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Forge Hood with Wall Exhauster


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Hi All,

I'm new to the forum and have recently acquired a coal forge (Centaur Forge 30" wide model).  I'm planning on installing the forge indoors, and had a question about using the super sucker side draft hood in conjunction with a wall-mounted exhauster.  I've attached a few sketches to illustrate. 

The exhauster is roughly 3 ft square and is louvered.  If I had to guess at the CFM rating, I'd guess it to be at least 1500 CFM.  It's powerful enough to where it will suck the man door closed in my 1500 sq ft shop unless I crack a window open.

So the question is, will using a side draft hood in combination with the wall exhauster cause any issues?  Will the wall exhauster negate the effects of the side draft hood, making it pretty much useless?  I would think if anything the super sucker would help direct the smoke up to the wall exhauster.

I've also attached a sketch showing two different duct exit details in front of the wall exhauster.  I would guess the 45 degree exit may be more preferable to a cut off duct?

Thanks for the input,
Ben

ForgeExhaust-Overall.JPG

DuctExitDetail.JPG

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3 hours ago, bbecker said:

using the super sucker side draft hood in conjunction with a wall-mounted exhauster

Why? Did the super sucker not move the smoke from the forge into the chimney and out of the work area on its own? 

If you put a 12 inch diameter pipe into a 3 foot by 3 foot opening you will need to close off the excess open area. Otherwise it will draw room air into the open area of the exhauster. 

The chimney top being open will work. If needed add a chinaman's cap, or a high volatility stack which is just a larger pipe around the chimney. Cutting the chimney at an angle as shown only works if the wind blows against the tall side of the cut, W to E in your drawing. If the wind blows E to W in your drawing, it will create a pressure that may kill the draft, or even reverse the draft of the chimney.

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I have not actually set up the hood for the forge yet, I wanted to get some input before I start fabricating the super sucker.  

My concern is that the wall exhaust fan may cause some of the smoke from the forge to bypass the super sucker, which may allow some soot, etc. to end up on equipment in other parts of my shop.

I don't have any issues with the exhauster drawing in room air along with the forge smoke, so long as it doesn't create an issue with the super sucker being bypassed at the forge.  The exhauster is used to ventilate the building for other activities in the shop such as welding, or when I use one of the heat treat furnaces. 

Quote

If the wind blows E to W in your drawing, it will create a pressure that may kill the draft, or even reverse the draft of the chimney.

I'm confused on why the wind would be of concern inside the building?  Maybe I'm missing something.  The view of the drawing is looking at the back wall from the inside of my shop. 

Thanks for talking this through with me.

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Welcome to IFI... I always suggest reading this to get the best out of the forum. READ THIS FIRST

I've also learned that exhausting smoke/fumes into the shop is not a good idea because air currents will spread the smoke even with an exhaust fan. You would be better off to run the super sucker box straight through the wall then run the stack up the outside. Like this.

https://www.iforgeiron.com/topic/23197-bp1048-side-draft-chimney/

 

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If you have the forge going you MUST have a window or door open to allow as much air into the room as you are exhausting out of the room. With just the supersucker in use this is a natural draft moving the forge smoke out.

If you kick on the exhaust hood, you MUST have enough air entering the room to replace the air that both the supersucker AND the exhaust hood move out of the room or the stronger of the drafts will pull any extra air needed from the weaker draft sources.  For instance is you have the exhaust hood running and all the doors and windows closed the only source for exhausted air is the forge chimney and it will move air DOWN to chimney to supply air to the exhaust hood.

For the top of the chimney, think of putting a 90* elbow on the very top of the chimney, and pointed to the East. Any wind blowing from N, S, and W will blow around the 90, create a partial vacuum, and draw smoke up the chimney. If the wind is coming from the east, the 90 will collect the wind and channel it down the chimney.  A cut off portion at the top of the chimney will, to a certain extent, do the same thing.

