One of more crucial elements of investigating treated wood combustion properties with the use of a cone calorimeter is a proper selection of heat flux (HF). The HF level is directly reflected in time to ignition and a thermal degradation degree. The ignition of raw wood or of wood ineffectively protected against fire occurs at a low HF level, i.e. 10-20 kW/m2. By contrast, the ignition of wood which is effectively protected against fire, may occur no sooner than at HF 50 or even at 75 kW/m2. The aim of the paper was to analyse the problem of the selection of heat flux intensity in both experimental and standard fire testing with the use of a mass loss calorimeter (MLC). The subject of the analysis was wood treated with protective agents of various durability and fire resistance. On the one hand, the high HF value for wood samples of low fire resistance restricts or even excludes practical applications of an MLC as a tool for the evaluation of fire protection efficiency. On the other hand, too low value of HF prevents wood either from ignition or from determination of its thermal degradation degree. It especially applies to the situation when wood is effectively protected against fire. In both cases, the problem arises at the interpretation stage of obtained results. The identified problem was presented on the example of wood treated with chemical compounds which are ingredients of commonly used fire retardants, i.e. MAP – monoammonium phosphate, DAP – diammonium phosphate and PC – potassium carbonate, well known for their fire retardant properties. They are also known for their high solubility, which proves their high leaching from wood resulting consequently in a significant decrease in fire retardant properties. Unfortunately a versatile research procedure was not proposed in the performed study. However, the necessity for the individual selection of physical properties of a combustion process was emphasised. The above forces an unconventional method of result interpretation.

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