secondary chimney vent?

[GianniFocaccia]

Re: secondary chimney vent?

Thanks Shuboyje and Al. I just did the Natural Draft Calculator on the Engineering Toolbox this morning and confirmed that when comparing flues of equal volume, the taller one wins. I actually have a 48" (hot water heatere rigid duct) smokestack on my 30" smoker. It looks funny but works a treat.

I think I'm gonna go with a 36" 304 stainless steel flue that starts out 10"x8" where it attaches to the top of the vent and tapers upward to finish at 8"x8". My bro works in steel and can get it welded up for me.

[benguilford]

Re: secondary chimney vent?

Its a blast door, there are lots of oven users using them..

Thanks Brickie! (I've been wondering what the heck a blast door was, lesson learnt! )

[brickie in oz]

Re: secondary chimney vent?

I am currently designing my flue and in particular, the size and shape of the flue gather, or vent. I understand that the longer a given flue is the higher the pressure and draw it will provide. Is it incorrect to assume a shorter but larger diameter flue would not provide the same amout of pressure that the longer/narrower flue could?

Not sure if thats correct, if you read up on fluid dynamics you will see that the longer the pipe the more friction and resistance you will get and thats with pressurized fluids.
Smoke from your oven isnt pressurized so has to coaxed to find the flue.

My last oven had a very short flue of about 400mm and worked fine, my current flue is over 2M tall and works fine.

It comes down to internal sizing over door opening size plus any restrictions like incorrect flue gather.

[brickie in oz]

Re: secondary chimney vent?

Simple and effective.

Its a blast door, there are lots of oven users using them..

[shuboyje]

Re: secondary chimney vent?

The second calculator on this page is perfect for your question. Simply plug in some numbers and find out.

Air Flow and Velocities due to Natural Draft

I am currently designing my flue and in particular, the size and shape of the flue gather, or vent. I understand that the longer a given flue is the higher the pressure and draw it will provide. Is it incorrect to assume a shorter but larger diameter flue would not provide the same amout of pressure that the longer/narrower flue could?

[benguilford]

Re: secondary chimney vent?

Being the impatient soul that I am, I usually start a pretty decent sized fire straight away, and I was definitely having issues with smoke giving my decorative arch the 'rustic look' that I wasn't really going for!

My neighbor Dave has a very simple solution for this problem; Cut a piece of solid timber as though you were making a door for your oven - mine looks like a semi-circle (see attached sketch). Then, to allow fresh air to be drawn in, cut decent sized gaps in the base of this semi-circle. If you sit this in the doorway just inside your decorative arch, it will ensure the smoke goes straight up the flue without staining the front of the oven. This door can then be removed once your flue has heated to the point that it is drawing good and hard. Simple and effective.

[brickie in oz]

Re: secondary chimney vent?

No Al, I have not tried a blast door what are the benefits?

It increases the draw dramatically at start up, a piece of cardboard with a cut out at the bottom across the opening will give you an idea of the effect.

[Faith In Virginia]

Re: secondary chimney vent?

No Al, I have not tried a blast door what are the benefits?

[GianniFocaccia]

Re: secondary chimney vent?

You can increase the height or the cross sectional area, or both

I am currently designing my flue and in particular, the size and shape of the flue gather, or vent. I understand that the longer a given flue is the higher the pressure and draw it will provide. Is it incorrect to assume a shorter but larger diameter flue would not provide the same amout of pressure that the longer/narrower flue could?

[brickie in oz]

Re: secondary chimney vent?

My chimney works great for the most part. Sometimes as the fire just starts to get big and just the right gust of wind feeds the oven I get smoke out the door.

Have you tried a blast door?

[Faith In Virginia]

Re: secondary chimney vent?

My chimney works great for the most part. Sometimes as the fire just starts to get big and just the right gust of wind feeds the oven I get smoke out the door. I think if you don't like that look you can always tile the opening so that if it does get dirty it will easily clean(like what David S said). personally I don't mind the markings I think it adds charm to my oven

[david s]

Re: secondary chimney vent?

It is the volume of the flue that creates the draw. You can increase the height or the cross sectional area, or both. Using the top down method also eliminates smoke at start up. I have a mosaic decorative arch (glossy tiles) occasionally I give it a rub with damp newspaper- no more smoke staining.

[SCChris]

Re: secondary chimney vent?

Brickie,

I agree with you, smoke always finds the easiest way out, but it needs to be said that as the chimney heats, the ability to pull the smoke out of the entry increases.

An analogy would be sink with and without an attached drain pipe.. The added mass of the water flowing out the drain pipe helps to pull the water out of the sink. A siphon is another example, water won’t flow above the water supply, but it can be pulled higher, with a siphon, as long as the end of the hose is lower than the source supply.

Once the chimney is hot, the higher the chimney, the better the draw all things being equal.. Many of the problems with smoke escaping the front of the entry relate directly to not having enough area to service the quantity of smoke for the given chimney height. I’m not advocating a 30 foot chimney on a 42 inch oven, I’m just saying, your right “Smoke always finds the easiest way out”

gtofani,
Time to time I have to remind myself.
"Keep it simple stupid"
"Don't be penny wise and pound foolish"
"Do it right" and
"If it's important don't compromise"

Chris

PS.
One more thing; "She is almost always right and she knows it and will remind me of it"
Best to all..

[brickie in oz]

Re: secondary chimney vent?

The only reason the fronts get sooty is because the flue isnt big enough, or the flue gather isnt built correctly, smoke always finds the easiest way out.

[gtofani]

Re: secondary chimney vent?

I like the stainless steel baffle idea until the fire gets going hotter.
I thought about a permanet baffle by dropping the outter arch a little but then I would not be able to get my door in and out.
Hope this link works.

Snapfish: Share Photo:Registration

the black represents the chimney inside, the outter arch prior to my outside finish look is where I wanted to cut the arch brick thinner to create more area for the smoke to go up instead of out.
Maybe I can incorporate a stainless baffle that slides up into the arch area that I can pull down when needed.