6"to 8" adaptor?

[kjsnooks]

I am wondering if it affects draft very much to put an adapter going from 6" to 8" right off the stove? We are thinking about replacing our stove and have an 8" stack, how ever finding a stove we like with an 8" exahust is proving hard.

thanks
Jacob

 

[Hanko]

not a problem at all

 

[fossil]

I am wondering if it affects draft very much to put an adapter going from 6" to 8" right off the stove?

Yes, it does...and you won't like it. That transition increases the cross-sectional area of your exhaust flue to 178% of what it started out at the stove. The mass flow rate of your exhaust gases will slow way down, resulting in decreased stove draft and more time for the flue gases to cool and condense out creosote on their upward journey to daylight. Start thinking about a full 6"chimney liner along with your new stove. Rick

 

[wahoowad]

I use a 6"-to-8" and have decent draft and no creosote issues. If you have poor draft to start with though I suspect this will make it worse. If you have good draft now I think you will be fine.

 

[Hanko]

Oh for Christ Fuc--- sakes cross sectional 178% Bull Sheet. Go put the 8 inch adapter in, Ive done it with no problems what so ever.

 

[fossil]

Don't get upset with me, Hanko, I didn't invent Pi. A 6" diameter pipe has a cross-sectional area of 28.27 square inches, while an 8" diameter pipe has a cross-sectional area of 50.27 square inches. That means the larger pipe area is 178% of the smaller pipe area. I understand fluid flow, and I understand that an increase in the conduit cross section of that magnitude is going to slow the fluid significantly. I'm not saying it won't work, I'm just saying that those are the facts, and it's something to consider. Rick

EDIT: And a lot has to do with the configuration and total height of the chimney, as well...is it internal or external, insulated, or not...etc., etc.

 

[Hanko]

Im not upset with you at all. You obviously are very smart when it comes to these wood stove issues, I ve read your post and I agree with you most of the time. But Ive done that 8 to 6 deal and its worked for me. I say try it, ands ee what happens. I thnk out of my three class a chimneys the one I have reduced down for the most part works better than the other two 6 inch jobs I have. Keep up the good reserch Rick

Henry

 

[kjsnooks]

Thats why i was wondering if i put the adapter right off the stove to my pipe. I do have good draft. I cant imagine the flow being to fast right there(the way i understand it is the gases spiral faster and faster as they go up)is that it wouldnt have gaine much momentum to begin with and second it has the rest of my chimney to pick up speed. How much different can it be than a bend halfway up or a 90 to go out your wall?

 

[kjsnooks]

and 178 dont make sense. thats greater then 1.5 times. and from 28 to 50 isnt 1.5 times. (if that makes sense)

 

[fossil]

50.27 = 1.78 x 28.27 An 8" pipe has 178% of the cross-sectional area of a 6" pipe, an increase in area of 78%.

 

[North of 60]

Heres another to support fossil. A 1" pipe has 4 times the cross sectional area as a 1/2" pipe to understand the magnatude of a little change. It does affect things. Some situations more some less. Now dont argue with a pipefitter

 

[kjsnooks]

Im realy not trying to argue with anyone. Im just wondering if those that have done it had good luck or bad. if nothing else ill just find myself a new stove with an 8" hole. no biggy, just less of them out there.

 

[North of 60]

No worries Jacob just given Fossils angle on the math end. Hankos had success so it may work for you. Worse case scenario is drop a 6" liner down it
if it gives you trouble. With all that fancy math it should fit.
;-) CHEERS N of 60

 

[countrybois]

and 178 dont make sense. thats greater then 1.5 times. and from 28 to 50 isnt 1.5 times. (if that makes sense)

Ummm. 28 x 1.5 = 42

28 x 1.78 = 49.84 so actually a little more than 178%.

 

[BrotherBart]

Since trying it will cost around nine bucks for a plain old single wall adapter there sure isn't any reason not to try it and see how well it does. If a liner is in order after you try it the extra for that is probably less than the difference in whatever stove you really want and the prices of the very few stoves out there with eight inch flue requirements.

 

[kjsnooks]

Thats kinda what i was thinking Brother Bart. I didnt mean to stir the pot so though. All this is mostly hpythetical till next year anyway. I just happen to have to light the stove and got to thinking about how nice one would be with a window and started looking around. It would be next year before we did anything with it. Thanks for all the responses guy!

Jacob

 

[countrybois]

removed

 

[BrotherBart]

It MAY work, but everything you can find on the topic will tell you NOT to increase the flue diameter as it WILL negatively affect draft. You can go from 8" to 6" but not from 6 to 8.

So....that is text-book. Do what you like.

Pray tell what "text book" or anything else for that matter says that it is OK to neck down a flue to a smaller size?

 

[fossil]

Im realy not trying to argue with anyone. Im just wondering if those that have done it had good luck or bad. if nothing else ill just find myself a new stove with an 8" hole. no biggy, just less of them out there.

