The Danger of Heat Spikes and Stove Temps

[soupy1957]

I'm currently in a one-on-one discussion about this, but I'll put it in here as well, hoping for some additional input on the subject of "heat spikes" and "stove temps" during the night and while away (for a few hours).
Obviously, a fire will have a MAJOR heat spike when first lit (say, within the first 1/2 hour), but would you expect that a "heat spike" in the middle of the night?
What if you lit your fire, ........got it settled down to "cruising temp" and left to go grocery shopping. Would you expect that the fire will "heat spike" as high, an hour and a half later, as high as it did initially after you added wood? (This is a discussion based on the assumption that we are talking about those like myself, that burn 24/7). What experiences have YOU had with "heat spikes" when away for brief periods, or during "overnight" burns? (Other than the obvious, "I burned my house down" responses).
As for "stove temps" in general, I have yet to be certain that my Rutland, mounted on the front overhanging lip of my Avalon, is really in the best place for temperature evaluations of the fire box, and will be zealously comparing my temp readings on the sides and top of the firebox, with my Laser, to the Rutland's info..
-Soupy1957

 

[oldspark]

Other than a malfunctioning stove why would you have a heat spike in the middle of the night or when yu are gone, after the stove has settled down after loading you should have a steady burn.

 

[Corey]

My air control is probably a little different than a modern stove, but this is what my 'heat spike' looks like:

(go high def for clear image of the time/temp)


In this case, I loaded pretty full at 10:40pm and cut the air back pretty quick afterward. Temp gradually rose to ~880º by 12:15am and gradually wen back down over the next few hours. I have never noticed the heat to spike within the first minutes of a reload.

 

[ckarotka]

I'm gonna go out on a limb here. Last year I noticed these "spikes"about a half hour after I turned the air down and the fire settled in. I believe it was because the wood I had was not dry enough. In the begining the heat was being use to boil off the water, once the h2o was gone the wood burned better and hotter with the same air settings creating a spike and made me re-adjust the air multiple times during the burning phase. I don't think my stove is at fault because I could snuff out the fire if I wanted to with the air control. The flames would die and not get pulled to a corner to suggest an air leak.

I hope to rid myself of this with much, much better fuel this year. IMOP

Charlie

 

[oldspark]

I'm gonna go out on a limb here. Last year I noticed these "spikes"about a half hour after I turned the air down and the fire settled in. I believe it was because the wood I had was not dry enough. In the begining the heat was being use to boil off the water, once the h2o was gone the wood burned better and hotter with the same air settings creating a spike and made me re-adjust the air multiple times during the burning phase. I don't think my stove is at fault because I could snuff out the fire if I wanted to with the air control. The flames would die and not get pulled to a corner to suggest an air leak.

I hope to rid myself of this with much, much better fuel this year. IMOP

Charlie

Sounds reasonable to me, so two reasons for spikes, a funky stove and funky wood.

 

[vvvv]

I'm gonna go out on a limb here. Last year I noticed these "spikes"about a half hour after I turned the air down and the fire settled in. I believe it was because the wood I had was not dry enough. In the begining the heat was being use to boil off the water, once the h2o was gone the wood burned better and hotter with the same air settings creating a spike and made me re-adjust the air multiple times during the burning phase. I don't think my stove is at fault because I could snuff out the fire if I wanted to with the air control. The flames would die and not get pulled to a corner to suggest an air leak.

I hope to rid myself of this with much, much better fuel this year. IMOP

Charlie

Sounds reasonable to me, so two reasons for spikes, a funky stove and funky wood.

a draft enhancing wiind can cause a spike?

 

[oldspark]

Now we have three reasons, I have never had these spikes happen to me that I noticed in 30 years of burning but I do know the old stove would draft a lot more with the right wind conditions, I wonder if the new EPA stove will react any differently to these conditions.

 

[soupy1957]

I dunno.........maybe it's just me..........but when I fire up a load, I'm there watching it til it settles in, and then I let it go and walk away. Any "spikes" in the burn process, typically take place within (as was said above) 1/2 hour of the lighting.

In the case of a 24/7 fire, I'm still burning (showing some flame) by say 8 PM, and have room for a couple of big logs for the overnight. So I would put those big logs in, and let em go (full open damper til they really take off). Because there was already a fire going when I added them, I find that the time for these big logs to catch is shorter than if it was a cold start (and I would never use a BIG log for a cold start, anyway).

