12.6.3.1 (2) "For sizing an individual chimney venting system for a single appliance with a draft hood, the effective areas of the vent connector and chimney flue shall be not less than the area of the appliance flue collar or draft hood outlet or greater than seven times the draft hood outlet area." -National Fuel Gas Code
To determine if a flue liner is needed you need to follow this simple equation-
Measurement of the inside of the chimney that the appliances vent into. Measure the width and the depth and multiply them to determine the square inches of the space. Next you will need to determine which appliance has the smallest diameter flue pipe (usually the water heater) and use that diameter to determine the square inches of the flue pipe. If the square inch measurement is more than 7 times the size of the smallest flue pipe then it is determined that a flue liner is needed. Here’s an example:
Chimney= 12”x12”= 144 sq.in.
Water heater flue pipe=3” (1.5x1.5x3.14) = 7sq. In.
7x7= 49 sq.in.
In this example the chimney is more than 7 times the square inches of the water heater flue pipe so a flue liner would be required.
If a furnace is installed in a masonry chimney and that furnace is draft induced (all furnaces are now) then it would automatically require a flue liner regardless of the “Seven Times Rule”.
Why is a flue liner needed?
Gas appliances produce a lot of condensation. This condensation is mildly acidic and can damage the interior of a masonry chimney. A large chimney will allow the flue gases to cool down which then causes the condensation. The job of the liner is to minimalize the flue size to keep the flue gases from condensing down before they are exhausted to the outside. Any condensation that is present will not effect the material of the flue liner.