Test it out

“If you do shut your flue on a regular basis but still feel like you might be loosing heat out of the chimney, do a quick match test.

Light a match, hold it around the seal of your fireplace damper to check for leaks. If the flame is drawn up to the flue then you have a leak. And now that you know you can get your chimney company to come seal it for you.”

https://www.howellsac.com/blog/top-10-energy-saving-tips-for-fall