During the pilot trial for ignition, what must occur to allow the main fuel valves to open?

For the main fuel valves to open during the pilot trial for ignition, a proper flame signal must be received from the flame detector. This is crucial because the flame signal indicates that the pilot flame is established and stable. The flame detector, often using a sensing method such as UV or infrared, confirms the presence of a flame by detecting the characteristics of the combustion process. If no flame is detected, the system thinks there is a risk of unburned gas accumulating, which could lead to dangerous conditions.

The correct operation ensures that the appliance operates safely and reliably. Without this signal, the system will not permit the main fuel valves to open, preventing any unburned gas from entering the combustion chamber, thereby enhancing safety protocols. Other factors like temperature, pressure, or fan engagement may be relevant to the overall functionality of a gas appliance but do not directly control the opening of the main fuel valves in this specific scenario.