In this paper, an experimental study of five different types of "turbulator" inserts for fire tube boilers is presented. The experimental setup was constructed in the Department of Mechanical Engineering Laboratory of Karadeniz Technical University. It was tested to evaluate the boiler efficiency according to TS 4040 standard, and to evaluate the second law efficiency of thermodynamics for the cases with and without inserts under the same operating conditions. Four new types of turbulator consisted of truncated hollow cones. They were placed in tandem parallel to the gas flow direction to allow no contact with the fire tube wall. The half-apex angles of the conical turbulators used in the experiments were 14o and 20o. The number of the turbulators inserted in the of the boiler was 200, their installation arrangements in the fire tubes were periodic, and their enlarging positions were in accordance with the gas flow direction. It was found that turbulators increased the boiler efficiency from 8% to 12% and the second law efficiency of the boiler from 24% to 27%. It is also shown that the one with the half-apex angle of 20o gives better results, about 4% enhancement compared with the other ones. A fifth new turbulator, consisting of a truncated half-cylindrical surface and placed in tandem with flow direction, also periodically interrupted and transposed in the fire tube boilers, provided a 4% increase in the boiler efficiency. The second law efficiency of the boiler was improved from 24% to 25.2%. It was also shown that there was no need to use an excess fan for the flue gas in the chimney because of the very low pressure drops in the new types of turbulator.