need some advice on what appears to be a unique creosote issue

[mabrown3]

let me start by staying I am not new to wood stoves. I grew up in a home with a VC defiant encore. when I built my new home 4 years ago, I wanted to go back with what I grew up with. so I purchased a 1986 VC defiant encore. now I would like to provide specifics for my installation and hopefully someone can advise why I have a creosote problem. the stove is mounted in a newly built home. coming out of the stove I have about 4 feet of vertical single wall duct that elbows into a 6" clay thimble (with code required clearances) and enters into a fully external chimney that is a clay 8x8 flue pipe and red brick outer "shell". outside dimensions of the chimey are about 27" square. the chimney is about 35' total height, with about 22' of height of the chimney being above the griddle surface. draft is not a problem. without a fire, a match will blow out when held close to the open damper. never had a backdraft nor smoke plume in the stove. I burn wood that has been seasoned, typically 18-20% MC. I do have a few that measure 22%, and those are split small to about 2" thick. I am currently burning the stove at about 600 deg F as I type. about 4 weeks ago I had a chimney fire. it was put out fairly quick with an extinguisher. I had a certified sweep inspect it and he indicated there was no visible signs of damage, but there was a lot of 3rd degree creosote where the thimble entered the chimney. obviously what creosote was at the top burned off quick. because of the fire, all creosote was brittle and was removed mechanically. I relit the stove and had a continuous fire for about 2 weeks until the temps picked up outside. when I let it go out, I decided to disassemble the single wall flue inside and just have a visual look to make sure everything was OK. to my surprise, I had 3rd degree creosote big time. being that I had no idea why I would have this being that the lowest maintained temp of the stove was about 375, with typical temps of 500 at the griddle, I put it back together and relit the stove with an extinguisher no more than a few feet away. I let it warm for about 4 hours, maintaining 500-600 degrees. I did not engage the Cat yet, and decided to climb a ladder to the top of the chimney and check the temp. the flue at the top of the chimney measured 110 degF. but there was very little smoke. I could stick my hand down the chimney with no issue. with the cat engaged and warm, the temp went down to 100 deg F. again, with a griddle temp of about 500-600. I have no creosote in the stove, glass is clean and clear. so I know my fire is clean. I do see some thicker smoke when I freshly load it, but typically see light white smoke or just heat vapor at the top of the chimney. why on earth do I have such a low flue temp. why do i have so much creosote so quickly. needless to say there is a glaze about 1/16" thick at the top of the flue right now. at this point, unless someone can offer some insight beyond the contractor that built the home, the sweep, and a local stove vendor, I may be forced to tear down the chimney and rebuild with an insulated SS flue. I have also wondered about insulating the chimney and adding another layer of brick veneer. any insight is greatly appreciated and I can add any other info that is needed if requested. thank you.


let me also add, I have been using the stove for 3 seasons, this season being the 3rd. stove was installed after house was built, but chimney was always there. I ran a wire brush for an 8x8 flue up and down the chimney each year, removing what I assumed was typical solids of creosote, but nothing I thought was major. I am sure the brush did nothing for the 3rd degree stuff, but the 3rd degree that built up over the first 2 years was either burned in the fire or mechanically removed after (drill and cable combo by the sweep). I believe the majority of the creosote I am seeing now developed over the couple weeks after the fire. I also leave a window cracked during burns to make sure I sustain a good draft. new houses are built too tight it seems

 

[rwhite]

I think you'll find your situation is not unique. You have a 6" (32 sqin) outlet venting into an 8x8 clay flue (64sqin). It needs a stainless liner in it. And because it's exterior it needs to be insulated which won't fit and the tiles will need to be busted out. That's probably the only solution other than wait for the next fire.

 

[mabrown3]

I think you'll find your situation is not unique. You have a 6" (32 sqin) outlet venting into an 8x8 clay flue (64sqin). It needs a stainless liner in it. And because it's exterior it needs to be insulated which won't fit and the tiles will need to be busted out. That's probably the only solution other than wait for the next fire.

so your thoughts are similar to what are my thoughts are if no one had a better solution, which is an expensive solution. replace chimney with a SS insulated flue...and maybe brick around it for cosmetics. man, that sucks. but this is what I feared would be the solution. do you happen to know if the flue can be removed/busted out without tearing down the chimney to the wall thimble. also, are you thinking because a 6" thimble entering into an 8x8 flue is creating an orifice affect, meaning a pressure and temp drop when the flue gasses pass thru the thimble into the flue. didn't know if this could be contributing. if so, maybe I could replace the thimble with a larger one. do you think a viable option would be to fill the cavity between the brick veneer and flue with a vermiculite to insulate the flue. thanks for your reply

 

[rwhite]

@bholler , he can tell you for sure.

 

[blacktail]

@bholler , he can tell you for sure.

I bet he'll say to break out the clay tiles and install an insulated 6" liner inside the chimney.
The current 8" flue causes the gases to slow down and cool.

 

[Babaganoosh]

Yeah you don't have to rip the chimney down. Just get a liner in there.

 

[coaly]

6 inch round is actually 28.26 square inches increasing to 64 square inches. Allowing the hot flue gasses to expand causes them to cool. You need 250º at the top to prevent water vapor from condensing that allows smoke particles to stick. A less efficient or larger stove with 8 inch outlet allows more heat out, which is what the current chimney needs, or decrease the diameter of flue for your stove.

 

[rwhite]

I bet he'll say to break out the clay tiles and install an insulated 6" liner inside the chimney.
The current 8" flue causes the gases to slow down and cool.

Almost 120% positive and 111% chance I need to work on my math!

 

[Easy Livin’ 3000]

Almost 120% positive and 111% chance I need to work on my math!

Looks like bholler has his staff trained up! I was thinking exactly the same thing.

I believe they use a special tool to break out the tiles, image an unbalanced weight spun around at the end of a shaft. Probably not the end of the world. A chimney fire might be.

 

[bholler]

You guys have it covered pretty well but I would bet that you have air getting into that chimney somewhere below the crock which is cooling the exhaust very quickly causing the creosote buildup. Make sure the cleanout is sealed up well and that there are no open block cores or other gaps in the chimney anywhere that could allow dilution air into the smoke stream.