In Kraft recovery boilers of pulp-mills very serious problems of fouling are still actual due to the high slagging tendency of fly ash in those boilers. For optimizing of the soot-blow regimes the information about real situation on fouling dynamics is necessary. There is no linear dependence of deposit's thickness and their thermal resistance of considered ash deposits because during the fouling process some tightening of deposits takes place at the surface. The results of fouling dynamics and its effect to the heat transfer investigation in different parts of recovery boiler Mitsubishi 315 of Kehra Pulpmill Horizon (Estonia) are presented in this paper. Special calorimetric probes with autonomic cooling system based on heat pipe technology were used for the measurements.