Out of control flue temps

[Dgrant09]

Hi all,

First season with a stove - we’re running a lopi evergreen with about 25’ of outside straight up chimney from t cleanout that connects to a 90* bend inside. We have double walled pipe inside so I was just using an IR gun on the stove top for the last month of heating (usually is around 5-600 when the blower isn’t on - 350-400 with blower on medium). Just installed a probe flue thermometer and it goes up to about 1400 after a hot reload. When I completely shut the air it still cruises around 700-800 until the coal/ember stage when it then drops to 3-400.

I’m also only getting about 4-5 hours out of a full load with the air control all the way shut. Wood for this year was purchased (☹️) and is at 15-22% for the most part. I know over 20 isn’t ideal but workin with what I got. I have about 6 cords split/stacked for next year already.

Any insight would be greatly appreciated.

Also, does running this hot risk damage to the pipe (duravent states they’re good for 1000 continuous) or is it just a waste of heat?

 

[EatenByLimestone]

You can try turning the stove down earlier to get a longer burn. Do you know what species of wood you’re burning?

 

[begreen]

Looks like there is about 4 ft of stovepipe inside. The draft may be too strong in which case a stovepipe damper may be necessary to tame it.

 

[DBoon]

I used to run a Lopi 1750 in a 27' vertical pipe chimney. The Lopi stoves seem to be really easy breathers and they will run away quickly and burn hot and short with a tall chimney if you don't shut them down early. Even if you do shut them down early, they will still run very hot with a tall chimney. A pipe damper didn't do much to help, for me at least.

 

[Jotel me this]

Hi all,

First season with a stove - we’re running a lopi evergreen with about 25’ of outside straight up chimney from t cleanout that connects to a 90* bend inside. We have double walled pipe inside so I was just using an IR gun on the stove top for the last month of heating (usually is around 5-600 when the blower isn’t on - 350-400 with blower on medium). Just installed a probe flue thermometer and it goes up to about 1400 after a hot reload. When I completely shut the air it still cruises around 700-800 until the coal/ember stage when it then drops to 3-400.

I’m also only getting about 4-5 hours out of a full load with the air control all the way shut. Wood for this year was purchased (☹️) and is at 15-22% for the most part. I know over 20 isn’t ideal but workin with what I got. I have about 6 cords split/stacked for next year already.

Any insight would be greatly appreciated.

Also, does running this hot risk damage to the pipe (duravent states they’re good for 1000 continuous) or is it just a waste of heat?

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Less efficient, but can you just run smaller fires? Its definitely a waste of some heat, but some of that heat going up the chimney is also heating the room. I dont think there would be damage to the pipes. You could keep more ash in the box to make a smaller area thus leading to a smaller fire but that may be a pain opening the door.

 

[mdocod]

Lots of insulated pipe will result in very strong draft...

When we first installed a stove at our new home here I ran all double wall stove pipe up to the support box (because I thought it was a good idea... nope), and that resulted in stoves wanting to run-away. Had problems with both a steel stove installed initially (temporary stove) and our current Mansfield. The draft force is so strong that if I ever let the temps get to 900F or so on the steel stove, it would run away from there. The steel stove was more problematic as it depended entirely on secondary combustion to meet EPA requirements, and in those stove designs, the secondary combustion is often given very little air inlet restriction, so a strong draft force can take the secondaries nuclear.. I saw 1200F+ a few times on that stove. Had to be careful about how hot it was allowed to get on startup, and load less wood.

The Mansfield, would also push higher than ideal EGT's with the double wall, but I think it only ever went over 1000F a few times as we learned what works and what doesn't. It has more thermal mass to dump the hot part of the fire into, and the cat acts as an additional restriction so this was better suited to this setup.

I eventually switched to single wall, 13' of it inside the house up the support box. This solved the issue, as it provides enough cooling/condensing of the exhaust that the air control of the stove can "win" the battle and drive the temps back down where we want them even if they are allowed to run up around 900-1000F after startup. It also puts more heat in the house - bonus.

In your install location, I see that isn't going to be an option. I think you're going to need a damper, or some way to extract more heat off of the stove pipe before it goes into the chimney to regain control of the burn rate. I would also explore using larger diameter splits to reduce the surface area of fuel in the box, as well as less fuel per load.

