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Flue size
Aug 14, 2019 17:42:11 GMT
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Post by theboss on Aug 14, 2019 17:42:11 GMT
Just wondering if there are any specific sizes of flue needed for size of oven? WFO 750mm dia. Door opening W450mm / H350mm
Was going to go for 125mm dia. Flue do you think this is enough. Also does it need to be twin wall. I understand it will be hot to touch, so I wound touch it, but would there be an excessive risk of cracking the dome due to expansion.
Just wondering what other folks have used and how these have held up?
Thanks in advance for any response.
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Post by albacore on Aug 18, 2019 20:48:07 GMT
125mm will be fine. I used 150mm for an 800 diameter oven, but ended up fitting a restrictor to throttle it down.
You can use single wall, but remember you will always be in fear of touching it! - just make sure you leave a gap round the flue when you render. the trick (courtesy of downunderdave) is to get a strip of corrugated cardboard (preferably the bendy type) and wrap it in clingfilm. Then you wrap this round your flue where the render is gong to make contact. Secure in place with the absolute minimum of tape. Then render and once the render has dried, pull out the cardboard and clingfilm. Now you need to fill the gap with Envirograf 1200C silicone (available on Ebay).
I would do this even with twin wall - I didn't and the render cracked!
Lance
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Flue size
Aug 19, 2019 18:07:21 GMT
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Post by theboss on Aug 19, 2019 18:07:21 GMT
Thanks for the advice and tip
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Post by downunderdave on Aug 19, 2019 20:07:29 GMT
For a 750 mm diam oven a 6” flue is generally required although it depends a little on whether the flue is inside or outside the oven chamber, You can increase the draw of the 5” flue by making it taller if need be. The height of the oven door should be approx 63% of the internal height of the dome, so if your dome is a hemisphere yours would be only 47%, a bit low and may affect the way it will draw.
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Post by theboss on Aug 20, 2019 6:52:40 GMT
For a 750 mm diam oven a 6” flue is generally required although it depends a little on whether the flue is inside or outside the oven chamber, You can increase the draw of the 5” flue by making it taller if need be. The height of the oven door should be approx 63% of the internal height of the dome, so if your dome is a hemisphere yours would be only 47%, a bit low and may affect the way it will draw. Thanks My dome height is 555mm with a door height of 350mm, so I’ve got the 63% Where exactly should the flue be located? I have a 150mm “tunnel” from my birch arch door, or should it be placed at the beginning of the dome itself?
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Flue size
Aug 20, 2019 6:58:41 GMT
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Post by theboss on Aug 20, 2019 6:58:41 GMT
*brick arch
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Post by downunderdave on Aug 20, 2019 10:48:56 GMT
For a 750 mm diam oven a 6” flue is generally required although it depends a little on whether the flue is inside or outside the oven chamber, You can increase the draw of the 5” flue by making it taller if need be. The height of the oven door should be approx 63% of the internal height of the dome, so if your dome is a hemisphere yours would be only 47%, a bit low and may affect the way it will draw. Thanks My dome height is 555mm with a door height of 350mm, so I’ve got the 63% Where exactly should the flue be located? I have a 150mm “tunnel” from my birch arch door, or should it be placed at the beginning of the dome itself? The first oven I built 12 years ago was based on the design of ancient Roman kilns that I have built. It has a central flue at the apex of the chamber. It performed very well but had the drawback of not being so efficient with fuel. Because the chamber is empty, unlike a kiln which is full of wares, the flame wants to jump straight for the flue. The big advantage though is that apart from being simpler to build, it provides great access because there's no tunnel to work past. The draw is also fantastic and there are no smoke issues. Heat retention with this design is poorer. There are a number of ovens of this design in Pompeii, all small and presumably in private homes, indoors where lighting an oven frequently can create smoke issues at start up, rather than commercial bakeries which use the cross draft design and which probably never cooled down being fired daily.. Oven 2 of which I built two was an attempt to improve circulation and while it does that is still not a patch on design 3, which just happens to be the most popular design developed by the Ancient Romans more than 2000 years ago. They must have had many generations trying different configurations and we front up thinking we can build a better oven at the first attempt, ha ha.I now stick with design 3 exclusively, but build the tunnel as shallow as possible. Apologies for the rough drawings.