I guess I was surprised to hear that (like the video above) anyone would go to bed BEFORE the spike had occured (assuming the use of good wood of course). In any case, I'd assume the firebox could handle the temps. I think I noted a peak of about 880ºF in that particular video example, although I don't recall how long it held there before cooling down.
Some pretty sophisticated equipment he has there next to the stove!!

I noted that in the video, the high temp started about 11:45 PM and started declining by 12:18, so he had a HOT burn spike for about a 1/2 hour max., unless my old eyes are decieving me. If I were maintaining that peak temp for an hour or more, I'd be concerned, but I'm not sure I need be concerned about the 1/2 hour. (Not sure).

I can see how an acceleration of wind would have an effect on draft........

-Soupy1957

 

[ckarotka]

Any pressure changes inside or out could could change the draft conditions. If the temp drops outside that would increase draft also. Fast out the flue means fast in the stove, = bigger fire=more heat=temp spike. I know wood burns in cycles and there is always raises and falls in the temp of the fire box, I just want to minimize it as much as possible for a longer constant burn. Small spikes I would consider normal, it's the large ones that I think the OP is worried about.

I would imagine also that a cat stove regulates these temps spikes better than a secondary comb stove.

 

[Pagey]

My stove peaks during the first couple of hours after a fresh charge on a well-established coal bed. I've only had 2 instances where it's spiked higher than what makes me comfortable, but there are so many variables involved in burning wood that I have no solid explanation for what made those 2 times any different than all the others.

 

[stoveguy13]

what do you consider a heat spike to be and how long would it last for?

 

[Pagey]

I consider a "spike" to be when a fresh charge of wood begins strong outgassing/secondary combustion. I would say on average that about 20-30 minutes after I load up on a hot, well-established coal bed, my surface temps on the lower plate go from about 400F to 650-700F for anywhere between 1.5-2 hours. After that "spike" it settles into the 600F range for a while, then the 400F range for a much longer time. Then 8 hours later it's at the 250-275F mark and ready for a reload.

 

[Battenkiller]

I have dozens of professionally produced burn charts from older stoves. Virtually every one of them shows a sharp spike about 1/2 hour into the burn and a secondary (usually smaller, but not always) spike from 2-3 hours after the stove was loaded. These charts were produced from heat output data gathered during controlled burns in a lab using properly seasoned red oak, so I believe that puts the "boiling out the water" explanation to rest. The only exceptions were in strictly thermostatically controlled stoves, like the Riteways and Downdrafters. I have posted scans of a few of these charts here on Hearth.com before. Below is one of the more dramatic examples, showing a secondary peak hours later that was significantly higher than the initial one.

I have experienced similar results in both of the stoves I've burned in over the years, and as well, in the ancient furnace I used to use about 25 years ago. That furnace would get like Dante's Inferno four hours into a burn with dry oak, locust, or hard maple. When I opened the door to check on it during those times, I had to stand back because the radiant heat was so intense I really worried it would shatter my glasses. No idea what the temps were in that beast, but the view inside the firebox was awe-inspiring to say the least.

I think that load configuration, load size, chunk size and shape, wood density, and state of coal bed when reloading will all contribute more or less to this phenomenon. Stove type will likely be the most important contributing factor. Air intake will also hugely affect this. I'm assuming folks here are referring to times when a spike occurs when we're not around, so intake air opening is constant. That doesn't mean intake air volume is constant, however, since the chimney will pull harder as it continues to warm and will draw in more air.

 

[mikepinto65]

Well Soupy, I figure I may as well respond here, rather than the PM conversation we were having, as there are a lot of good things being said here. As the fire posted by Cozy Heat and graph posted by Battenkiller point out, there tends to be a "spike" mid way into the burn cycle. Someone posted a less sophisticated version of Cozys digital probe method, a year or two ago using a piece of aluminum foil placed on the thermometer top. The needle would push the foil to the hottest temperature being hit throughout the burn cycle and leave it behind. I tried it out and found the same results: Sometime, during the burn cycle, I found my temps to peak about 100 degrees hotter than the stable cruising temperature I had been experiencing for an hour or two before bed. I would see proof that every once in a while my night time burn peaked between 700-750 on occasion.

