6" flue to 8" chimney?

[Rickmo]

New to your town, folks, so be gentle!! We're about to replace a big Aurora with an Andersen 10--found a good deal on the latter, and it fits the room a lot better. The Aurora has an 8" outlet, and the Andersen has a 6" outlet. The 8" i.d. chimney's only about ~12' long above the ceiling, which is ~6' above the top of the stove. I gather that there's no problem going from the 6" to the 8", but--any advice on where the reduction should occur, and what the best strategy is? (We've been burning the Aurora for 15 years with good results except that when the outside temperature is, say, above 25F, it has always been very hard to start. Any thoughts on why that would be, and whether there's anything I should consider doing about it?)

Thanks!

--RIck

 

[Grisu]

Hello and welcome to the forum!

Your problem with the difficult start of the Aurora may be the same problem you may encounter with the Scan 10: You have difficulties establishing a good draft and/or wet wood. Given that, going from a 6" stove outlet to a 8" flue could make things really difficult for you. If you do that you may have a poor performing stove and run the risk of serious creosote accumulation. I would look into either replacing the chimney with proper 6" double wall. Maybe you could also drop a 6" liner down there if code allows that.

The Scan 10 had a recall a few years back: https://www.cpsc.gov/en/Recalls/201...called-by-SCAN-Andersen-Due-to-Injury-Hazard/
I would check whether that stove is one of the ones mentioned and has been fixed. There were also some reports that Jotul may discontinue importing Scan stoves. I would check with the company and see if there is a local dealer not too far from you. May be important when you ever need some parts.

Be also aware that a modern, EPA-approved stove like the Scan 10 absolutely needs dry wood with an internal moisture content of less than 20% to perform well. That means the wood needs to be split and stacked in a sunny and windy spot for one to three years depending on the wood species.

 

[Simonkenton]

Yes, grisu is right. You don't want to go from a 6 inch pipe to an 8 inch pipe. People do it, but it can cause big problems.

 

[begreen]

I would put the increaser at the ceiling and run 6" connector pipe up to it. Use double-wall connector to keep the flue gases hotter. With 18' of straight up chimney it may work fine. Check the cap screen if there is one to see if it is getting clogged.

 

[Kosmik]

Yes, grisu is right. You don't want to go from a 6 inch pipe to an 8 inch pipe. People do it, but it can cause big problems.

If I wasn't renting, I'd have changed out the chimney.
But I had 99 problems with my setup, size being one.

I did as stated above, reducer at top, 6" down to stove.
I used a good bit of creosote remover, and had sub-par wood.

Still was able to heat the house, and after putting a key damper above the stove, was able to control my runaway situations before the point of no return.

Still, if you own, it's a real investment to not do that. Had so many problems that it was hard to tell where exactly they were coming from...But 6" to 8" did aid problems (and should have done a double wall to keep it hotter, but sw was cheaper).

 

[begreen]

There are several folks successfully running their 6" vent stoves on 8". It is not ideal, but within mfg. recommended spec as long as the chimney is tall enough to still draft well. Burn dry wood and don't choke the stove if you want to keep the chimney clean.

 

[Kosmik]

My draft was always good, but unpredictable (mainly because of bad chimney placement/height) but also in the process of heating up the 8" it just felt like it might go easier and be more predictable if there wasn't that expansion and slowing of gases there.
Which could have meant an easier time sweeping.

 

[begreen]

I just noticed your altitude Kosmik. I think that's what you are fighting. At sea level this probably wouldn't be an issue. The stack needs to be taller to draft well at high altitudes. If you want to go 6" all the way, by all means do it. It should improve operation. And if you can add a couple more feet, even better.

 

[webby3650]

I have a short 8" flue in my front room. I have used all of my stoves on it before moving them to my primary flue. All have have worked as well if not better on it than the 6" flue, both are the same height. In fact, the Leyden and the Cape Cod smoked out like crazy on a minimum height (15' from the floor)6" chimney. Neither stove smoke at all on the 8" chimney.
The Ashford on the other hand, did great on either flue!

 

[begreen]

Cat stoves can be a bit more forgiving of low draft. They don't need to pull air through the secondary manifold. Then again, what's your altitude?

 

[Rickmo]

This is a lot to think about and research. Altitude-550', begreen. And Grisu, the stove is outside the scope of the recall, but the parts issue is a risk we'll take (and there is a dealer in the neighborhood, which is where we're getting the stove--a floor sample). We do have a good supply of dry and drying wood--we live with the sadness of the Emerald Ash Beetle and so have a lifetime supply of white ash. I wasn't aware that the EPA approved stoves were more finicky about moisture content, though--good to know. Thans.

The state code doesn't appear to have any restriction on lining a flue. Assuming I can confirm this, am I right thinking that I can run 6" double-wall, which is 7" O.D., up the snoot of the 8"? Would there be any advantage to replacing the 8", other than that I'd know exactly what I had then (a prior owner put the Aurora in, in the mid-90s)?

Thanks for the great responses, folks.

 

[webby3650]

what's your altitude?

About 760 here

 

[Ashful]

My first Jotul was run with 6" pipe into an exterior clay 8" flue, and very similar heights to the OP for 20 years. No problems with creosote, as Grisu and simon speculate.

The draft was okay (not fantastic, but okay) up to 40F, and I just wouldn't burn when temps went above 40. I actually never had trouble with starting, but I would have trouble with occasional back puffing when I'd shut down the air real tight (downdraft cat stove).

About a year ago, I slid an insulated stainless liner into the 8" clay, and I think it improved draft a little, but not drastically. With the liner, I have draft inversion problems on startup, that I never had before, but we've also done some other things to tighten up the house, so maybe there are other factors at play.

 

[begreen]

VC has always included an altitude chart with their stoves. Perhaps this will be helpful to Kosmik. Note how it goes from recommending 16' at sea level to about 24' at 7200ft.

 

[Ski-Patroller]

We put in a Jotul Castine last fall, to replace our old Garrison. We had 12' of 8" single wall inside the house, and 12' of 8" insulated double wall from the ceiling out. We had quite a bit of difficulty getting the stove really hot. We replaced the interior 8" pipe with 6" double wall Duravent pipe this spring, and the stove draws a lot better and burns hotter. We still have 12' of 8" Metalbestos in the attic and the chimney chase, but don't see any reason to change that part. I suspect that if the lower 12' had been double wall Duravent we would not have had to make the change to 6"