Purely because of the low price, some people opt to use galvanised or mild steel flue pipes. However, for closed combustion fireplaces these materials should be avoided at all costs.
Closed combustion fireplaces produce a higher concentration of corrosive gases than open fireplaces and the Acetic Acid produced as the wood burns will eat through these types of flues in a short time. In an instance where the flue runs in the cavity between your ceiling and roof, it is very difficult to visually inspect the condition of your flue. Corrosion could lead to openings in the duct which, if left unattended, could in turn lead to a fire. Therefore, we recommend for that you use 304SS or 316SS grade stainless steel for any closed combustion fireplace in order to ensure safe exploitation and long life of your chimney duct system and the fireplace itself.