I really dont feel any need for concern with this happening BUT, as mentioned in our PM, If your stove is left cruising at 750 an hour or two after loading; than one can only assume that if the "spike" occurred 3-4 hours in, your probably playing a little too close to the overfire range.

Glad you posted this though so we could get input from all the others.

Oh ya...
I've yet to respond to a house fire (nor ever remember my father in his 15 years as a fire fighter) that had been caused by a wood stove simply burning too hot. Its only going to happen with faulty installation of the stove ( + clearances), faulty installation of the chimney ( + clearances), or a clogged up chimney resulting in a chimney fire FWIW.

 

[oldspark]

I'm baffled as I never noticed any on the old stove or the new one (only 2 months with the summit) but I will keep an eye out this winter.

 

[soupy1957]

Those who have read my postings in here, realize that I'm a "detail freak" and very (almost ad nausium) much a disector of data. I appreciated the "chart" that was placed here. I can't say I've seen a spike during my burns, beyond the initial spike when the wood goes into it's second stage of burning. Dunno why that is, but........I haven't.

This quote was quite comforting to me: (with reference to a house fire)
"Its only going to happen with faulty installation of the stove ( + clearances), faulty installation of the chimney ( + clearances), or a clogged up chimney resulting in a chimney fire FWIW."

"Comforting" because I know my installation was professionally and correctly done, (clearances included) and I know I'm burning wood that is "good wood" and no trash, and because I'm determined to stay on top of the creosote issue. I can sleep well at night.

Since it would seem that some types of stoves may be more prone to heat spikes (sounds like that is what's being said in some responses), then I'd wonder if there is any sort of consesus out there about the new EPA-Rated stoves like mine, -vs- an older stove, or catalytic stove?

Obviously it would be a mistake to allow any fire of mine or yours to stay at 750ºF for a long period of time, but fire naturally tapers off as the fuel source is consumed..............in other words, a fire can only go down, in controlled conditions, so "spike" or "no spike," the issue for me is really the potential of those "spikes" if they occur at all. The evidence presented so far, seems to indicate they do.

BTU or "heat intensity" is another aspect to the burn, isn't it. Has anyone done any comparisons with regard to the heat I feel when I open the door, -vs- what the thermometer says? Not that I'm planning on sticking my head or my cat in there, but..............

-Soupy1957

 

[Battenkiller]

Sometime, during the burn cycle, I found my temps to peak about 100 degrees hotter than the stable cruising temperature I had been experiencing for an hour or two before bed. I would see proof that every once in a while my night time burn peaked between 700-750 on occasion.

Interested to know if you feel the same thing happened with your Vigilant. I know mine does, usually about 2-3 hours after I shut the bypass and put her into "secondary combustion" mode. I feel that with the across-the-bottom flame path in that stove, the spike is mostly due to the growth of the coal bed as the burn progresses, which finally generates enough off-gassing in the wood above it to kick off a real secondary burn. Plus, by then the stove is hot enough to do a proper job of heating the secondary air. BTW, my flue temps also rise during the second spike. This isn't necessarily the case with the initial spike, which usually shows declining flue pipe temps as the stove rises during the first half hour after shutting the bypass damper. Not sure why yet, but it's an observation I've had.

I've yet to respond to a house fire (nor ever remember my father in his 15 years as a fire fighter) that had been caused by a wood stove simply burning too hot. Its only going to happen with faulty installation of the stove ( + clearances), faulty installation of the chimney ( + clearances), or a clogged up chimney resulting in a chimney fire FWIW.

Which was likely caused by not burning hot enough. ;-)

I'm a major believer in running stoves very hot. Still, Soupy, I'd tame that stove down just a shade if you really love it. Stove top temps that high on any portion of the top are at the extreme end, even for an old smoke dragon. Occasional brief spikes will probably be fine, but the stove eventually loses the battle if you run it that hot routinely. Parts burn out if nothing else.

 

[firefighterjake]

Well Soupy, I figure I may as well respond here, rather than the PM conversation we were having, as there are a lot of good things being said here. As the fire posted by Cozy Heat and graph posted by Battenkiller point out, there tends to be a "spike" mid way into the burn cycle. Someone posted a less sophisticated version of Cozys digital probe method, a year or two ago using a piece of aluminum foil placed on the thermometer top. The needle would push the foil to the hottest temperature being hit throughout the burn cycle and leave it behind. I tried it out and found the same results: Sometime, during the burn cycle, I found my temps to peak about 100 degrees hotter than the stable cruising temperature I had been experiencing for an hour or two before bed. I would see proof that every once in a while my night time burn peaked between 700-750 on occasion.

