Overfired my Insert - A Case Analysis for Hearth Members

[op_man1]

Jotul states, for the C450, that the unit is overfiring if any part of it is glowing. I had the top of my unit glowing last night, all too easily (which means over 900F). Here's what happened...

The start up
I did a start up fire with kindling, bark, small pine splits and chunks of silver maple. Within 30 min, the fire was going strong so I dampered down to 50%. The fan was going about 15 min later. No problem so far. I let it burn down to minimal flames, good coal bed.

The reload
I then proceeded to reload with two medium to large pine splits and some smallish chunks of silver maple and elm. As soon as I closed the door, it took off right away. Within a minute, I dampened down to 50%. It was still increasing in intensity so 5 min later I reduced it to almost the absolute minimum (about 1/4 inch from the minimum). I walked away, thinking that I have never overheated the stove with it dampened down to the near minimum. Besides, the firebox was not full, even if there were two splits of pine.

The overfire
I checked things out about 45 min later. The fire was blazing and I could see the the top of the unit was glowing - not bright, rather dull, but glowing nonetheless. I immediately dampened down to the minimum (not much difference there) and put the fan on high rather than low. I could see a spark here and there on the top of the unit (what was that?). Within 15-20 minutes, the unit was not glowing anymore, although the flames were still going very strong (full, intense secondaries for another 30-40 minutes). I measured the hot air coming out of the insert, with the fan set at high, at about 310F...

Why?
So what did I do wrong? The box was not full of wood. I have burned pine before and have never overfired - plus, this was not solely pine, it had other types of wood in there. I had the fire dampened down way before I approached overfire temps... And the fan was working all along. I don't want this happening again but at the same time I don't want to be too shy with the unit - I want some good heat! So what's up?

Damage?
Did I damage the unit? My sense is probably I did not - these things are pretty tough and I have never had it this hot. However, I do not want to make this a recurring event - I would like this unit to last many, many years. Should I modify the primary air control so that I can dampen it down to airtight levels (good or bad idea)? Any other suggestions?

Anyways, thanks for any input.

 

[woodgeek]

I'll try: gaskets? Dollar bill test?

 

[op_man1]

I'll check tonight - the insert is new from last year so that should not be it but I'll check anyways.

 

[branchburner]

Agree, sounds like air getting in - perhaps the gaskets themselves are not faulty, but not sealing due to a small piece of wood, or other obstruction?

 

[Battenkiller]

Sounds like a runaway fire caused by using fast burning wood. Once the flue gets hot enough, it can start to draw harder and harder as the temps continue to rise. This will pull more air into the intake, increasing air velocity, which increases turbulence, which makes the fire burn faster, which... you get the picture - a classical positive feedback loop.

At some point (depending on your stove's design), this will level off because there starts to be an actual reduction in draft as temps go beyond a point. With your insert and your install and that particular load, that point was probably not reached until after an overfire event occurred.

Try using even larger splits of pine, or mix in a good dense wood like ash or oak if you have them. Don't think you can't get a stove too hot with a small load. I just started my stove about an hour ago. I used some old poplar from a busted dresser to start the fire, with a large split of very dry cherry opposite the draft opening. All of the wood was extremely dry. I was just sitting there in front of it, spaced out, feeling the warmth of the stove when I happened to look up and see my flue pipe temp was nearly pegged at 900º. Less than 10 pounds of wood in a 2.5 cu.ft box, and not five minutes after I struck the match. Glad I didn't walk away from it.

 

[Todd]

Pitch in Pine? Highly flammable stuff. How tall is your chimney? If on the tall side I think it may be a good idea to tweak the air control, infact I think it's a good idea anyways, gives you more control if fire gets out of hand.

 

[op_man1]

Sounds like a runaway fire caused by using fast burning wood. Once the flue gets hot enough, it can start to draw harder and harder as the temps continue to rise. This will pull more air into the intake, increasing air velocity, which increases turbulence, which makes the fire burn faster, which... you get the picture - a classical positive feedback loop.

At some point (depending on your stove's design), this will level off because there starts to be an actual reduction in draft as temps go beyond a point. With your insert and your install and that particular load, that point was probably not reached until after an overfire event occurred.

Try using even larger splits of pine, or mix in a good dense wood like ash or oak if you have them. Don't think you can't get a stove too hot with a small load. I just started my stove about an hour ago. I used some old poplar from a busted dresser to start the fire, with a large split of very dry cherry opposite the draft opening. All of the wood was extremely dry. I was just sitting there in front of it, spaced out, feeling the warmth of the stove when I happened to look up and see my flue pipe temp was nearly pegged at 900º. Less than 10 pounds of wood in a 2.5 cu.ft box, and not five minutes after I struck the match. Glad I didn't walk away from it.

Interesting... I was planning on using mostly silver maple this winter to heat the place, but I think that is also considered fast burning, although perhaps not to the extent as pine. Could a box full of silver maple also lead to a runaway fire, given similar circumstances?

Also wonder why this has not happened before - just a fluke combination of dry, fast burning species? I will definitely be monitoring the fire more frequently in the immediate future.

 

[op_man1]

Pitch in Pine? Highly flammable stuff. How tall is your chimney? If on the tall side I think it may be a good idea to tweak the air control, infact I think it's a good idea anyways, gives you more control if fire gets out of hand.

Now that you mention it, this pine was probably very pitchy (if that's a word) - flames were shooting out of it (like small explosions) very early on in the burn cycle.

The chimney is not particularly tall - maybe 20 feet or so.

 

[begreen]

Exactly what part was glowing? The burn tubes and baffle in the firebox or the actual top of the stove? Was the blower on during this time?

