By increasing the effective height of the flue, the Flue Stretcher addresses two problems.
1) Negative House Pressures
When you have two fireplaces. One upstairs and one downstairs the smoke from the upstairs fireplace can get pulled into the downstairs fireplace flue causing smoke to enter the home. By Adding the Flue Stretcher to the upstairs fireplace flue, this will distances its smoke from the downward pull of the downstairs flue.
2) Increase Draft
By incresing height and putting the flue opening above foors and other nearby structures this will help for a better draft of the chimney
Measure the exact outside dimensions of your flue tile sticking up above the crown of your chimney.