#11  
Old 12-25-2013, 06:48 PM
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Default Re: Smoke

Quote:
Originally Posted by Tscarborough View Post
Flue size really has no direct relevance to oven size, it is related to door size. The flue has to be in proportion to the door, 10-15% is the normal range, depending upon the tallness of the stack.

A round 6" i.d. flue gives about 10 SqIn of cross section, so it should be fine for an opening of 100-150 SqIn, again, depending upon the height of the stack.
But bigger would be better in any situation, no?
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  #12  
Old 12-25-2013, 07:04 PM
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Default Re: Smoke

Max,
I have been waiting for you to fill in some more details of your oven which is "loosely based on the Pompei"i. You might want to reply with some pics and the actual design that you have built. There may be other factors at play, here. I looked at your profile. I see that you intended to build an 850 mm/33.5" oven. I am very sorry to see that you asked about the "flu size" question in the designing stages twice with no responses. I will take my share of the responsibilty for that . But, for the size oven that you were intending to build, you may be in specs.

Quote:
Can anyone offer advice? Will extending the flue make any difference? Note that I do not have a smoke chamber. The flue comes straight out of the arch.
A smoke chamber, that transitions from the narrow depth rectangle to the round (the "upside down funnel" that David S. mentioned) is the key to this. Going sharply from gathering the smoke from your door width to a 6" opening is a stretch IMO.

For the mean time though, experiment with a cheap section of 6" stove pipe (or two) added to the top of your flu. That will help with the draw, and may help the smoke to clear your house. (note: smoke will rise faster in cold weather and when the barometer is reading higher. It will lay close to the ground when the barometer is lower) When we see the smoke laying close too the ground, we think that rain is in the forecast.

My oven is not “under flued”. But, we do have weather extremes. Windy, is a given. We also have temperature swings. It can be in the 20's F. at daylight and in the 60's F. by around noon. That results in a reverse draw due to the chimney cooling the ambient air. Cool air will fall in warmer ambient air.

Try some of the cheaper solutons (preheat and or extend the flu) before you do something drastic.............. like you mentioned here.

Quote:
The way it is going I don't think I will use this oven much, if at all
.

That is not an option .
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Last edited by Gulf; 12-25-2013 at 07:09 PM.
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  #13  
Old 12-25-2013, 07:11 PM
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Default Re: Smoke

Bigger is almost always better, Brickie, for certain.
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  #14  
Old 12-25-2013, 07:44 PM
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Default Re: Smoke

I consider my 6 inch flue "adequate" for my 750mm/29.5 in oven. I do have a nice fabricated s/steel hood funnelling the smoke from a rectangular opening in the arch to the round section of the flue though.
I would go 8 inches for anything bigger than 30 inches though.
If the oven is pumping that much smoke it's getting into the house, that might be a combination of things. e.g. damp firewood, damp oven, short chimney, etc.

Last edited by wotavidone; 12-25-2013 at 07:48 PM.
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  #15  
Old 12-25-2013, 11:40 PM
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Default Re: Smoke

Quote:
Originally Posted by Tscarborough View Post
Flue size really has no direct relevance to oven size, it is related to door size. The flue has to be in proportion to the door, 10-15% is the normal range, depending upon the tallness of the stack.

A round 6" i.d. flue gives about 10 SqIn of cross section, so it should be fine for an opening of 100-150 SqIn, again, depending upon the height of the stack.
A flue is the engine that creates the draft. The larger the flue the bigger the draft. You can increase the diameter or the height to do this, but increasing the diameter is more powerful in obtaining more draft. A large oven needs a bigger draft than a small one, because the volume and fire is larger.

A round 6" flue has a cross section of 28 sq in not 10

The 10-15% rule may be ok for a 42" oven, but for a small oven you need to go more like 20%

Restricting the air intake from say 1500% down to around 25% of the flue area actually creates an extremely efficient combustion atmosphere, as some members have found by using a blast door.

My own oven has a 5" flue, a 21" internal diameter and an opening of 100 sq in which makes the flue cross section 20% of the opening. No smoke staining issues after four or five years.

Last edited by david s; 12-26-2013 at 12:12 AM.
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  #16  
Old 12-26-2013, 06:29 AM
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Default Re: Smoke

Hey Max

Nothing is beyond redemption. I am sure that with a bit of tweaking that you will get that oven drawing acceptably well and if a bit of initial smoke stains the entry then you will probably have plenty of company.

