The invention relates to a process for operating a condensing boiler in which the fuel is a Fischer-Tropsch product which boils for more than 90 wt % between 160 and 400.degree. C. and contains more than 80 wt % of iso and normal paraffins, less than 1 wt % aromatics, less than 5 ppm sulfur and less than 1 ppm nitrogen and wherein the density of the Fischer-Tropsch product is between 0.65 g/cm.sup.3 and 0.8 g/cm.sup.3 at 15.degree. C.