If an existing chimney is showing signs of wear and tear due to its age or no longer fulfills the requirements of a modern heating system, then a restoration of the chimney is necessary.
Usage of a modern heating system in combination with an old chimney leads to soot in the chimney as its cross section is too big for the low temperature flue. The acid condensate corrodes the chimney stones/bricks as they, too, are not made to withstand the side effects of modern flue systems.
A restoration of the chimney system is made by introduction of a stainless steel liner to line the chimney and create a pressure tight and corrosion proof flue. The choice of the flue system depends on the type of appliance, and a sizing should be performed by an expert. The designers at Jeremias are the venting experts.