HOT chimney

[Montana Blaze King]

We have a Blaze King woodstove that has been our main source of heat in our house for about 5 years. The King is in the basement. Off the top of the stove, we have double wall black pipe. About 6' above the woodstove there is a 90 elbow, then a short piece of horizontal pipe until the transition to SST insulated double wall pipe. The insulated pipe then passes through the exterior wall of the house and into a chimney chase framed on the outside of the house. At the wall penetration where the insulated pipe passes through the wall, an insulated wall thimble is used. The pipe then turns 90deg upward at the wall support via an insulated Tee and Plug. There is about 25' of vertical insulated pipe in the chase, then a standard cap. We have been burning standing dead lodgepole since the stove was new. All of our firewood comes off some nearby property and our wood storage, wood splitting, wood species has all been consistent for the last 5 years. I mention that to rule out the fuel as being a variable.

We have recently had a couple close calls with the insulated pipe getting so hot inside the chase that it has charred the framing that makes up the chase. The chase itself is 2x4 framing, with batt insulation. The insulated pipe has more than 2" of clearance to any of the wood framing and a fire stop joist shield is utilized at the floor penetration in the chase. There is only evidence of charring of the framing at the floor of the chase where the fire stop joist shield is located. Our standard burning procedures are to do a hot burn in the morning getting the stove well into the active stage by the temperature gauge that is located on top of the blaze king. We burn it hot for at least 20 min, then stock it full of wood, engage the catalytic bypass, damp the air and allow the stove to burn all day. When we return to the house after work, the stove is easily still in the active zone on the temperature gauge. We sweep the chimney every year and get very minimal build up - a small pile of dust is about all we get.

Year after year, the stove has performed great: good draft, easy to start, nice controlled burn. Until the day after christmas this December. We had allowed the fire to die completely the day prior and started from scratch that morning. We got a small fire started in the stove using our standard procedure of newspaper/kindling. Once the fire got started we filled the firebox and started the hot burn - catalytic was not engaged. We started to smell a burning plastic smell. Upon investigation, the firebox appeared normal with a vigorous hot fire, the temperature seemed reasonable with it registering in the active zone, but not pegged, the chimney cap appeared normal - no flames, and no whooshing sounds of air that I have heard a chimney fire can be explained by. Even though all seemed normal, the inside of chimney chase got really hot. We ended up opening up the sheetrock on the inside of the house to gain access to chase for further investigation. That is when we discovered the charred framing along the floor of the chase. Although the charring was minor, it was enough that you could smell smoke inside the chase and the batt insulation.

We kept the wall open to the chase to allow monitoring of the outside wall of the pipe. Using a laser infared temp gun we have kept track of the chimney pipe temp inside the chase and note that with the condition of the stove (i.e. temp guage reading on the stove, is the catalytic engaged, is it damped, etc). On a normal burn, the exterior of the chimney was measuring between 120 and 160 deg. One other time, under very similar circumstances to the day after christmas (hot burn, no catalytic engaged), the pipe got real hot and measured 384. At this temp, the framing in the chase was just starting to smolder and when I sprayed the framing down with water, it steamed and sizzled.

Things we have done to trouble shoot:
1) hired a local certified chimney installer/sweeper to inspect the entire installation. All clearances and proper installation were verified

2)We have recently replaced the temperature gauge just to ensure we were reading proper temperatures on the stove.

3) inspected the insulated pipe for damage, it all looks clean, straight, no warpage, no creosote build-up.

We can't find anything wrong with the installation or our process of burning the stove, but yet we are obviously experiencing some extreme heat inside the chase.

On both instances where the chimney got HOT, the plug at the bottom of the clean-out tee was loose and creating about a 3/4 inch opening around the perimeter of the plug. The bottom of the chase is open to exterior ambient air until the floor joists. Is it possible that this opening was a fresh air source and that due to the fact that the catalytic was not engaged that there was enough combustibles left in the flue gas/exhaust that it was reburning the flue gas inside the chimney? This would explain why there was no "whooshing" sound of air, but yet still explains how a chimney fire was occuring with no creosote present.

Since that time, we have secured the plug tight into the cleanout and not had any issues. We have deliberately created some really hot fires to test this theory and the hotest the chimney measured was about 254, with no smoldering of the framing and no smoke smell inside the chase. Our ultimate goal is to build our confidence that we have located the problem, fixed it and can move on with patching the sheetrock in our house.

Thanks for reading this long post and offering up your insight and suggestions.

 

[pen]

I'm thinking that you are letting things get too hot for when you are in bypass and flames are essentially shooting right up the chimney.

