Placement of damper with wood stove flue which increases in diameter?

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Fractal20

New Member
Oct 27, 2021
12
Colorado
I've been having what I still think are some overdraft issues with a quadrafire 4300 wood burning stove and I've decided to at least give installing a damper a try (those details are here: https://www.hearth.com/talk/threads/should-i-install-a-damper.189015/#post-2543084). I figure I can always remove the damper, or just keep it open.

The stove has a 6 inch flue, but in my case it immediately increases in diameter to 7 inches. The attached picture shows this. A few questions

1) Should I continue with a 6 inch flue, rather than having it immediately increase to a larger diameter, and install a 6 inch diameter flue, or should I keep it as is and install a 7 inch diameter damper?

2) If I should change it up to have the 6 inch flue continue, how far should I go? A response in my previous thread, suggested that it might be better to run a 6 inch diameter all the way to the ceiling where it becomes double walled. Financially, I don't want to consider replacing the double wall with 6 inches as well, so at this point I'd only consider running the 6 inch up to the ceiling of the 1st story. My understanding is that an increase in a diameter can contribute to the draft. Would running the 6 inch higher and having the transition be further up the flue help alleviate any increase in drafting from the diameter change?

Any advice would be welcome. Thank you!

Photo Oct 27, 9 35 36 PM.jpg
 

begreen

Mooderator
Staff member
Nov 18, 2005
92,795
South Puget Sound, WA
I'd move the increaser up to the ceiling support box. Add the key damper at about 6-12" above the stove top to the new 6" stove pipe.
 

Fractal20

New Member
Oct 27, 2021
12
Colorado
I'd move the increaser up to the ceiling support box. Add the key damper at about 6-12" above the stove top to the new 6" stove pipe.

Then when sweeping I'd just do it with a 7 inch sweep for the top half and then a 6 inch sweep for the lower half?
 

begreen

Mooderator
Staff member
Nov 18, 2005
92,795
South Puget Sound, WA
Depends on what you are using. Do you use brushes or a Sooteater?
 

Fractal20

New Member
Oct 27, 2021
12
Colorado
Depends on what you are using. Do you use brushes or a Sooteater?
Brushes currently, but I hadn't heard about the Sooteater before. Can you sneak the Sooteater past dampers? I'd probably switch if it meant I wouldn't have to take the damper out to clean the flue.
 

begreen

Mooderator
Staff member
Nov 18, 2005
92,795
South Puget Sound, WA
Yes, with the damper open it should pass by it ok. The advantage is that with the rotary brush uncut, it will conform to the increase in pipe size.
 
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Fractal20

New Member
Oct 27, 2021
12
Colorado
Yes, with the damper open it should pass by it ok. The advantage is that with the rotary brush uncut, it will conform to the increase in pipe size.

Two more questions

1) Can you point me to some literature that discusses flue diameters and drafting? I've come across some conflicting stuff that 6 inch will have a smaller draft but also that larger diameter might have less of a temperature differential and then a similar draft in the end. Mainly I'm interested just in deciding whether it is worth running the 6 inch up to the ceiling or if I should just put a damper in the 7 inch diameter.

2) If I do run 6 inch to the ceiling, I'm a little unsure how to do the transition. I'm used to having a telescoping piece at the support box, but I haven't found any combined telescoping pieces that are also increases. So would you have a telescoping piece that runs into an increaser, or would it be better to have the opposite order?

Thanks for your time!
 

Dave_in_ABQ

Member
Oct 27, 2021
81
New Mexico
Draft is largely a function of chimney height. The flue diameter corresponds to the volume of gas transported. A really large stove will need a larger diameter flue to vent the larger volume of gasses from burned wood. There is a small reduction in draft for a smaller diameter flue due to friction on the walls, but this really is very small unless the flue is way undersized.

 
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coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,516
NE PA
The increase in diameter only gives the chimney more capacity. Like a higher horsepower engine needs more fuel to create the horsepower, the larger chimney requires more heat to cause the same draft. A larger stove or higher BTU output stove would need more capacity and loose more heat up the chimney to fuel the larger chimney. A 6 inch stove is governed by the outlet size. So you’re not using the extra capacity available.

Height is one factor that creates draft. (the higher you go, the less gain from adding more height due to cooling as gasses rise)

Temperature differential between inside and outside of flue is another important factor.

Increasing diameter allows cooling as hot gasses expand. I’ve tested 6 inch to 8 and found flue gas temperature dropped by 1/2 expanding into the larger area. Huge temperature drop. Less heat in a larger area decreases NET draft as well as slowing the velocity decreases NET draft. So draft is decreased from temperature drop and slowing velocity, and increased slightly due to less friction in the larger flue.

Velocity is controlled with flue damper by increasing the variable resistance just like every other resistance added in the venting system.

Everything is factored together to calculate NET draft. NET draft is what is most important.
 
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Fractal20

New Member
Oct 27, 2021
12
Colorado
The increase in diameter only gives the chimney more capacity. Like a higher horsepower engine needs more fuel to create the horsepower, the larger chimney requires more heat to cause the same draft. A larger stove or higher BTU output stove would need more capacity and loose more heat up the chimney to fuel the larger chimney. A 6 inch stove is governed by the outlet size. So you’re not using the extra capacity available.

