Overheating in Green Mountain 40

[ThereAreGoats]

Hi all,

A question about my Green Mountain 40 and overheating. Let's say I reload my stove from coals (stove top around 350-375), keeping the air shut down all the way and reengaging the cat right after. If I add, say, three to four decent sized Ash splits, the stove usually eventually gets hot enough to where the cat temp starts approaching too hot and the surface temp gets past 600. It burns fairly slowly at first, but at a certain point, the fire gets pretty big and there's nothing I can do to turn it down aside from disengage the cat. Even with the air shut down, it rages a bit.

My burn times have naturally been quite low under these conditions (4 hours max, coals to coals), as it's quick-burning ash and there are only a few splits. I know that theoretically, I could increase burn time by really packing the firebox and using harder wood. But I'm terrified to do this given how hot it already gets. I understand, of course, that tightly packed wood will burn much more slowly and not all at once, but this seems impossible to achieve with the way my splits are cut and, as I said, I'm scared of trying this given the fear of overheating.

I don't seem to have any door leaks. The only slightly suspect thing is that the air control doesn't seem to affect the flames as much as it seems it should, yet I can't see how it would be damaged or disconnected or anything. But it's almost as if there's a governor of sorts on it.

Any thoughts on this? I'd really love to get my burn times up, but I also don't want a meltdown.

Robert

 

[Caw]

Reloading at 350-375 is too hot and the wood will start off gassing too quickly. Id try a few reloads at 250-300 STT and see if your results improve. Turn the air down early and often to keep it under control. If this doesn't help I'd be checking for air leaks.

Also, 600 STT is not too hot. A modern steel stove can cruise at 650-700 STT with no issues unless otherwise stated in the manual. My steel stove lists 840 STT as over fire.

 

[ThereAreGoats]

Reloading at 350-375 is too hot and the wood will start off gassing too quickly. Id try a few reloads at 250-300 STT and see if your results improve. Turn the air down early and often to keep it under control. If this doesn't help I'd be checking for air leaks.

Also, 600 STT is not too hot. A modern steel stove can cruise at 650-700 STT with no issues unless otherwise stated in the manual. My steel stove lists 840 STT as over fire.

That makes sense, since sometimes the secondaries start going nuts before the wood is burning much. But it's very tough to reload that cool. After moving the coals to the front, the small firebox makes it tough not to smother at least some of them and it can take many minutes for the lip of the wood to catch at those temperatures. I do probably need to use thinner/smaller pieces in the front. But there's no way I could reload at 250. Of course, this could be a case of cast iron lag; I feel like sometimes the stove top temp doesn't indicate the state of the coals due to how good it is as holding heat. I sometimes feel like just looking at the color of the coals is the better way. But TBD.

As for 600-650 degrees, that tends to correspond with the cat thermometer getting pretty up there, close to the too hot range (or close enough to where I start paying attention). The folks at Hearthstone told me to use that cat themometer as the general stove thermometer, which I took to mean that the stove is too hot (not just the cat) when it gets into the "too hot" area. THe manual says "The Green Mountain 40 typically operates in the 800 ⁰F to 1300 ⁰F range." I'm not sure if that's referring to the firebox temp or the cat. In any case, a conundrum. I think I should look again for leaks.

 

[Caw]

I didn't realize that was a cast iron stove. I can't comment on the max STT temps for that, I've never burned one. My stove is steel with a cast iron door so it's quite different.

However I'm surprised reloading at 250 STT is a problem. Perhaps it's lag like you said. I can reload on coals in my 1.85 cu ft steel box down to about 200-250. I can resuscitate with kindling from like 150. If there are even a few coals it's restart-able. You just need to use appropriate sized pieces on the coals and sometimes let that burn a little bit before reloading.

There has to be a balance at 375 you're getting what I expected you were with instant secondaries and a bit of a runaway stove. That is just too hot for reloading. Might take some practice and experimenting to find the ideal temps. Might take some kindling and TLC with the smaller firebox but youll find it

 

[ThereAreGoats]

I didn't realize that was a cast iron stove. I can't comment on the max STT temps for that, I've never burned one. My stove is steel with a cast iron door so it's quite different.

However I'm surprised reloading at 250 STT is a problem. Perhaps it's lag like you said. I can reload on coals in my 1.85 cu ft steel box down to about 200-250. I can resuscitate with kindling from like 150. If there are even a few coals it's restart-able. You just need to use appropriate sized pieces on the coals and sometimes let that burn a little bit before reloading.

There has to be a balance at 375 you're getting what I expected you were with instant secondaries and a bit of a runaway stove. That is just too hot for reloading. Might take some practice and experimenting to find the ideal temps. Might take some kindling and TLC with the smaller firebox but youll find it

This all makes sense. I'm going to try using a small 2-3" diameter starter piece on the coals in front of the splits next time, and I'll wait for it to get down lower. I might also try to take note of where the cat thermometer is and start trying to go by that when reloading.

 

[webby3650]

I have a hybrid Jotul Oslo. It has the same issue. To combat it I’ve been using larger splits of wood that’s not been split very long. It’s around 30% MC, with seasoned wood I can’t get more than 6 hours without reloading. With the greener wood it acts like my old version of the same stove did with very dry wood. No smoke, cat stays very active throughout the burn, clean glass. It’s the only way I’ve found to tame the beast!