I really dont feel any need for concern with this happening BUT, as mentioned in our PM, If your stove is left cruising at 750 an hour or two after loading; than one can only assume that if the "spike" occurred 3-4 hours in, your probably playing a little too close to the overfire range.

Glad you posted this though so we could get input from all the others.

Oh ya...
I've yet to respond to a house fire (nor ever remember my father in his 15 years as a fire fighter) that had been caused by a wood stove simply burning too hot. Its only going to happen with faulty installation of the stove ( + clearances), faulty installation of the chimney ( + clearances), or a clogged up chimney resulting in a chimney fire FWIW.

Agree . . . with one correction and one explanation . . . add in the improper disposal of hot ashes as one of the big causes of fires caused by "woodstoves" -- I see at least one of these in the area each year.

And an explanation . . . while an over-heated stove in itself will not fall apart and cause everything in the house to burst into flame it can certainly become an issue especially if there are other issues -- i.e. getting the chimney hot enough to ignite any creosote, catching nearby combustibles on fire if they are too close, etc. If nothing else, it's just a bad idea to over-fire the stove . . . assuming you like the stove and want to keep it in good working order for a long time.

 

[mikepinto65]

Sometime, during the burn cycle, I found my temps to peak about 100 degrees hotter than the stable cruising temperature I had been experiencing for an hour or two before bed. I would see proof that every once in a while my night time burn peaked between 700-750 on occasion.

Interested to know if you feel the same thing happened with your Vigilant. I know mine does, usually about 2-3 hours after I shut the bypass and put her into "secondary combustion" mode. I feel that with the across-the-bottom flame path in that stove, the spike is mostly due to the growth of the coal bed as the burn progresses, which finally generates enough off-gassing in the wood above it to kick off a real secondary burn. Plus, by then the stove is hot enough to do a proper job of heating the secondary air. BTW, my flue temps also rise during the second spike. This isn't necessarily the case with the initial spike, which usually shows declining flue pipe temps as the stove rises during the first half hour after shutting the bypass damper. Not sure why yet, but it's an observation I've had.

I've yet to respond to a house fire (nor ever remember my father in his 15 years as a fire fighter) that had been caused by a wood stove simply burning too hot. Its only going to happen with faulty installation of the stove ( + clearances), faulty installation of the chimney ( + clearances), or a clogged up chimney resulting in a chimney fire FWIW.

Which was likely caused by not burning hot enough. ;-)

I'm a major believer in running stoves very hot. Still, Soupy, I'd tame that stove down just a shade if you really love it. Stove top temps that high on any portion of the top are at the extreme end, even for an old smoke dragon. Occasional brief spikes will probably be fine, but the stove eventually loses the battle if you run it that hot routinely. Parts burn out if nothing else.

Yes I did notice that when I was burning in the Vigilant. Actually, I would often times notice a greater increase in temps. I'd find my thermometers would routinely spike up to 800-850 at some point in the cycle. Again, I really wasnt TOO concerned by it but i'd be lying if I said I wasnt a little.....that is, untill, I thought about all the nights I lay next to my dads stove glowing red as a kid, and him not giving it a second thought!

 

[mikepinto65]

Good addition Jake, I posted pics of one I went to last year for that very reason. The flames missed making it to his house by about fifteen feet (even closer to the detached garage).

 

[branchburner]

After primary combustion has driven off enough moisture and released enough volatiles to begin secondary combustion, I would expect an early heat spike. But this is still early in the burn, so there is still significant primary combustion occurring. It makes sense that later in the burn there is a point at which the balance of primary/secondary combustion hits another "sweet spot." At this stage of burning smoke, perhaps the heat from volatiles gets maxed, and all the primary combustion is done. The secondary combustion suddenly becomes optimal for the most complete, efficient and intense period of the burn. The second spike then slowly declines into the coaling stage.

 

[branchburner]

BTW, my flue temps also rise during the second spike. This isn't necessarily the case with the initial spike, which usually shows declining flue pipe temps as the stove rises during the first half hour after shutting the bypass damper. Not sure why yet, but it's an observation I've had.

