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The Goodman B1370109 Stack Overtemp Switch is the OEM high-limit safety control that monitors flue/vent temperature on the GUN100-4 gas furnace. Mounted near the stack (flue outlet), it opens the control circuit and shuts down the burners if vent temperatures climb past the safe threshold — a condition typically caused by a blocked flue, failing inducer draft, or a restricted heat exchanger.
The genuine Goodman B1370109 carries the correct trip temperature and body dimensions for the stack mounting location on this furnace. Generic high-limits with mismatched set points either nuisance-trip during normal firing or fail to protect against genuine overheat conditions. For a flue-temperature safety, match OEM exactly.
Easy (2 / 5). Remove the mounting screws, disconnect the terminals, and install the replacement in the same orientation. If the switch has been tripping, have a technician diagnose the venting or draft problem first. Power off before servicing.