Because I have a low ceiling over my gas forge I built a 3' x 4' sheet metal hood with a 3" lower lip. I then added a double wall chimney pipe.

It contains the heat and I no longer worry about the ceiling catching fire.

It doesn't want to draw properly.

Googling around seemed to indicate an in-pipe fan for a zillion bucks.

Anyone cure a similar problem in your shop?

Thanks-

Dave

 

 

Using an old metal exuast fan thats like the one you'd use over a stove? it would draw smoke and not burn up like a plastic one, or better yet fabricate a old boat propellar with a 1hp motor with speed control?

I've got a big exhaust fan that I have no use for.  It was working when I moved it in '05.  It's yours if/when you're in the area.  How about a bathroom vent fan?

 

Geoff

You could put a positive pressure fan in the garage rather than exhaust. I don't know how well that'd work in winter, how cold does it get in Tenino?

 

Frosty The Lucky.

I usually run my forge at about 2100. The forge is about 18 inches below the lip of hood. The mouth of the forge sits directly under the pipe so anything mounted right there would have to withstand the dragon's breath. I have an air dam just outside the forge door that blows most of the dragon's breath almost straight up.I think any kind of general use fan would bite the bullet pretty quickly.

It doesn't get 'cold' here, much, Frosty. I was fetched up in Juneau so I know the difference :) If the forge is lit I have the shop standing open for air, year-round. My shop is about 20 x 30 but carbon monoxide is a concern.

I was toying with the idea of a fan blowing into a tube that crossed the stove pipe near the top hoping that would draw better. Been thinking on this problem for more than a year.

After the forge is up to temp, if I light a loosely wadded full sheet of newspaper and hold it right under the pipe it will draw like crazy until the paper is burned up, then back to poor draw again.

 

Geoff, i would take you up n your offer, maybe, would not be so worried about ruining a free fan, but you  have not put your location in your post info.

 

Still pondering in Tenino,

Dave

Sorry, confused by multiple forum traffic.  I live in Duvall, east of Seattle, up in the hills a bit.  The fan is a great big beast, three speed, from a large furnace.  It's yours if you want to come get it.  I used it in a much smaller shop to avoid CO poisoning, the current shop is so leaky that it's not a problem.

 

Geoff

Hi Dave.  Were you by any chance at last year's NWBA fall conference?  Bob Kramer has a fan under his forge that blows air straight up in front of the opening that he says does a good job keeping separating the hot innards of the forge from a cooler outside atmosphere.  (If I remember correctly, he compared it to the blast of warm air that greets many PNWers as they enter grocery stores in the wintertime.)  That may blow the heat into the hood/chimney effectively...

all those numbers, but I dont see where ya told us the diameter of the flue? to small a flu causes problems no matter the hood size.

Geoff that is a far piece from where we live and my truck gets 6 MPG. Don't get up your way that I ever remember but I appreciate the offer.

 

BillyO I was there this year and last, learned a lot and forgot much more:) I have that same setup on my forges as well.

 

Steve the pipe is 8 inch inside.

 

Still pondering in Tenino

 

Dave

How high is the outlet of your pipe in relation to your roof/surrounding obstructions? You can increase draft by making it taller.

Years ago I vaguely remember reading an article or book that talked about brick chimney design. There was a ratio of diameter to height that tended to draw better than others. The book was written when they used coal to generate steam in most factories and a good draft was critical to the proper generation of steam or heat. I've hunted thru the books I can locate on chimneys but I haven't located the reference I'm looking for.

 

Best I could come up with was this tonight. I'm betting a bit more time with google might locate what I was looking for.

 

http://www.gutenberg.org/files/22657/22657-h/chapters/chimneys.html

On my coal forge I dropped a second pipe down to about 12" above the fire pot, draws great, much better than before.

You said your stack draws well with paper right under the pipe. Drop another piece of pipe down as close as you can get. It doesn't have to be solidly attached, mine is hanging on a wire with about an inch gap between it and the main pipe. I think that is why side drafts can work well, the heat is very close to the chimney opening.

BillyO I was there this year and last, learned a lot and forgot much more:) I have that same setup on my forges as well.

 

Re-reading, I see the mention of the air-dam, sorry.

Put a "Y" in your flue pipe with the "y" part pointed down connect a fan & vent pipe where the air flow will blow up the flue. it should draw hot gases like a siphon up the flue. i'm not a rocket scientist, but that should work.

Lots of really good stuff here.

I found a couple places that sell 'Draft Inducers' that fit on the top of the pipe for under $200. I will do some more searching on Google but will probably go with one of those. They make several other kinds that require cutting into the pipe [double wall stainless] but because the pipe is located in such a cramped area I have to lie on my belly to get to it I think the one on the top of the pipe, outside the building with lots of room to work will suit me best. This piece replaces the chimney cap and can be wired to a simple switch to operate. It is rated at 1,000 degrees so should work and last and comes with a lifetime guarantee.

Another type, with NO moving parts is installed in the pipe and generates heat in the pipe to induce draw.

I have WAY too many ideas and pictures in my head to work on it any more right now. When I get the challenge solved I will post some pics of the actual setup, working properly and the total cost and time involved.

Thanks again for all the input, experience and ideas :)

Dave

Seems like I'm brainstorming exhaust stacks today, okay. My suggestion of a positive pressure fan is to increase the pressure in your shop and so increasing the volume of air and exhaust gasses moving through the stack. I'm thinking 8" is too small for a natural draw great enough for a gas forge.

 

The air curtain is a good feature, I've been thinking about putting one on my forge. Perhaps if the air dam source was from outside the shop it'd allow the stack to draw better? I don't know if I'd like doing that myself, it'd mean some kind of duct work on the floor. It would in my shop anyway, my forge is center floor so any ducting would be a trip hazard.

 

I've been planning on ducting exhaust to the gozintas and may have to bite the bullet and just do it this winter. That might require refabbing the forge entirely and I don't know if I want the hassle.

 

Frosty The Lucky.

Are you going to the NWKC show in Kelso, http://nwkc.org/news_page.html?  I've got a table and I could throw the fan in the car and swap it off in the parking lot.

 

Just a thought

 

Geoff

Frosty is right. an 8" flue is just too small. It needs to be 12". Also what the chimney does on the outside relates to the draft, too. It needs to be higher than the peak by 24" if within 10 feet of the ridge to get a good draft. Also if you have a cap over it it needs to be as high as the diameter of the pipe, not 2" as they come from the store.

 

The air curtain is a great and simple tool. I just used flexable pipe and squished it down to make a rectangular shape. Stops the heat from getting to you and from running up the bar..I love it!

 

             

I did not even see the air curtain Mr McDaniel,  I looked straight at that bick anvil! It looks very nice.

 

 

But yes a air curtain is a handy thing for large gas forges 

How does air get into your shop?  If it leaks Like a sieve then no problem.  If on the other hand it is virtually air tight then try it with a window or a door open.  All that hot air you want to go up the chimney  has got to come from some where.

 

Bob

How does air get into your shop?  If it leaks Like a sieve then no problem.  If on the other hand it is virtually air tight then try it with a window or a door open.  All that hot air you want to go up the chimney  has got to come from some where.

 

Bob

I just noticed the bit about the door being open.   sorry about that.