Method of cleaning a furnace heat exchanger and/or a flue gas heat exchanger in a boiler of an incinerator, the heat exchanger comprising bundles of tubes or a membrane wall formed by or containing tubes, the method comprising the steps of attaching a pyrotechical charge to the end of a lance, inserting the charge into the incinerator, positioning the pyrotechical charge in an ally between bundles or between the tubes in a bundle, and initiating deflagration thus removing deposits, such as slag, from the tubes.