A study is presented where factorial design is used to find how some selected economic and technical factors affect the profitability of an investment in a combined heat and power plant. The study is performed on a Swedish district heating system. The minimal cost for supplying the demanded heat is calculated with a developed energy system optimisation model, MODEST. The effects on the resulting parameters, such as system cost and optimal size of steam cycle, are calculated from a series of experiments performed using high and low levels of the most relevant factors. The conclusion of the study is that both the main factors and the interactions between them have to be analysed to establish an accurate ranking of the technical and economic factors.