 

[jmb6420]

I have a Lopi Endeavor and was having problems similar to yours. I installed a flue damper, turn the fire down sooner and build smaller fires. It’s working out great. I have 21’ of double wall.

 

[MongoMongoson]

Yes, a damper is the way to go. That solved my issues with a non-cat stove wanting to run away. Trying to cool flue gasses with a long run of single wall just seems like a bad idea. It might be OK when you are running nice and hot but what about when you are not?

bigger splits, turn it down sooner. Mine never sees 900... why would I want that? That is just BTUs up the stack. Turn it down in the 500s, maybe. Try to keep it under 700 as your hard limit. we run in the 500-600 range.

 

[mdocod]

Yes, a damper is the way to go. That solved my issues with a non-cat stove wanting to run away. Trying to cool flue gasses with a long run of single wall just seems like a bad idea. It might be OK when you are running nice and hot but what about when you are not?

Trying to keep flue gases hot with a short run of double wall just seems like a bad idea. It might be OK when you're running nice and cool but what about when you're not? Hehe....

Goes both ways... There are lots of variables in any stove install and utilization; size, fuel type, burning methodology. This configuration works better for us. Double wall caused over-fire on 2 different stoves installed in this location. In a install where double wall is required for clearances like Dgrant09's install, the only option is a damper to force a lower burn rate. When we have clearances for single wall we have more options to experiment with, including the one where we extract more thermal energy from the system into the house.

Lots of installs out there have 1.2-1.5 cubic foot stoves with 4-6+ feet of 6" single wall. Manual for this 3.2 cubic foot stove does indicate that it can be used with 8" pipe/chimney systems with an adapter, just has to be more carefully observed for creosote. I've seen tons of installations of 2-3+ cubic foot stoves in rooms with tall vaulted ceilings with long runs of single wall.

Stoves larger than the ~3 cubic foot mark jump up to 8" flue...

 

[begreen]

I've seen tons of installations of 2-3+ cubic foot stoves in rooms with tall vaulted ceilings with long runs of single wall.

Which is generally discouraged in many stove manuals and warned against in some catalytic stoves due to their already cooler flue gas temps. As a rule, stovepipe runs >8ft should be double-wall.

 

[mdocod]

Which is generally discouraged in many stove manuals and warned against in some catalytic stoves due to their already cooler flue gas temps. As a rule, stovepipe runs >8ft should be double-wall.

Dampers are also generally discouraged in many modern stove manuals and installs!

How does that "rule" apply for a 1 cubic foot stove vs a 3 cubic foot stove? They will both produce similar EGT's, but the 3 cubic foot stove will drive about 3X the volume of exhaust, so it could theoretically drive a single wall stove pipe about 3X as long and have about the same cooling/condensing characteristics... If a 1 cubic foot stove can drive say, 5 feet of single wall safely, then a 3 cubic foot stove will probably drive 15ft of single wall safely. That would make sense to me anyway.

I've run both double wall and single wall and for our fuel type, stove size, and burning methods the single wall allows the stove to operate at steady continuous burn rates with set-it-and-forget-it air settings without over-firing and puts more heat in the house, the double wall required constant fussing with the air control in a range of control with far less resolution to keep it from running away. The stove saw "too hot" territory on the built in meter several times on double wall, but operates perfectly at the top of the recommended operating range without any fussing or babysitting required now on single wall.

The local Hearthstone dealer/installer here wanted to do single wall for this installation because it's a large stove and he said with this straight shot through the roof (no elbows) a fully insulated pipe/chimney system would likely over-fire the stove. I went against his recommendation, for the same concerns you're expressing, did double wall, and it worked badly and put less heat in the house. I actually owe him an apology as his intuition for these stoves was correct.

 

[begreen]

Actually, we've noted that some stove manuals are encouraging dampers as a solution for strong draft. Considering that these stoves are tested at 16' flue system spec, it makes the best sense for a 30' flue system. This is not a large vs small stove issue.

This is from the Hearthstone Mansfield 8013 manual:

We do not recommend long runs of stovepipe to increase heat dispersal. Longer lengths of stovepipe, or more connecting elbows, than necessary increase the chances of draft resistance and the accumulation of creosote buildup.

In general, you do not need to install a stovepipe damper with the Mansfield. Some installations, however, could benefit from a stovepipe damper, such as a tall chimney which can create a higher than normal draft. In such cases, a damper can help regulate the draft. The Mansfield requires a draft between -0.06" and -0.1" WC. For drafts above -0.1" WC, install a stovepipe damper.

