A rotary regenerative air preheater (10) having a linear scan hot spot detection system (38). This linear scan hot spot detection system (38) has a linear rail assembly (44) generally extending from the center to the periphery of the rotor (14) cavity. The rail assembly (44) preferably supports two carriage assemblies (54) for linear motion thereon. A drive mechanism (58) located on the exterior wall of the air preheater (10) shuttles the carriages (54) between the central portion and the peripheral portion of the rotor (14) cavity. Each carriage assembly (54) supports a single infrared sensor assembly head (66) for detecting hot spots on the air preheater rotor (14). The linear reciprocating motion of the two carriages (54) results in complete detection coverage for the rotating air preheater rotor (14).