It is easy to post he pictures of the perfect things and we all appreciate that but the reality is sometimes a little different a little short of perfect.

Perfect will be 'Build II' for all of us on build 1.
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  #17  
Old 12-27-2013, 02:15 AM
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Default Re: Smoke

Hi everyone
Thanks for your valuable suggestions.
I have persevered and today we had our first wood-fired pizzas.
I have come to the conclusion that I have/had two separate issues:

1. Horrible smoke; and
2. Not all of the smoke going up the chimney.

Smoke
I was originally using gum tree. This created noxious smoke. I have since switched to New Zealand Ti Tree. This still smokes during ignition but is nowhere near as acrid as the gum smoke.
Another design flaw is that I built the oven upwind of my house - that was just plain dumb!

Chimney issue
I think my major problem is that there is no 'throat' or funnel from the arch into the flue. Therefore, only smoke that is directly in line with the flue gets extracted, whereas smoke that comes through on the shoulders of the arch continues on straight out the front. See attached photo

To answer some of your queries:
1. I have attached some photos of my build. For more details go to Wordpress.com and search for claypizzaoven and you should come across my blog, which tracks my build.

2. My oven is 850 cm or 34" in diameter and 425 cm or 17" high.

3. The entry is 185 sq" in area. The 6" flue is 28 sq ", therefore it is 15% of the entry - probably a little small according to David's calculation

4. I wanted to go for an 8" flue, but couldn't see how I could do that, given that my arch is one brick length in width. That would leave only 1/2" on either side of the arch, which seems to be insufficient to me. I found the Pompei instructions a little vague on that point (or maybe I am just stupid)

So, it seems that my major problem is that I don't have a 'throat' or 'funnel'.
It would be major surgery to retrofit one, so I guess my options are:
1. to try and carve a throat (albeit shallow) into the underside of the arch. I am not sure how I will do that. Probably with an angle grinder; and
2. Extend the flue. I have a piece of ss flue left over, so I will try and attach that to see if it makes any difference
3. Put a 'blind' extending down about 2.5" at the front. I experimented using the plywood arch former (until it caught fire). This worked quite successfully by redirecting smoke back into the arch and up the flue (see 3rd attached photo).

All further comments are welcome.
Attached Thumbnails
Smoke-flue-arch.jpg   Smoke-final-plastered.jpg   Smoke-plywood-blindf.jpg  
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  #18  
Old 12-27-2013, 02:22 AM
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Default Re: Smoke

Quote:
Restricting the air intake from say 1500% down to around 25% of the flue area actually creates an extremely efficient combustion atmosphere, as some members have found by using a blast door.
Davis
I am not sure what you mean by "reducing the air intake. Do you mean cut down the entrance size?

Also, what is a "blast door". I can't find any reference to one on this site
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  #19  
Old 12-27-2013, 02:45 AM
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Default Re: Smoke

Quote:
Originally Posted by Massimilliano View Post
Davis
I am not sure what you mean by "reducing the air intake. Do you mean cut down the entrance size?

Also, what is a "blast door". I can't find any reference to one on this site
It is a door that blocks off all the air from the outer arch bar about 25% of the flue cross section at the bottom of the door. I'm not a real fan of this technique because it can lead to too rapid a rise in temperature which can be damaging to the refractory. Apart from not having any funnelling to the flue pipe I think your 6" flue should be adequate, but the alterations you suggest should all give an improvement. I think your oven is probably still wet and or your wood was too wet or green. Judging by the water dripping from the bottom of the floor in the third photo, this is most likely your problem. Usually you need to keep wood for 12 months before burning it. My guess is that if you keep firing and make sure your wood is well seasoned and dry, your oven will fire up fine. The other thing to check is that your oven opening height should be around 63% of the internal dome height, give or take an inch or so either way.

Last edited by david s; 12-27-2013 at 03:52 AM.
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  #20  
Old 12-27-2013, 02:58 AM
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Default Re: Smoke

To illustrate how inappropriate the 10% rule can be, take a medium sized oven say a 36" neapolitan oven with a 14" internal dome height. With a semi circular inner oven door opening 9" high and 18" wide (127 sq") !0% of this (12.7sq") would suggest a flue diameter of only 4", which is clearly way too small for a 36" oven. Even at 15% of door opening (19 sq") would suggest a 5" flue. Again this would be inadequate IMO, but a 6" should do the trick.

Last edited by david s; 12-27-2013 at 03:35 AM.
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