Problems with smoking AND over-firing in my Green Mountain 40

[ThereAreGoats]

Hi all,

My frustrations with my Green Mountain 40 continue—now because of over-firing. I've elsewhere described the incessant smoking that seems to always happen when I load a new load for my secondary burn. I wait until the stove cools to around 300 degrees, open the air/bypass, rake the coals forward, and pack the firebox with ash and maple. I leave the air fully open. It generally takes many minutes for it to really catch and stop smoking. It could be that I need to load at a hotter temperature. It could be that my wood still needs drying but I doubt it; the ash, which I always put on the bottom, has been stacked and facing south for 10 months. But that brings me to the next problem: over-firing.

Once the load (let's say it's all 4-6 inch splits of maple this time, packed) does get going, it isn't long before the catalyst is too hot and the stove is over 700 degrees (measured on top) and I have to stop it all the way down and open the bypass. I've also heard that dreaded roaring sound around this point. This just happened on a 50 degree day.

All of this is frustrating because it makes it very hard to relax and just enjoy the stove. I'm constantly having to watch it. And if the over-firing problem is happening with maple, I'm assuming it means I definitely shouldn't try burning something harder, like oak. Yet I have a rack full of it out back just waiting to be burned...

Any advice on any/all of this?

Robert

 

[bholler]

Hi all,

My frustrations with my Green Mountain 40 continue—now because of over-firing. I've elsewhere described the incessant smoking that seems to always happen when I load a new load for my secondary burn. I wait until the stove cools to around 300 degrees, open the air/bypass, rake the coals forward, and pack the firebox with ash and maple. I leave the air fully open. It generally takes many minutes for it to really catch and stop smoking. It could be that I need to load at a hotter temperature. It could be that my wood still needs drying but I doubt it; the ash, which I always put on the bottom, has been stacked and facing south for 10 months. But that brings me to the next problem: over-firing.

Once the load (let's say it's all 4-6 inch splits of maple this time, packed) does get going, it isn't long before the catalyst is too hot and the stove is over 700 degrees (measured on top) and I have to stop it all the way down and open the bypass. I've also heard that dreaded roaring sound around this point. This just happened on a 50 degree day.

All of this is frustrating because it makes it very hard to relax and just enjoy the stove. I'm constantly having to watch it. And if the over-firing problem is happening with maple, I'm assuming it means I definitely shouldn't try burning something harder, like oak. Yet I have a rack full of it out back just waiting to be burned...

Any advice on any/all of this?

Robert

How tall is your chimney? What type of chimney? What moisture content is your wood at?

 

[ThereAreGoats]

Moisture reading from middle of a freshly cut piece of maple is 12-14%. Same with the ash. Chimney is a double lined 6-inch flue I think. It has a 45-degree bend about three feet above the stove so it can get into the wall, unfortunately. Not sure how tall, but it sticks up maybe four feet above the very top of my roof.

 

[ThereAreGoats]

I should add that my burn time (to coals and stove around 300) will probably by hardly 4 hours.

 

[bholler]

I should add that my burn time (to coals and stove around 300) will probably by hardly 4 hours.

Have you had the draft measured?

 

[begreen]

When you say smoking, is this spilling out of the stove or visible smoke out of the top of the chimney?

After the cat is engaged, it's ok to turn down the air sooner. Sounds like this may be happening too late.

 

[ThereAreGoats]

Have you had the draft measured?

I haven't! I didn't know this could be done. What's strange is that it seems like it's both too weak (wood not catching) and too strong (very short burn times—under 4 hours with packed maple—and getting super hot).

When you say smoking, is this spilling out of the stove or visible smoke out of the top of the chimney?

After the cat is engaged, it's ok to turn down the air sooner. Sounds like this may be happening too late.

Just lots of smoking from the wood inside the firebox. It doesn't spill out really. Regarding turning down the air, I have to keep it open for a long time until the wood really catches (which is tough to really measure). I guess I should just stop it down almost all the way after that. But even when it is down completely all the way, the fire is still pretty rippin'.

Not sure how I square all of this with the idea, espoused by some, that you should really control temp by load size and not air, though. Seems like this might be moot for me since the secondary combustion stays strong even after the stove is stopped down.

 

[begreen]

Lots of smoke in the firebox with a fresh load of wood is not uncommon. Is there a cat thermometer on the stove? When the cat is in the active zone it will start chewing up the smoke. At that point, the bypass can be closed and the air can be turned down more, maybe about half of the wide open setting.

I think what you are seeing is the hybrid function where secondary combustion is also burning up the smoke which can lead to a lively fire if the air is not turned down enough, soon enough. Practice turning it down once the cat is active and the bypass has been closed. Close it down in increments of maybe 50% at a time or until the flames get lazy, but not out. Then wait until the flames gain strength and close it down 50% more.

