jester

Chimney/Flue Topper

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Good morning learned iForgeIron peoples...

I need some advice

I am trying to decide on a chimney/flue topper for my forge chimney/flue.

It's a side draw hood design with no fan.

I am looking at making one as I can't seem to source a decent one for the 14 inch flue where I am.

Which one of these two designs do you think would be better? It's mainly to minimise water getting in while keeping a good level of air draft.

 

Thoughts, other suggestions would be appreciated.

 

Gratitude

Chimney.JPG

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first one, level top, make sure the clearance is 1.5x the flue diameter and you will be fine

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Ditto. The one on the left as per Steve's advice.

Frosty The Lucky.

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1.5x the diameter of the flue? OK. Will do. Cheers

I'd read 1x the diameter, but I'll take experienced people knowledge over internet googling!

Thank you gentlemen! I might even put a pic up when I'm done :)

 

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The weather head on our coal forge was easy to make out of thin stainless steel opening is 1.5x the diameter of the flue. It works very well and only lets water in if the wind is blowing hard in the right direction. The pictures you have would let water in from any direction with wind.

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No disrespect intended guys, but 1.5x the diameter? Where is that coming from?

I have a 12" diameter flue, so by your calculation I should have a 18" clearance between the lip of the flue and the weather cap?. I have 4".

20180617_131904.thumb.jpg.47322b94f843ccfa245feb1c17e4dc12.jpg

The reasoning I used was the area of the flue, Pi x (radius squared). Call it 113 square inches. Now the circumference of that flue is Pi x diameter, call it 37.7 inches, divide that into the area 113 and I get 3 inches to have the same area as the pipe. I went 4 inches.

It draws fine.

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20180617_135740.thumb.jpg.639592f567cd93756f6dc7e234f798f2.jpg

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Here's my take on chimney caps, for what that is worth:  I like the TEE fitting, but it is vulnerable to high crosswinds.  With a conventional hat type (what they used to be called doesn't bear repeating), I would consider making the free surface area of the annular gap 1.5 times the crossectional area of the flue (if I can work this simple algebra in my head with no paper: 1.5 x D/4 = gap, i.e. for a 10" flue the gap should be at least 3 3/4", with a bit of extra allowance for the blockage of the supports).

The type I really like takes advantage of the fact that rain and snow rarely fall straight down.  A concentric outer flue, about 2 inches larger in diameter than your flue jackets the actual flue, connected with an annular spider.  It projects a couple of feet above the main flue.  Precipitation hits it and runs down outside the forge flue with no obstruction to draft.

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1 hour ago, Latticino said:

... The type I really like takes advantage of the fact that rain and snow rarely fall straight down.  A concentric outer flue, about 2 inches larger in diameter than your flue jackets the actual flue, connected with an annular spider.  It projects a couple of feet above the main flue.  Precipitation hits it and runs down outside the forge flue with no obstruction to draft.

I considered that design but just couldn't quite get past the "rarely" as rarely ain't never.

And then there's keeping critters and debris out of the flue. 1/4" hardware cloth will work to keep most critters out but what to do about leaves and acorns? Add an inner 'hat' made from hardware cloth so debris slides off?

Decided if I'd have to do conical hat anyway, the traditional solid 'hat' with a vertical ring of hardware cloth under the cap would suffice.

 

The free surface area of the angular gap needing to be 1.5x the cross sectional area of the flue makes more sense to me than 1.5x the diameter. In my case, with a 12" diameter flue, that would be roughly 4.5 inches of height between the lip of the flue and the weather cap.

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