Contending with resin heavy wood in a "Hybrid" Stove.

[mdocod]

Hello Hearth!

My wife and I have been enjoying a new Mansfield 8013 Hyrbid stove through the later part of last burning season and the beginning of this burning season as one of the centerpieces of our new custom home in Black Forest Colorado.

We have been very happy with the stove. I'll probably do a more thorough review with photos and such some day, but for now I have a question about wood, specifically, those occasional pieces that are hiding a ton of filthy black smoke that leaves residue all over the interior of the stove and plugs up the cats!

We burn almost exclusively ponderosa, most of which has been split and stacked for many years, downed from trees killed in the forest fire we had almost a decade ago here. Problem is... when a tree is killed by a forest fire, the standing trunk does weird things. The "pitch" of the tree all seems to drain back down into the lower parts of the tree, so the bits from the top, dry out nice and have a nice "hollow" ring to them. Those burn clean. The bits from the middle have a bit more pitch in them, but they usually burn pretty clean too as long as they are given enough air and make a ton of heat. The lower parts of the tree, especially in knotty sections, can sometimes be so heavy with pitchy/resin, often "hiding" in the wood, that it completely plugs up the cat and coats the entire inside of the stove with black sooty stuff. The chunks of wood that are the really bad offenders are pretty rare, but we've run across a few now that are a mess! When one of these logs is burning, you can watch it just spewing streams of heavy black sooty smoke. It reminds me of the black smoke that comes from burning plastics, which often needs much hotter fire and a lot more air to burn off cleanly than would be appropriate for a wood stove. (smelting temps will burn this stuff off pretty clean...)

I usually try to set aside pieces that I expect are going to be troublesome, to burn mixed with other "cleaner" logs on top of a bed of hot coals in a lighter load, with the air intake running more open, so that it can burn off all that black soot and make useful heat, but sometimes one of these buggers sneaks into the stove without proper preparation or situation to deal with it or is just so heavy with resin that it doesn't matter, the result is an unexpectedly plugged cat, and a bit of smoke spewing out between some of the top-stones on the stove (as I understand, these aren't cemented, rather, just sitting on a grid up there, so the positive pressure from the now plugged exhaust stove forces gases out between the stones?) The smell of that black nasty smoke in the house is sometimes our first clue that things have gone pear shaped. At that point I'll bypass the cat and turn up the air to let it finish out the dirty burn, but the cats at that point are totally plugged.

It's not too hard to "fix." I just wait for the coals to burn down, prop up the telescoping section of stove pipe, and lift out brick-shaped cat sections and vacuum all the soot out of them, as well as the soot on the walls of the stove, secondary inlets, top of the baffle and bypass.

Have you had similar experiences burning resin-rich woods? How do you contend with this particular challenge? Any tips or tricks you have learned along the way to minimize the negative impacts from these chunks of nasty wood? Any way to recover without having to wait for the stove to cool down and clean it out manually? Do you have a trick to spotting and separating these pieces of wood? At the end of the day this is just one of the hazards of burning junk wood, but I have probably 20 cords piled up here that I intend to use over the next many years... Hopefully I get better at it!

Thanks!
-Eric

 

[EbS-P]

I think a cat stove owner may have more relevant feed back but I’m going to ask give my feedback and ask questions. I’ve burned plenty of pitchy splits from the bottom of big pine trees. Some times the whole split looks like fat wood. Couple years ago I set one aside for a really cold night and it sent the stove nuclear. Parts were glowing.

I have never had in the inside of my stove coated in soot. I have seen raging secondary combustion. Soot burns off at 750 degrees so any thing in the firebox was a temp below that while soot was being deposited. (Welders use a sooty to torch fame to coat a work piece then Clean the flame up and preheat once the soot starts to disappear the temp is 750.)

To me it sounds like firebox temps are too low to get a clean burn. Smoke leakage is another bit worrisome sign telling me the draft is weak and the stove is no longer under negative pressure.

Question time what stove top temps to you regularly run at? Do you have a flue gas temp probe?

Interested in others experiences with this stove or cat stoves in general.

 

[mdocod]

Hello EbS-P!

Great information/questions/concerns!

I didn't know about this soot burn-off temp. That's very helpful information. That explains why the soot collects more on the air inlet ducting in the stove, as well as up above the baffle on the bottom side of the stove top. These surfaces are cooler normally in this stove...

I have a FlueGard thermometer 18" above the top surface of the stove. I try to operate the stove with EGT's ~600-800F when there's active primary/secondary flame combustion in the box. Occasionally it might hit 900-1000F temporarily. Once the gases are out of the wood it settles down to ~400F during coaling.

