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It is important that furnace flues are sized properly or vent into chimneys which have chimney liners in them. Many older heating systems were oversized. From the furnace to the chimney used to vent the exhaust gases many older systems are oversized. These older systems were also inefficient. Modern furnaces and boilers are far more efficient than the older inefficient systems. The older systems had flue gases that contained more heat then the newer modern heating systems. It is therefore important that flue size be sized properly for the newer furnaces because the flue gases are highly corrosive when they condense. Condensation occurs in the flue gases when they cool too fast. This can happen on very cold nights or with chimneys or flues that are oversized. Corrosion inside a chimney causes the masonry to crumble and crack. Even terra cotta lined chimneys are not immune to this problem as the terra cotta has masonry joints which can be corroded by the condensed gases.
If you install a new boiler or furnace that has a higher AFUE of 80% or more and these furnaces or boilers use the chimney for primary venting a liner should be installed. If you install a high efficiency furnace or boiler that uses PVC for venting combustion gases and the old furnace or boiler shared the chimney with a gas fired water heater a liner needs to be installed in the chimney for the gas fired water heater.
