I have a chimney being rebuilt that has 3 flues and one
needs a gas liner for a furnace(NTI high efficiency boiler that is 5 years old). and the other 2 liners are for 2 wood burning fireplaces that are not presently in use and may just be capped for now.
My fireplaces are currently old coal burning fireplaces.
You can use the same type of liner for both the gas furnace and also for the wood burning fireplaces. The flexible stainless steel 316ti liner works for wood, gas, oil, pellet or coal type of fuel. A flexible liner will be easier and less expensive to install than a rigid liner. Depending on the height, roof pitch and your ability, a liner can be installed in as little as 3 hours.