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

[coolin415]

opening is 1066 sq. inches. The top mount damper opening is 144 sq inches, and the run is about 18 ft from hearth to cap. According to several sites this more than meets the suggested 1/10 rule for proper ventilation.

My issue is still a slight down draft. The original flu is an 18" round. At some point somebody set some steel screen atop the flu and then placed a 12" top mount damper on top of that. Could this be the reason I'm getting smoke in my house? Because the new top mount damper is choking off the the very top of the flu? According to the numbers above my new flu opening is a little over 1/7th, which, in theory, should draw air fine.

Any feedback/suggestions would be greatly appreciated, Because I was just quoted over $850 for a new 18" rectangular top mount damper with a custom fabricated round fitting to attach to my round flu.

Thank you!
View attachment 150718 The fireplace

 

[DAKSY]

Wow. Who decided THAT was a good idea?
I'd get up there & pull the cap & look at that screen thru the damper.
I bet it's not completely clear.
You need to make that right if you're gonna burn...

 

[begreen]

Why the top mounted damper? Is there a negative pressure issue where the fireplace stinks when not burning? Or is this just the way it was originally designed.

 

[coolin415]

Wow. Who decided THAT was a good idea?
I'd get up there & pull the cap & look at that screen thru the damper.
I bet it's not completely clear.
You need to make that right if you're gonna burn...

Pulled the cap off and had a look and there is nothing blocking the flow up top. With the specs included, does it look like a setup like this should draw air ok? (I mean besides ugliness of it all)

 

[coolin415]

Why the top mounted damper? Is there a negative pressure issue where the fireplace stinks when not burning? Or is this just the way it was originally designed.

Not sure, this is how it was when we bought the place. But yes, every once in a while the area around the fireplace does stink. Not totally sure but I think it may have a stove originally, or maybe just in the recent past. I say this because when I moved in there was some insulation that I removed from just above the hearth/shelf.

Any ideas what could be causing the negative pressure? The flu is clear. Should I consider installing a throat damper?

Thanks for the anymore feedback you can give

 

[bholler]

Installing a throat damper might be very difficult what is the size of your fireplace opening?

 

[coolin415]

Installing a throat damper might be very difficult what is the size of your fireplace opening?

The fireplace opening is 41" x 26". But seeing how I just found out I may have negative pressure issues, it seems that adding another damper may make it worse.

 

[bholler]

No adding a second damper would have zero effect on negative pressure issues

 

[begreen]

Not sure, this is how it was when we bought the place. But yes, every once in a while the area around the fireplace does stink. Not totally sure but I think it may have a stove originally, or maybe just in the recent past. I say this because when I moved in there was some insulation that I removed from just above the hearth/shelf.

Any ideas what could be causing the negative pressure? The flu is clear. Should I consider installing a throat damper?

Thanks for the anymore feedback you can give

Negative pressure can be caused by a number of things like a basement install, a tight house that gets negative pressure from fans running, etc.

http://www.woodheat.org/all-about-chimneys.html

 

[Jo191145]

Are you getting an occasional back puff or is the smoke just refusing to go up the chimney? If you get a hot fire going does your problem persist or is it just on startup?

As for that black cap and topside damper. Rule of thumb I always used in masonry was the height of the cap should equal the opening of the flue. A 12" x12" flue should have the cap 12" above it.

Without a damper in the bottom you cannot easily build a proper smoke shelf as the damper flap acts not only to allow smoke out but to curl any downdraft back up into the heated exhaust gases and not into the house. I won't say it's impossible because nothing is impossible in my book but much more difficult. Most smoke shelves and throats aren't built all that well in the real world. Remove the damper flap and things can go bad fast.

Check out your throat and smoke shelf.
Pull off that entire topside damper and cap and make a fire. See if things improve. If they do perhaps your $850 would be worth it. If it still smokes it will be a waste almost guaranteed, but not quite.

As a guy who's built hundreds of fireplaces I'm not ashamed to say. If you want heat from your burnt wood put a wood stove in it

 

[bholler]

Without a damper in the bottom you cannot easily build a proper smoke shelf as the damper flap acts not only to allow smoke out but to curl any downdraft back up into the heated exhaust gases and not into the house. I won't say it's impossible because nothing is impossible in my book but much more difficult. Most smoke shelves and throats aren't built all that well in the real world. Remove the damper flap and things can go bad fast.

Smoke shelves in general are a horrible design fireplaces work much better with out the at all it just adds totally unnecessary turbulence. Smoke shelves are an outdated and pointless design feature left over from the late 1700s and early 1800s. Rumfords work way better and have no smoke shelf. And no the cap does not need to be 12" high it needs to have 150% open area in reference to the flue opening.

 

[Jo191145]

Respectfully disagree. Not about the Rumford though, I'm a fan. IMO the Rumford works well at stopping down drafts because of the intense velocity of exhaust gases traveling through the small opening at the top of the firebox. Very difficult for a downdraft to overcome that velocity. Of course given the right conditions at start up they can be just as nasty as any fireplace.

Finding damper systems and flues designed for a Rumford build is a daunting task.

