New chimney, downdraft or poor draw

[Nick]

I just finished my new forge chimney, and the first couple days using it I didn't have any trouble with it, really.  But today it was hardly drawing at all, almost all the smoke was coming out the side, mostly to the left and the opposite side from me. 

 

The chimney is 12" diameter, 20' in length straight up and the top is about 4.5' above the ridge.  There are no obstacles within 10'.  The rain cap is 12" above the top.  The hood is overhead, the bottom edge is 14" from the firepot.

 

What really gets me is that it was working fine.  There was a very little bit of smoke escaping every now and then, but hardly any.  I preheated the flue with a piece of burning newspaper in each instance.  I walked around the workshop with a piece of smoldering newspaper looking for drafts and couldn't find any that would account for the apparent draft pushing the smoke out from under the hood.  I stuck my hand up the hood and I think I felt a cold draft.  The direction of the smoke could have been from a downdraft, though there was still some going up the chimney. 

 

So what exactly did I do wrong?  Is it the weather?  Chimney too big?  Hood not big enough?

 

The picture is from the first fire with the new hood, heating up a big ladel, not doing any smithing.

[MLMartin]

How was the weather today? I use a "super sucker" 12" and when the wind outside starts blowing real hard sometimes I will get a good puff of smoke pushed back down the pipe and into the shop.

Also shine a light up the chimney, I guess its possible something is in there.

 

As a side note I have been thinking about climbing on top of my shop and painting the top of the chimney exposed to the sun black. I would hope this would cause the pipe to heat up faster and cause better updraft.

[Nick]

Rainy (the other two days weren't), but only a breeze, no real wind.  I'll check up the chimney and see if something's in there.

 

Would it be worth trying a smoke shelf in the pipe just above the hood?  Or would it not be effective since it's a hood and stack instead of an enclosed box?

[Nick]

I just checked, there's nothing in there.  I also put some burning newspaper in the hood and shined the light up the chimney. All the smoke went straight up, I couldn't see any evidence for a downdraft, so that's probably not it.  I still can't figure out where a strong enough crossdraft could be coming from, though.  My shop isn't sealed tight, it's a barn and there's a bit of space between the boards.

 

One thing is that the ceiling isn't there at the moment, it was just boards and sheetmetal over the rafters that I moved over to put in the chimney and haven't put back yet.  Maybe that's effecting it, but it doesn't account for when it was working fine.

 

This bad Paint drawing shows about how the smoke is behaving.  I could fabricate a back to go on the hood to the forge, I guess, but I'd rather not.

[Frosty]

That's a head scratcher Nick. A cold draft coming down the stack on a rainy day. Did you try re-priming it with burning news paper? Your forge is putting out enough flame it should be plenty to keep the flue hot but if the heat isn't going up it. 

 

If it happens very often even with a re-prime with the forge going I'd be tempted to maybe make a little pipe thing to sit directly on the burning coal and aim up the stack. Nothing heavy or large, say 3" dia. exhaust pipe just long enough to extend up into the hood a bit. Literally blow hot smoke up the stack to get it drawing.

 

I wish I had something solid to say, those are just ideas.

 

Frosty The Lucky.

[Fe-Wood]

It sounds like you might be experiencing a temperature inversion. The temperature in your work space might be colder than the outside temp. If this is the case, warm air from outside will flow down the pipe because of the cold air in the shop. I just built a 12" flue for my forge and have a hood on the forge that allows more air to come into the flue than the 12" will carry. My solution for this was to use a short section of 10" pipe right off the hood then step up to a 12" that continues up to the roof. My flue goes out a window before going to the roof. This system has worked flawlessly for me even on stormy and windy days. The expansion of the flue after the initial opening insures that as the flue get hot I will have a draw no matter what. 

[Nick]

I'll give that a try tomorrow, Frosty.  I'm going to close the ceiling tomorrow and stop up some of the spaces and holes in the walls, to reduce any crossdrafts that may be there. 

 

Fe-Wood, as a matter of fact I think it was about 42F in my shop while it was around 50F outside.  I might try putting in a piece of 10" (given my setup, maybe a piece with a flange put up inside the existing 12"?), but I was wondering if maybe I have too much chimney.  At 20', being single walled, I'm guessing it's not warming up at the top.  It was warmer on the days it was working, but the snow is going to fly soon.

 

Frosty, is your chimney single walled?

[Glenn]

My chimney is 24 inch diameter. The fire puts out a lot of heat and drives the smoke up and out. On heavy weather days I have seen the smoke struggle to just clear the top of the chimney and then fall back and be sucked into the fire. (this is on the outside forge). The smoke will crawl along the ground and sink into the bottom land below the forge.  Good days the smoke will clear the top of the chimney and another 20-30 feet before turning into the prevailing wind or breeze.

 

You have two problems that I would address. Pull the chimney cap off the top of the chimney. This acts as an obstruction to the smoke. As I recall it is suppose to be 1.5 times the diameter of the chimney above the chimney. So for a 12 inch chimney it would have 18 inches of clear space between the top of the chimney and the bottom of the cap.

 

The second problem would be the height of the hood above the fire. You can put a skirt on the bottom of the hood to extend it say 6 inches toward the fire. Also try 9 and 12 inches, and clear on down to 6 inches or so above the fire. This would restrict the room air from being sucked up the chimney and increase the amount of heat from the fire and the smoke that is sucked up the chimney. You could make it with a 45* or 90* cut out on the working side so you can still see the fire and get full use from the forge. If you make it attachable with small s hooks you could put it on or take ti off as needed.  Call it a hood skirt for lack of a better term.

