Final answer:
An irregular flame pattern when the blower starts is an indication of a possible crack or hole in the heat exchanger. Staining in the combustion area could accompany this, while short cycling often points to other furnace issues.
Step-by-step explanation:
One indication of a possible crack or hole in the heat exchanger is an irregular flame pattern when the blower starts. The heat exchanger in a furnace is a critical component where combustion occurs and heat is transferred. If it is compromised, it can lead to dangerous situations such as the escape of toxic gases like carbon monoxide. An irregular flame, accompanied by other symptoms such as staining in the combustion area, could be a sign of a defective heat exchanger. Other issues, such as a clogged filter, usually do not cause irregular flame patterns but can lead to reduced airflow and efficiency, making it crucial to maintain regular filter cleanings or replacements.
Short cycling, another symptom, involves the furnace turning on and off more frequently than normal, which could also indicate different problems, including thermostat issues or overheating.