The Mansfield has a temp probe installed on its side that has been "dumbed down" to 3 zones... "inactive, cat active, and too hot." It's installed such that it tries to measure the temp just over the CAT's, but I believe that because the probe is installed through a thick chunk of soapstone, it is very slow to react and may tell more about the temp of the soapstone than the cat at times. Whatever this probe is measuring, we routinely operate in the top half of the "cat active" zone, and most heavier fuel loads will drive this probe to the top of that obfuscated range.

I often use a infrared temperature reading "gun" to check surface temps of stove pipe and stove. The design of this stove runs hotter on the sides than top... Side temps are generally ~400-600F, top only runs ~250-350F on this stove.

Clarification on the draft issue: The stove only leaks smoke when the CAT-Bypass is closed and the cats have plugged up with soot. As soon as the bypass is opened our usual strong draft force returns. We have outside air supplied to the stove through a short 4" insulated pipe, 13' of vertical stove pipe connected direct to a support box/chimney system, which is probably another 8' of insulated chimney. No elbows.

I think what's happening here, is a negative feedback loop problem. In a cat-stove, the wood starts to spew black sooty smoke from resin, and immediately begins to smother the fire and the catalyst with an over-rich condition, this probably kills off the "fire" in the cat, which leads to the beginning of deposits on the cat, which causes more air restriction which cools the whole fire down, snowball effect from there plugs up the cats rapidly and coats the inside of the box with soot.

In a non-Cat stove, the over-rich condition caused by all the resin also leads to a feedback loop snowball effect, but without a cat in the exhaust to plug up, the snowball goes nuclear instead....

I should probably just try to be more careful about Identifying and setting aside these pitchy chunks. Perhaps they should only be burned off without the cat engaged.

 

[moresnow]

Guess i'd stop bringing that wood home.

 

[EbS-P]

Hello EbS-P!

Great information/questions/concerns!

I didn't know about this soot burn-off temp. That's very helpful information. That explains why the soot collects more on the air inlet ducting in the stove, as well as up above the baffle on the bottom side of the stove top. These surfaces are cooler normally in this stove...

I have a FlueGard thermometer 18" above the top surface of the stove. I try to operate the stove with EGT's ~600-800F when there's active primary/secondary flame combustion in the box. Occasionally it might hit 900-1000F temporarily. Once the gases are out of the wood it settles down to ~400F during coaling.

The Mansfield has a temp probe installed on its side that has been "dumbed down" to 3 zones... "inactive, cat active, and too hot." It's installed such that it tries to measure the temp just over the CAT's, but I believe that because the probe is installed through a thick chunk of soapstone, it is very slow to react and may tell more about the temp of the soapstone than the cat at times. Whatever this probe is measuring, we routinely operate in the top half of the "cat active" zone, and most heavier fuel loads will drive this probe to the top of that obfuscated range.

I often use a infrared temperature reading "gun" to check surface temps of stove pipe and stove. The design of this stove runs hotter on the sides than top... Side temps are generally ~400-600F, top only runs ~250-350F on this stove.

Clarification on the draft issue: The stove only leaks smoke when the CAT-Bypass is closed and the cats have plugged up with soot. As soon as the bypass is opened our usual strong draft force returns. We have outside air supplied to the stove through a short 4" insulated pipe, 13' of vertical stove pipe connected direct to a support box/chimney system, which is probably another 8' of insulated chimney. No elbows.

I think what's happening here, is a negative feedback loop problem. In a cat-stove, the wood starts to spew black sooty smoke from resin, and immediately begins to smother the fire and the catalyst with an over-rich condition, this probably kills off the "fire" in the cat, which leads to the beginning of deposits on the cat, which causes more air restriction which cools the whole fire down, snowball effect from there plugs up the cats rapidly and coats the inside of the box with soot.

In a non-Cat stove, the over-rich condition caused by all the resin also leads to a feedback loop snowball effect, but without a cat in the exhaust to plug up, the snowball goes nuclear instead....

I should probably just try to be more careful about Identifying and setting aside these pitchy chunks. Perhaps they should only be burned off without the cat engaged.

It still think extra fuel should be consumed in the secondary combustion IF you are running high enough firebox temps. I agree with your negative feedback assessment. Ina stove with unregulated secondary air I just don’t see how one pitchy splits chokes a fire. Over whelm the cat maybe.
I wonder what the moisture content of them is. The probe moisture meter probably isn’t design to measure accurate moisture with that much pitch.

I propose a test. Find a pitchy piece measure moisture content on a fresh split room temp face and record the weight. Let those pieces dry by/near the fire for a week and re weigh.

If I come across one I will do the same. And report back.

 

[ABMax24]

Try putting the sappy pieces on the bottom row when you load the stove, and put non sappy on top.

 

[Highbeam]

I burn Doug fir and big diameter stuff. At the butt ends it can be just saturated and pouring out this crisco looking pitch. These, when dry, make the stove run rich. Just like a sooty diesel rolling coal before the turbo catches up. Fir me, it all gets burnt off the next time the stove gets hot. Except the glass, the glass never really comes clean for me by burning. The cat especially burns off easily.

