Jump to content
I Forge Iron

Chimney isn’t drawing well


Recommended Posts

For a while I’ve had a smoke problem in my shop. My shop is too smokey almost all the time. I have a large Centaur Forge hood and an 8” stove pipe. My chimney doesn’t go straight up through the roof as I’ve seen a lot of blacksmith shops do. It goes through the side of my shop then straight up. I really am not sure what to change to make my chimney draw better. It’s like it just can’t draw all the smoke, only some of it. That makes me wonder if I just need a larger stove pipe but I wanted to ask opinions before I just start trying everything. Hope someone has some suggestions. It is much appreciated!

Thank you!!

Merry Christmas

Link to comment
Share on other sites

Is this what they call a half hood?

First, open a door or window to make sure you are getting enough air INTO the shop to make up the amount of air that is going up the chimney.

A 8 inch circle has an area of 50 square inches

A 10 inch circle has an area of 78 square inches

A 12 inch circle has an area of 113 square inches

The suggested chimney size for a solid fuel forge is 10 inch and 12 inch in diameter.

It is suggest that you use 2 ea 45 degree bends instead of a 90 degree bends so there is less restriction in the air flow in the chimney.  A straight chimney is preferred as a horizontal run can kill the draft.  

The top of the chimney should be 4 feet above anything within 10 feet of the chimney.  Check for any air currents or eddies that may be caused by buildings, trees, or other objects that would affect the air flow around the chimney.

As frazer said, we need photographs before we can make any suggestions.

Link to comment
Share on other sites

My forge had a half hood on it when I bought it (not from centaur forge). I took it off since the fire would have been so far from the chimney opening that there would have been a long way for the smoke to travel and cool down before it (hopefully) reached the hood. I think it was intended to be used outdoors or in a very open shop space where if not all the smoke was captured it wasn't a big deal.

8" isn't ideal, but with the right setup it can be made to work. If it is the half hood I'm inclined to think that's the biggest factor, but I'll hold out for pictures before I just start making assumptions.

Link to comment
Share on other sites

Is that a cap on the top?  If so is it choking the flow some?   Also that hood is mainly drawing in cold shop air rather than hot air that will drive a chimney flow.  The 90 deg bends have already been discussed.

Have you looked at the design of the super sucker for  forge use?

I have very little escaping smoke and have done pretty much nothing; but 10" diameter chimney with the inlet close to the fire. (I really need to build a super sucker end now that I have electricity to my arc welder!)

forge1.jpg.5aca21adda72a79bb73e45cdc5e651f8.jpg

 

Link to comment
Share on other sites

Ok, so I think the hood is the main culprit. You could probably modify it somehow to bring the opening down closer to the fire. Hotter flue gasses, more movement of air up the chimney, stronger draw. However, you might be better off leaving it in one piece and building a new hood. I have a modified super sucker design that works extremely well, but it's not the only good design out there. Basically, you want to bring the fire closer to your hood and make it so the path of least resistance for the smoke is up through it, not around into the room. 

Even if you did increase the diameter of the pipe, you would have to replace the hood since that opening is 8". Easiest thing first, start with the hood.

I also can't tell what's going on with the cap. If it's an improvised setup you want to make sure it's not overly restrictive. I've read some specs on caps, but I can't remember them at the moment.

Here's my modified super sucker. If you're keeping the 8" pipe (which you are better off bumping it up to 10") you'll have to further reduce the size of the rectangular opening in front. The cross sectional area of the inlet should be smaller than the cross sectional area of the outlet (chimney).

IMG_3640.jpg.5d1f27c8756e591fdceac7cb5ebd6c3e.jpgIMG_2021-05-28_19-34-41.jpeg.b0f729ed8df2f6ac9ab7a08a0b23bbc2.jpeg

 

Link to comment
Share on other sites

I think between those updates you'll be in pretty good shape. One other thing I'll add... As long as you're working on it now would be a good time to rethink that support system you have there. You're asking a lot from that one little strap and I would hate to see the whole thing come down suddenly and unexpectedly. They make telescoping roof brackets for pipe of various diameters as well as standoffs you can use to mount the pipe to the exterior wall.

12'' DuraTech Extended Roof Bracket - 12DT-XRB 6'' DuraPlus Adjustable Wall Strap - 6DP-AWS

Link to comment
Share on other sites

Suggestions for minimum of space between the chimney and cap is 1-1/2 to 2 times the diameter of the chimney.  Draft can be improved by an inverted cone under the cap (with the 1-1/2 to 2 diameters stand off).  This helps divert the air flow out and away from the cap. 

There is a thing called a high velocity chimney rain cap which is a larger pipe placed over the existing chimney.  I do not recall the suggested increase in diameter and length of the outside pipe.  The idea is to allow the chimney to produce draft and the outside pipe to act as a booster, sucking and introducing additional air into the draft.  By the design it keeps rain from getting inside the inner pipe.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...