 

[rudysmallfry]

Dampers are also generally discouraged in many modern stove manuals and installs!

How does that "rule" apply for a 1 cubic foot stove vs a 3 cubic foot stove? They will both produce similar EGT's, but the 3 cubic foot stove will drive about 3X the volume of exhaust, so it could theoretically drive a single wall stove pipe about 3X as long and have about the same cooling/condensing characteristics... If a 1 cubic foot stove can drive say, 5 feet of single wall safely, then a 3 cubic foot stove will probably drive 15ft of single wall safely. That would make sense to me anyway.

I've run both double wall and single wall and for our fuel type, stove size, and burning methods the single wall allows the stove to operate at steady continuous burn rates with set-it-and-forget-it air settings without over-firing and puts more heat in the house, the double wall required constant fussing with the air control in a range of control with far less resolution to keep it from running away. The stove saw "too hot" territory on the built in meter several times on double wall, but operates perfectly at the top of the recommended operating range without any fussing or babysitting required now on single wall.

The local Hearthstone dealer/installer here wanted to do single wall for this installation because it's a large stove and he said with this straight shot through the roof (no elbows) a fully insulated pipe/chimney system would likely over-fire the stove. I went against his recommendation, for the same concerns you're expressing, did double wall, and it worked badly and put less heat in the house. I actually owe him an apology as his intuition for these stoves was correct.

This is an interesting insight about the more steady operation with the single wall. I had single wall on my older Heritage for about 5 years before I switched to double wall. I found the stove over fired easily in both cases , and never did find a "set it and forget it" mode on my stove until recenly when I added a damper. Just a tiny 1/4 turn is enough on my stove to fine tune a lazy flame when I can't do it completely with the air shut down completely. I switched to double wall because I'd rather have a second layer of pipe between me and the over heated air passing through the pipe. It's just no fun sitting on the couch with a fire extinguisher helplessly watching the temp climb since I didn't turn down the air soon enough. The outer wall of my double wall sits over 100 degrees, so there is still plenty of heat coming off the pipe into the room. At least if it over fires with double wall, I know that pipe can handle it where the single wall would just stink and occasionally glow red.

 

[MongoMongoson]

Dampers are also generally discouraged in many modern stove manuals and installs!

I measured my draft, showed Pacific Energy my numbers (and a video showing how I measured my draft), and Pacific Energy recommended that I install a damper.

 

[kennyp2339]

I also suffer from high draft, actually install 2 key dampers on my black pipe, really only use 1 but the 2nd gets used when it’s stormy like a nor’easter, the damper has improved my quality of stove burn, before the damper, I was blasting through loads of wood in 4-5 hours with tons of coals, at first I thought I had a seasoning issue, a moisture meter proved otherwise, installed a damper and my burns slowed down with the same stove top temps, I went from 5 hour hot loads to 12hr hot loads, this really helped me out, I did buy a manometer and tested my draft .18 was the result, my stove calls for .05 wc” I now run between .05 -.08 which is almost perfect for my setup.

 

[mdocod]

We do not recommend long runs of stovepipe to increase heat dispersal. Longer lengths of stovepipe, or more connecting elbows, than necessary increase the chances of draft resistance and the accumulation of creosote buildup.

"...Than necessary." I believe they are referring to installs where people intentionally run the stove pipe extra distance to extract more heat, like horizontally across the room. They aren't going to say what a "too long" run is because there isn't a definitive answer. It depends on many factors....

What is a long run for a 1 cubic foot stove? 5ft? 8ft? 10ft? Canada says 10ft without any concern about stove size. I would estimate that 10ft is too long for a 1 cubic foot stove to drive without risk of creosote buildup. Would you agree?

What is a long run for a 3 cubic foot stove? Is it the same? Is this a clipboard warrior issue or a practical issue? I would argue that there isn't a clipboard we can point to with a definitive answer. only "rules of thumb/guidance/recommendations."

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Other members of the household should be able to operate the stove with basic instructions. The addition of more dampers makes instructions more confusing and more difficult to remember for non-enthusiasts....