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Glen,  I'm afraid you are misunderstanding his sketch.  As I see it, he has the top of the duct terminating inside his building.  Bbecker, this is not a good idea.  The sidewall fan will not reliably extract all the exhaust from the duct, and combustible materials may be too close to the  outlet.  Please ensure that the duct from the hood terminates outside your building, typically at least 3' above the roof peak.  Also you likely will need a roof thimble for the penetration.

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I saw the top of the stack terminating inside the exhaust hood and the exhaust hood being a power booster to get the smoke on up and out of the building, I agree that moving the smoke just close to the exhaust hood is not a good idea.  

Thank you for the correction.

 

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12 hours ago, Latticino said:

Glen,  I'm afraid you are misunderstanding his sketch.  As I see it, he has the top of the duct terminating inside his building.  Bbecker, this is not a good idea.  The sidewall fan will not reliably extract all the exhaust from the duct, and combustible materials may be too close to the  outlet.  Please ensure that the duct from the hood terminates outside your building, typically at least 3' above the roof peak.  Also you likely will need a roof thimble for the penetration.

You're correct, the duct was planned to terminate right at the face of the exhaust fan.  I don't really have the option to create another penetration through the wall or roof.

I've attached a picture showing the wall exhauster.

As I do not use the wall exhauster for any other activities in my shop, I have an alternative idea:

Not using or completely removing the wall exhauster, and propping open the louvers permanently.  The current fan cavity could be capped off and sealed, and the flue could pass through the cap and terminate in the fan cavity.  I'd put a draft inducer in-line rated for use with coal stoves, and have it vent through through the sealed wall exhauster opening.  So essentially this would be a sidewall / direct vent chimney which I believe there is no issue with this configuration from a building code / safety standpoint.

I've attached another quick sketch, this time showing the view from the side.

I should also mention there are no doors or windows of any kind of the side of the building where this would vent.  It appears that sidewall / direct venting is up to code here as well.

Thanks guys!

35258181-CCE4-4F4A-A51F-EE68665D1C24.jpeg

ForgeExhaust-Alternate.JPG

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With sufficient exhaust vent duct rise you should not need an inducer fan.  I prefer not to use them as they provide another point of failure in your system.  

Removal of the exhaust fan and capping of the opening with non-combustible material, then centering the new flue in that cap and bringing the outlet all the way to the outside of the building is my recommendation.  You will have to consult your local code for allowable location of termination for chimneys and vents as well as the required separation between the metal flue, method of construction for the flue, and any combustible materials you have in the wall it penetrates.

I strongly recommend you get the local authorities having jurisdiction involved, since installation without their approval may void your insurance (and some of your design ideas appear dangerous, particularly as regards lack of CO extraction and potential fire hazards).  Careful review of the International Mechanical code and NFPA 211 should be made.

11 hours ago, Glenn said:

terminating inside the exhaust hood

Unfortunately there is no secondary exhaust hood, just a sidewall fan...

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On 1/21/2019 at 11:33 AM, Latticino said:

Careful review of the International Mechanical code and NFPA 211 should be made. Unfortunately there is no secondary exhaust hood, just a sidewall fan...

I agree with review of the applicable codes, which I have been working through in parallel with this discussion.  I thought the best route, before contacting the fire chief, was to educate myself prior to talking the application over with him so that I give the best solution for this situation.

I think the alternate design, with the flue at or extending past the face of the building meets NFPA low-heat application.  Single wall, non-galvanized pipe would be suitable.  Obviously the preference would be extending the flue beyond the building, and I think this could be done without too much hassle.  As far as clearance for air intakes, pedestrians, and nearby windows is no issue.

The issue gets a little muddy whether a class A chimney is required.  If that were the case, a double-walled pipe and termination above the roof line would be required.  But, because this isn't a device used as a primary heating source, I'm not sure where that falls exactly.  The local codes, of course, mention to refer to the manufacturer's instructions, which there are none.

I've considered a propane forge, and would use an extraction hood with it for the CO (along with a monitor), but the issue of storing and using a propane cylinder indoors comes up.  OSHA allows storage indoors, but use is another issue.  Storing a tank outside and piping through the wall is not possible, either.  And, I don't like the idea of a bomb in the same area while forging.  Having a concrete-encased acetylene tank is a different story.

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