With more experience, this will become easier. The first fires with any new stove are a learning experience, even with seasoned pros. Having instrumentation on the stove and flue (or cat) helps a lot.

 

[stoveliker]

But 15 percent wood should not take a long time to catch.and smoke a lot.

It seems to me (burn time) that draft may be too high, and wood too wet. I know you measured, but 15 percent wood on coals should just take off.

 

[ThereAreGoats]

Lots of smoke in the firebox with a fresh load of wood is not uncommon. Is there a cat thermometer on the stove? When the cat is in the active zone it will start chewing up the smoke. At that point, the bypass can be closed and the air can be turned down more, maybe about half of the wide open setting.

I think what you are seeing is the hybrid function where secondary combustion is also burning up the smoke which can lead to a lively fire if the air is not turned down enough, soon enough. Practice turning it down once the cat is active and the bypass has been closed. Close it down in increments of maybe 50% at a time or until the flames get lazy, but not out. Then wait until the flames gain strength and close it down 50% more.

Good to know that smoke in the firebox isn't extremely uncommon. I do have a cat thermometer, and I always just close the bypass again right after reloading since it's always still hot enough. It just never seemed like a good idea to close the air by that much right after reloading since the wood takes 10 or more minutes to really catch (it burns around the edges, but comes and goes, smolders, etc), and it takes much longer than that for the temperature to start going up again from 300. The secondary combustion doesn't kick in again after a reload for 20 or so minutes if not longer.

But 15 percent wood should not take a long time to catch.and smoke a lot.

It seems to me (burn time) that draft may be too high, and wood too wet. I know you measured, but 15 percent wood on coals should just take off.

I'm using a moisture meter intended for construction materials, I think. Does that change things?

 

[begreen]

It's a judgment call based on the wood species, wood dryness, and draft strength which increases when it gets colder outside. If the wood is slow to start then wait 5 or 10 minutes until it does start burning well and then turn down the air incrementally as described. If the cat temp drops below 600 then it's been turned down too early.

 

[stoveliker]

The moisture meter should be fine.
I have never had issues with starting to burn when it's below 20 pct. It takes off even before I'm done reloading (filling) the firebox.

 

[ThereAreGoats]

The moisture meter should be fine.
I have never had issues with starting to burn when it's below 20 pct. It takes off even before I'm done reloading (filling) the firebox.

Do you rake your coals forward or just put the wood on top? And what temp is the top of your stove when you do it? I've been waiting until it's around 300 degrees. That might be too cool.

 

[stoveliker]

If I have ANY glowing coals left, a 15 pct piece gets going far before I am done filling up the firebox, regardless of where they are. I do normally rake them to the front.

I don't measure the top of my stove as I have a convection top and a catalyst affecting the reading there.
But even if I have one coal of 1" left, it (maple, ash, cherry, line - with oak to be seen this year) takes off before I'm done reloading.

 

[stoveliker]

Obviously only if you can make the new split in good touching contact with the coal.

 

[outdoorguy864]

A big trick to the GM when loading it, is the air intake is in the front on the bottom (in the box). When you go to load, move coals to the left and right, leaving a valley in the middle all the way to the back. Then put your wood in, you will have a valley under neath the wood and keep a small slight gab between the back wall and the wood. This will create a perfect flame roll from bottom to back to top to front every time and keep the secondary engaged and hot burning up smoke. You will get take off's a lot faster.

 

[neverstop]

A big trick to the GM when loading it, is the air intake is in the front on the bottom (in the box). When you go to load, move coals to the left and right, leaving a valley in the middle all the way to the back. Then put your wood in, you will have a valley under neath the wood and keep a small slight gab between the back wall and the wood. This will create a perfect flame roll from bottom to back to top to front every time and keep the secondary engaged and hot burning up smoke. You will get take off's a lot faster.

^^you definitely need to keep a path in the ash from the air intake to the back of the stove.

I had similar problems to what you've described with my hearthstone shelburne. A realization that I just came to thanks to @begreen is that the air intake on the GM design line (incl. shelburne, crafstbury, manchester) is not linear. So 50% travel in the lever doesn't equate to 50% reduction in primary air. As pointed out in the manual the last ~0.375" of travel in the lever accounts for 50%+ reduction in primary air.

