To add to thread, I just got through with an inspection of my fireplace
this morning. My chimney takes a 90 deg bend and he was recommending a
draw fan on the roof to assure draft and keep down cresote buildup.
This would certainly solve the op and others' problems with down draft.

We decided to ditch the whole thing and have an electric insert
installed. My wife never liked the wood smell from the downstairs
unit.

Frank