New Coal Forge - Any luck with 8" chimney pipe and half hood?

[LemonAden]

This is my first post here and I figured I could start my adventure into learning to forge by getting some advice on proper forge ventilation.

I purchased a great coal forge for myself from Centaur Forge.  It came with a half hood and also blower and I have been practicing outdoors while I sort out the location I want to install it in one of my workshops.

This morning I had some contractors out that deal with wood stove installations and to make a long story short they are quoting me between 4-5k to install 14' of pipe through the roof of my unfinished workshop.  The hood that came with my forge was designed for an 8" pipe and the contractors said that this would be sufficient.  Everything I have read is saying to use no less than 10" but 12" is preferred.

I am not going to pay that much money for this install and can do it myself but I want to make sure I get the right size pipe.   The folks at Centaur Forge say that they use 8" pipe in their workshop with this hood and it works great.  I was hoping to get some advice about this so I don't have to redo this project later on. I don't mind a little smoke in the shop as its a farm and I am used to it but I want to make sure I have a great draft so when I am burning fresh coal down it doesn't billow into and fill up my whole shop.  I am including a photo of the forge from Centaur.  This is not the final location where I will be installing it.   Thanks in advance for any into you can give.

[Glenn]

Contractors said 8 inch chimney would be sufficient.  He is a contractor, NOT a blacksmith.  The blacksmith suggests 10" but 12" is preferred for a reason.    

50.265  square inches for an 8 inch chimney.

78.540square inches for an 10 inch chimney or 56% larger than the 8 inch chimney.

113.10square inches for an 12 inch chimney or 125% larger than the 8 inch chimney.

A hood tries to draw smoke and exhaust from the entire room due to the large opening size. A side draft hood only draws smoke and exhaust from the side of the forge due to the small opening size. 

 

Think of it as a production issue. Making the forge with a large opening, small hood to chimney conversion coupling, and a small chimney, is a matter of a lower price for the materials and construction. Lower price means more profit on each sale. 

Since you already have the forge and hood, build a fire and watch the smoke.  If it is working properly, there should be no smoke escape and go into the room.  Play with the hood opening by making it smaller.  Clamp some light metal to the top of the opening.  You most likely will be operating the forge from the side which means you can lower the top of the opening and still see your fire and the metal in the fire.  Next use light metal to close in the sides a bit, reducing the hood opening.  Watch the how the smoke acts and how much smokes escapes into the room.  The lungs were designed to breathe CLEAN air.  Anything else can cause problems, especially when you inhale smoke and airborne debris. 

Once  you have things working, go outside and look at how much smoke and exhaust is coming out of the chimney.  THAT is the amount of air that must be allowed to enter the room to make up what is being up and out of the room by the chimney.  Open a window or door to allow the make up air to enter the room.

[LemonAden]

Thank you Glenn. I tried to explain this to them and they ended up giving me an analogy about how a straw works great when its a smaller diameter but when you go with a wider straw you have to exert more effort to use it.  That didn't work for me.

To confirm what you said, you think I could use a conversion coupling and size up from the 8" hood to either a 10 or 12?  I am pretty amazed at the percentages you mentioned and it makes sense.

[Stash]

Well you still would have an 8" throat that will restrict the flow. If you are marginally handy with sheet metal work, you have another option.
 Cut your hood down so you have a 12" opening, then fab up your adapter to go from there to the 12" flue pipe. You should be able to do it with sheet metal screws and metallic tape if needed. The chimbly guys were probably quoting you for double wall stainless pipe, which is very spendy. You just need single wall galv ducting and watch your detailing where you pass thru the roof. You won't get hot enough to burn off the zinc. For a handy person this should be a simple diy.

Steve

[Glenn]

Go with the hood opening first.  Then make a circle at the 12 inch diameter on the hood. Cut slices from the 8 inch end to the 12 inch mark and fold the slices up and make a crown so you can screw the 12 inch chimney in place into the slices.  This will create a upward funnel and a secure connection.

[mutant]

If you close in the opening around your forge, less cool ambient air will mix with the hot air from the fire and you will get a good draw. I did this with my rivet forge. My pipes go from 8" down to 6" (insulated pipe that passes through a wall) then back to 8". I also have multiple bends and have no problem with draw. 

[George N. M.]

OK, I have to be something of a contarian here.  Bigger is not always better.  This forge was designed for an 8" chimney and I would assume that a certain amount of experience went into that design decision.  The problem with too large a chimney is that the fire may not be heating enough gas to rise in the chimney and stay warm enough to make it out the top.  An oversized chimney will not draw as well.  This is more of a problem in colder climates where the chimney will be cooling faster in cold weather.  

Sometimes you can overcome the cold air plug in the chimney in cold weather by lighting some news paper and letting it start the warm air flowing up.

If it were me, I'd put a section or 2 of 8" stove pipe on the hood and fire it up outside.  If it is drawing well then I would install an 8" chimney in the shop.

Remember, the top of your chimney should be higher than the peak of your roof so that wind blowing over the peak doesn't create turbulence that will result in back pressure down the chimney.

"By hammer and hand all arts do stand." 

[Irondragon Forge ClayWorks]

I have basically the same hood as yours. The hood had an 8in connector like yours. I cut it off and the 10in pipe just sets on the opening then runs up and fits inside a 12 inch double wall, that goes through the roof with a weather head on the top. It draws fine, after I preheat it with a propane torch. If I try building a coal fire when it's cold it will smoke a lot till heated up. I'll get a couple of pictures for ya. I found this one on the computer from inside. The large hood was going to be my only one, but it didn't work at all. The 12 in double wall through the roof rests on it.