 

Go outside on heavy weather days and look at the top of the chimney and watch what the smoke is doing. Compare this to the smoke on a good day and it should give you an idea as to what the problem might be..

 

Overall I think it is weather related.

[Frosty]

The stack on our wood stove is triple wall but I've been burning wood for almost 40 years now and messed with lots of stoves and stacks. I'm not a frequent solid fuel forge guy but what experience I have tells me a coal forge puts out less exhaust heat than wood so draw is more of an issue.

 

I'm thinking inversion sounds the most likely now.

 

Frosty The Lucky.

[Nick]

Glenn, the chimney pipe is supported by a 2-piece collar suspended from the rafters, so I can loosen the collar and adjust the height of he hood by moving the whole chimney down and still have 4' of clearance (I have to go up to take off the braces on the outside, but if I have to take the raincap off anyway I'll need to drop the chimney down so I can reach it.) 

 

Thanks for the help, gents.  Might take a few days, I'll need my brother to help out moving things around.

[Frosty]

To try a lower hood just make a skirt around it. That way you can see if it helps without rearranging the whole set up. Then again, if you installed the stack so it's easy enough to move that's less an issue.

 

Frosty The Lucky.

[Fe-Wood]

Nick,

The way I did mine was to use 10"coming off the hood, turn 90 deg. with a 10" elbow and place the 10" to 12" bell reducer on a short section of 12" then I used another 12" 90 deg. elbow. That got me going verticle up the outside wall. I think I am about 8' of pipe after that. The more important rule for chimney pipe is to be 2' taller than any obstruction with-in 10' of the pipe. In my case, I come on the peak of the roof and my opening is 2' taller. I didn't get a rain cap on it yet and we have had several major downpours and its been fine.

[ThorsHammer82]

sounds like the smoke is cooling by the time it reaches the top of the pipe and wanting to fall back down. but the heat from the fire is preventing it from going down and the cooled smoke is preventing more smoke from going up the pipe. You said on the sunny day it worked fine, because the top of the stack was warm from the sunlight, but when it was cloudy it was cool and thus cooling the smoke as it rises up the stack. painting the top part black may help on cold days. Other than that I don't know enough to comment on. Frosty's suggestion of a blower sending hot air up the pipe sounds promising. You could also put in a fan at the top that you could turn on on cold days to pull the draft for you. but that my cause problems on warm days.

[Woodsydad]

Try opening a door or window a crack. When i worked for a chimneysweep, we frequently had people who experienced that type of problem but, as soon as we told them to crack a window the problem went away after the stove or fireplace ran for a bit and they could shut the window but not always. Usually the wordt problems were in basements because pressure differences would cause a vacuum in the house. For those cases we would install a permanent outside air supply. So you might try a drier vent or similar and run it to near the forge.

[Nick]

I think I have enough black paint to try that.  I'll lower the whole thing a bit when I get a chance, a fan I'll see if I can find something.  There's no need to open a door or window, in fact the workshop's a bit more open than I'd like, though it's closed enough that there isn't much draft. 

 

It's been fine the last couple days, some occasional spill but not much.  The draw seems to be inconsistent, but when it's drawing well it'll suck up a whole column of green coal smoke, and even some of the steam from my quench tank.  I left a space around the chimney in the ceiling, and any spill goes up through that (and other spaces, too) into the roof and out through the wall, so even with the occasional escape the shop isn't smoky.

[ThorsHammer82]

I'd give the turbine top from stuart's thread a try. unless it's high temp paint you run the risk of it just peeling off once you get the chimney hot.

[MLMartin]

If your chimney is getting hot enough to peel paint 10 or 20 feet in the air you have some real problems.

[ThorsHammer82]

The fire place in my family's cabin has a metal chimnet that is 18"x24". It goes through the second floor before exiting the roof some 20+' total length and even the high temp paint peels on that on a regular basis. We can get the cabin cooking though. 80+ degrees when it's sub freezing outside. and it's a log cabin that was built in the EARLY 1900's. we did a repair on the addition and found news papers that were used as insulation announcing that Lindberg had crossed the Atlantic.

 

yes I understand thatthe size of a forge fire is drastically smaller than the 3'x5' fire place at the cabin but it's still a concern that should be mentioned.

[MLMartin]

WOW sounds like a lot of heat escaping the building!

 

My 12 duct is straight up and has a simple super sucker side draft hood on the bottom. It is 15 feet long. Even after forging 1 1/2" Square all day in the Georgia summer the duct at the top was not so hot that I could not handle it. Very hot yes, but I could grab it for a moment and slide it around without burning myself.

[ThorsHammer82]

WOW sounds like a lot of heat escaping the building!

 

My 12 duct is straight up and has a simple super sucker side draft hood on the bottom. It is 15 feet long. Even after forging 1 1/2" Square all day in the Georgia summer the duct at the top was not so hot that I could not handle it. Very hot yes, but I could grab it for a moment and slide it around without burning myself.

I assume you're referencing the size of the chimney in the cabin, and to that I just point out the usefulness of a damper.

[MLMartin]

I was not thinking about the size of the chimney in any way, just that you are saying that the paint is peeling all the way at the top. 

[ThorsHammer82]

yeah, we've had some pretty crazy fires in that beast at times. but usually when we're cranking it to warm the place up when no one's been there for a week+ is when the fire is the biggest. But the fire is going pretty much full bore for 2-3 days straight on average if not longer even in the summer. so the heat has plenty of time to work it's way up the chimney.