Flue too small?

[AdamG]

Hi guys,

 

I live in Calgary and am about to move into a new house why an awesome garage, but it brings up a few questions.  First, I am thinking of building a hood to gather the combusted gasses from both a charcoal brake drum forge and a propane forge. The guy at a bbq shop is suggesting an 8" double walled chimney.  The good thing is that it will be pretty close to code (wall penetration and 6 ft above the gutter, about the same height as the peak).  He says that because I am not burning wood in a direct vent system a hood is ok.

However, on this forum people keep saying 8" might not be enough.  I'll post some pics for context.  Any informed opinions?

Btw, the garage is in a lower area with a hill behind it (fairly protected) and I bought a cheap window fan that I can use to pressurize the garage a bit.

The forging area will be tucked on the left side of that garage door, right next to the cinder block wall.  There is virtually no overhang (no soffit) as you can see in the pictures.

Still looks like over a grand just for the parts, and I need to get a hood custom made by a local sheet metal place.

[Mark Ling]

If you can get ten inch than do it. also look into making a super sucker SIDE DRAFT forge. I use one, and it was simple to make and actually creates a draft, unlike the hood that you were planning on making. It wont create a draft, and so it wont get the smoke out as well.

                                                                                                     Littleblacksmith

[AdamG]

Thanks, I figured that would be the response.  

Has anyone tried to vent two forges like this before?  How do gas forges typically get vented?  I have only seen them operate with big doors open, but I would like to keep the garage door closed for heat and noise.

[Steve Sells]

You really need to read more before you kill yourself.  Now before you get upset, know that I am not picking on you . So far you posted you read something about flue sizes but want to ignore that and do a small one instead, then you say you want to run a forge with no fresh air coming in.  If you do not understand the basics if fire,  you can and will kill yourself using either of your forges in a closed garage.

So rather than yell at me,  either read as I suggest, or go try it and if you survive, then tell us about your trip the ER.

 

[JHCC]

31 minutes ago, Steve Sells said:

[...] then you say you want to run a forge with no fresh air coming in. [...]

I think AdamG is asking if a smaller flue with a fan pulling fresh air in through the window (and therefore pushing more air and smoke out through the flue) would work. I don't know if it would, but I think you may have misunderstood his question. Or I did. 

[AdamG]

JHCC, you've got my intention.  I planned to slightly pressurize the room by blowing air in to help create the right draft conditions.

It seems to be harder to find 10-12" chimney so I was asking about 8".  I'll keep looking.

I understand the best way for a single coal/charcoal forge is a side draft and wide chimney.  I was trying to find ways to accommodate both the charcoal and gas forges from the beginning so I don't ace the charcoal one but totally ignore the monoxide from the gas one.

[Steve Sells]

He is the one that said he wanted to keep it closed up for heat and noise, an open widow does not do that.  There is a major risk to people in the house as well.   Forges and fire need air, the black smith needs air and the flue needs make up air,  also there is the pinned post at the top explaining flues and size things.  good luck

[JHCC]

5 minutes ago, Steve Sells said:

He is the one that said he wanted to keep it closed up for heat and noise, an open widow does not do that. 

True, but he did also say:

11 hours ago, AdamG said:

I bought a cheap window fan that I can use to pressurize the garage a bit.

 

[AdamG]

Thanks, JHCC.  

What I said was that I was having a hard time sourcing chimney larger than 8" and wondering if a fan blowing air in from the open window would make up for it enough to work.  I want it to meet code, even if it's not hooked up to a wood stove.  The neighbours across the alley from the garage door look down from the slope into the garage from their yards/decks and are unlikely to enjoy the hammering noise, so a closed garage door sounds like a good idea.

Ideally, the solution would accommodate both forges, but even one that works would be great.  

The stickies in this section have been helpful, and I have read them.  I'm hoping not to reinvent the wheel.

[Steve Sells]

I have already mentioned how you can use two 6 inch pipes to make into one 12 inch pipe in the pinned thread up top  so I guess I just am not able to understand anything you are posting today...... I really was trying to point you in correct direction but then if he read that already why ask how to make larger flue diameters?

