The external bypass valve is set by determining the:

The external bypass valve is crucial for maintaining appropriate conditions in a gas system. When setting this valve, you need to determine the temperature rise across the appliance. This is essential because the temperature rise indicates how effectively the appliance is operating and whether it is achieving the desired heating effect. By measuring the temperature before and after the appliance, you can ascertain if the bypass valve is set correctly to allow the right amount of flow, ensuring optimal operation and efficiency.

In contrast, while factors such as pressure drop and flow rate provide valuable information about the performance and capacity of the system, they do not directly relate to the temperature performance of the appliance in terms of service delivery. Efficiency rating contributes to understanding overall performance but is more a characteristic of the appliance's design and construction rather than a setting for the external bypass valve. Focused on temperature rise reflects the direct impact of the bypass valve's setting on appliance efficiency and operational capabilities.