replacing burned-out Defiant - 6 or 8 chimney?

[GreenMountain]

I'm replacing an original non-cat Defiant. The back is burned out; I'd rebuild it, but there's a crack in the bottom plate so I think the stove is a total loss.

Essentially everything I'm thinking about as a replacement (BK Boxer is the leading candidate) takes a 6" flue. The existing chimney is 8" and in pretty good shape. There's about 9 feet of double wall inside, straight through a thimble to 6 or 7 feet of triple wall outside.

It would be nice if I could avoid messing with the existing roof penetration. Is it kosher to connect a 6" flue inside to the existing 8" thimble and outside chimney? Or is there a way to line the existing chimney?

Or should I just take the path of least resistance, and get a new Defiant Flexburn and preserve the existing 8" setup?

 

[SpaceBus]

What about a BK King that uses an 8" flue?

 

[begreen]

Cat stoves are going to be a bit more sensitive to the reduced draft and additional cooling from the transition to 8". They run at cooler flue temps to start with. Going to an outside chimney is going to make it worse, especially if this is to a larger area masonry flue. An insulated 6" liner in the chimney will be the best solution. What is the ID of the current chimney?

 

[GreenMountain]

I thought about the King, but the bigger box gives me some clearance issues with the existing hearth.

The chimney is metal all the way up. So the simple solution would be 9 feet of 6" flue inside the house, then an increaser to the 8" thimble through the roof, and 6 feet of 8" triplewall above the roof. A local installer suggested that approach, but a slowdown in draft at the 6->8" transition is exactly what I was worried about. If the existing 8" thimble/chimney assembly can be lined, that would be a nice compromise. But otherwise I'd have to change the roof penetration.

The Defiant's draft through the existing 8" system is super strong. It practically sucks the paint off the walls.

 

[begreen]

BK specifies that the connector (stovepipe) be 6" so that is correct. Make the stove pipe double-wall to keep the flue gases warmer.

It sounds like you have a ceiling support box and not a wall thimble. That was throwing me. With a straight-up, interior, flue system it's worth giving it a try. It might work fine.

 

[Woody Stover]

How tall is the chimney? Taller is better for more draft in the 8" but it might be harder to get a draft going if it's not too cold out. If you have backup heat, you could just wait for cooler weather so you don't have to fight to get the draft going.

 

[GreenMountain]

@begreen - correct - the flue is straight up through the roof. @Woody Stover, it's reliably cold here so we never have draft problems (except on another stove, where some bonehead ran the pipe horizontally into a cold chase). The stove's also on the second floor, so I think the house's chimney effect is augmenting the natural draft. I think I'll try it - after all, if it doesn't work well, the only part I'm out is the 6->8 increaser.

Thanks for the thoughts.

 

[Woody Stover]

it's reliably cold here so we never have draft problems (except on another stove, where some bonehead ran the pipe horizontally into a cold chase). The stove's also on the second floor, so I think the house's chimney effect is augmenting the natural draft. I think I'll try it - after all, if it doesn't work well, the only part I'm out is the 6->8 increaser.

I was in Bozeman years back. Lost my backpack by leaving it in a hotel lobby and asking the desk guy to keep an eye on it for me whilst I visited a local watering hole. Young and foolish I was.
The altitude there will cut your draft some, but the fact that the stove is on the second floor should counter that I think, as you say. We were there at the end of July, and it still wasn't all that warm, especially on the mountains. I agree, give it a go and see what happens, you have little to lose (but the stove is a big investment if it doesn't work satisfactorily.) You could go to double-wall chimney outside, which holds heat better than triple, or build a chase around it maybe, but that's gonna cost. My cat stove breathes well but I have an insulated liner in a sealed masonry chimney, at low altitude. I don't think the BKs breath quite as easily, but I'm unsure how different the Boxer is from their other stoves..
Wow, this is getting hard to wrap my brain around..lotta factors at play.

 

[GreenMountain]

Surprised about your pack - this place is usually pretty safe. Years back there was more of an air of economic desperation though.

In July we're usually more worried about the fires outside than the fires inside.

The outside chimney actually is double - I misspoke. I have two other air-cooled triplewall chimneys that I've basically decommissioned because they don't work worth a damn in our weather.

Add to the complexity the fact that I've been busy air-sealing the house. But I have a feeling it's going to work.

 

[GreenMountain]

Just a quick after action report:

We picked up the Boxer on Thursday and installed it Fri/Sat. Getting the old Defiant out was surprisingly easy. Getting the Boxer in was surprisingly hard. I think it's about the same weight, but the large size of the box, particularly the height, made it a lot more awkward to maneuver.

The fit and finish on the stove is impressive. Zero issues there. The close clearances allowed make up for the size of the unit.

I'm underwhelmed by the DuraVent DVL pipe we used. LOTS of issues:
- There's a huge size gap between their pipe and their stove adapter. We hoped to fit a pipe directly to the stovetop, but no amount of crimping seemed to work; it didn't help that the pipe was somewhat out of round to start with. By contrast, the adapter was way too small, so we had to uncrimp it to get it to fit.
- Some of the flow direction stickers were backwards - it's not really a mistake you could make anyway, but why bother with the stickers then?
- One of the predrilled holes was exactly aligned with rivet, making it unusable.
- The adjustable elbows are pretty cranky (this may be inevitable for a double wall).
I think minor pipe issues were responsible for 95% of the cursing on the job.

Draft seems fine with about 18' of chimney total (half 6", half 8"). It's only about 30F out; hopefully that will continue to be true below 0. First fire produced an impressive amount of heat from a small amount of wood.