Reducing 8" chimney for Jotul Oslo

Getting closer and closer to pulling the trigger on an Oslo to replace my VC Encore. The Encore has an oval to round adapter (not sure of the right term) and flows into my 8" chimney pipe.

The Oslo is 6" so I'd need to step the chimney down to 6". Is there anything I need to know here in terms of impacts to draft or anything? Is this a common thing? The shop I'll be buying from said it was no big deal but wanted to check with you guys.

Thanks.

My Firelight 12 manual stated the same as your Oslo manual, "If a round fireclay liner is to be used it must have a minimum inside diameter of 6” (157mm) and not larger than 8” (208mm) in diameter." My chimney was on an exterior wall, all stone and brick with an 8" round clay liner, and not exceedingly tall. It worked okay in cold weather, but drafted poorly on rainy days and slightly warmer days, and prevented me from running the stove as low as I'd like without stalling.

I slid a blanket-wrapped 6" smooth-wall flex liner down inside the 8" clay tile, and it made a noticeable improvement. I wouldn't hesitate to recommend the same to anyone in a similar situation.

I've never measured it but it's class A chimney pipe and somwhere around 20' I'd ballpark. No bends, straight shot.

Oh, and not a liner in a masonry chimney. The home was built for a wood stove, never had a fireplace.

With 20 feet height of 8", you may have more draft than you desire. I'd try installing a flue damper to control that draft, and running with the 8" for a season to see how it works.

I'm not sure about that. Wouldn't the 8" pipe potentially decrease the draft?

As I understand it, height and temperature differential create the pressure. The pipe diameter simply dictates how much volume can flow thru the pipe, for a given amount of pressure.

The electrical analogy would be height & temperature diff create voltage. The pipe diameter is your conductivity. Air volume is charge, so flow (CFM) is current, or charge per time.

The only potential flip side I could see is if you went way too big, and then your flue gasses may cool due to the reduced velocity. I imagine you could get a stalled draft, in this circumstance.

That was what I was worried about of course. I was thinking it might but I don't pretend to understand such things.....

And 20' is a guess. No more than that but I'd say at least 15'. Theres about 6' from the stove to ceiling and maybe 8-10' in the attic then about 3-4' above the roof.

You better put a damper in place - never hurts to have some extra control. Same setup i ran into, 8" flex liner in place and bought an Oslo. My chim is probably 20' or so, straight up. Hooked it all up, draft was strong (chim is internal), i use the damper sometimes to slow things down. Despite going from 6" stove outlet to 8" liner - strong draft at 20+ feet.

I went from an encore to an Oslo. Exterior chimney, 8" ss liner, 24', insulated.
I would grade the first year at ok to good.
A few months ago I dropped an uninsulated 6" ss liner inside the existing 8" liner.
I have noticed a difference. The Oslo seems to run better. I'm waiting until the end of the season to post a final opinion.
So far, I'm glad I did it. I'm guessing it will only get better with colder temps.

Can you tell me what differences you're noticing? Better draft? My wife is in strong favor of a brown enamel so the Jotul or T5 Classic seem to be the main choices but both are 6".

all other things being equal, you will NOT see an improved draft with 8" (compared to 6"). But with other offsetting strengths (examples: good gaskets, good wood, tall chimney, good liner, no horizontals, etc) you will find the difference is insignificant. Mine is working perfectly well, because all of those other strengths are in place. That said, I do like BV's solution above, too

So far, the main difference I am seeing is improved draft, especially on start ups and reloads. I am able to start shutting the air down sooner, saving time. Last year, with the 8" liner, I had to wait longer for temps to go up to start shutting down the air. This year, with the 6", it seems like I can dial it in sooner, saving time.
I think I am also getting better burn time. I am hesitant to give a detailed informed opinion just because it is still early in the season.
One thing I am positive of, the glass stays cleaner, longer. I'm not talking about black glass, never had that. I'm talking about the white haze that accumulates over time in the shape of a tombstone on the front glass. I had more of it, more frequently with the 8" liner. I still get it now but it takes longer to show up and the air wash gets rid of it quickly with a hot fire.

Went from a Defiant to an F600. Put 6" black pipe from the stove to a 6"-8" increaser at the ceiling box. We kept the original 15 or so feet of 8" class A from the ceiling box to the cap. Haven't had a problem in two years with this setup.

Quoting myself, but Begreen... can you comment on this? I've always read large pipe = more air at same pressure differential, that differential being governed by height and temperature. There's a reason big fireplaces had big chimneys, and small fireplaces had small chimneys.

The oft-quoted overfiring created when a door is cracked, versus wide open, is a case of increased velocity created the accellerating air thru the crack. Not any indication of greater mass air flow.