Fouling on to convective heal transfer surfaces in boilers using solid fuel is a big problem. To investigate the fouling dynamics the probes with similar thermal properties as the tubes of the convective heat transfer surfaces must be used. To investigate the fouling in the laboratory circulating fluidized bed unit in Thermal Engineering Department of Tallinn Technical University the probes with autonomic cooling system based on heat pipe were developed, constructed and tested. Screens and detector surfaces surround the heat pipe of the probe. The heat flow through the probe was mathematically modeled and comparing measurements were carried out. Dynamic behavior of the probe was investigated also. Tests showed that the fouling of convective heat transfer surfaces could be investigated with this kind of probe.