In case of hot-gas welding, the parameters of welding influence the strength of the welded joint. A welding portal was designed and built to make the welding parameters of manual hot-gas welding controllable and independent of human factors. Large series of experiments were performed with the automatic welding station to explore the effect of welding parameters (welding temperature, flow rate, welding speed, and welding force). The strength of the welded joints was determined in a standard tensile test carried out according to ISO 527-3. Using two-way analysis of variance it was concluded that all four parameters influence the strength of the welded joint significantly, and the strength of the welded joint exhibits a complex dependence on all four parameters. Based on a thermodynamic approach the welding energy was determined. The measured tensile strength values were plotted against the welding energy and the welding force and it was found that the results can be approximated by a quadratic surface function well.