Hi everyone, I rebuilt a chimney for a customer, when he had his first fire in the fireplace on the main floor it drew some smoke into the basement fireplace. I swapped out the concrete rain pots you see in the picture as the ones i used (8" x 12" flue size) where open on the 12" side. Swapped them out for pots open on the 8" side and it improved it quite a bit but still minor downdrafting into basement flue. Flues are offset in height of each other by 4". Basement flue is no longer in use. I could extend the basement flue by 8" or so but it wouldnt fit under the lid of the rainpot and they dont want to loose the look of the chimney with removing the one rainpot lid. Since the basement flue is not in use i would just like to seal it off at the top with a sheetmetal cap. I cant see any downside to this but the homeowner said he read online somewhere that that is not good to do but im not sure why. Any other suggestions? Thanks!