[LemonAden]

Thank you all very much.  I am on the fence about it.  It would  be much easier to size up to 12 if the 8 doesn't work out.  I can DIY the install.  Closing up the sides won't work for me as the things I want to make are long (garden stakes and such).  If I do size up I have no problem using some mapp gas to get it warm in winter. I am in RI so it does get cold.  I am curious to see your hood Irondragon.  I won't cut into mine myself but there is a guy who I can take it to locally. 

[JHCC]

35 minutes ago, LemonAden said:

Closing up the sides won't work for me as the things I want to make are long (garden stakes and such)

Removable hood extensions are an option. Just some sheet metal bent into shape with a handle would be fine.

[Glenn]

When you first start an engine (gas or diesel, and particularly small engines like lawn mowers, chain saws etc) you should give them a bit of time to warm up and let the parts to get to know each other.  The same is true for a forge chimney, give it some time to get a draft started. This can be as simple as a couple sheets of burning newspaper, torch, whatever.  Once the draft is established, you can build a small fire, then add fuel to get a larger and working fire.  The fire will then take over and keep the draft going.

You are getting some good ideas from several different directions.  Each has merit in their own right.  The problem then becomes cross linking the ideas that were not meant to be add together.  Take one idea and put it into use to see how it works.  When I suggested clamping sheet metal to close down the opening, it was meant to be temporary until you figured out what worked. 

Then hinge the sheet metal to the hood in case you want to revert back to a larger opening, or just open a section of the opening. 

All this starts with using the forge and hood as is, outside, to see how the original works. Trial run should be for a week (40 hours).   If you are satisfied, then good.  If not, choose ONE idea and put it in gear.  Run the forge with the modification for a week (40 hours) to see if you like it.  Tweeking your set up to please you is a challenge.  Accept the challenge and MAKE it work for YOU.  Please let us know what works best for you, in your location.

[Irondragon Forge ClayWorks]

Boy my memory must be shot. I thought the hood flange I cut off was 8in, but when I looked at it, I hadn't cut it off   and I thought that's not 8in so I took calipers and measured the pipes. The flange I thought I had cut off, been about 30 years since I installed and patched the system is 6in. The pipe it goes into is 8in and the inner pipe of the double wall through the roof is also 8in. I have been planning to reduce the opening of the forge hood for years now but haven't gotten around to it and it works well after pre-heating a little. So basically my stack is 8in. Here are the pictures, hope they help.

Here is an old one of it running and no smoke after the preheat got the draft going. So I don't think you will have a problem with 8in.

[Irondragon Forge ClayWorks]

I decided to close up the opening some and it works better now. Draws a much stronger draft without preheating it.

[LemonAden]

Thank you very much for sharing this! I am still undecided about what size pipe I will go with. I am using my forge by moving it outside while I get comfortable with it and decide where I want to put it and what size pipe I will use.  A local welder said he could convert the hood to fit a 12" flue should I decide to go in that direction.  

I am having some issues with my coal bed getting very wide and burning through coal very fast so I might try containing the coals in a smaller area using some fire bricks or something.  I am only working with small round and square stock right now and would like to keep the are burning thin rather than fill up  my entire fire box to help use less coals.

[JHCC]

You may be putting too much air into the fire, if it's getting bigger and hotter than you need. If you're burning coal, you can also keep the fire contained by wetting the outside perimeter with your sprinkler can.

[LemonAden]

It is getting much hotter than I need! I did try wetting some of the coal but it doesn't take long to burn off.  Even with my electric blower on the lowest setting it gets very hot, very fast.

[JHCC]

Try adding an air gate to restrict the blast or to dump some of the excess air. No point in burning excess fuel or increasing the risk of damaging your workpieces.

Also, consider adding a deadman switch to your blower, so that it's only running when you're standing on it and waiting for the workpiece to heat up. I added one to the blower on my coal forge, and it uses a lot less fuel than previously. Also, since the fire dies down a bit while I'm hammering and therefore has to come back up when I'm not, it lessens the risk of an overheat.

[LemonAden]

Hmmm.  I will look into this. I am not sure where I could put this on my particular blower/forge but I will ask Centaur Forge if they sell anything

[JHCC]

The simplest thing to do would be to rig something to hold your ash dump open slightly, either by pulling up slightly on the handle or by wedging the flap open. The perfect thing would be to attach a length of sash chain to the underside of your forge and put a wire hook on the handle of the ash dump to grab onto the chain. That would allow you to adjust the opening as needed, and you could still dump the ash without worrying about resetting the chain. 

It's also possible to build a gate valve; here's one I made for my old JABOD forge: 

 

[LemonAden]

My ash dump actually has a lever on it and I can control how open or closed it is. It stays put also. You think this would be enough to pull off some of that heat?  Awesome suggestion! Thanks

[JHCC]

That's good! Play with it a bit and let us know how it works out. Remember, fuel doesn't make the fire hot; oxygen makes the fire hot! If a quarter of your air is spilling out the ash dump, then you'll only get 3/4 as much heat.

[LemonAden]

Thanks so much! I will try it out this week

[Glenn]

Fuel does not make the fire hot. Air makes the fire hot.

Dump some of the air going to the fire. 

Make the fire smaller by using bricks, or a circle of metal to contain the fire.

 

[ThomasPowers]

BTW when you say coal are you meaning bituminous, anthracite or charcoal? 

[LemonAden]

Thomas, we are burning coal.  We are working with anthracite only.