DO insulate your existing masonry chimney, especially if it's located on an outside wall.
Because insulation helps create a proper draft and prevents icing, which can block your chimney.
DO insulate the space between the existing masonry chimney and the flue, but do not use loose pack insulation.
Because loose packed insulation can enter the flue and block the flue gases.
DO use the manufacturer's recommended flue diameter. For woodstoves vented through an existing masonry fireplace chimney, use solid pack insulated lining to fit the stove flue into the chimney.
Because an improperly sized flue will not provide the draft needed to operate the stove. Also, smoke may leak into your house through the air inlets without proper draft.
DO make certain that all seals connecting the stove to the flue, and within the flue, are as airtight as possible.
Because tight seals will prevent smoke from leaking into your house and contribute to good draft.
DO determine the correct amount of draft for your stove. Draft is the force that moves hot gases up through the flue. Draft depends on the length of the flue, local geography, nearby obstructions, and other factors. Consult certified installers and chimney sweeps (they may have draft gauges to measure draft).
Because too much draft produces an uncontrolled burn and may cause the stove parts to overheat. This can damage the stove. Too little draft will cause "backpuffing" of smoke into the room, or will accelerate the deposit of soot in the stove or flue.