I've never once received a call from one of my customers complaining that my fireplaces built to code aren't drafting properly. Those codes include smoke shelves with definitive guidelines in their construction. YMMV.

150% may seem reasonable. Of course if there's a prevailing wind that 150 gets quartered. I preferred to use empirical data.

 

[coolin415]

Are you getting an occasional back puff or is the smoke just refusing to go up the chimney? If you get a hot fire going does your problem persist or is it just on startup?

As for that black cap and topside damper. Rule of thumb I always used in masonry was the height of the cap should equal the opening of the flue. A 12" x12" flue should have the cap 12" above it.

Without a damper in the bottom you cannot easily build a proper smoke shelf as the damper flap acts not only to allow smoke out but to curl any downdraft back up into the heated exhaust gases and not into the house. I won't say it's impossible because nothing is impossible in my book but much more difficult. Most smoke shelves and throats aren't built all that well in the real world. Remove the damper flap and things can go bad fast.

Check out your throat and smoke shelf.
Pull off that entire topside damper and cap and make a fire. See if things improve. If they do perhaps your $850 would be worth it. If it still smokes it will be a waste almost guaranteed, but not quite.

As a guy who's built hundreds of fireplaces I'm not ashamed to say. If you want heat from your burnt wood put a wood stove in it

It's the occasional backpuff. It looks as though a throat damper used to be installed. Might have been removed for a stove in the past? I've got a chimney sweep coming by next week to have a look and will look into a re-install. Thanks for the feedback!

 

[bholler]

I've never once received a call from one of my customers complaining that my fireplaces built to code aren't drafting properly. Those codes include smoke shelves with definitive guidelines in their construction. YMMV.

I am not saying they cant draw properly but i am saying that fireplaces work much better with out smoke shelves that create unnecessary turbulence in the smoke path. I am curious what your reasoning behind needing a smoke shelf it i have heard many explanations and really none of them make much sense to me. The best preforming fireplaces i have built or worked on are all based off of the rumford design and use no throat damper just a top sealer. And by the way smoke shelves and traditional throat dampers are a real pain to clean as well which is why many guys don't do it properly. But to get back on topic the numbers work for your chimney so i think it is probably either bad fireplace design or a negative pressure issue with the house. But i am not there to check things out in detail so i could be wrong.

 

[branchburner]

If you want heat from your burnt wood put a wood stove in it

Just what I was thinking. If I was going to spend close to $1k to redo the cap/damper, I'd think about spending a little more and putting in a stove and liner. All that money might come right back in heating-bill savings within the first year.

We did love the fireplace, but I think it was giving us a net heat loss, rather than gain. My wife was really afraid she'd miss the fireplace if we put a stove in it. Happily, no regrets... she prefers the stove!

 

[bholler]

Just what I was thinking. If I was going to spend close to $1k to redo the cap/damper, I'd think about spending a little more and putting in a stove and liner. All that money might come right back in heating-bill savings within the first year.
We did love the fireplace, but I think it was giving us a net heat loss, rather than gain. My wife was really afraid she'd miss the fireplace if we put a stove in it. Happily, no regrets... she prefers the stove!

I agree totally but some people really like their open fires so to each their own

 

[coolin415]

I am not saying they cant draw properly but i am saying that fireplaces work much better with out smoke shelves that create unnecessary turbulence in the smoke path. I am curious what your reasoning behind needing a smoke shelf it i have heard many explanations and really none of them make much sense to me. The best preforming fireplaces i have built or worked on are all based off of the rumford design and use no throat damper just a top sealer. And by the way smoke shelves and traditional throat dampers are a real pain to clean as well which is why many guys don't do it properly. But to get back on topic the numbers work for your chimney so i think it is probably either bad fireplace design or a negative pressure issue with the house. But i am not there to check things out in detail so i could be wrong.

Thanks a ton! Very helpful info.

 

[coolin415]

Just what I was thinking. If I was going to spend close to $1k to redo the cap/damper, I'd think about spending a little more and putting in a stove and liner. All that money might come right back in heating-bill savings within the first year.

We did love the fireplace, but I think it was giving us a net heat loss, rather than gain. My wife was really afraid she'd miss the fireplace if we put a stove in it. Happily, no regrets... she prefers the stove!

Thanks, for us, (mostly my wife), it is an aesthetic thing.

 

[Jo191145]

I am not saying they cant draw properly but i am saying that fireplaces work much better with out smoke shelves that create unnecessary turbulence in the smoke path. I am curious what your reasoning behind needing a smoke shelf it i have heard many explanations and really none of them make much sense to me. The best preforming fireplaces i have built or worked on are all based off of the rumford design and use no throat damper just a top sealer. And by the way smoke shelves and traditional throat dampers are a real pain to clean as well which is why many guys don't do it properly. But to get back on topic the numbers work for your chimney so i think it is probably either bad fireplace design or a negative pressure issue with the house. But i am not there to check things out in detail so i could be wrong.

