(even more) Draft questions

[spason]

Hi, the box at the top of the screen has been urging me to post for the first time for so long that I thought I would so it will go away. That, and I have questions that I haven't seen asked. Here goes:

I have a buffalo forge and champion blower. I am setting it up in a 16x32 shop with cathedral ceiling. I have had a woodstove (wood/coal) set up in there with a 7" stovepipe that goes about 12' up and then horizontally out, then one more 90 up above the eave. To test out this side draft thing, which seemed impossible to me, I ran the pipe down next to the forge fire. It sucked it up a little bit, but not nearly enough, and only intermittently. I realize that a 7" hole is smaller than most people write about on here, but I was wondering two things:

1.) Do I need bigger pipe than that, or is the problem the 2 bends in the pipe?

2.) Can I run the side draft from the forge up into the woodstove pipe and have them both going at the same time? I live in Maine- very cold!

I just want the optimum way to make sure it is venting ALL the smoke it can.

Thanks, and sorry if this has been answered already.

[dablacksmith]

yes to bolth i think... 7 in is small for a forge hood of any type ... and the heat from the woodstove can be used to help the draw of your forge hood ... you will probably have to go to a bigger pipe for the woodstove also . ime not a expert on this but it should work real well if done right good luck!

[Steve Sells]

in any size flue, each of your 90 degree turns causes restriction as well,

Min 10 inch diameter is recommended.

[jimbob]

coal smoke or suit is about 100 times as heavy as wood smoke thats why the larger diameter pipe...I think you need a bigger pipe

[TASMITH]

Steve has it right..... a 10inch minimum for the forge is best to get a proper draw off it. You could run the 10 inch right up from the forge thentie the 7 inch woodstove into it. I would run the forge pipe straight up... no elbow.

[Glenn]

Some one comment please on the following:

10 inch dia = 78 sq inches
7 inch dia = 38 sq inches

So if you put the 7 in volume into the 10 stack that leaves you only 40 inches of diameter for the coal stove.

This is not an exact correlation as the stronger draft will draw from the weaker draft rather than the stronger heat source. There is also the issue of putting a smaller diameter chimney into a larger chimney (no second chimney involved) and the diameter change reducing the draft. I don't have documentation for this, just real time experience.

If some one has experience or references that will help us understand how this works, please post them.

Edited November 30, 2008 by Glenn
math corrected

[spason]

Thanks for the replies. Much appreciated.

[unkle spike]

I ran into that with my shop, running the pipe straight out the roof was not an option, so I did a hood over the top, with a 10" flue out the side of the shop then up. One of the things that I found to be true is the longer your "Up" pipe the better the draw. In winter I "preheat" my pipe by lighting one sheet of newspaper balled up and toss it in the flue. 10" Pipe is available at most Home Depot store, it is located near the floor registers and dryer pipe. It may be kind of hidden on an upper shelf, so you may need to ask. I also took a piece of 1 inch conduit off one of the elbows on my forge fan and bent it on a pretty long radius and put it in the chimney pointing in the direction of the chimney flow. It does not push a lot of air, but it does help.

[jayco]

Some one comment please on the following:

10 inch dia = 78 sq inches
7 inch dia = 38 sq inches

So if you put the 7 in volume into the 10 stack that leaves you only 40 inches of diameter for the coal stove.

Actually, if I added correctly, you would have 40 sq. inches of diameter for the coal stove.
Two 7 in. pipes (38 sq. in. each) equals 76 sq. in.....very close to the carrying capacity of ONE 10 in. pipe.

The 10 in. pipe would not be restricting the flow, and would not be so large as to allow the gases to cool too quickly in the chimney.

The problem with my above statement is that it would only be true when fires of equal heat were in both the stove and the forge.......which probably wouldn't happen most of the time.

I've never set up a forge and a stove in the same chimney because of all the stories of heard over the years of stove/ fireplace combinations........2 stoves into the same flue, etc........that didn't work (draw) correctly.
I'm not saying it can't be done, merely saying that I would not know how to do it correctly.

I have tried both hood/straight vertical forge flues and am currently using a side draft flue in my new shop.

In all fairness, I must say that the forge 'hood' I had was a makeshift affair with too small piping, but I noticed this........that since the hood was 18 in. above the forge, it didn't really draw unless the forge fire was really hot.
The hood sort of guided the smoke into the pipe, but it didn't really draw.
That 18 in. of open space between the fire and hood let a lot of cooler air into the bottom of the stack, which lessened the draft considerably.

I once lowered the hood to about 12 in. above the fire, and had a great draft.........only problem was, no room to manage the fire or get large pieces in and out.

I really like my new side draft.......10 in. sq. horizontally from the forge for 30 in.....then into 10 in. round for the vertical piping

I think the key difference is that the fire can be quite close to the side draft entrance........keeping cooler 'room 'air from entering the stack.
As I understand it, the more temperature difference between the bottom and the top of the chimney, the better the draft. Edited November 30, 2008 by jayco
mistakes!

