How to correctly char wood load before closing primary

[neverstop]

I've realized my problem of uncontrollable max STT has most likely been due to choking down the air and closing the by-pass without properly charring the load of wood. This is particularly easy to do on a reload; STT can be 450+ and cat active shortly after reloading at 300 STT. In the past I shut the by-pass if it was still in the active range immediately after reloading and choked down the air to keep the flames lapping the baffle. Eventually secondaries would get out of hand and the entire load would off-gas.

I've played around with allowing it to burn hotter longer initially to burn off this large off-gassing. During this time the by-pass is open. Depending on the load size leaving the primary 100% open results in 600+ STT, or 450 before there is a noticeable reduction in aggressive flames. For smaller loads I can close the by-pass and shut the air down to ~85% and cruise with secondaries. No run-away scenario. Larger loads I'm guessing I have to shut the primary down sooner to keep STT lower.

The stove loads E/W and the primary is in the bottom front. So for the larger loads the pieces closest to the door catch and burn aggressively but the pieces behind them don't catch as well.

So my question is, how do you know when a load of wood is sufficiently charred so that there won't be a massive out-gassing and a run-away secondary combustion scenario?

 

[EatenByLimestone]

I try to close down the primary as quick as I can. I want to burn those volitiles that are off gassing. I don’t fret about the charring.

I start turning the primary down when the flames are reliably licking the baffle.

 

[Todd]

Stove top temps really lag behind if your trying to judge when to turn down stove. I prefer watching my flue temps.

 

[begreen]

Stove top temps really lag behind if your trying to judge when to turn down stove. I prefer watching my flue temps.

Agreed. I rarely look at SST anymore except to inform folks here. Flue temp via the Auber and visuals are what I go by. I turn down the air as soon as possible and load large splits to keep off-gassing under control. Leaving the bypass open would seem to be letting a ton of heat up the flue.

That said, I am burning N/S in a different stove and with large softwood splits. The OP's stove is an E/W hybrid burning hardwood. Another variable is draft strength. There can be a big difference in stove management from a 16' flue system to a 30' flue and/or draft strength when it's 40º vs when it's 10º.

 

[neverstop]

I try to close down the primary as quick as I can. I want to burn those volitiles that are off gassing. I don’t fret about the charring.

I start turning the primary down when the flames are reliably licking the baffle.

When I do this I inevitably get a scenario where the load starts of gassing uncontrollably. Stove will be cruising along nicely with secondary combustion then the secondaries start picking up strength, I close the primary all the way but secondaries take over and Temps spike.

Stove top temps really lag behind if your trying to judge when to turn down stove. I prefer watching my flue temps.

I use flue Temps as well. I've closed it down when Temps on single wall are 300 until flames visually are weaker. Continue this until secondaries are sustained and "mild". Eventually what I described above occurs.

Agreed. I rarely look at SST anymore except to inform folks here. Flue temp via the Auber and visuals are what I go by. I turn down the air as soon as possible and load large splits to keep off-gassing under control. Leaving the bypass open would seem to be letting a ton of heat up the flue.

That said, I am burning N/S in a different stove and with large softwood splits. The OP's stove is an E/W hybrid burning hardwood. Another variable is draft strength. There can be a big difference in stove management from a 16' flue system to a 30' flue and/or draft strength when it's 40º vs when it's 10º.

Temps have been 20-mid 30s. Rear exit Stove, external brick Chimney with 6" insulated liner to class a extension; total height ~21 ft from Stove exit. Adjustable elbow off the stove and then ~2 ft Horizontal run maybe 1/2"rise/ft. Haven't measured draft but it's probably borderline minimum spec because depending on the scenario smoke may rollout on reloads with few coals.

So you all close the bypass and turn down the air before the wood turns black? When it runs away I see smoke coming off the non charred logs. E/W loading makes it hard to judge flame activity on reloads. The entire bottom of the load may be burning but the flames only extend a couple of inches off the load vs hitting the baffle. Or the flames are just visible on the edges/front of the load.

The one positive is I'm getting good at managing run away fires by opening the primary, bypass and door to flood the firebox with cool air.

 

[begreen]

I have not run this stove, or any hybrid for that matter, so I trust your observations. When reloading, try burning down the coal bed a bit more. Put on a couple thin (2") splits and open up the air to say 50% to do this while still providing good room heat.

 

[neverstop]

I have not run this stove, or any hybrid for that matter, so I trust your observations. When reloading, try burning down the coal bed a bit more. Put on a couple thin (2") splits and open up the air to say 50% to do this while still providing good room heat.

from the manual

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 14). 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 1/2”) 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.

The air control has maybe 3" of travel, but the "medium high burn" setting is 2.5" pulled out from the lowest setting. I think this may be where I was screwing up. I would close the primary by % of total travel; so I moved the lever 1.5" (50%) in then to 2.25"(75%) in which is still not at what is considered a "medium high burn". Basically I've been operating in error. This also implies the stove doesn't have great control over air supply.

I reloaded on a hot bed of coals (STT ~300+) with 2 large splits. Left control on high until flames caught and then shut it down to the "medium high burn" setting immediately. Left it like this for 5 minutes then closed the by-pass. I had dancing secondary flames and the STT didn't go above 380 over 30 or so minutes. I checked the cap and saw smoke so I increased the burn rate slightly. Still had control over the temperature. When time/weather permits I'll have to try this again with progressively larger loads.

I feel like I need to design a useable gauge to attach to the primary air lever. Something that gives a visual cue as to % of travel and that I can see even if the lever is pushed underneath the stove.

 

[tebenhoh]

I feel like I need to design a useable gauge to attach to the primary air lever. Something that gives a visual cue as to % of travel and that I can see even if the lever is pushed underneath the stove.

