This experimental research was focused on the investigation of heal transfer augmentation and pressure drop of various turbulator inserts in gas-heated channels. The work was conducted directly in a convective part of a two fire-tube boiler. Twisted-tape with twist ratio y = 4.12, straight tape, and combined turbulator insert were investigated. The working fluids were combustion products of light fuel oil and wood pellets. For comparison, additional experiments without any inserts were performed. Despite of relative large data scattering obtained in these experiments some qualitative and quantitative conclusions were made.