If a technician finds that the burner flames waver in one corner after starting a 5-year-old natural gas furnace, what could be the likely issue?

The observation that the burner flames waver in one corner of the furnace is a critical indicator of a potential issue. In this case, a faulty heat exchanger could be the likely culprit. A heat exchanger that is compromised may disrupt the airflow or the combustion process, leading to unstable burner flames. The wavering flames suggest that there is an inconsistency in gas combustion or airflow, which could be due to cracks or blockages in the heat exchanger preventing even heating and causing irregular flame patterns.

The heat exchanger plays a vital role in the safe and efficient operation of a furnace. If it is not functioning properly, it can not only affect performance but also lead to dangerous conditions like flue gas leaks. Therefore, it's essential to consider the integrity of the heat exchanger when observing such abnormal burner behavior.

While other issues like a clogged flue pipe, insufficient gas pressure, or a malfunctioning thermostat could also affect furnace performance, the symptoms specifically described point more directly towards the heat exchanger as the issue causing wavering burner flames.