First time poster. You’ve all (well most) have been highly recommended as the Answer Gurus.
Question: Is there a ratio for the flue size of a chimney?
Facts: Our chimney is 25′ high or tall. The fireplace opening is 39″ wide, 30″ tall and 27″ deep. The side and rear walls are straight up and the rear wall is 26″ wide has a bit (1/4″) bow to it. The ceiling of the firebox domes (lopsided) to meet the flue with the flue opening towards the back of the dome. The dome is 25″ tall and has a little ledge where it meets the walls. It is not a very smooth transition. There are even a few mini-domes or dimples in this transitional dome.
We recently got the flue relined. Appearently the Chimney Guy ran some sort of flexable tubing up the old flue. It looks like an 8″ diameter tubing, but the opening (where it meets the fire box) is only about 6″ in diameter.
Should any of this be a concern? Gotta tell you that unless we build the fire in the VERY back of the fireplace the house fills with smoke. The Chimney guy says this is due to how well the house is insulated. Note: This house was built in 1789. Yes, we have installed/ replaced a few insulated doors and windows, but most are still single pane. Some panes have broken and have been replaced with cardboard taped to the doors, until we do the big renovation push. The house also has vinyl siding, but we are slowly removing it. So far three sides are done.
What do you guys and gals think? Any experience with all this? Any info would be greatl appriciated.
Thank you.
Replies
This may be complicated to find an answer.
Assuming the thing worked fine b/4 it was relined would suggest that the new lining somehow screwed up the draw.
Did it work fine b/4?
Any changes to the top of the chimney? new cap etc?
Any large trees near the chimney or in line with the prevailing wind?
Smoke just during start up?
Any additions to the structure up top?
What happens if you crack open a door or window in the room?
And of course, what are you burning?
Remodeling Contractor just outside the Glass City.
Quittin' Time
You've all (well most) have been highly recommended What a way to start off your first discussion. Makes a person wonder which catagory they are in.
Welcome, and congratulations on providing substantially more info than many people, old & new, provide in 2 or 3 postings.
I have to agree with the initial diagnosis of the other responder ... if it worked fine before the re-line, then that would be a great clue. Sounds vaguely like the chinmney guy is trying to deflect the blame.
Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
Yes, there is a formula, but I don't have it. I let my mason figure it out, and the height of the stack is one of the variables - that and the openning size ahd height. Bends reduce the flow and have to be accounted for also.
I compliment you on what thorough information you have provided with the question. This is rare. It Might even get you a gold star in lieu of an answer.
;)
Seriously, though, My standard openning size uis a bit smaller than you mention, 36"W x 30" H, and he uses an 8x12 for those, if my memory serves me. Some get as big as 14" x 16"
I wouldn't use an 8" round for anything bigger than a large wood stove controlled conbustion unit. Can't imagine what the installer was thinking here - other than his profit. When I have seen flue liner systems like that installed on an open fronted fireplace, they always included a top mounted fan in the price. It won't work right without it, as you already now know.
Makeup air is part of the scenario, but if it was working fine before and not now, your only choice is probably to spring for some extra to get that powered smoke sucker mounted on top.
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Bottom line... if the house smokes up... it is wrong (unless we are talking just a small puff at start-up).
Get the chimney guy back out to remedy the problem.
A number of things affect draft, as other posters have pointed out. The size of the flue is usually dictated by the spec that comes with the appliance being vented. 8" is a standard size for many applications.
The liner your contractor installed was probably a contiguous stainless steel liner - a good idea in an old flue for safety reasons. The smoke entering the fireplace plenum shouldn't be a problem for a 8" flue, although a 6" reduction at the plenum could be an issue for draft. BTW, having a big flue isn't neccessarily good, as you get better efficiency and less creosote if you can keep the flue temperatures up during venting. A properly sized flue does this.
Based on your information in the post, I would start with having the contractor resize the entrance to the flue to be the same size as the rest of the lining. Enjoy the day! h.
I'll agree with you that the neck down at the damper throat is the weakest pooint, but the eight inch is absolutely too small. You have ansered as though this is a controlled air combustion device like a wood stove, not a free burning open front fireplace - a good size one at that. There is no plenum, as with some such appliances. There is a need for a larger sized flue. When have you ever seen a wood stove with an opennin door this big? So I'd ask you to take note of that fact, you must have missed it.
