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Chester
08-17-2001, 08:19 PM
<font face="Verdana, Arial" size="2">Originally posted by fahnoe:


The basement fireplace had the same opening, just a taller chimney, so I figured I could either add height to the chimney (expensive) or reduce the aperature with a set of doors (less expensive). The addition of doors solved my problem: even when they're open, the frame reduces the opening enough to draw properly.

--Larry</font>


Interesting comments, but from my experience it goes even further. In my basement I have a wood burning stove with two cast iron doors. All performs well, and it will draw perfectly with the doors open for the first couple of hours or so. When the chimney reaches operating temperature there is a hugh reduction in the draft effect and you can have wisps of smoke coming out into the room with both doors open. Close one door and it will operate with out smoking again. But, to return to the same draft effect that was there upon start-up (everything cold) you need one door closed and the other just cracked open. Have been told that in industry a similar thing happens to their boilers, so it looks to me like the chimney height is not the only consideration effecting the draft available.

rockrat
03-02-2006, 07:58 PM
Ok, I have just about had it.

I have a nice big fireplace. It is a big wall of rock with a fire box constructed in it. I clean it out every year in the fall before the big cold.

So, I have learned that any outdoor temp over 40ºF results in a bit of a backdraft which puts some smoke in the house. It is just enough that you can't see it but your eyes will let you know.

Tonight, its about 38ºF outside and falling. I stoked up a fire around 6:00pm and enjoyed a beverage with a book near the heat. The cats were happy warm and I was fine.

About 8:30 I threw a log on and noticed it was smoking a bit. So I moved the log to the back to keep the smoke drawing up the flue. Seemed to work well.

But within about 10 min, I smell a bit of smoke. I poke the logs and move things aroun a bit. I even go and crack open a window at the far end of the room. I feel a draft comming in and I watch the smoke roll up the flue.

It all clears out and I shut the window. Back to life and it will probably fine now.

So what gives? Did I let the flue temp drop? I try to keep a good fire burning. Normally, it gets so hot I have a bit of trouble getting near it without gloves on.

It seems to do this every once in a while.

What am I missing?

rock

aboard_epsilon
03-02-2006, 08:07 PM
For heat and fumes to go up the chimney it needs to carry some room air with it.
so the room must be vented .
the fire also consumes air from the room .....this must be replaced from somewhere
some people put a vent in from the ouside with a tube that comes out underneath the grate..so you dont get drafts of air being replaced in the room.

also caution many people have died from having unvented fires burning in houses.

all the best...mark

deltap
03-02-2006, 08:10 PM
Does the fireplace have its own outside air supply? If it worked when a window was opened, maybe the house is too tight. All that volume going up the chimney may create a negative pressure in the room causing draft to be lazy.

railfancwb
03-02-2006, 08:13 PM
Our fireplace seems to draw better when I've kept a bed of ash (about 2") under the grate -- unless I forget to open the damper. Charles

snowman
03-02-2006, 08:28 PM
your chimney may not be tall enough either.

my brother in law's chimney needs another two feet added to it, it doesn't stick above the top of the house high enough.

-Jacob

big-dipper
03-02-2006, 08:38 PM
Lots or air going up the chimney, needs to come from somowhere (like your open window). I don't believe the flue temp is the problem, your house may be too "tight", the fire needs to breathe. I got a leaky house, opening the glass doors on the fireplace, it's almost like there's an air hose balsting the fire! Smoke goes away fine with the doors closed.

HWooldridge
03-02-2006, 09:05 PM
We have the same problem on occasion and you can sometimes see the smoke rolling out the front of the opening. Soon, there is a haze of smoke thoughout the entire house. I have found that good healthy flames help - a big log will smoke unless there is a real good bed of coals under it to keep the flames going. The fire also needs to be against the back of the fireplace so the box heats up and causes it to draw. I made a grate that keeps the coals about 3 inches off the hearth and push the logs well back in the grate.

You might try a couple of things - a lot of small logs will make more flames. Another suggestion is a fireback, which is a cast iron or steel plate that gets warm and radiates into the room plus that heat helps carry smoke away.

rockrat
03-02-2006, 09:07 PM
It's an old house and I always thought that there were enough gaps around to pull air from the outside. If I shut the front door with the back door not latched, it would not move. My buddies house is tight. Do the same there, and the back door will fly open. I know, there are volumetric claculations that could probably explain the difference.

I do have the ability to get under the firebox. I might try and install a quick little air supply from outside and see what happens. Might make all the difference in the world.

Ok, major spelling errors tonight, maybe lack of oxygen. Edit....

Yes, I opened the little window back up and the fire is almost out. I think we are safe tonight. http://bbs.homeshopmachinist.net//biggrin.gif I know how you tool hounds are.
Just waiting for me to give it all up.

Ha ha ha..

[This message has been edited by rockrat (edited 03-02-2006).]

