Fireplace opening to flu opening (top mount damper) choked off at top, not drawing air well

[Jo191145]

yeah they are pretty but what a waste of wood. I am not telling you you are wrong by any means and i am not trying to insult your expertise either. I can lay brick but i am sure you are a much better mason than me. But i am wondering what function you have been taught that a smoke shelf has. I am really curious because all of the explanations i have heard make no sense other than the one you gave that i need to to pass inspection that i can absolutely respect we don't have that problem here though.

Two things I've learned in the union. No mason is better than the guy on his left and right and you will not expect any consistency in your mortar

Ok I see where your coming from, the big question WHY? Besides the manual says you must. Now cut me some slack because it's been thirty years or so so since I've read this stuff. As I remember it the theory went that the hottest gases will ride up the throat towards the front of the throat and flue ( that does make sense especially if your throat is deep enough for a smoke shelf) the theory goes on that cooler air/smoke mixtures can be forced to travel downwards in the back of the flue. The smoke shelf depth, shape and position of the flue behind the open damper plate is supposed to effectively spin that down draft around and get it mixed into the hot gases rising and allow it to once again mix with the hottest gases and exit.
Now you can take that with a grain of salt. I'm sure we've both been on tops of chimneys with a fireplace running and I've never seen any evidence of anything traveling backwards.
The other theory goes that sudden wind changes can create a downdraft effect on a chimney. The smoke shelf in that instance is supposed to act as both a reservoir and as a place the down draft hits first and if properly built spun around and aimed back up.
Obviously were not contemplating changing any sustained gusts or reverse in flow. I'm not sure if nature could produce them. It's the brief puffs that a smoke shelf is supposed catch and reverse without disrupting the fireplace.

 

[bholler]

Ok I see where your coming from, the big question WHY? Besides the manual says you must. Now cut me some slack because it's been thirty years or so so since I've read this stuff. As I remember it the theory went that the hottest gases will ride up the throat towards the front of the throat and flue ( that does make sense especially if your throat is deep enough for a smoke shelf) the theory goes on that cooler air/smoke mixtures can be forced to travel downwards in the back of the flue. The smoke shelf depth, shape and position of the flue behind the open damper plate is supposed to effectively spin that down draft around and get it mixed into the hot gases rising and allow it to once again mix with the hottest gases and exit.
Now you can take that with a grain of salt. I'm sure we've both been on tops of chimneys with a fireplace running and I've never seen any evidence of anything traveling backwards.
The other theory goes that sudden wind changes can create a downdraft effect on a chimney. The smoke shelf in that instance is supposed to act as both a reservoir and as a place the down draft hits first and if properly built spun around and aimed back up.
Obviously were not contemplating changing any sustained gusts or reverse in flow. I'm not sure if nature could produce them. It's the brief puffs that a smoke shelf is supposed catch and reverse without disrupting the fireplace.

Yeah those are 2 of the reasons i have heard and the one of stopping downdrafts from sudden gusts could apply but honestly in my years experience actual down drafts are very uncommon while an appliance is running. And i am not sure how a smoke shelf would stop them anyway but i could absolutely be missing something there. And the other one about mixing the air is the most common reason i have heard and it really makes no sense with out the smoke shelf there is no air to mix it is all going up. And that theory was diss proven by an engineer (whose name escapes me at the moment) who worked on improving the rumford design right after ww2.