Method for determining and monitoring the degree of material depletion of the pipes of heating areas in steam generating plants fired with fossil fuels by means of at least one device for recording a thermal image, - the walls of the furnace cavity being divided into n segments - at least one thermal image of the furnace cavity being produced, - an average surface temperature being determined for each of the segments on the basis of the thermal image, - the incident heat flow being determined by measurement for each of the segments for various operating parameters in the start-up phase of the steam generator, - the steam temperature of each pipe of the steam generator being measured at a measuring point, - the useful heat flow being approximately calculated for each of the segments as a function of the incident heat flow of the segment concerned, of the average surface temperature of the segment concerned, and of the measured steam temperature in the pipes of the segment concerned, - the steam temperature in the pipes of the segment concerned being calculated by approximation calculation over the length of the pipe, starting from the measuring point, for each of the n segments with the aid of the calculated useful heat flow, - the useful heat flow being calculated exactly by using the calculated steam temperature of the segment concerned, - an average pipe wall temperature, averaged over the operating time of the pipes of the segment, being determined for each of the segments as a function of the incident heat flow, of the calculated useful heat flow and of the average surface temperature, and - a theoretical lifetime consumption of the pipes of the segment being calculated from the average pipe wall temperature while taking into account material stresses. - the walls of the furnace cavity being divided into n segments - at least one thermal image of the furnace cavity being produced, - an average surface temperature being determined for each of the segments on the basis of the thermal image, - the incident heat flow being determined by measurement for each of the segments for various operating parameters in the start-up phase of the steam generator, - the steam temperature of each pipe of the steam generator being measured at a measuring point, - the useful heat flow being approximately calculated for each of the segments as a function of the incident heat flow of the segment concerned, of the average surface temperature of the segment concerned, and of the measured steam temperature in the pipes of the segment concerned, - the steam temperature in the pipes of the segment concerned being calculated by approximation calculation over the length of the pipe, starting from the measuring point, for each of the n segments with the aid of the calculated useful heat flow, - the useful heat flow being calculated exactly by using the calculated steam temperature of the segment concerned, - an average pipe wall temperature, averaged over the operating time of the pipes of the segment, being determined for each of the segments as a function of the incident heat flow, of the calculated useful heat flow and of the average surface temperature, and - a theoretical lifetime consumption of the pipes of the segment being calculated from the average pipe wall temperature while taking into account material stresses.