My reasoning is fairly simple. Unless you build a Rumford to printed design specifications it's not a Rumford. If it's not a Rumford building codes require a smoke shelf designed to building code specifications. I've done what I refer to as hybrids, a little mix of both, but you had better know your inspector and he'd better trust your judgement or they could make you rip it down. In 25 years of residential masonry I never went in reverse, never picked up a brick I had laid. I tore down quite a few half finished condemned fireplaces but not my own. It wasn't until I hopped in the union and got into commercial work that I learned the term demo. I hate it with a passion.

I dislike cap top dampers probably as much as you dislike firebox dampers.

 

[bholler]

I have never had a fireplace fail either and i very rarly build a smoke shelf the only time i do is if i am redoing an existing one and there is no room to do it the other way. I am curious as to what you have against top sealers. I also find it hard to believe you have never picked up and relayed a brick. If that is true i am amazed

 

[Jo191145]

Just what I was thinking. If I was going to spend close to $1k to redo the cap/damper, I'd think about spending a little more and putting in a stove and liner. All that money might come right back in heating-bill savings within the first year.

We did love the fireplace, but I think it was giving us a net heat loss, rather than gain. My wife was really afraid she'd miss the fireplace if we put a stove in it. Happily, no regrets... she prefers the stove!

Glass doors might help some but fireplaces are for aesthetics. Although I have lived in front of mine for seven days on occasion when the power is out several times.
Of course it's a big honkin beauty we called "the wall of flame" back in the day when parties were frequent in the house. Back you up twenty feet with no eye brows and wilt every plant in sight. But damn when everyone goes home and your out of wood it's like having a garage door open in your living room.

 

[bholler]

. But damn when everyone goes home and your out of wood it's like having a garage door open in your living room.

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.

 

[Jo191145]

I have never had a fireplace fail either and i very rarly build a smoke shelf the only time i do is if i am redoing an existing one and there is no room to do it the other way. I am curious as to what you have against top sealers. I also find it hard to believe you have never picked up and relayed a brick. If that is true i am amazed

It requires more room to build a smoke shelf. Now I'm curious to figure out where you were lacking room to not build one?
Top sealers, well to be honest I think I've only seen one in my life. Called out to fix the chain that drapes all the way down the chimney to activate it. Middle of winter, not my fondest memory in life.
I can only imagine during those humid dog days of summer when the crystalline structured rocks in my fireplace are dripping sweat on the hearth how much more creosote I would smell if my chimney were exposed to the inside of the house vs the outside.

Dry laid sidewalks and driveways I've relaid plenty of brick. Landscapers do that now though. Fireplaces and walls not so much. I'm sure I exaggerated a smidgen on that, especially whilst learning the trade. But it's rare. I had a great laborer who was extremely consistent in his mortar mixing skills. Within reason all a mason needs is consistency in his mud, adjust technique to fit.
What I really meant by that was every brick/block laid in a fireplace, seen or not, was placed in exactly the place I wanted it to be. Thought through, pre planned long before I got to it. Had it down to a science if you will. A science I could do in my sleep which left me bored with the whole idea of building fireplaces.
Now I'm in the union and bored out of my skull. Not much room to be creative there. They frown on creativity.

 

[bholler]

It requires more room to build a smoke shelf. Now I'm curious to figure out where you were lacking room to not build one?

It takes more height to build a rumford and many times on a firebox rebuild there is not enough height to do it it is also very hard to do a proper rumford smoke chamber when you are rebuilding just the firebox. so those are usually a hybrid one like you were describing. But i always try to minimize the smoke shelf as much as i can to smoth out the smoke path and get it to flow better.

Top sealers, well to be honest I think I've only seen one in my life. Called out to fix the chain that drapes all the way down the chimney to activate it. Middle of winter, not my fondest memory in life.

If you have only seen one how do you have such a poor opinion of them?

I can only imagine during those humid dog days of summer when the crystalline structured rocks in my fireplace are dripping sweat on the hearth how much more creosote I would smell if my chimney were exposed to the inside of the house vs the outside.

That is why many people crack them open a bit to let out the hot moist air in the summer. It is not necessary with most throat dampers due to the fact that most do not seal well to begin with so there is already a little air moving

 

[bholler]

Dry laid sidewalks and driveways I've relaid plenty of brick. Landscapers do that now though. Fireplaces and walls not so much. I'm sure I exaggerated a smidgen on that, especially whilst learning the trade. But it's rare. I had a great laborer who was extremely consistent in his mortar mixing skills. Within reason all a mason needs is consistency in his mud, adjust technique to fit.
What I really meant by that was every brick/block laid in a fireplace, seen or not, was placed in exactly the place I wanted it to be. Thought through, pre planned long before I got to it. Had it down to a science if you will. A science I could do in my sleep which left me bored with the whole idea of building fireplaces.

Like i said i have no doubt what so ever that you are a better mason than me. I am sure t takes me longer than it would take you but yeah i always plan every thing out ahead of time and have yet to have one fail inspection or fail to preform properly.

Now I'm in the union and bored out of my skull. Not much room to be creative there. They frown on creativity.

Yeah i have some friend in unions also and I couldn't put up with allot of the crap they have to deal with