[TheForge] forge flues

[Peter Hirst]

OK .  The thimble is the horizontal penetration of the wall.  That is at the 
top of the hood, about 4 feet above the fire.  From there, I have 8 feet of 
8-inch pipe vertically, for a total of abut 12 feet above the fire.    The 
shop is otherwise well ventilated.  S0unds like I may need another four feet 
of pipe at least.  maybe 8?

PGH
----- Original Message ----- 
From: "GRAF" <adveniam at att.net>
To: "Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Wednesday, January 16, 2008 10:52 PM
Subject: Re: [TheForge] forge flues


> 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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