Is the size of your flue important?

[barney]

I have read elsewhere (i.e. fornobravo) that the cross-sectional area of the flue should be 10% of the cross sectional area of the door. Does anybody have any views on whether this is important or experience of too small/too big flues or chimneys?

On this premise my flue should be 4"/100mm and I don't really want it to be over 6"/150mm, obviously the smaller you can get away with the cheaper it is, but is this likely to impact on draw if the flue is too small?

As always all thoughts appreciated.

Thanks.

[tonyb]

Barney, a flue isn't actually necessary for a functioning wfo, it just makes it easier to operate, and you might not loose your eyebrows A flue helps particularly in the early firing phase when there can be smoke about and fuelling the fire ain't no fun with a lungful of smoke.

In this case at least, bigger and longer is better, but as all things wfo it becomes a trade-off between cost/benefit, and generally we can't quantify the marginal benefits of design differences, just guess/gut feel.

[turkey]

tony raises a good point, length is as an important a factor as diameter really for creating draw, you do need more than a token stub to get a good draw.

Personally I think FB take exact calculations a little too seriously, to me 6" for a 32" oven and 8" for a 42" oven seems to be what folks use.

don't forget to check ebay for bargains, I think more than a few of us have brought flue parts there on the cheap.

[h12rpo]

Double entendre alert !

[barney]

As the jokes section is currently closed:

A woman walked into a bar and asked for a double entendre,
so the barman gave her one.

More seriously, thanks for the comments guys. As a scientist I find absolutes appealing but I suppose here things are not quite so rigidly defined. I'll see how I get on and post back my results.

[slowfood]

Flu sizing

Flue size. The US underwriters Laboratories state that it is best to use a flue as large as you need and no larger;
So if you were burning 12 lbs/hour of wood you would be generating 97,000 btu's and therefore need a 6" Flue (My 41" oven would burn less than this)
16.5 lbs/hr=7" according to The US underwriters Laboratories.
21.6 lbs/hr=8"
27.3 lbs/hr=9" and so on
Also I found this on an italian Oven building web site
Diametro del forno diametro minimo canna fumaria in cm
da 60 a 80 ----cm 20
da 80 a 120--- cm 25
da 120 cm in--- su 30

[h12rpo]

Surely the answer is: it doesnt actually need one, but whatever size looks nice will be best

[nixie]

I wonder if a restriction plate of some kind fitted to a larger flue would work well (I'm building an 80cm oven and was planning to use a spare bit of twin wall flue pipe however its much wider than 20cm)

[barney]

Well those last three posts cover all the options:

Some clearly defined numbers, which I like.
A plea for aesthetics, which is equally if not more important!
And good use of available materials which appeals to my miserly side.

I am happily going to go with the calculated option, which will also look good and cost less money. If only it were always so simple.

Nixie: I'd try it with the pipe you have, making later modifications in the unlikely event that a smaller flue would be more efficient shouldn't be too difficult. I think the previous posts by Tony and Turkey on the importance of length were probably the most perspicacious (as well as being in keeping with the crude undertone of this thread).

[h12rpo]

Ahhhh, the old girth versus length debate rears its head again

[faz]

It's not just the width of the flue, but also how well the entry to collects the smoke. Ideally a wide onry to the flue covering most of the width of the entry will maximise smoke collection, or some kind of smoke box which is much wider than the flue.

When the oven is going full bore the exhaust gases come out of the door at a decent speed and if the flue entry is narrow it is easy for them to zoom past one side or the other then out of the front.

I am planning on getting my grinder out to try and make my flue entry wider to try and reduce this effect, as the front of my over gets quite badly stained with smoke.