If the chimney liner is narrower than usual for the chosen appliance, its venting effects will not be as good as it should; the liner will fail to carry smoke and combustion products properly up the flue that might eventually cause inefficient burning of fuel, leakage of the gases into the house and accumulation of moisture and creosote in the chimney liner. Similarly, if the chimney liner is too large compared to the normal size, it might inhibit the process of proper venting as it would not be able to heat up well from its bottom to the top to create a stronger updraft through the chimney.

The chimney liner must be insulated from its top to the bottom in case if it has been built on the outside walls of the house. However, if the chimney has been surrounded by some insulated rooms, it would be important to have the chimney liner properly insulated for that particular potion of a chimney that has an extension leading to the un-insulated roofline.

Effective chimney liners would allow all the combustion gases to vent up the chimney to its top from where it would leave the house. This would slow down down-draft that is often caused as a result of cooler gases at the top of chimney to provide healthier updraft to pull smoke and gases out and keep the house cleaner.

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