We report here the development, validation, and application of a reliable analytical approach to calibrate gaseous formaldehyde (HCHO) using gas chromatography coupled with flame ionization detector and methanizer (GC-FID-methanizer). Contrary to the classical GC-FID, GC-FID-methanizer allows HCHO measurements and presents a higher detection sensitivity for other oxygenated products. Therefore, it becomes a common analytical technique used in the combustion community. However, a common challenge encountered while using GC-FID-methanizer is that the conversion efficiency reduces over time of use and consequently the initial response factor of HCHO based on methanizer is no longer applicable. Therefore, we have developed a direct HCHO calibration method that consists of using of an aqueous liquid mixture with known concentration of formaldehyde and acetaldehyde as an internal standard. A detailed protocol of this proposed method was established. The method was tested and evaluated in (1) two GC-FID-methanizers (one with low and one with high conversion efficiency, respectively) and (2) a calibrated GC-Mass Spectrometry system. Finally, the method was successfully applied in a study devoted to the characterization of furanic biofuel flames and the impact of biofuel combustion on pollutant formation, including HCHO.