#1  
Old 08-19-2010, 05:44 AM
Serf
 
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Location: Solihull, West Midlands, England.
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Default 4" insulated flue?!?!?

Hi All,

I'm building a small WFO which will have an internal diameter of approx 28"...would a insulated flue with inner diameter being 4" be suitable or should I get a minimum of 6 inches? Also, if I were to utilise the flue by removing the inner section, i.e. making it 6" then would this be suitable?

Many thanks

Rob
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  #2  
Old 08-19-2010, 09:25 AM
Nic The Landscaper's Avatar
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Location: Newberg, Oregon
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Default Re: 4" insulated flue?!?!?

What is the size of the opening? Your Flue should be about 10% of the area of your opening.

So lets say your opening is 12" by 18" for a total of 216 Square inches. 10% of that is 21.6 square inches. Take the square root of 21.6 and you get 4.65" by 4.65" for a square flue dimension.

I you want a round flue you still need 21.6 sq in. Divide 21.6 by 3.14 (Pi) and then take the square root of that (SQ rt of 6.88 = 2.62) and this is the radius of your flue which means you need a minimum diameter flue of 5.25 inches.

So in conclusion you really need a 6" flue, be it square or round, insulated flues are nice but not necessary and that is a judgment you need to make on your own.
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  #3  
Old 08-19-2010, 01:13 PM
david s's Avatar
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Location: Townsville, Nth Queensland,Australia
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Default Re: 4" insulated flue?!?!?

A bloke up our way makes ovens with a 4" flue and all of them smoke up the front of the exterior arch quite badly. My oven is 21" and has a 5" flue which is perfectly adequate. It is quite important to funnel the smoke up to the bottom of the flue otherwise it will smoke up a fair bit at start up. The 4" flue will draw OK once the oven is hot, but not too well at start up.
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Old 08-19-2010, 03:11 PM
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Default Re: 4" insulated flue?!?!?

Thanks for the replies, what you have both said makes perfect sense to me. As the flue is a freebie I will take the middle 4" section out and use the 6" outer flue which should be ideal. The opening is about what has been mentioned but I have also obtained an aga door with built in thermometer for 99p on fleabay so I will use this also. I also appreciate the comments about making the base of the flue high enough and angled enough to draw the smoke out.

I must also say this is a great forum with some really useful information, I just hope my WFO works out to plan!

Many thanks

Rob
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Old 08-19-2010, 08:18 PM
david s's Avatar
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Default Re: 4" insulated flue?!?!?

Rob,
Be careful, the inner flue is usually stainless steel while the outer flue is usually only galvanized, which will burn off and corrode quickly if it gets too hot. Better to do stainless now than have to replace it down the track.
Dave
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Old 08-20-2010, 03:58 AM
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Default Re: 4" insulated flue?!?!?

Quote:
Originally Posted by david s View Post
Rob,
Be careful, the inner flue is usually stainless steel while the outer flue is usually only galvanized, which will burn off and corrode quickly if it gets too hot. Better to do stainless now than have to replace it down the track.
Dave
Dave, I've worked out that my opening will be approx 12 inches high by 14 inches wide (due to the aga door I'm installing) however based on this I have worked out that I would need a 4.62" diameter flue, i.e. 5" minimum. Could I use say two of the 4" insulated flues next to each other giving a total of 8" or is this a bad idea as the draw of air will be too great? Also, could I use a 5" clay chimney instead without the worry of it cracking from the heat?

Cheers

Rob
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Old 08-20-2010, 04:35 AM
david s's Avatar
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Default Re: 4" insulated flue?!?!?

Having the 4"flue for free is a good incentive and I guess would work OK I don't think you can overdo the flue, but you can certainly under do it. A clay chimney would also be OK, but you need to insulate around it if it is thin otherwise the temp difference will lead to it cracking.
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Old 08-20-2010, 04:56 AM
Serf
 
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Default Re: 4" insulated flue?!?!?

Quote:
Originally Posted by david s View Post
Having the 4"flue for free is a good incentive and I guess would work OK I don't think you can overdo the flue, but you can certainly under do it. A clay chimney would also be OK, but you need to insulate around it if it is thin otherwise the temp difference will lead to it cracking.
Thanks for the response Dave, I guess I can try two of the 4" next to each other as they are free and see how it goes as replacing it with say a singly 5 or 6" would be a fairly simple job. I trust there is no minimum hight which the chinmey needs to be...just high enough above head height I presume?!?!
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  #9  
Old 08-20-2010, 12:52 PM
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Default Re: 4" insulated flue?!?!?

Rob,
The greater the height the more the draw. Let aesthetics be your guide.
Dave
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  #10  
Old 08-22-2010, 12:46 PM
Serf
 
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Default Re: 4" insulated flue?!?!?

Well as it happens, my mate dropped off the flue for me and he clearly can't measure as it turns out to be a 5" internal diameter, perfect for my needs! All I need now is a cap to stop any rain water getting in.
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