An 8" will rarely work for aan open fireplace, even one I am familiar with where the flue is 8x8 and the firebox openning is 28" x 28".
Fireplaces also almost never build creosote also. it is the wood stoves that experince that problem. The fire burns hotter and the exhaust air moves faster with fireplaces.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
>> Fireplaces also almost never build creosote also.
It isn't common, but it isn't rare either, based on the hundreds I've looked at. Especially with an exterior chimney.
I think the place to start is with some sheetrock (or durarock - but for short periods sheetrock should be ok) and temporarily reduce the firebox opening, say a couple of inches at a time on the sides and the top and do some test burns.
See what works, if anything.
The alterntive is rebuilding the chimney with a new, full sized flue liner.
You'll need a couple of extra couch cushions to look under to pay for that.
"It is as hard for the good to suspect evil, as it is for the bad to suspect good."
-- Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)
Sheetrock is covered with papre, Bob. If I were to play around with adjusting openning size, I would use sheet metal or Durock
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
As I mentioned, for short term tests I think it would be OK.
I see plenty of sheet rock that is way to close, or even in contact with, furnace and water heater flues, which should show scorching signs because of the paper, but which doesn't.
But I should have made it very clear that it should be closely watched and used short term only.
Sheet metal or durock would be better, of course.
"It is as hard for the good to suspect evil, as it is for the bad to suspect good."
-- Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)
Piff--
You're not often wrong (think this is the first time I've caught you on something technical) but this time you goofed.
There is no way an open fireplace can ever hope to burn as hot as a well-designed air-tight woodstove (we're not talking Franklins, here, LOL). And I don't believe the air moves faster through one, either--although I'll grant you there is a larger volume of air moving through a big fireplace into a big flue and dragging all that heat out of the house and blowing it out to Mother Nature....
The most likely cause of a poor draft is the fact there's a six incher opening up to an eight incher. This causes a loss in draft velocity--like a reversed venturi. You can choke down from 8 to 6, but not the other way around.
That said, a 25'-tall stack of cold air is pretty heavy, and it may indeed require opening a door or two or a few windows while flash-burning a few sheets of newspaper to warm up the chimney before it will start to draw. Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
I'm not arguing that the 6" choke isn't a problem. That is definitely the worst of it, but even if it weren't for that choke, the 8" flue is simply not big enough for that large of an openning.
Like I said, my basis for that opinion is that I have never seen my mason build a standard penionning ( smaller than this) with such a small flue, the fact that the 89" rpound is a standard size for your basic large wood burning stove with controlled combution, and the fact that I know of a 28" x 28" small fireplace openning with an 8x8 flue that has a poor draft, with all other circumstances being acceptably normal on it.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
We don't have any argument about the inversed reduction being the problem; I caught that in your first post but was addressing the question of what burns hotter--fireplaces or woodstoves.
Our standard here for pre-fab insulated metal chimneys for woodstoves is now 2" thick insulation; virtually all recent (last 15 years) woodstoves use a 6" ID/10" OD pipe. Fireplace inserts use an 8" ID pipe--and some of them are homologated to run on the older 1" thick insulation (grandfathered only; new installations have to conform to the 2200-degree standard).
I'll grant you that the draft through a fireplace insert is not the same as an open fireplace. It's more akin to that of a big woodstove, say a Jotul #8. But it's still not as efficient a heating device as a cast-iron stove unless electric blowers are added to get the heat out into the house.
I haven't had an open fireplace since I moved out of my Manhattan apartment 25 years ago. But there, I had two! Top floor of a 5-floor walk-up railroad-flat, circa 1885 construction. One FP in the front room, one in the kitchen at the other end. Each was about the size of the unit mentioned at the head of this thread, and each was running on 8" square fire-bricked flue. Very basic fireplaces, both of them (it was a tenement, after all!); no smoke shelf, no mechanical damper, just a 16"-deep depression in the brick 5-flue chimney. I had heard these fireplaces were originally intended for coal; don't know for sure. It would be logical; the building was piped for gaslights too. I burned scrap pallet wood scavenged on the streets of the Lower East Side and cut up in the downstairs hallway with an electric chain saw belonging to a neighbor. (Betcha I felt like Paul Bunyan, LOL!!)
Front fireplace drew like a champ.
