BK Ashford 30.2 creosote question

[Fly Navy]

New to this forum. Long time wood burner, burn only well seasoned oak, hickory, and a little maple or cherry. Replaced my 40 ish year old VC Defiant with Blaze King Ashford 30.2 in 2022. Defiant was a heat monster but harder to control at lower settings. BK is a definite improvement there, and in efficiency. While the BK flue is 6” inside (vice the large oval pipe the VC had) it transitions to the VC’s 8” Class A stainless insulated pipe to the outside. Is this an issue? I ask because I am concerned with a large increase in creosote in the BK flue pipe, which I never had in the old VC. I always ran the VC quite hot at least daily when it was running low and am quite aware of how creosote forms. I do the same on the BK as needed. I have noted a large temperature differential between the base of the single wall flue pipe and the top (about 4.5 ft up) where it goes into the collar and then transitions to the Class A. For example, the IR temp right now reads 490F at base, 280F 8” up, and 160F at top. Cat about 900F. Don’t recall this extreme differential with the 8” flue piped VC. Is this normal? Wondering if the 6” to 8’ transition is a problem. Any ideas?

 

[EatenByLimestone]

Going from 6” to 8” is a large increase in area. This slows down the velocity of the gasses and allows them to cool. That said, if the flue was run hot enough to keep the moisture from condensing creosote wouldn’t form. It’s also possible that there’s too much moisture still in the wood.

 

[Fly Navy]

Thank you, that was exactly my concern. But there is such a dramatic temp drop from the base of the flue pipe to the collar at the ceiling, about 4.5 ft; the temp now is 550 at base to barely 200 at ceiling (cat around 1300). Air control at around the 85% point. I am able to do that today because it is around 0 degrees, but cant run it that hot for long on warmer days. And the 6" to 8" junction is where I really noticed the creosote when I cleaned it out a few weeks ago.

I will recheck the wood but it is all >2.5 years aged and tested around 12-15% at start of season. I will check the flue as weather permits and be certain to spend some time at high temps daily (although I had been doing that). I just rarely had any creosote issues when I was running the VC and the bigger internal flue pipe. It was easy to get the temps up there every day.

Wondering if this warrants switching to 6" external flue.

 

[EatenByLimestone]

That’s the recommended size. The BK probably sends much less heat up the flue than the old VC.l, complicating the issue.

I don’t know that the base is the best place to take a good starting reading from. It’s probably getting heat transferred from the stove. Starting with the 8” figure might be more accurate.

If your pipe buildup is good until the transition from 6 to 8 happens, it seems like that might be the culprit. You’re going into Class A, so you’re insulated. Can you increase the heat going up the flue a bit by adding more air? Did this buildup occur during the shoulder season when you might have let the Class A cool down more than it is now? If it’s kept warm now, you won’t be getting any more buildup.

 

[Fly Navy]

Thanks for the 8" suggestion, it makes good sense. After moving it the temp differential isn't quite as extreme but the issue of low temps at top unless the stove is running quite hot remains. Earlier this year was rather warm outside so wasn't running as hot. Right now isn't an issue because it is quite cold and BK is cranking. Concerned when temps return to more normal.

As to exact location of creosote buildup, kind of hard to tell when looking down chimney. The 8" insulated section was pretty clear, minimal dry soot. The issue appeared to be worst near the junction but kind of hard to tell how much accumulated below that spot.

While researching other forum questions I noted a comment that an air leak in the uninsulated pipe might cause a lower temperature downstream. My inside pipe is a 2 piece and where they come together there is a small gap of about 5/16" in one area, gap does not extend around pipe. Wonder if that causes issues. Picture attached.

 

[BC_Josh]

Thanks for the 8" suggestion, it makes good sense. After moving it the temp differential isn't quite as extreme but the issue of low temps at top unless the stove is running quite hot remains. Earlier this year was rather warm outside so wasn't running as hot. Right now isn't an issue because it is quite cold and BK is cranking. Concerned when temps return to more normal.

As to exact location of creosote buildup, kind of hard to tell when looking down chimney. The 8" insulated section was pretty clear, minimal dry soot. The issue appeared to be worst near the junction but kind of hard to tell how much accumulated below that spot.

My BK manual (for my smaller stove) states that air gaps in chimneys is definitely a problem. This gap should not be there.