It depends on what condition your current liner is in (if any). Some older flues did';nt have them installed at all, and in some cases, the old liners may be rotted and corroded away. A liner is necessary, especially on gas and oil fired furnaces, because the gases are corrosive and can eat away at the brick and mortar and other material within the flue, even the original liner. This can cause serious problems, not least of which is possible CO and other gas released through cracks in the flue into your walls and hence into your living quarters (needless to say a SERIOUS health hazard), on top of other serious problems such as moisture collection, etc. If the condition of the inside of the flue deteriorates to the point that a collapse or blockage results, you can have large amounts of flue gas enter the living quarters.
Have the flue inspected and replace the liner if necessary!
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D.M.
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