Kitchen fireplace smoked abominably; never could get it to draw no matter what.
Both had exactly the same construction, flue size, flue length, and all.
The only difference was, a newer building had been put up wall-to-wall with ours to the east sometime in the 30s, and the rear (kitchen) chimney on that one was three feet taller than ours--so my kitchen fire's draft was blanketed by the newer building's chimney structure.
Damn! I haven't thought about that old apartment in many a moon, Piff--you've gone and got me all nostalgic now....
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Kindling wood ( Pallets) does burn hot enough to create a draft for sure, but I'm not suprised that you got a draft in Manhattan. That's 'cause citys suuck. LOL
I believe those were coal burners, I've seen a few. Wouldn't be suprised if they were only 22-24" wide and about 28 - 30" high opennings, and they may have been lined with Cast iron in the box at one time too.
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ROAR!
Cities only suuk if you wanna live in the country, and vice versa. It's SUBURBS that really suuuuukkkk: faults of both, benefits of neither....
No sign of any cast iron liners in those fireboxes; but the very shallow hearth stone (about 12") would indicate a small coal scuttle or something similar used to live in there....Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Three years ago, middle of the night: Watched a neighbors place burn due to a chimney fire. 200 Y/O stone construction, walk in fireplace, big chimney, NO wood stove. All creosote needs is a surface cool enough to condense on. If enough condenses and ignites, you'll think theres a subway train in your chimney. Those folks used that fireplace a lot, and I know they weren't real picky about dry wood.
Also: In older construction, it is not unusual to find joists or rafters seated in masonry pockets in a chimney. Add old, soft mortar to that and you've a real potential problem, fireplace or stove. I like relining with stainless and pouring an insulating concrete mix around it. I used perlite for the aggregate. I've done this twice for OLD chimneys to be used with wood stoves, and it worked well.
The rule of thumb I always use is the flue area needs to be about 10-12% of the opening size. A little more if a shorter chimney, and conversely, you can lean toward the lighter side if a taller chimney with no outside obstructions. I think it's written into the code now that the inside dimensions of the flue are what's used - not the common dimensions.
Your opening's approx 1170sq in, so you need about 120sq in of flue area. The 8" pipe is about 50sq in (3.14xrxr), so you're running a little light there. The 6" stepdown or whatever it is just makes things worse.
I don't know anything about relining chimneys with pipe, but if the original flue size was approx 8x12 they should have made it to fit, or at least be a lot closer. Stepping down the size by 50% is too much.
My curiousity - Is this chimney original to the house?
Medusa,
DON'T LOOK AT ME!!! I got a mirror!!!
Anyway, I found this site in my collection of favorites. It came reccommended by the Chimney Safety Istitute of America.
WoodHeat.org and this page should get you started understanding the issue, maybe give you some hints on how to live with what you've got or ammunition to hit the chimney guy with.
They also have a contact page and answer individual questions.
SamT
Good words from RVillaume.
I'll duplicate a bit of his words and add a few more:
Yes, there is a ratio between flue size and the size fo the fireplace opening, but it depends on the shape of the flue, square or round.
When using square (or rectangular) flue the flue should be 1/10th the size of the fireplace opening. Round? then 1/12th. Round flues vent more efficiently because gasses will not swirl and stagnate in the inside corners of the square flues, and that swirling can reduce the effective size of the flue.
Realize that flue tile can also be measured differently. This means that a 12" by 12" flue may not neccessarily have a 144sq in air space. There are "standard" and "modular" sized flues, and each measures differently. Round and square measure differently as well. What I'm getting at is that the 12" square tile may actually be 11-1/2" square at its exterior dimensions, and that the free air space inside the tile may actually be just 10" square, or 100 sq inches.
Your opening was 39" by 30", 1170 sq inches. A round flue should have a free space of 10%, or 117sq inches. A square tile needs 12%, or about 140 sq inches. A qualified mason or a supply house can set you up with the right sized liner.
As the others have said, the 8" round with a 6" choke point is simply too small.
If you keep what you have, your easiest fix may be to get a set of glass doors with dampers for the front of the fireplace opening.
The rule of thumb is the cross sectional area of the flue should be at least 10% of the fireplace opening. Some fireplaces work that are less than that and some don't that are more. There are other variables, but a 6" dia. flue won't carry much of a f/p.