[Glenn]

Let me rephrase that.
one 10 inch diameter stack = 78 sq inches
one 7 inch diameter stack = 38 sq inches
two 7 inch diameter stack = 76 sq inches meaning it would fit in a 10 stack by volume.

But 7 inches is about half the volume needed for a coal fire as 10 inch is usually recommended. This means there may be draft problems.

If you add a 7 inch diameter (38) and a 10 inch diameter (78) you need (116) and may be able to use a 12 inch diameter (113) chimney. As you said, both fires being of equal heat, etc, etc.

I made a post a while back on Chimney size and height. Interesting reading and some relation to this thread.

[habu68]

Take a look at Master Hofi's solution to the need for elbows in a side draft forge. This is found in the blueprint section of this site. His use of a side draft box to exit the building beyond the eves and then a strait chimney pipe is the best solution I have seen yet. I would think that an elbow placed out side the building below the eve with a 10" pipe through the wall at forge level would produce the same effect. I would also experiment with restricting the inlet of the pipe at the forge to increase the velocity of the air flow were the smoke is the hottest. Side draft beats a hood 5 to one because of the cold air collected by the hood.

BP1048 Side Draft Chimney

Edited November 30, 2008 by habu68

[jayco]

But 7 inches is about half the volume needed for a coal fire as 10 inch is usually recommended. This means there may be draft problems.

Yes, I agree.......there may be draft problems.

It seems there are a bewildering number of variables involved in how flues operate.

I have to get information into bite-sized chunks so I can wrap my brain around the idea. Maybe we can isolate one problem at a time and arrive at a fact.

I have an old pot-bellied coal stove in my shop with 7 in. stove pipe.......and it draws fine.

I've tried a verticle 7 in. flue with a hood on my forge.....and it didn't work.

Same fuel.......same pipe........what made the difference?

The forge and the stove did not share the same flue.
Stack heights were aprox. the same.

The only answer I can come up with is that little unheated 'room' air manages to get into the coal stove..........while a lot of unheated 'room' air manages to find it's way into the forge. ( maybe 50% by volume)........reducing the draft.

If this is correct, then the 10 in. pipe is necessary to overcome the inertia caused by the cooler air entering the bottom of the stack.

I must stop now before I get a headache( I normally don't use words like inertia):)

[element]

If you took a side draft like hofi's and you got a 10 or 12 inch pipe for the chimney, How high can you go before negative reactions occur? Will it stop drawing or something. A low velocity stack might help or a blower in the chimney. The trees around my shop are 30 feet or more and the smoke tends to twirl around before leaving and most time it comes back down. If it wasn't for the neighbors i wouldn't give a hoot.

[Glenn]

Jayco, where did you get the 50% by volume room air number? Please site a reference as I want to study on this one.

Element, the major concern with height would be gravity and securing the stack from falling over.Taller stacks have greater issues. Watch the (solid fuel) smoke as it is pushed about by the air currents. Many times this will tell you what is happening in your location. Coal fires are a manually adjusted and require good fire maintenance, small hot fires, and some attention in order to get a good burn rate with little smoke.

[jayco]

Jayco, where did you get the 50% by volume room air number? Please site a reference as I want to study on this one.

Glenn, I have to admit that the '50% by volume' was just a guess!
There are no references to site.

You may have helped me discover a fatal flaw in my logic. I observed that the closer the fire is to the flue.....the better the draw. I came to the conclusion that drafting had something to do with allowing or excluding room air from the flue entrance.

This may.....or may not.........be true.

Conclusions from observations are not always accurate.

The ancients observed the sun 'going down', and concluded that the sun was moving.

Oh well, back to the drawing board! :)

[Glenn]

Putting the flue next to the fire is better then putting the flue on the other side of the room and hoping the smoke will find it. The smoke will find the flue on the other side of the room eventually, and the flue will draw, some, but does not work nearly as well as placed near the fire.

Think of the old pot belly stove, the flue was directly on top of the fire, and went straight up. Air came in from the bottom of the stove and when you opened the door to add more fuel, additional air was sucked in through the door, usually with little or no smoke escaping into the room. (if you remembered to open the damper, yes, been there and did that - once upon a time).

Relate it back to the forge. Draft causes suction (draw) and the ratio of the chimney diameter to the size and location of the fire, times the height of the chimney, factored by the temperature and humidity, multiplied by the day of the week plus your attitude results in the bottom line, an enjoyable time at the forge.

Ok maybe writing formulas is not my strong point, but the bottom line is what is important anyway, an enjoyable time at the forge.
.

[element]

I like your formula.

What if your chimny was your flue? I mean take a 12 inch chimny
that goes strait up and put a hole in it at the same hight as your
forge. No bends, one strait pipe. If i am correct i saw something
like this in apprenticemans pics but the pic was to low to see the rest
of the top flue part.

[rthibeau]

In essence, that is what my side draft forge is............A straight run of stovepipe from the forge table up through the roof. The bottom of the stovepipe is cutout to make an archway next to the firepot. My portable forge is setup the same way, but uses barrells instead of stovepipe....works the same.