 

[begreen]

One possibility is that the insulation in the pipe may have failed. Perhaps there have been a chimney fire or two in the past that were small enough to not be noticed? Or perhaps the insulation settled leaving a part of the pipe less protected? Was the chimney professionally installed when you got the stove? If so, usually the pipe is warranted.

 

[jatoxico]

I would have thought the open tee would tend to cool the flue gases but maybe a small leak was introducing fresh air low in the system and acting as an afterburner.

Don't really have anything to add but I am wondering if the ignition point of the framing in the chase has been lowered now that it has been charred a couple times.

It's worrisome that all clearances were honored and there was this problem. I would have thought there was more of a margin of error. If the pipe is damaged or failed as BG suggested perhaps replace with triple wall?

 

[begreen]

Triple wall is usually the value line pipe. It would need a new chase because the pipe still needs 2" clearance and it is larger in diameter. It would need new flashing as well. If the chimney needs to be replaced perhaps consider shielding the wood in the chase with sheet metal on spacers. I'm also wondering about venting the chase if the insulation is allowing it to build up heat within.

 

[DougA]

The first thing I would do is buy a proper flue thermometer. I like the Auber unit because it is digital, very loud alarm that you can set the max. alarm and it's super easy to install. I was posting just a few hours ago to agree with another member who was speculating on the number of overfires he had prior to installing his flue thermometer. That together with a stove top thermometer will tell you what your stove is doing and what your fire is doing. It's the best way to know what is going on inside your stove and flue. You probably far exceeded the temperature limit on your chimney. Many of us have come very close. I've had stove top of 700 on my VC and a flue of over 1300.

It's the best $65. you will spend. http://www.auberins.com/index.php?main_page=product_info&cPath=17&products_id=292

 

[Montana Blaze King]

@pen: We were instructed to conduct our "hot" burn with the catalytic in bypass, are you suggesting that we keep it engaged during the hot burn?

@begreen: I wondered about the insulation failing in a chunk of pipe. How would we go about investigating it? I installed the whole system myself, so Im guessing no warranty applies.

 

[blueguy]

I'm thinking that you are letting things get too hot for when you are in bypass and flames are essentially shooting right up the chimney.

I agree. It was mentioned the only times it has done this is with a hot burn, bypass open. The OP mentions using the thermometer on the stove top to measure temps, I wonder if they're using the cat probe to measure stove top temps? If so, this will not give an accurate measurement and I'm willing to bet they were in close to overfire territory with a full box, air wide open and bypass open.

 

[blueguy]

@pen: We were instructed to conduct our "hot" burn with the catalytic in bypass, are you suggesting that we keep it engaged during the hot burn?

Your manual gives specific direction on hot burns (bypass closed)

 

[jatoxico]

Triple wall is usually the value line pipe. It would need a new chase because the pipe still needs 2" clearance and it is larger in diameter. It would need new flashing as well. If the chimney needs to be replaced perhaps consider shielding the wood in the chase with sheet metal on spacers. I'm also wondering about venting the chase if the insulation is allowing it to build up heat within.

OK did not know that. Thought maybe the triple ran cooler outside wall temp so less clearance required while solving the already charred wood issue.

 

[begreen]

You could be correct. I've never measured the outside temps of modern triple-wall chimney.

 

[jatoxico]

You could be correct. I've never measured the outside temps of modern triple-wall chimney.

Doesn't matter if the manufacturer still calls for 2" clearance gotta honor it, I was just assuming. Not to derail but then what's the purpose of triple, maintain higher internal temps?

 

[begreen]

Just guessing, but maybe so that a less expensive insulation can be used in the first layer? That would be worth a call to DuraTech support.

 

[DougA]

Even if you install it yourself the warranty should be honored but an overfire may void it.

I had the same question to a chimney inspector with my old SS chimney. I was told that there is no way to inspect the inside to see if the insulation has fallen. The only way they can tell is if there are obvious signs of charring or warping. I replaced it due to it being 30 yrs old and of unknown quality. The only reference I can provide is a double wall SS chimney which I used to run directly off my old VC (for closer clearance than stove pipe). Even with a hot stove, the SS chimney was never so hot that it would burn my hand when I touched it. There is no way it was ever close to 384 as yours measured.

 

[Montana Blaze King]

Overfire might be an option, the only thing that doesn't explain in my mind is the fact we have been using this wood stove for 5 years and just now we have had two overfires in one month.