Operating Instructions (and observations) with double wall: Bypass CAT to open door and start fires. For cold starts, leave door open until firebox is ingulfed in strong flame and draft produces thunder/roar sound. Open Air control all the way then close door. Engage the CAT when target EGT's are reached on start-up (600F), and any time door is closed thereafter. Run with air control wide open until 600F EGT is achieved on startup and subsequent fuel loads. Adjust down to 1/8-1/4" open for large fuel loads, 1/4-3/8" for smaller fuel loads. Check and adjust air control every 30 minutes to prevent EGT's from going over 800F otherwise the stove may run-away and overheat regardless of air control position, If <1/8" on large load or <1/4" position is selected on the air control to prevent over-firing the stove, continue to check and adjust air every 30 minutes to prevent primary/secondary combustion from being extinguished during the gaseous phase of the burn, which can lead to back-puffing and chimney deposits which could be dangerous. Good Luck. Don't enjoy. Be nervous and paranoid that the stove could overheat or explode. Babysit it constantly. Good luck sleeping or leaving the house.

Current operating instructions (and observations) on single wall are simple: Bypass CAT to open door and start fires. For cold starts, leave door open until firebox is ingulfed in strong flame and draft produces thunder/roar sound. Open air control all the way then close door. Engage the cat when target EGT's are reached on start-up (600F), and any time door is closed thereafter. Run with air control wide open until 800F EGT is achieved on startup and subsequent fuel loads. Adjust down to 1/4-3/8" open for large fuel loads, 3/8-1/2" for smaller fuel loads. EGT's will self-regulate from ~800F down to ~600F throughout the gaseous part of the burn cycle with no adjustments required. 800F+ EGT's will be more aggressively cooled and condensed by the single wall pipe, providing a self-correcting draft-control for excessive EGT's. No manual corrections required. The minimum air setting of ~1/4-3/8" (depending on fuel load) is sufficient to maintain primary/secondary combustion through the gaseous part of the burn, which prevents back-puffing and creosote/soot deposits in later stages of the burn as the fuel transitions from gaseous to coaling burn phases. Enjoy the fuss-free clean fire. Sleep well.

I don't know what the damper on double wall situation would look like specifically for controls, if it would require frequent adjustment or not... I do know it requires manually controlling and adjusting a 3rd thing and I question whether it would produce the sort of fuss-free results we get now on single wall. I know I would get less heat out of the system with double wall and a damper.

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The double wall was so likely to cause over-fire conditions, the "solution" was often to use smaller fuel loads and more restricted air settings, which can also lead to cooler fires and creosote/soot deposits. We generally tried to prevent the stove from ever going over 700F EGT otherwise it could snowball out of control.

The double wall that I removed from the system had about 12 weeks of burning on it since it was previously swept (end of last season, beginning of this season), and had about a 1/4-5/16" layer of soot (black ashy) deposits. Not unexpected for the resin-rich wood we burn here, definitely needed to be swept, but trying to keep the stove from over-firing was likely contributing to worse conditions for deposits in the chimney system.

Since transitioning to the single wall 8 weeks ago the soot deposits on the single wall are 1/16"-1/8" thick. We are getting more and more comfortable allowing the stove to run higher EGT's now, as we learn that it can do so safely without snowballing out of control.

Given the type of wood we have here, some full of resin, it probably makes more sense to aim for stronger, faster, hotter burn cycles with over 750F EGT's during the most active part of the burn. This would be the key to keeping soot deposits under control. If I'm going to run the stove hot enough to properly process the resin, I'd like to recover some of that heat!

Lots of variables... ALL things considered, I think that I'm better off driving 600-900F EGT's into single wall than 400-700F EGT's into double wall, and while there's likely a way to drive high EGT's into double wall with a damper to better process the resin without over-firing, that seems like a waste to me. Time will tell.. My house my experiment. I have a very handy Wohler Viper chimney sweeping tool and will keep a close eye on the deposits. If I need to sweep a few times a season that's not a big deal... Based on current observations I would say that the single wall driven hotter will require less sweeping than what we were doing. I think I might install a temp probe on the pipe up near the support box so that I can observe the drop in EGT's over the length of pipe and report back findings to the forum. It would be easily visible from up on the loft..

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Apologies to Dgrant09, this has gone off topic a ways. Though I believe still applicable to the larger picture here. I think the conclusion for your particular stove situation, with the clearances and such involved, is that a damper is the only good option to help pull your stove temps down into the reasonable range.