Height is one factor that creates draft. (the higher you go, the less gain from adding more height due to cooling as gasses rise)

Temperature differential between inside and outside of flue is another important factor.

Increasing diameter allows cooling as hot gasses expand. I’ve tested 6 inch to 8 and found flue gas temperature dropped by 1/2 expanding into the larger area. Huge temperature drop. Less heat in a larger area decreases NET draft as well as slowing the velocity decreases NET draft. So draft is decreased from temperature drop and slowing velocity, and increased slightly due to less friction in the larger flue.

Velocity is controlled with flue damper by increasing the variable resistance just like every other resistance added in the venting system.

Everything is factored together to calculate NET draft. NET draft is what is most important.

So for my situation (more details in a previous post https://www.hearth.com/talk/threads/should-i-install-a-damper.189015/#post-2543084) where it seems like I am having issues with an overdraft and I currently have an increase from 6 inch to 7 inch right when the flue leaves the chimney, would you suggest running 6 inch up to the ceiling with a damper in the 6 inch (and having it increase to 7 inch at the ceiling). Or should it be just as effective to put a damper in the 7 inch? Just putting a 7 inch damper would be the simplest and cheapest for me.
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,516
NE PA
I like you coaly but I am meager with my mind cells so in a few words what does that all mean..?
So for my situation (more details in a previous post https://www.hearth.com/talk/threads/should-i-install-a-damper.189015/#post-2543084) where it seems like I am having issues with an overdraft and I currently have an increase from 6 inch to 7 inch right when the flue leaves the chimney, would you suggest running 6 inch up to the ceiling with a damper in the 6 inch (and having it increase to 7 inch at the ceiling). Or should it be just as effective to put a damper in the 7 inch? Just putting a 7 inch damper would be the simplest and cheapest for me.
If you reduce it right at the stove, the gasses are moving faster in the 6 and will be hotter entering chimney where it increases size and temperature drops. If you needed the extra heat for better draft, that would be best, and would keep it the cleanest. But since you don’t need to make a better draft, increasing right at stove will allow the gasses to move slower in the larger pipe, increasing dwell time inside, allowing a little more heat to radiate from pipe, ending up with slightly lower temp entering chimney. If you don’t think you need more draft, and the chimney stays reasonably clean, do it the cheap easy way with a 7 inch pipe and damper all the way.
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,516
NE PA
I like you coaly but I am meager with my mind cells so in a few words what does that all mean..? clancey
It means there are a lot of things that cause a chimney to make draft, (Low pressure area like a vacuum in the stove) and a lot of things that decrease draft. It doesn’t matter how you go about it, the important thing is NET draft which is the final product. Just like what you make vs. what you spend is your net. That’s what counts.
 
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clancey

Minister of Fire
Feb 26, 2021
2,314
Colorado
Wow I think I understand now..Its a matter of what kind of draft you want verses the first pipe in the house that would reflect more heat to the house if you have a smaller pipe verses a larger pipe that would increase the draft so that the chimney remains warmer which would be better if you do it that way and I think less creosote would form...I hope I got this right and explained it okay so that you can understand me without the professional technical language. I would go a larger pipe so that I would get more draft if needed---and not make it smaller in the house....thanks..Please don't nobody judge by this because I know very little about stoves and just trying to duplicate what a experience person said about these stoves and heating...old clancey
 

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,516
NE PA
Yes, the same amount of exhaust is traveling up the pipe, so in a smaller one it has to go faster than a larger diameter to move the same amout. So it has more time to cool and give up more heat inside. That's not great for keeping the chimney as hot a possible, but in his case it may not be necessary to keep the vent system at efficient as possible. Always think of it as cars on a highway. If the same about of cars go through a narrow one lane road, they have to go twice as fast as the same amount on a two lane road to get from point A to point B. You got this!

The only correction, is the chimney creates draft, and everything else, including inside pipes take it away. The larger the diameter, the more CAPACITY the pipe and chimney flue has. It doesn't make more draft being bigger. It can only do that by feeding it more heat. Like a water pipe with larger diameter can carry more volume of water, but the water will be at a lower pressure, so it needs a bigger pump to move what the larger pipe can carry. In the case of a chimney, it uses heat to make it work. So to move more in the larger diameter, it needs more heat left up to create the same low pressure (draft) needed at the stove. You can't get more heat up the chimney limited by the smaller stove and small outlet, so bottom line is making the stove size and outlet match the chimney.

That is where most people buying stoves go wrong. They need to buy a stove for the existing chimney size first, and it must also need to be the right size to heat the area it's in. People with an 8 inch chimney end up with a 6 inch stove and the chimney soots up an loads up with creosote because the little stove can't heat the larger area inside the chimney. With smaller stoves like a Baby Bear people will tell me their stove doesn't heat as well as they were told it would and are not happy with it. When I find out they are using a chimney too large, it is because they hav veto let so much heat up just maintain enough draft to keep the smoke from coming in when door is open, the little stove doesn't have enough heat left over to heat the home. Install a liner the same size and insulate it, so they don't need to leave so much out, and suddenly the stove heats up and drives them out. It's all about the chimney, not the stove.
 
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