What could be a sign of blocked vents in a gas appliance?

The presence of yellow or orange flames instead of the typical blue flames is indeed a sign of blocked vents in a gas appliance. When a gas appliance is functioning correctly, it should produce a steady blue flame, indicating complete combustion of the fuel. A deviation from this, such as yellow or orange flames, suggests that there is incomplete combustion occurring, which is often the result of insufficient oxygen due to blocked or obstructed ventilation. This change in flame color is an important indicator of potential issues with the appliance's safety and efficiency, as it can also lead to the production of harmful carbon monoxide.

Other choices, while they may relate to gas appliance performance, do not directly indicate blocked vents. For instance, reduced energy bills might suggest increased efficiency but wouldn’t signal a vent blockage. Similarly, an unusual noise could indicate a number of issues without specifically pointing to vent obstruction, and increased flame size might indicate a variety of problems, including too much gas flow, rather than directly linking to the state of ventilation. Therefore, the presence of yellow or orange flames is the most relevant and significant indicator of blocked vents in gas appliances.