What may cause a furnace to emit yellow or orange flames?

When a furnace emits yellow or orange flames, it is often an indication of incomplete combustion, which can be directly linked to inadequate airflow. For combustion to occur efficiently, the proper mix of fuel and air is necessary. If there is not enough air supplied to the combustion chamber, the result is incomplete combustion, which produces a yellow or orange flame instead of the typical blue flame that indicates efficient burning. This coloration is a result of soot and other unburned materials being present due to the insufficient oxygen levels, leading to the formation of carbon particles that glow.

In contrast, while excessive gas pressure can certainly contribute to a range of operational issues in a furnace and potentially affect flame characteristics, it typically results in a larger, more vigorous flame rather than a shift to yellow or orange. A faulty ignition system may affect the starting of the furnace but would not generally produce a change in flame color once the combustion process is underway. Moreover, while dirty burners can impact efficiency and combustion, they don't singularly account for the yellow or orange flames without the broader context of inadequate airflow contributing to combustion issues. Thus, inadequate airflow is the key factor in this scenario.