BURN RATE This wood heater has a manufacturer-set minimum low burn rate that must not be altered. It is against federal regulations to alter this setting or otherwise operate this wood heater in a manner inconsistent with operating instructions in this manual LOW BURN: Move the air control lever all the way inward toward the stove. (See figure 13). This closes the air shutter to its minimum opening. A low burn rate over extended periods is not advisable as it can promote the accumulation of creosote. Inspect the venting system frequently if using low burn rates consistently. MEDIUM LOW BURN: With the primary air control in the low position, pull the air control forward (a little less than 1/4”). A medium-low burn rate is the typical setting and is preferable if the stove is unattended. At this burn rate, a little goes a long way. MEDIUM HIGH BURN: With the primary air control in the low position, pull the air control forward (a little less than 3/8”) HIGH BURN: Completely open the primary air control by moving it all the way outward toward the operator. Fully load the firebox with wood on a bed of hot coals or on an actively flaming fire. To minimize creosote accumulation, run the stove on high once or twice daily for 35 to 45 minutes to fully heat the stovepipe and chimney.

So when turning down the air in increments you may want to treat 0.375" from fully closed as 50% open, 0.25" from fully closed 25% open. I've tried this approach on a fire and noticed that I have much more control and it didn't run away from me like it did in the past. I've also noticed a lag in the cat temp gauge, so I also kept the by-pass open for 5+ minutes after the new load catches in order to ensure the cat is back up to temp.

As @outdoorguy864 said the flame(s) you want to be adjusting the air by are the ones you see coming from center back/middle of the wood stack. With the path in the ashes this is where you'll be feeding the most oxygen. If the flame is aggressively hitting the baffle you want to back off the air until it is barely touching; that's where the art of it comes in.

I by no means have this stove figured out, but I feel like I'm getting closer.

 

[cabinwarmer]

Good Day, something I learned with my GM60 - I installed a flue damper. It has been the most improvement that I have made. I add wood when the flame is low and flue temp is between 375-400. Always keep a flame, works for me. the main air intake is very very sensitive. I think you may be waiting too long before adding your additional load. Maybe because you are afraid of the run away? I had the same feelings at one point. I end up with the flue damper at @50% closed, and the main air intake open just an 1/8 of an inch for a long burn. Get a flue probe if you don't have one already. I think you are close and do not give up hope. This GM is a much different stove (hybrid). This is my third season, and with help from everyone on here, I have got a stove I am extremely happy with.

 

[begreen]

This is my first season with it. Any opdates from OP or others?

I am having same issues as OP with occasional overfiring when putting in roughly more than 4 splits which causes sleepless nights on overnight burns. This stove has all the same known issues I've read on this board. So far it seems like it's fiddly and looks better than it works. It was not my first choice but was recommended by the installer who I don't want to come back out ever again.

@ThereAreGoats

The stove is not going to run like the BK. It is a hybrid and secondary combustion occurs with the cat just cleaning up the smoke. Pack the wood tighter using thicker splits.

 

[bmikeb]

The stove is not going to run like the BK. It is a hybrid and secondary combustion occurs with the cat just cleaning up the smoke. Pack the wood tighter using thicker splits.

Thanks. I deleted my post because I realized that the OP was referring to a 40 instead of the 60 which I have.

I will try that more. I've had better overnight success with some larger pieces and rounds.

 

[GM80NY]

A big trick to the GM when loading it, is the air intake is in the front on the bottom (in the box). When you go to load, move coals to the left and right, leaving a valley in the middle all the way to the back. Then put your wood in, you will have a valley under neath the wood and keep a small slight gab between the back wall and the wood. This will create a perfect flame roll from bottom to back to top to front every time and keep the secondary engaged and hot burning up smoke. You will get take off's a lot faster.

Also to add to this. Load your pieces East to West fairly tightly, directly in the center. Having a good tight mass in the center is key to having more control with the air on my GM80. This is more important on the real cold days.

Also your first log can rest on the front lip over the hump in the middle (as long as you cleared out the ash/coals behind the hump so air can flow.)

In my experience raking the coals foreward has done nothing to help my burns. Just make a path behind the hump and try to level out the coal bed somewhat.

Read the manual repeatedly and keep listening to some advice and make sure it lines up with what the manual says. All of our setups are unique. Some of the suggestions in the manual need tweaking based on your setup and experience.

I used to worry about overfiring and too much smoke in the firebox. #1 thing you want to do with smoke is get heat up the chimney. The manual has good instruction on this. You will see a lot of smoke when you close the door and maybe after engaging the cat. The smoke does subside and flame should start building slowly depending on the coals under the load. Tickling 700 is ok on really cold days. I used to think I needed to see more flame before I shut the air down more. Now I have my loading technique I can close the air sooner and leave it alone more. Keep experimenting. This my third year and I had many troubles my first two years. I am still experimenting but It is so much better now. This is my first wood stove.