How much of a thermopile must be covered by the pilot flame for optimal performance?

For optimal performance of a thermopile, it is essential that it is adequately covered by the pilot flame. When one third to one half of the thermopile's tip is enveloped by the flame, this coverage ensures that the thermopile receives enough heat to convert thermal energy into electrical energy effectively. This electric energy is what powers the safety and control systems that prevent gas leaks while keeping the appliance operational.

Having this specific amount of coverage allows the thermopile to reach its operational threshold, generating sufficient voltage to maintain the valve in an open position, thus ensuring proper combustion and appliance safety. If the coverage were less than one third, as in some of the other choices, the thermopile might not generate enough heat to function properly, which could lead to inefficient operation or even a shutdown. Similarly, covering more than half or just the end of the tip would either create excess heat that may damage the thermopile or insufficient heat, failing to activate the necessary safety mechanisms. Therefore, the optimal coverage is critical for maintaining both efficiency and safety in gas appliances.