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Flue size
Aug 21, 2019 7:21:28 GMT
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Post by theboss on Aug 21, 2019 7:21:28 GMT
Thanks for the pic!!
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Post by lorettaf on Aug 25, 2020 7:21:20 GMT
I’m going to try to recreate the romans’ design number 3 as proposed by downunderdave. It’s my first build so it might be a tall order! I’m trying to work out how you physically build it. It’s going to be clay/sand mix thermal layer. I presume i need to build as two separate chambers ie dome first, then flue chamber, as it looks like the flue chamber is in part built on top of the dome entrance. If that’s correct, how long do you leave the dome to dry before you start with the flue chamber? I guess it needs to be malleable to get the two to seal. Or, as soon as you have cast the dome do you approach with a pre made form for the flue chamber and crack right on? I was hoping to use sand for all the form work but maybe for this design I cannot. Thanks
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Flue size
Feb 27, 2021 23:18:43 GMT
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Post by theboss on Feb 27, 2021 23:18:43 GMT
Uncovered my oven for the 1st time in a couple of weeks and now the suns out I was going to fire her up, but.... some water has got in under the flue and some of the clay inner around the arch has slipped off revealing only the inner side of the vermicrete.
I’m going to take this opportunity to change my single wall steel fiue with a twin wall stainless steel option (the salty air here in West Cornwall will aggressively rust anything).
Ideally what I’d like would be to cast in a sleeve into the oven, which I can then insert and remove my new flue when needed. This would let me properly cover the whole oven if we’ve got heavy rain forecast etc.
Anyone got any advice, experience fitting something similar?
Love to hear any tips?
Also has anyone ever replaced their flue?
Cheers in advance Tim
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Post by oblertone on Feb 28, 2021 0:05:39 GMT
Most twin-wall systems come with a separate register plate that can be fixed (bolted/screwed/glued) to the top of the arch; then the section of twin wall is seated by a eighth-turn twist and can be demounted for storage.
My build thread below shows just such a system but might not display on a phone screen.
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Post by downunderdave on Feb 28, 2021 21:45:22 GMT
Uncovered my oven for the 1st time in a couple of weeks and now the suns out I was going to fire her up, but.... some water has got in under the flue and some of the clay inner around the arch has slipped off revealing only the inner side of the vermicrete. I’m going to take this opportunity to change my single wall steel fiue with a twin wall stainless steel option (the salty air here in West Cornwall will aggressively rust anything). Ideally what I’d like would be to cast in a sleeve into the oven, which I can then insert and remove my new flue when needed. This would let me properly cover the whole oven if we’ve got heavy rain forecast etc. Anyone got any advice, experience fitting something similar? Love to hear any tips? Also has anyone ever replaced their flue? Cheers in advance Tim Hi Tim, I've built quite a few with a sleeve to take a removable flue. I don't like building an oven with the flue attached if it is to be transported. The attached pics should tell the story. It results in a nice sliding fit. The back plastic you see creates just enough clearance to produce a nice sliding fit once removed, as well as allowing the mix to adhere to the sleeve, but not the pipe.
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Flue size
Mar 24, 2021 15:59:35 GMT
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Post by theboss on Mar 24, 2021 15:59:35 GMT
Thanks for all the advice gents. Always appreciated.
So I’ve removed the old flue and cast is a new twin wall adapter, which I can then attach the new flue to.
It’s essentially a cob oven so I’ve used that mix to put the sleeve in but it has cracked quite a bit already, and I’ve only had one very small fire in there to help dry things out.
If I patch in the cracks do you think it will be ok to render to? Problem is I just don’t have the space to get much else in there
Cheers
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Flue size
Mar 26, 2021 3:12:25 GMT
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Post by downunderdave on Mar 26, 2021 3:12:25 GMT
Because clay shrinks considerably it is normal for the cob mix to contain 2 parts sand to 1 part clay by volume. This high sand proportion reduces shrinkage. When I make a flue sleeve I always leave a gap of at least 10 mm so the sleeve does not make a full circle. This then allows it to expand without cracking the refractory that surrounds it. As all potters know, you can’t fire wares until they are perfectly dry. Depending on the weather around a week is normal. As your casting will be way thicker than a potters pots you need more time for the drying process. Filling the cracks with more wet mix is a poor fix and usually results in failure, although cob is virtually free so it’s really only you time that’s costing you.
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