 

[mdocod]

It still think extra fuel should be consumed in the secondary combustion IF you are running high enough firebox temps. I agree with your negative feedback assessment. Ina stove with unregulated secondary air I just don’t see how one pitchy splits chokes a fire. Over whelm the cat maybe.
I wonder what the moisture content of them is. The probe moisture meter probably isn’t design to measure accurate moisture with that much pitch.

I propose a test. Find a pitchy piece measure moisture content on a fresh split room temp face and record the weight. Let those pieces dry by/near the fire for a week and re weigh.

If I come across one I will do the same. And report back.

I supposed I should get a moisture probe...

All the wood I'm bringing in the house this year is from a side of the pile that was cut and split many years ago. Some of the pieces look very aged, with that grey seasoned look. The ones that still look like a fresh split, with lots of yellow/orange color still on the surface, are the ones to watch out for... Still heavy, they look and feel like they would still be wet... but what they are soaked with isn't water! With that said I agree a measurement of moisture here would be interesting.

I think you're definitely correct about the fire not being hot enough to burn it off.... I think the problem occurs mostly when doing continuous feed to maintain the stove at temp, rather than when a full load of wood is inserted, setting off a very hot "cycle" that can contend with the soot better.

Try putting the sappy pieces on the bottom row when you load the stove, and put non sappy on top.

Yep! I need to carefully mix it with other wood. I suspect that a lot of these pieces are already getting burned up this way at random...

I burn Doug fir and big diameter stuff. At the butt ends it can be just saturated and pouring out this crisco looking pitch. These, when dry, make the stove run rich. Just like a sooty diesel rolling coal before the turbo catches up. Fir me, it all gets burnt off the next time the stove gets hot. Except the glass, the glass never really comes clean for me by burning. The cat especially burns off easily.

I'm glad to hear I'm not the only one to have observed the effect of these heavy resin laden chunks. "Diesel Rolling Coal" is a great description of what's happening here... Sometimes when I put a piece of this in the stove I will sit and watch the grease start oozing out and burning

Guess i'd stop bringing that wood home.

I didn't have to bring any of this home, it was all fell, bucked, and split here on the property.... We have many years worth still piled up here. When I begin gathering wood from elsewhere again I will seek out alternatives for sure, but ponderosa is the dominant tree for miles around here so this is what most people here burn.

 

[mdocod]

I picked up a moisture meter from the rain forest folks.

If I take a piece of wood from either the indoor wood storage area or the woodpile outside (often covered in snow this time of year), split fresh to take an "internal" reading, I am getting ~8-10% on several samples whether plucked from indoor/outdoor doesn't seem to matter for the internal reading.

If I stab pieces from a non-fresh split, like, the end-grain or old-split surface, the indoor wood is all reading ~6-9%, the outdoor wood reads 10-20+% depending on whether there was snow on that face of the wood, and other outdoor variables.

I took readings from lots of different pieces, some of which I expect to be resin-rich, and haven't seen any meaningful difference in moisture.

Either way, the wood is very dry, as expected. It's been split and stacked out in the dry Colorado front-range air and sun for years.

 

[EbS-P]

I picked up a moisture meter from the rain forest folks.

If I take a piece of wood from either the indoor wood storage area or the woodpile outside (often covered in snow this time of year), split fresh to take an "internal" reading, I am getting ~8-10% on several samples whether plucked from indoor/outdoor doesn't seem to matter for the internal reading.

If I stab pieces from a non-fresh split, like, the end-grain or old-split surface, the indoor wood is all reading ~6-9%, the outdoor wood reads 10-20+% depending on whether there was snow on that face of the wood, and other outdoor variables.

I took readings from lots of different pieces, some of which I expect to be resin-rich, and haven't seen any meaningful difference in moisture.

Either way, the wood is very dry, as expected. It's been split and stacked out in the dry Colorado front-range air and sun for years.

That’s good it’s dry. Seems all that’s left short of not burning then is to intentionally burn a hotter fire so that all/most the soot is burned before it gets to the cat. I can see the continuous feeding method not creating high enough firebox temps to ensure complete combustion. Try loading one pitchy piece on a full load and run it and see what happens.

 

[mdocod]

That’s good it’s dry. Seems all that’s left short of not burning then is to intentionally burn a hotter fire so that all/most the soot is burned before it gets to the cat. I can see the continuous feeding method not creating high enough firebox temps to ensure complete combustion. Try loading one pitchy piece on a full load and run it and see what happens.

Yep! This is absolutely the issue!

Plenty of pitchy pieces have burned through the stove when they are burned alongside other pieces.

It's when a pitchy piece goes in as a solo/duet (only done on a very hot coal bed) that this has been an issue. It has never plugged the cat on a traditional "full load" of wood, only on light loads / continuous feeds.