 

[op_man1]

Exactly what part was glowing? The burn tubes and baffle in the firebox or the actual top of the stove? Was the blower on during this time?

The outside, top of the stove was glowing, mostly from the center to the left side (where the fire was most intense). the burn tubes did not seem to be glowing but frankly there were lots of flames and I could not really tell (and I did not look very hard). The blower was on at low from the moment I refilled - as soon as I noticed that it was overheating, I increased the blower speed to high. Just to reiterate, I had the air down to almost the minimum WAY before I approached any kind of overfiring temp.

 

[begreen]

It would be good to do the dollar bill test on the door gasket and to look for any possible air leaks. Also make sure the door latch is snug and not sloppy.

How is the insert vented? How tall a flue?

 

[branchburner]

I get a hotter, more intense "runaway" type fire burning small splits/branches of hardwood than I do burning large splits of pine, but it also depends on the moisture content. I think hitting 900F stovetop with virtually no primary air would be more a function of wood size/dryness than of wood species.

I avoid burning large amounts of very dry, very small wood, whether hard or soft, except when starting a fire.

 

[maverick06]

glowing"????

I have had the air tubes (oup top) glow once or twice , but the actual top baffel plate wasnt... I didnt like that too much... not sure if that counts as an overfire.

 

[Battenkiller]

Interesting... I was planning on using mostly silver maple this winter to heat the place, but I think that is also considered fast burning, although perhaps not to the extent as pine. Could a box full of silver maple also lead to a runaway fire, given similar circumstances?

Also wonder why this has not happened before - just a fluke combination of dry, fast burning species? I will definitely be monitoring the fire more frequently in the immediate future.

I wouldn't go jumping to any conclusions about the wood until you eliminate any problems there may be with the stove itself. You should have no problem with burning silver maple in any if these stoves. Heck, they are tested using Doug fir. But it's always a good idea to monitor your stove often, so don't give that up. ;-)

 

[ckarotka]

I've had similar burns happen to me, actually quite a few times. Come to find out the pieces were to small in too great a quantity with too large of a coal bed. I still have some issues because I split to small. This will be my last year for that.

 

[krex1010]

You saw sparks on top of your unit because it got hot enough to ignite dust another particles that were in the air and came in contact with the stove or were already resting on your stove.

 

[ckarotka]

I've had similar burns happen to me, actually quite a few times. Come to find out the pieces were to small in too great a quantity with too large of a coal bed. I still have some issues because I split to small. This will be my last year for that.

I should clarify that I managed to stall the fire by covering the secondary air intake with my gloves till it settled down, so the hottest it ever got was around 650, but I knew if I didn't do something it was going nuclear.

 

[op_man1]

It would be good to do the dollar bill test on the door gasket and to look for any possible air leaks. Also make sure the door latch is snug and not sloppy.

How is the insert vented? How tall a flue?

I will test with a bill tonight.

I have an uninsulated 5.5 inch flue - about 20 foot run.

 

[op_man1]

Interesting... I was planning on using mostly silver maple this winter to heat the place, but I think that is also considered fast burning, although perhaps not to the extent as pine. Could a box full of silver maple also lead to a runaway fire, given similar circumstances?

Also wonder why this has not happened before - just a fluke combination of dry, fast burning species? I will definitely be monitoring the fire more frequently in the immediate future.

I wouldn't go jumping to any conclusions about the wood until you eliminate any problems there may be with the stove itself. You should have no problem with burning silver maple in any if these stoves. Heck, they are tested using Doug fir. But it's always a good idea to monitor your stove often, so don't give that up. ;-)

I thought I had this down pat but apparently not... More monitoring is definitley in...

 

[op_man1]

You saw sparks on top of your unit because it got hot enough to ignite dust another particles that were in the air and came in contact with the stove or were already resting on your stove.

Makes sense... I have heard of cases of plant explosions due to dust igniting. Thanks for th info.

 

[op_man1]

I've had similar burns happen to me, actually quite a few times. Come to find out the pieces were to small in too great a quantity with too large of a coal bed. I still have some issues because I split to small. This will be my last year for that.

I should clarify that I managed to stall the fire by covering the secondary air intake with my gloves till it settled down, so the hottest it ever got was around 650, but I knew if I didn't do something it was going nuclear.

Maybe I need to figure out where the air intake is for the secondaries. Will review the diagram.

 

[pmac]

Does the C450 have an ash pan door that maybe wasn't closed all the way? I left mine open once accidentally on my Castine and the stove really got cranking before I realized what happened.

 

[vvvv]

I've had similar burns happen to me, actually quite a few times. Come to find out the pieces were to small in too great a quantity with too large of a coal bed. I still have some issues because I split to small. This will be my last year for that.

I should clarify that I managed to stall the fire by covering the secondary air intake with my gloves till it settled down, so the hottest it ever got was around 650, but I knew if I didn't do something it was going nuclear.

amazing how letting air into the stove can result in overfiring in the right conditions. Ashley thermostatic stoves used to shut down air when it got hot but the result was unburnt smoke & creosote

 

[op_man1]

Does the C450 have an ash pan door that maybe wasn't closed all the way? I left mine open once accidentally on my Castine and the stove really got cranking before I realized what happened.

No ashpan, thank god... That's just what I would have needed.

 

[oconnor]

I have done exactly the same thing with my stove. I find that once it is warmed up, you can close the damper almost fully and it may still climb to over 800*F stove top.

I don't think you'll find anything is wrong with the stove, and I doubt you have hurt anything. Do you have a stove top thermometer? Once my stove hits 500*F, I close the air to the minimum that will allow for constant flames. If yours is like mine, it makes different expansion/contraction noises when cooling than heating. Between the noise changes and the flame, you should be able to close the air earlier and avoid the overfire.