We are using the catalytic temperature probe to get a rough idea of how hot the fire box is. I understand it doesn't tell exactly what the temp is, but due to the fact it is a bi-metal coil thermometer, it should react to temperature change in a linear and repeatable manner.

No takers on my theory of the plug being loose and providing a fresh air source. That part I know is one thing that has changed from years past.

 

[begreen]

It's possible that there was enough unburnt gases passing through that they were flaming up the chimney. Whether some air leaking in the tee cap made it worse is unknown. What is known is that the damage is done. Pyrolysis has started in the wood. It now has a lower ignition point. That would make me nervous. I would be thinking about replacing the pipe and rebuilding the chase, running without a chase or at least shielding combustibles in the interior of the chase with metal on spacers.

 

[pen]

We are using the catalytic temperature probe to get a rough idea of how hot the fire box is. I understand it doesn't tell exactly what the temp is, but due to the fact it is a bi-metal coil thermometer, it should react to temperature change in a linear and repeatable manner.

Unfortunately, they are not as clear a measuring device as we'd hope they would be in this situation.

As a farmer is good in his field, so are certain thermometers.

Here's an example of one I bought and was used in my flue and the results were questioned,



Long story short, while the probe was obviously way off for reading a pot of boiling water, it still met factory standards for being used on double wall pipe (it's a long story and an old thread here, but that thermometer may read wrong for what is shown but still reads right to this day in the double wall system of my cousins stove, where it lives now)

I'm concerned that in leaving the bypass open on a "hot fire" you were letting things rip too much straight up the flue and the probe, not being designed to measure that, couldn't let you know it was happening.

pen

 

[alforit]

Our standard burning procedures are to do a hot burn in the morning getting the stove well into the active stage by the temperature gauge that is located on top of the blaze king. We burn it hot for at least 20 min, then stock it full of wood, engage the catalytic bypass, damp the air and allow the stove to burn all day. When we return to the house after work, the stove is easily still in the active zone on the temperature gauge.

Your technique is suspect...........if you burn it hot with flames for 20 minutes.. and then load it full with wood ? Without allowing that new load to catch flame and char ??

If that is the case you are setting yourself up for massive offgassing of that wood. That would explain the gases going up the chase.
That full load you put in needs some flame action and needs to char , otherwise that load is sitting on a bed of hot coals and will smolder it quickly , creating too much offgassing, and possibly choke the cat from the cold load . creating more unburned gasses passing through the cat.

 

[jeff_t]

You are running too long with the bypass open. Once it is in the active range, there is no reason not to close it. If you're not monitoring flue temps, this is asking for trouble. On a reload, mine is closed as soon as the load is well involved, regardless of what the cat probe says. I would never run it to the top end of the active range with the bypass open.

At five years, the cat is nearing the end of its life. Perhaps this is leaving a buildup of creosote that is ignited with the high flue temps?

 

[blueguy]

Overfire might be an option, the only thing that doesn't explain in my mind is the fact we have been using this wood stove for 5 years and just now we have had two overfires in one month.

We are using the catalytic temperature probe to get a rough idea of how hot the fire box is. I understand it doesn't tell exactly what the temp is, but due to the fact it is a bi-metal coil thermometer, it should react to temperature change in a linear and repeatable manner.

Just to add something relatable, back in the fall I loaded up the stove and got distracted making supper while waiting for the new load to char with the bypass open. When I remembered (oh sh*t), my flue probe was showing a hair over 1000*. The stove top thermometer was barely to 400 and the cat probe was just into the active range. The cat probe shows how hot the cat and the gases passing through it are. When the bypass is open, all of that is bypassing the cat and heading straight up the flue, which is why you cannot use the cat probe for anything other than an indicator of when to close the bypass. Add to that you were starting from a cold stove - sometimes there is a delay / discrepancy in the probe registering how hot things really are.

If you search the site, you will find threads with damage done to bypass retainers from running the stove bypassed at high heat. If this way of running a high burn has been done over the past 5 seasons, I would be shutting the stove down and having the bypass and its retainers inspected for warping.

 

[Montana Blaze King]

I am real interested in procuring a flue thermometer and haven't decided if I will go the digital route with the Auber one suggest by DougA, or a basic bimetal thermometer probe. My question is where should this probe be installed to measure flue temp? Do you want it just above the stove, or up the stack a couple feet? I searched around on this site for flue temp posts but don't seem to find anything that indicates properly placement. Thanks for your help, this has been educational

 

[blueguy]

usually about 18" up from the flue collar is where most companies recommend it to be placed. Can't say for sure about the Auber though....

 

[CRE10]

How loud is the standard internal buzzer on the Auber?