Abstract: A flame-sensing system for a tangentially fired broiler in which a plurality of scanners are positioned on the side walls of the combustion chamber adjacent respective burners, and oriented to sight the flame from the respective burners, rather than the central fireball. Closely spaced coolant-carrying tubes lining the boiler walls are oriented with at least one tube in a serpentine pattern and with two or three tubes displaced laterally and rearwardly of the flame zone to provide space for a viewing port approximately the width of two tubes. The displacement pattern tends to dissipate the heat load on the scanner port to prevent scanner or boiler case overheating.