Carl
03-02-2006, 09:54 PM
Some nights our wood stove burns so easy and so hot that I have to close the damper quite soon to keep it from getting so hot that you can't get near it. Other nights it is extremely hard to keep hot. Some nights it is the wind backdrafting it. On some still nights though I always thought it might be the barometric pressure causing even thinner air than we normally have up here at 9500 ft. elevation.

J Tiers
03-03-2006, 12:13 AM
<font face="Verdana, Arial" size="2">Originally posted by rockrat:
Ok, I have just about had it.

I have a nice big fireplace. It is a big wall of rock with a fire box constructed in it. I clean it out every year in the fall before the big cold.

So, I have learned that any outdoor temp over 40ºF results in a bit of a backdraft which puts some smoke in the house. It is just enough that you can't see it but your eyes will let you know.

Tonight, its about 38ºF outside and falling.

But within about 10 min, I smell a bit of smoke. I poke the logs and move things aroun a bit. I even go and crack open a window at the far end of the room. I feel a draft comming in and I watch the smoke roll up the flue.

rock</font>

A fireplace needs a draft.

Natural draft comes from HOT smoke.

A big wall-o-rock tends to cool off the smoke going up, until there is less draft than required to pull well. Any adverse breeze may cause a roll-back into the room.

Opening a window may allow easier air flow, OR it may let in a breeze that pressurizes the room and helps the smoke out.

After a while, when the wall-o-rock gets warm, it will draw well and suck out smoke like crazy (and room air for 3 hours after the fire is out too).

Been there....

darryl
03-03-2006, 02:30 AM
When I was using my woodstove, I found that often I would get smoke in the room. It turns out that my chimney was in a bit of a high pressure area what with the orientation of the house, trees, and the wind. I had a pretty good draft coming down the chimney a lot of the time, so that put a damper (sic) on the use of the stove. If I kept it hot, it wasn't so bad.

railfancwb
03-03-2006, 06:04 AM
One Thanksgiving week at Outer Banks the winds were so high that we couldn't have a fire in the fireplace -- the smoke was forced into the room. Grabbed the burning logs with tongs and threw them out into the rain one by one. The house had a wind speed gauge, which those winds blew off... Charles

bob308
03-03-2006, 08:55 AM
check the hight of the chimney. also the trees are they over hanging or are the in front of it.{on the upwind side}

WLW-19958
03-03-2006, 09:10 AM
Hi There,

How old is the fireplace? Was it built using Count Rumford's principles?

Good Luck!
-Blue Chips-
Webb

rockrat
03-03-2006, 04:06 PM
Ok, good stuff here.

I know that the height of the chimney and its distance to the roof, trees, and other items makes a big difference.

So, what are the ratios and calculations? I need to search, but you all seem to know from experience.

Also, as far as the "Count Rumford's principles" the best I could do is look the subject up in my Foxfire books. I seem to remember a calculation of sorts in there.

rock-

sailor69
03-03-2006, 04:30 PM
I have to tell you all a similar story, I too was having trouble with the draw and smoke would wisp into the room, I did not think too much about it until I walked by the furnace room and smelled burning paint. I opened the doors and the front of the furnace was very hot and a hand on the flue told me it was very cold. The draft of the fireplace was causing a downdraft from the furnace flue. Can you imagine? all that carbon monoxcide coming into the house. We are lucky to still be here. Opening a door restored the proper venting. I use a gas log now with glass doors.

Your Old Dog
03-03-2006, 04:59 PM
as for chimney height, insurance codes call for 2feet higher than anypart of the roof within 10 feet.

However, if it's random, tell the family not to turn on the exhaust fan in the kitchen or bathroom if the house is very tight. That solved our problem. The exhaust fans need air from somewhere and it's easiest place to get it is the chimney.

I'm haveing a similar problem in my shop but it has to do with the direction of the wind. I put a stake out in teh yard with a ribbon on it so I can see the wind direction and when the wind comes from over the barn roof I get a backdraft. Anyother time it seems okay. I'm considering puttina dunce type cap on the chimney to help create uplift as the wind blows past the chimney. It worked good on my old shop.

aboard_epsilon
03-03-2006, 05:29 PM
you have to create a streamlined narrowing of the chimney ..just above the fire.
not too narrow..
this will speed up the velocity of the air ..in that area ..so preventing down drafts.
all the best..mark

[This message has been edited by aboard_epsilon (edited 03-03-2006).]

fahnoe
03-03-2006, 05:55 PM
I don't have a formula to offer, but along with chimney height and its cross sectional area, also consider the area of the fireplace opening relative to the chimney dimensions.

When I bought my house (a 1940's rambler) I thought it was odd that the fireplace in the living room was unused while the one in the basement showed lots of use. As soon as I made a fire in the living room fireplace I knew why it had hardly been used: it smoked. The basement fireplace had the same opening, just a taller chimney, so I figured I could either add height to the chimney (expensive) or reduce the aperature with a set of doors (less expensive). The addition of doors solved my problem: even when they're open, the frame reduces the opening enough to draw properly.

I got lucky with my guess, might work for you too?

--Larry