[TheForge] forge flues

[GRAF]

By thimble, you mean where it penetrates the roof? (That flashing isn't 
what I meant by thimble.)
How far is it from the top of the draft hood to the ceiling?
If the top of the forge to the top of the flue is 18-20 feet, that 
should already be working. Adding another two feet would be cheap and 
easy. 8 inch would be a bit skimpy for many folks. 10 inch would be 
better. The amount of lift is a function of height and cross sectional 
dimension with any given fire.
If you are talking 18-20 feet total I might suggest looking at a few 
other things.
Is there someplace for air to enter the shop? A flue can't exhaust once 
it has effectively put the room into a negative pressure.
What kind of forge hood do you have? It seems a properly designed side 
draft works best. That is not what I have , but I have seen some pretty 
impressive fires get sucked right into a side draft.

Mike Graf

Peter Hirst wrote:
> Good points.  Any idea how to calculate sufficient stack size and 
> height? Mine is 8" pipe eight feet high from the thimble and that is 
> inadequate. It draws just ok when the fire is at its hottest, but any 
> thing less and  it spills smoke.   I know I have to add to it, but how 
> much?
>
> pgh
> ----- Original Message ----- From: "GRAF" <adveniam at att.net>
> To: "Sponsored by ABANA" <theforge at mailman.qth.net>
> Sent: Wednesday, January 16, 2008 7:13 PM
> Subject: Re: [TheForge] forge flues
>
>
>> If the flue is set up with enough size and height it will draw like a 
>> banshee with just the room ambient heat to make it work.
>> Kitchen vents in restaurants don't work without fans because they 
>> lack the stack height usually and have filters which restrict the flow.
>>
>> Mike Graf
>>
>>
>> Peter Hirst wrote:
>>> One more thought if you can stand it.  Something to watch out for: 
>>> "positive ventilation".  I had a fire and health inspector come by 
>>> recently who asked me if my forge was positively (power) vented (it 
>>> isnt).   There is no specific standard in this jusridiction for 
>>> forges (as there is for coal stoves, wood stoves and everything 
>>> else), and the concern was that the flue gases  are NOT HOT ENOUGH 
>>> to draw well.  Just like over a stove or gas grill in a commercial 
>>> kitchen, the heat from an open fire mixes with a lot of ambient air 
>>> on its way toward the hood, and is not very hot at all as it travels 
>>> up the flue.  That's why commercial kitchen hoods are POWERFULLY 
>>> positive vented.  With a coal forge,   this usually means that 
>>> significant smoke escapes the hood and the least back pressure stops 
>>> the draft altogether.  How many of us with coal forges have shops as 
>>> smokeless as a commercial kitchen?  Or as any room with a properly 
>>> vented wood stove? Anybody?  I didn't think so.
>>>
>>> So I would soft pedal the idea that the flue gases don't get very 
>>> hot. This could work against you and lead your inspector to conclude 
>>> that the forge should have a high powered exhaust fan.  Which it 
>>> probably should, but why press it?
>>>
>>> pgh
>>>
>>>
>>>
>>>
>>> ----- Original Message ----- From: "john ortery" 
>>> <bluestoneforge at gmail.com>
>>> To: <theforge at mailman.qth.net>
>>> Sent: Monday, January 14, 2008 7:36 PM
>>> Subject: [TheForge] forge flues
>>>
>>>
>>>> Thanks guys,This seems to be the thought all around.  Most of the 
>>>> people I
>>>> talked to seem to think that as long as you have double wall pipe 
>>>> at the
>>>> exit point and are insulated from any combustible material all 
>>>> should be
>>>> well.
>>>> Thanks again Allen Ortery
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