The thermal signal produced by tubes in a heat exchanger is disturbed by the signal produced by the gas and the flame in a furnace. A new method is to record several hundred similar to15-second sequences by a infrared camera (50 frames/s, 12-bit signal), equipped with narrow-band optical filters, and to process them numerically and analyze by a specialize software to achieve a synthetic thermogram which contains the least-disturbed temp. values. The method was verified by the tests run at the PKN ORLEN, Plock, Poland