 

[begreen]

THe manual says "The Green Mountain 40 typically operates in the 800 ⁰F to 1300 ⁰F range." I'm not sure if that's referring to the firebox temp or the cat. In any case, a conundrum. I think I should look again for leaks.

That refers to the catalyst temperature.

It sounds like you are doing ok but agree the coals need to be burned down a bit further. Maybe wait another 30 minutes before reloading. Do you have any softwood like pine? Or some 2x4 scraps? If so, put a couple of them down on the coals and then add the regular wood.

How tall is the flue system that the stove is connected to?

 

[ThereAreGoats]

That refers to the catalyst temperature.

It sounds like you are doing ok but agree the coals need to be burned down a bit further. Maybe wait another 30 minutes before reloading. Do you have any softwood like pine? Or some 2x4 scraps? If so, put a couple of them down on the coals and then add the regular wood.

How tall is the flue system that the stove is connected to?

I don't have a good way to measure the flue but I'd say from the top of the stove, it's probably between 15-17 feet. I tried reloading at 300 degrees this time using a starter piece and it worked well and was much calmer, until I got nervous that it was too little air, opened it half way, came back a few minutes later, and the secondaries were cranked again. So that was user error. I'll keep experimenting.

 

[begreen]

The flue height sounds good. Give yourself time to get adjusted and used to the air control position in relation to the stage of the burn. You'll find that larger, thicker (4-6") hardwood splits will take off much slower than thinner (2-3") softwood splits.

 

[ThereAreGoats]

The flue height sounds good. Give yourself time to get adjusted and used to the air control position in relation to the stage of the burn. You'll find that larger, thicker (4-6") hardwood splits will take off much slower than thinner (2-3") softwood splits.

I was just thinking: could the abundance of air leaks in my cabin also be a factor, contributing to the air draw? It's not like there are gaping holes, but let's just say that the cabin is not air tight.

 

[EbS-P]

I like reloading with 2-3 pieces of kindling on bottom one on top. I find. Can get air turned down sooner.

 

[begreen]

I was just thinking: could the abundance of air leaks in my cabin also be a factor, contributing to the air draw? It's not like there are gaping holes, but let's just say that the cabin is not air tight.

I doubt it. It's most likely the size of the fuel and the way the stove is being run.

 

[neverstop]

some kind of flue temperature monitoring would be very helpful in this case.

I have the shelburne. The layout of these stoves can make things look deceiving. you have the wood E/W (unless you cut your wood to <12" and the flames are normally on the front facing piece of wood/up by the secondary air tubes; so they're very visible. On re-loads once the flames start rolling up/over the baffle I'll close the by-pass and start shutting down the air. I don't see a noticeable change in supplied air until I close it more than an inch (if you look up under the stove at the air handle; this position corresponds to the slot having probably 0.25" gap on the wooden handle side of the control).

how are you measuring STT? i have 2 bi-metallic gauges, an IR gun, and a custom thermocouple. the bi-metallic gauges are trash, and even the IR gun is off by 30+F.

 

[begreen]

how are you measuring STT? i have 2 bi-metallic gauges, an IR gun, and a custom thermocouple. the bi-metallic gauges are trash, and even the IR gun is off by 30+F.

Some bi-metallics are better than others. Has the IR gun been calibrated for the correct emisivity?

 

[ThereAreGoats]

some kind of flue temperature monitoring would be very helpful in this case.

I have the shelburne. The layout of these stoves can make things look deceiving. you have the wood E/W (unless you cut your wood to <12" and the flames are normally on the front facing piece of wood/up by the secondary air tubes; so they're very visible. On re-loads once the flames start rolling up/over the baffle I'll close the by-pass and start shutting down the air. I don't see a noticeable change in supplied air until I close it more than an inch (if you look up under the stove at the air handle; this position corresponds to the slot having probably 0.25" gap on the wooden handle side of the control).

how are you measuring STT? i have 2 bi-metallic gauges, an IR gun, and a custom thermocouple. the bi-metallic gauges are trash, and even the IR gun is off by 30+F.

I've been measuring STT with a Vermont Castings thermometer placed a bit off center.

 

[ThereAreGoats]

I didn't realize that was a cast iron stove. I can't comment on the max STT temps for that, I've never burned one. My stove is steel with a cast iron door so it's quite different.

However I'm surprised reloading at 250 STT is a problem. Perhaps it's lag like you said. I can reload on coals in my 1.85 cu ft steel box down to about 200-250. I can resuscitate with kindling from like 150. If there are even a few coals it's restart-able. You just need to use appropriate sized pieces on the coals and sometimes let that burn a little bit before reloading.

There has to be a balance at 375 you're getting what I expected you were with instant secondaries and a bit of a runaway stove. That is just too hot for reloading. Might take some practice and experimenting to find the ideal temps. Might take some kindling and TLC with the smaller firebox but youll find it

So do I have this right? In general, the runaway effect happens because too much gas is being released due to the hot firebox, and once the secondaries kick in and start burning it, it gets way hotter in a hurry, and all before the primary combustion has really even started? It seems like the ideal fire would be one where the primary flames can take care of a greater percentage of the gas. So maybe the best way to think about a runaway fire is one where too high a percentage of the gas is burned by secondaries and not primary combustion?

 

[begreen]

Slow the rate of offgassing by turning down the air sooner and by burning thicker splits.

 

[Caw]

Slow the rate of offgassing by turning down the air sooner and by burning thicker splits.

This. Begreen always so smart and succinct. Also mixing species can help depending what you need.