heat the space, or it may not be able to maintain the desired temperature.

In addition to low airflow, this condition can lead to other problems such as overheating of the heat exchanger, which can trigger safety shutoff mechanisms in the furnace. Therefore, monitoring airflow from the vents is a critical aspect of regular furnace maintenance and can directly point to the status of the air filter.

The other choices, while they may relate to issues within a furnace, are not direct indications of a clogged air filter. For instance, increased noise levels could indicate mechanical issues, rapid cycling might suggest a thermostat problem or other operational concerns, and unusual odors can be related to gas leaks or burnt components, but these do not specifically signal a clogged filter.