What is a possible exception for the lining requirement of a masonry chimney when replacing the appliance?

A masonry chimney typically requires a lining to ensure proper venting of combustion gases and to protect the chimney structure from heat and corrosion. However, there can be exceptions to this requirement when replacing an appliance.

When replacing an appliance with a similar type, it is sometimes permissible to retain the existing chimney lining if it is deemed to be in good condition and suitable for the new appliance's venting requirements. This practice is based on the idea that if the new appliance operates similarly to the previous one, the existing chimney lining may still efficiently handle the byproducts of combustion without the need for a complete overhaul.

This exception acknowledges that functionality and safety can sometimes be maintained with existing infrastructure, as long as the replacement complies with relevant codes and the local authority's guidelines. The key factor is that the existing lining must still meet the necessary performance standards, which is assessed during the replacement and installation process.