This makes sense. The first spike happens when you shut the bypass, so letting less heat up the pipe means lower flue temps. The stove is getting hotter, but you're now keeping an even greater amount of that heat in the house.
During the second spike, there is no change in stove settings, but since the stove is now throwing more heat, some of that extra heat will make its way up the flue, raising temps.

 

[Battenkiller]

After primary combustion has driven off enough moisture and released enough volatiles to begin secondary combustion, I would expect an early heat spike. But this is still early in the burn, so there is still significant primary combustion occurring. It makes sense that later in the burn there is a point at which the balance of primary/secondary combustion hits another "sweet spot." At this stage of burning smoke, perhaps the heat from volatiles gets maxed, and all the primary combustion is done. The secondary combustion suddenly becomes optimal for the most complete, efficient and intense period of the burn. The second spike then slowly declines into the coaling stage.

Interesting thinking. Problem is, this phenomenon occurs even more in stoves that don't have true secondary combustion, like the Jotul 602 in the chart I posted. That stove didn't even have the top baffle in place during the test. My Vigilant is supposed to achieve secondary combustion, but it's an act of faith to accept it since it all would occur behind the fireback where it can't be seen. If the very similar sized Jotul Oslo is, in fact, more efficient than the Vig (and everyone who's used them both say it is), I think that maybe the secondary combustion in the Vigilant ain't as much as was claimed by the manufacturer. Most older VC techs I've talked to think believing in secondary combustion in an old VC stove is like believing in Santa Claus. Even so, I sometimes experience a very significant spike well after I retire for the evening.... about first nighttime pee time for a 58 year-old bladder, otherwise known as "Pee-thirty" in the morning.

There are so many factors that can contribute to this that I doubt there is a single cause. Rates of pyrolysis can cause from 1/3 to 2/3 of the fuel content of wood to be given off as gases. A huge, hot coal bed loaded with small splits of fast burning wood like pine or popple at 10-12% MC might give off nearly all of fuel as gases very early on, while a half-filled box of fresh cut ash at 30-35% MC will undergo much slower pyrolysis and only convert 1/3 of it's potential fuel to gases before it enter the final coaling stage. We all tend to think along the lines of hard and fast rules, but what goes on inside the box is obeying the laws of physics and chemistry. If we change the conditions in any way, expect the stove to behave differently.

 

[Slow1]

For folks who don't recall/use this trick. A while ago someone (sorry, I don't recall who) on this forum made a really good suggestion on how to know what the peak temperature your stove hit while you were away from it if you are using a stove top thermometer. I've been using it ever since and it is simple and very low cost...

Simply take a piece of foil and fold it so that you have a small V shaped piece and place it in the groove of the thermometer on top of your stove ahead of the needle. Make this foil small and light enough to be easily pushed by the needle as the temps rise, but heavy enough to not be easily blown off (i.e. more than one thickness is my suggestion). Then before you leave the stove gently nudge it against the needle so that as the temps rise it will push the foil up. When the temps go down it leaves the foil at the 'high temp' mark and you will know what it was.

Anyway - using this method you can know what the peak temp was on your overnight/day burns. Clearly you won't know when or how long that peak temp was. But it can be interesting to know if you are peaking higher than you expect.

My experience with my VC was that it would peak much higher than I expected many nights and I was unable to predict when/why etc (part of why I lost confidence in the stove).

With my current FV I have found that it tends to peak early on and then stay pretty steady with a slow rise for the first hour or so. Like someone else posted I did find that I would get a second peak/spike about an hour or so after feeding when I was using less well dried wood. Then again last year was my first year burning with it and my wood was VERY mixed so things were changing constantly between my experimenting to try and figure out how best to burn and the variability in the wood I'm not sure I really had a predictable pattern. But in any case I never saw any high temperature peaks that alarmed me.

 

[branchburner]

Problem is, this phenomenon occurs even more in stoves that don't have true secondary combustion, like the Jotul 602 in the chart I posted.

I think anytime you put fire in a box (and control the air flowing into, and out of, that box) and burn a full load of good, dry wood, you are going to have secondary combustion to some degree. Obviously to a much lesser degree in a lesser stove, but with good burning practices in certain older stoves I expect the stages of combustion aren't as radically different as it might seem they would be.

I know a guy who burns his smoke dragon with no smoke - two years of seasoning wood is his secret. And without a good bed of coals, early in my learning curve, heaven knows how many times my new-fangled downdraft stove has been belching smoke instead of burning it!