I'm certainly not aguing with anyone either...just doin' the math and talkin' about principles of fluid dynamics. Hanko responded right off the bat that he's done it and it works fine in his setup. Might be just fine in yours as well. Give it a try and let us know how it works for you. Depends on the stove, the configuration of the installation, the height of the chimney, and how tortuous the flowpath is for the flue gases. To my knowledge, most, if not all, stove manufacturers recommend that for optimum performance, the connector pipe (stovepipe) and chimney all be the same diameter as the collar on the stove. Not every woodstove is installed that way, obviously. Likely they still all burn wood and put out heat. Rick

 

[countrybois]

It MAY work, but everything you can find on the topic will tell you NOT to increase the flue diameter as it WILL negatively affect draft. You can go from 8" to 6" but not from 6 to 8.

So....that is text-book. Do what you like.

Pray tell what "text book" or anything else for that matter says that it is OK to neck down a flue to a smaller size?

Wow, I stand corrected(and removed my posting). Maybe it was just the literature from that particular stove I was installing or maybe I was just dreaming. Decreased chimney diameter would lead to better draft and thus increased diameter would need more heat for the same draft. But I sure can't find anything to support that theory now. Goes to show you that you can't trust EVERYTHING on here. Humbling. I apologize.

 

[BrotherBart]

countrybois you may have been thinking about the few of us that use 5.5" liners in stoves with 6" flue collars. In our cases it isn't kosher but it is something we did knowing the possible negative consequences. In my case I did it knowing that the actual hole in the stove body under the flue collar of one stove was in fact 5.5" even though the collar was standard 6". With the other I knew that the same stove has been sold in Europe with a 5" collar.

And I also knew that if either one didn't work right I had tossed four or five hundred bucks down a well and would have very expensive 316ti stainless steel downspout extensions on my gutters. Not a chance your average stove buyer should take.

 

[fossil]

Decreasing the cross-sectional area of the conduit for fluid flow will do a couple of things, depending on the system. In a system with a constant supply pressure, the fluid flow rate downstream of the reducer will be increased (this is technically referred to as the "thumb over the end of the garden hose" phenomenon). In a system with a variable supply pressure, the fluid flow rate downstream of the reducer will tend to increase, but it will at the same time tend to increase the pressure in the system upstream of the reducer. So, in a stove flue exhaust system, the effect of a reducer is to increase the pressure differential across the system (stove box to atmoshpere), and would be expected to degrade the draft of the stove because the exhaust flow has been restricted. Perhaps not intuitive, but it's the science. Take it to the extreme in a thought experiment...what would you expect to see if you were to reduce a 6" flue down to say, 1/2"...better or worse draft? Rick

 

[BrotherBart]

What does it introduce into the process fossil when it is a negative pressure situation such as we have with a chimney. That being where the pressure differential is produced by pulling the gases through the pipe instead of something at the other end pushing it through the pipe?

Inquiring minds wanna know. I plan to live and learn. And then die and forget it all.

 

[countrybois]

Ok, so maybe not total draft 'volume' overall. But a smaller chimney would require less heat to keep a draft going.

With the same heat in the stove, a smaller chimney would have better (read - faster air movement) draft, than the same stove with a larger chimney. You are talking about pipe in which there is water pressure, chimney pipe is a different dog. Your chimney must be hot in order to create a good draft to vent your stove. Insulated chimney pipe is designed to keep the chimney hot just as much as it is to keep the area around the chimney cool.

Let's use your 'scientific' wisdom and apply it here. Compare a 6" chimney to a 6' diameter chimney. On the same stove what will be the difference in draft? You would have a difficult time keeping an upward draft in the 6 footer.

I would say it is safe to say that you want neither too small a chimney nor too large of one. Best bet is to use the same size chimney as the flue collar on the stove you are using.

 

[fossil]

Draft of a wood-burning appliance is actually produced by gravity. Combustion in the stove produces very hot gases, thus they are of relatively low density...they're lightweights, thus they tend to rise. They will be displaced by cool, heavier combustion air entering the stove. It's a natural convective flow of gases initiated by heat of combustion and driven by gravity. The flue gases want to go up (hot air balloon), and they'll find a way...so long as we've provided an appropriate path that contains and encourages the flow. There isn't a lot of horsepower behind this flow (we're not talking about pounds and pounds of weight differential...and it doesn't take much to slow it down. Horizontal or down-sloped runs are bad, changes of direction are bad, flow restrictions are bad, large increases in cross-sectional area are bad. Height of the chimney plays a role, too. I can imagine a chimney so tall that the flue gases cool to ambient and basically equalize in density with the atmoshpere at the chimney's terminus and flow up the chimney just stops, because there's nothing to make it keep going (smoke alarm time!). I know there is an optimal height for a chimney, but I'm not smart enough about it to give an example...a subject for further research on my part. So much to learn. Rick