[TheForge] flues -- Shot Gun Top

[David E. Smucker]

Rain never falls straight down -- or at least a heavy rain never falls 
straight down -- always at some angle to the vertical.  This depends on the 
amount of wind.  Because of this it is possible to put a somewhat larger 
section of pipe / flue at the top of your stack so that it has a small 
clearance at the bottom to let water drain out above where the main flue 
enters your roof.  In my case with a 15 inch flue I have an extension on the 
flue that is about 15 and 1/2 inches in dia. and extends 60 inches above the 
top of the flue itself.  It is shimmed out from the main flue by 1/4 inch 
shims so that there is a 1/4 gap all around the main flue.  The two section 
over lap by 12 inches.  Rain falling (or snow) at an angle hits the side of 
this "shot gun" extension  and run down the inside of the extension and 
drains out in the gap between the two flue sections.  Rule of thumb is that 
the extension above the main flue should be 4 times the diameter of the main 
flue. In my case 4 x 15 equals 60 inches.

This is a common industrial design today, but not really new -- some clay 
chimney pots in England were made this way with a ridge (rim) on the inside 
and drain holes.

Does it work?  Not perfect, in a light rain I do sometimes get a little 
water down the flue but less than I get from condensation when starting a 
fire with a cold stack.  But I have very very good draft.  Keep in mind that 
the common cone hat at the top of the stack is a very bad draft waster --  
most studies say it cut the draft by about 50 %.  (Depends on how much 
separation between the cone and the flue.  My biggest surprise is that it 
works best with heavy rain, heavy rain seems to always come with more wind, 
and therefore more angle to the vertical.

I don't have spark screen, but I do have a 20 foot stack (without the shot 
gun extension) and I am burning coal, not charcoal.  (Charcoal and some 
woods are bad for carrying glowing embers up the stack.)

I was going to do an article on this type of stack for the AACB Newsletter, 
but I retired (burned out) before I got it done.  May do one in the future, 
as it is getting time to do some more articles.

Dave Smucker
Brasstown, NC

----- Original Message ----- 
From: "GRAF" <adveniam at att.net>
To: "Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Thursday, January 17, 2008 5:36 PM
Subject: Re: [TheForge] flues


>
> davesmucker at hotmail.com wrote:
>
> "culvert with a shot gun top."
>
> I think I know what you mean. but please explain anyway.
>
> Mike Graf
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