[Mark Ling]

Or you could just use one forge at a time. Just have the forges on wheels so that you can roll them underneath and away from the hood. 

                                                                                                Littleblacksmith 

[arkie]

For the past few years, I've been trying to get by with an 8" flue pipe and honestly, it doesn't draw worth a cr*p!  Just finished making a new hood and going with 12" pipe this time.  Save some time and frustration and go with 12".

[AdamG]

Thanks, all.  I see that I can get simple black pipe and click them together, and I found a sheet metal place that can do up cheap 26 ga galvanized pipe if that's heavy enough.  

It seems that this should be an off-the-shelf product though.  Anyone know a product by name that has wall penetrations and attic/roof bits etc?  And that's affordable?!

[arkie]

I think that you'll find that the 26 ga. stuff will not last long right above the fire.  Maybe consider a heavier pipe gauge.  My plain old 8" black stove pipe is just about rusted and burnt through after maybe 2 yrs.  Go a little heavier and you may only have to do it once.

[AdamG]

Agreed.  Maybe I'll gather the materials and try to build a super sucker, or see if the same shop has any heavier sheet/plate to make one up from the sketch in the stickies.  

What this won't do is vent the gas forge though.  I called a local blacksmith shop and they just open the doors.  Fine if it's an open shed, but I don't really like that plan in a garage with the door closed.

BTW, does anyone know if the wall thimble would still need clearance between the pipe and wall, given that it's cinder block rather than combustible material?  I think they can make something up.  Just seems redundant if the wall is 'cinder' rated.  

It's also going to get darn cold in there in the winter with a 12" heat escape.  I'll try to get a damper for sure.

[Charles R. Stevens]

I think code would still want some stand off from the cinder block. But a 16" square hole cut grew the wall gives plenty of clearance. That would allow a Hofi style side draft threw the wall. Something on the line of a kitchen range hood would handle a small gasser as a separate workstation. 

[AdamG]

Getting closer.  Got a quote for proper stainless insulated chimney and a mild steel hood.  About $1200 before tax, permit etc.  That's 10".  

Im wondering a couple of things.  One: to meet code in a garage the combustion zone has to be 24" above the floor and made of non-combustible material.  Might do that.  Two: I'm hoping one hood will work for either charcoal or gas forges.  Any comments?  Unfortunately the super sucker doesn't look like it will work for gas, and I use both.

 

 

[Charles R. Stevens]

I would think "combustion zone" would refer to the forge fire, as most  solid fuel forges are anvil hight (30-32") I would think that is not an issue. 

[jimmy seale]

put in a sweep T and a blower.....

[FoxFire]

Just a quick thought, if your neighbors are close enough to see in your garage and possibly be bothered by the noise you might want to think about a gas forge. While we all love the smoke they may not...

[AdamG]

All up and running.  10" a-vent.

 

[gote]

It seems that many forget that blowing and sucking are two very different things. If you blow through a pipe you get a stream of air that can be directed. If you suck through the same pipe you get air or whatever coming from all directions. If you have a biggish hood above the fire it will suck air from everywhere in the room. Thus the hot gases from the fire are very diluted and thus not very hot and thus not creating much of a draft.

The side sucker works so well because the opening is very close to the fire so what goes through it is hotter creating more of a draft. Compared to the typical hood the opening is also smallish so there is not so much room air sucked in as in an old fashioned hood.

I am not using sidesucker because I want the forge away from the (wooden) wall, because I want the forge free in all direction and because I use forced draft anyway. BUT I have the hood as low as is practical and relatively small and with more than 45° slope so I catch the smoke as undiluted as possible. You want your exhaust to take out the smoke not the air in the room. A big hood works like a funnel on a vacuum cleaner would. That it works at all is of course because the smoke is hot and goes upward.

I would be very worried about the hood in the picture since it is so flattish especially in the left part. There is no slope to take the smoke to the opening of the flue. Steve makes a very good point. This can be risky business. A forge can easily create a lot of CO which is a deadly gas. Since this is the result of incomplete combustion of the carbon in the fuel, the temperature is lower and it is more likely to escape from the hood. Please get yourself a CO-monitoring device before attempting to forge with this setup.