I marked my “air lever”, or slider (not familiar with what is on your stove), with a metallic Sharpie marker. This was after my complete newbie learning stage with our Buck stove when I finally got tired of getting on the floor and twisting my neck to look at the actual plate covering the air intake. I hear the newer Buck stoves’ air controls are now marked from the factory. I also thought of removing the part from the stove and making a permanent mark with a chisel or grinder.
If you’re slider disappears under the stove too far, you could wire tie(?) tape an extension to it maybe? When mine is closed all the way, I can’t see my marks - the first mark on the left towards the stove is open/closed 50%, or half of its full travel.

 

[tabner]

I have similar stove and internal tech. I don’t reload until stt is more like 200, sometimes the cat is just out of active range, and the bypass stays open while the load chars and then it closes when the cat is back up to temp. I start closing down air almost completely based off visual flames in the box, I just kinda have to feel how much I can cut back without snuffing the flame. Sometimes the air is cut back before the bypass is even closed - if the cat isn’t up to temp but the load is taking off. I know what you’re saying about aggressive secondaries, I had this the other night with a big load of dry oak, that gas grill effect happens in the back and it sorta becomes self-sustaining and accelerating (positive feedback loop?). In that case I closed the key damper for 45 mins until it settled down a bit, then opened the damper and went to bed. This is main reason I constantly price out BKs, the peace of mind that must come with a thermostat sounds really nice to me.
I’m thinking maybe you want a key damper? And maybe you want to let the stove cool a little more before a reload? And maybe you wanna go to the other extreme and shut the air down sooner?
You could expirement with cutting the air quicker and just take a look at your smoke stack, as long as you’re not snuffing out the fire or puking out smoke, no reason not to shut down faster?

 

[begreen]

The air control has maybe 3" of travel, but the "medium high burn" setting is 2.5" pulled out from the lowest setting. I think this may be where I was screwing up. I would close the primary by % of total travel; so I moved the lever 1.5" (50%) in then to 2.25"(75%) in which is still not at what is considered a "medium high burn". Basically I've been operating in error. This also implies the stove doesn't have great control over air supply.

It implies that the air control is not linear. It won't be because draft will get stronger with a hotter fire.

 

[neverstop]

I have similar stove and internal tech. I don’t reload until stt is more like 200, sometimes the cat is just out of active range, and the bypass stays open while the load chars and then it closes when the cat is back up to temp. I start closing down air almost completely based off visual flames in the box, I just kinda have to feel how much I can cut back without snuffing the flame. Sometimes the air is cut back before the bypass is even closed - if the cat isn’t up to temp but the load is taking off. I know what you’re saying about aggressive secondaries, I had this the other night with a big load of dry oak, that gas grill effect happens in the back and it sorta becomes self-sustaining and accelerating (positive feedback loop?). In that case I closed the key damper for 45 mins until it settled down a bit, then opened the damper and went to bed. This is main reason I constantly price out BKs, the peace of mind that must come with a thermostat sounds really nice to me.
I’m thinking maybe you want a key damper? And maybe you want to let the stove cool a little more before a reload? And maybe you wanna go to the other extreme and shut the air down sooner?
You could expirement with cutting the air quicker and just take a look at your smoke stack, as long as you’re not snuffing out the fire or puking out smoke, no reason not to shut down faster?

I find that on reloads if my STT is above ~430 and I have secondaries going I have zero smoke from my cap. Below that regardless of the cat being engaged or not I usually see smoke.

Loaded almost a full load last night. Raked coals to the front, left a channel for the primary air to the back of the stove. Caught pretty quick, had the cat engaged in 5 minutes. Had the air shut down to less than 1/8" open by the time the STT was at 475, flue was at 280. Over the course of an hour the flue dropped to 245 then rose to 410; STT maxed at 730.

Now I've been leaving a pretty big "valley" behind the primary air that extends to the back of the stove (per the manual). 2" x depth of the ash. The bottom of the logs are being fed air and flames in the valley are visible even when I have the air turned down.

I'm wondering if I should just make sure that the ash/coals don't block the air inlet vs leaving a large valley underneath the entire load?

For the final air setting should I be turning down the air until I can no longer see active flames on the load and just secondaries?

 

[neverstop]

The answer to the above questions in black are a resounding "yes". Leaving that large valley underneath the entire load was causing the entire load to off-gas at once.

 

[neverstop]

I have noticed water dripping from the cap. There isn't smoke; I do see heat waves. This is even with running at 617 STT and 340 flue with secondary combustion and cat engaged. I don't test every piece of wood I burn for MC, but the ones I have tested recently have been good. I just split a 4"x4" red oak that was inside by the stove for 6 hrs and it read 15% along the grain on the fresh split. Stored stacked/covered since last October ("seasoned delivery").

 

[EatenByLimestone]

There will always be water (and carbon dioxide) as a biproduct of what we burn. Since we limit air to control the burn were never going to get rid of carbon monoxide either.

 

[neverstop]

There will always be water (and carbon dioxide) as a biproduct of what we burn. Since we limit air to control the burn were never going to get rid of carbon monoxide either.

So nothing to worry about then?

 

[EatenByLimestone]

It sounds fine. As long as it’s water dripping and not gooey black creosote.

 

[BotWoogy]

Try using different size splits. And packing them in certain ways. Sounds like you are putting them in a way that is really great for starting fires with kindling, with lots of air flow right over the intake hole. Next time try getting things good and hot, rake the coals all to one spot, I say over the front intake. Just have ashes at the back. Then put some big splits in and pack them in so there is not as much air flow, kind of flat and up against the walls. If it was hot enough they should off gas slowly like this. And last long time. Turn down the air and flames should just dance up top. The wood way down at the bottom at the back won’t even get hot for a long while.