Insulated 8" Chimney Performance On A Stove With a 6" Outlet

[Renovation]

Hi Folks,



In another thread, BeGreen posted:

[quote author="BeGreen" date="1292853494"]

From the Englander 30NC manual: "The design of your new appliance is such that the exhaust “smoke†is now at lower temperatures than in the past, therefore requiring proper chimney size to give adequate draft. If your chimney is too large, the heater will have a difficult time raising the temperature of the flue enough to provide adequate draft, which can cause a "smoke back," poor burn, or both."[/quote]

To avoid threadjacking, I'm posting my question here:

Is a 2" diameter chimney oversize (8" versus 6") a significant issue with a well insulated pipe--like an insulated liner or Class A in an insulated chase?

Asked another way,

Would a well-designed 8" chimney (straight, insulated, tall) likely cause significant performance issues with a 6" EPA stove, with it's lazy, cool exhaust?

Hint: I'm hoping the answer is "no".

In case you're interested, here's my motivation for this question.

Pending investigation of the new Woodstock I'm leaning towards a Blaze King King. The Woodstock requires a 6" chimney, the King an 8". I'd like excellent performance with whatever stove I get, with the flexibility to run other stoves in the future. I have to decide before putting in my chimney.

After much pestering, investigating and calculating, and no manufacturer response to my technical questions, I've come to the conclusion that running an 8" stove on a 6" chimney is safe, with the chief downside being increased smoke spillage on reload, and possibly reduced maximum output. (If anyone has more insights into that, please share!)

That has me leaning towards biting the bullet and putting in an 8" chimney as the best bet against future needs. But if an 8" will probably perform poorly with a 6" stove, I want to take that into consideration.

I know it's hard to predict hypotheticals, but I'd appreciate any insights!

 

[Hardrockmaple]

I cannot speak to an 8 in. chimney, what I can tell you, from personal experience, is that I ran my FDW small w/cat & 6 in. requirements through a 8x10 in. masonry flu for 15 years. This summer I installed a 6 in. SS liner down that masonry flu and hooked up the same FDW small w/cat to it. I have found there is no noticeable difference in draft between the two.

The chimney sits in the middle of the house and runs straight up 32 ft.

hope this helps.

 

[cmonSTART]

The stove will perform BETTER with a properly sized flue, but unless the manual says it MUST be hooked to a 6 inch round liner, you can probably get away with a well done 8 inch flue. It's better than undersizing a flue but some food for thought: An 8 inch round flue has almost twice the cross sectional area of a 6 inch flue. The difference is actually pretty significant.

 

[oldspark]

Not to hyjack this thread but my flue temps are not that much cooler than my old stove, in fact on start up and reload I run them hotter than the old stove.

 

[begreen]

+1
Note that the quote is not from me, it is out of the Englander 30NC manual. If there is a question for a specific stove installation, check that stove's manual. All stoves do not draw equally. And very few installations are identical. Confounding factors can be stove and flue location, negative pressure, wind, etc.

 

[LLigetfa]

Basic math would show that an 8 inch flue needs almost double the volume to maintain the same velocity as a 6 inch. If the velocity is too slow, the gasses have more time to cool along the way. If the flue runs up through the house, it most likely wouldn't be an issue but if going external, it may be more of an issue.

 

[Renovation]

+1
Note that the quote is not from me, it is out of the Englander 30NC manual. If there is a question for a specific stove installation, check that stove's manual. All stoves do not draw equally. And very few installations are identical. Confounding factors can be stove and flue location, negative pressure, wind, etc.

Thanks BeGreen, I've updated my quote from you to note that.

And thanks all, for your observations and comments.

I'm acutely aware that all installations are unique, which is why I'm being as diligent as I possible in selecting the most compatible compromise between 8" and 6" chimney, knowing that I may want to burn a different stove in the future.

Considering your comments, and the fact that Blaze King specifies an 8" flue for its sometimes-cool-and-lazy-drafting King (Smoke spillage on reload with that big firebox, big door and lazy draft is enough of an issue that the website dedicates a page to minimizing it.), I'm thinking the 8" flue is the more flexible choice.

Yes, there are a multitude of confusing and conflicting compromises, but I'm thinking/hoping that--with a straight, tall, well-insulated chimney--I'll have adequate draft once hot with either one, and that the difference I'm most likely to notice between the two is the almost-doubled air exhaust rate on reload with the 8", minimizing smoke spillage. Seem reasonable?

As some are painfully aware, I already have the piping for a 6" chimney on hand, so the 8" alternative is not the cheaper one.

Please keep your experiences and thoughts coming.

Happy burning!

 

[Renovation]

I cannot speak to an 8 in. chimney, what I can tell you, from personal experience, is that I ran my FDW small w/cat & 6 in. requirements through a 8x10 in. masonry flu for 15 years. This summer I installed a 6 in. SS liner down that masonry flu and hooked up the same FDW small w/cat to it. I have found there is no noticeable difference in draft between the two.

The chimney sits in the middle of the house and runs straight up 32 ft.

hope this helps.

It does, Hardrockmaple, thanks very much.

 

[branchburner]

I think "tall" may be important here - how tall is tall?

 

[Renovation]

I think "tall" may be important here - how tall is tall?

The stove will be on the first floor of a two story house. I'm figuring 6' of double-wall connector into a minimum of 16' of Class A, straight up, insulated chase, effectively interior. So about 22' minimum, maybe 24'. I can make it taller if need be. Thoughts?

 

[begreen]

You'll probably be ok with that plan. Straight up interior flues are more forgiving.

 

[TX-L]

You should be ok. I installed an 8" MetalFab insulated chimney some years back with a friend who hooked up a Timberline. Just this year he has upgraded to a Quadra-Fire (he's VERY happy, but that's another story). Anyway, we connected the stove with 6" single wall pipe up to the 8" ceiling connector, it works just fine. This particular install is straight up, no elbows, in a single story house.

 

[Skier76]

I have an 8" exterior chimney running off a Castine which has a 6" outlet. It works. I'm sure it's not "ideal". But I wasn't about to spend a few grand on a new chimney at the weekend house.

It sounds like you should be OK with your setup. But as others have mentioned, it will probably burn a bit better with the 6".

 

[Renovation]

Thanks folks, for the continuing insights and experiences! :coolsmile:

It looks like an 8" flue is more future-proof than a 6", which is the heart of my concern--I'm thinking of changing this thread title to that.

The manuals I've read are much more negative about running a 8" stove on a 6" pipe than a 6" stove on a 8" pipe. I've seen "no without special dispensation from your local inspector" to the first, and "may cause performance problems" to the second. And I'm guessing inspectors and insurers would be more approving of the second, as well.

Again, the 8" solution means I pay for new pipe, so I'm willing to bite the bullet and do it right.

To semi-hijack my own thread, has anyone had any insights into installing a 6" liner in an 8" pipe? Is it straightforward? Is termination a problem at either end? That's another future option with an 8" pipe.

Thanks again, and happy burning!