(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.

[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]

Originally Posted by 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.

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.

[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.