it is a safety valve designed to "pop" fully open once the 15 PSI set point is reached

Check for 120V at the circulator

or visible bulging in the associated run capacitor

Verify 24 VAC at the R terminal

while single-acting operation is ideal for solid fuels like wood or coal

Lochinvar & A.O. Smith 100265150 Ignitor 318820-201 it is a safety valveThe Lochinvar & A. O. Smith 100265150 Ignitor is a critical OEM replacement component engineered specifically for high output FB 1501 2001 gas boiler models. As a veteran grade hot surface igniter (HSI), this part provides the consistent electrical discharge required to initiate combustion within the burner assembly. This specific Lochinvar 100265150 kit includes the necessary high temperature gasket to ensure a pressurized seal, preventing flue gas