skeptical but it works!
Like many other reviewers, I was skeptical about how effective this would be. But our chimney damper does not close all the way and it was making our family room very cold in the winter. So I decided to try this, and it works! The key is to make sure the balloon is fully inflated, so it has good contact along the entire inside of the chimney (so make sure you purchase the proper size for your chimney diameter). It took some contortions to get it fully in place above the damper, but we no longer have a draft by the fireplace and the room stays several degrees warmer in the winter. An innovative and cost-effective solution to a common problem.