first of all I would like to say that the chips actually have a very pleasant smell. Also they burn quickly and well. However, then in our furnace developed such a strong black sooty smoke that the disc is heavily blackened very quickly. Cleaning the disc the next day (the soot is not burned away by the "airwash") then turns out to also be extremely difficult because this layer of soot is not so easily dissolves as those in the "normal" combustion arising.
Conclusion: We are no longer using the chips.