Replace draft hood with barometric damper to reduce stack heat loss?

D107

Yes, for that reason I had our chimney guy put a chain-operated damper on top of our adjacent fireplace flue so we can keep it closed when not in use. (Although in the process I found out that at the chimney base the fireplace and heating flues enter a common chamber, which doesn't seem right to me, like they should be sealed from each other.)

Steamhead

David107 said:

Yes, for that reason I had our chimney guy put a chain-operated damper on top of our adjacent fireplace flue so we can keep it closed when not in use. (Although in the process I found out that at the chimney base the fireplace and heating flues enter a common chamber, which doesn't seem right to me, like they should be sealed from each other.)

That is a dangerous situation. If the top of the flue gets blocked, combustion products from the boiler will enter the room through the fireplace. Get it corrected immediately.

captainco

A wood burning appliance and a gas appliance cannot share the same flue.

D107

Correction: Looking into the base chamber I can see that the fireplace flue IS sealed by brick; it is only the fireplace cleanout dumpbox that empties into the basement cleanout chamber that is adjacent to --and not sealed from--the boiler flue. I looked up my notes on the chimney sweep's advice the last time he was here: "stuff backless fiberglass rolled below boiler breeching in chimney to prevent airflow." Best I guess would be cementing in cinderblocks in at base to seal boiler flue from fireplace dumpbox. But now knowing that it's not the actual flues connected to each other, maybe the fiberglass suggestion would suffice.