Is it possible to overheat the stove and not the flue??

[drewboy]

Just curious, I only have a thermo on the flue pipe. Would you rather watch the temp on the stove top or flue temp??
I realize they are relative but it's just a question I don't recall having a direct answer to on the forum...

Rob

 

[Corey]

I think that would be the definition of a flue fire. But under normal operating conditions, the temps are mostly relative...hotter stove = hotter flue.

My preference is to watch the stove temp as that is where all the "magic" of combustion, secondary combustion, etc is happening. But really, this boils down to a scan with an IR thermometer every couple of days or when something interesting happens as opposed to watching the thermo like a hawk with every wood addition and air adjustment. The flue temp is what it is, and I can't really control that. Though I suppose in a perfect (and well instrumented) world, we'd shoot for the highest stove temp and the lowest stack temp...meaning most of the heat is in the stove and going into the house as opposed to up the flue.

 

[Backwoods Savage]

Definitely I'd watch the stove temp before the flue temp. It is relative in that when the temperature goes way up in the stove it will also go way up in the flue. However, that is not always true. For example, let's say I engage the cat. when the stove top is 250 degrees and the flue temp is 500 degrees. Within minutes, the stove top will go to 500 degrees while the flue will drop to around 300-350 degrees. So if I were watching the flue temp, I'd think the stove needs some adjusting while in fact, it would be working properly.

Or for an example, let's go back to when I was a boy and we heated with the old pot belly type stoves. Leave that flue damper open too long and you would have a roaring fire for sure and the stove pipe could even start turning red! Yet, the stove would not be kicking out that much heat. But shut that damper part way down and the stove kicks heat while the pipe cooled.

Hope this makes sense.

 

[edthedawg]

You're asking two different questions here...

"Is it possible to overheat the stove and not the flue??" Yes. If you damper your flue down, the right combination of woods and air settings can easily overfire your stove, leading to warped doors, cracked baffle plates, burned tubes, etc. The flue can often stand those prolonged temps where the stove can't.

"Would you rather watch the temp on the stove top or flue temp??" Answer here is both - they tell you different things. You want your heat in the firebox, but early on, you need it up into the flue to establish both draft and a good charred load that will keep burning well once you back your settings down to cruise. After those adjustments, your flue should cool down, and your stovetop temp should rise substantially.

For those with a well established set of practices, and who know their wood conditions are ideal, I doubt they ever more than casually glance at their thermometers unless something suddenly "goes wrong"...

 

[savageactor7]

Is it possible to overheat the stove and not the flue??

I'm opining yes it's possible, cause flues are designed to accept higher temps than stoves...also the somewhat closed nature of a stove v. open chimney flue. More critical tolerances in stoves v. flues too.

 

[Jags]

X2 with Ed. He nailed it.

 

[WES999]

Yes I think it is possible to overheat the stove and not the flue, the flue is rated to about 1000* and a stove is rated to arround 600*- 750* depending on model etc.
IMO you need to monitor both the flue tempand stove temp. The flue temp can go from RT to 900* or more if you let it, in 5 or 10 minuts.
I disigree with Cozyheat: "the flue temp is what it is", the flue temp is directly related to the air setting, you can see a change in flue temp almost instantly with a change in air setting. (At least on my setup)

 

[firefighterjake]

You're asking two different questions here...

"Is it possible to overheat the stove and not the flue??" Yes. If you damper your flue down, the right combination of woods and air settings can easily overfire your stove, leading to warped doors, cracked baffle plates, burned tubes, etc. The flue can often stand those prolonged temps where the stove can't.

"Would you rather watch the temp on the stove top or flue temp??" Answer here is both - they tell you different things. You want your heat in the firebox, but early on, you need it up into the flue to establish both draft and a good charred load that will keep burning well once you back your settings down to cruise. After those adjustments, your flue should cool down, and your stovetop temp should rise substantially.

For those with a well established set of practices, and who know their wood conditions are ideal, I doubt they ever more than casually glance at their thermometers unless something suddenly "goes wrong"...

I'm with Ed all the way on this one.

Yes, I believe you can overheat the stove and not the flue . . . namely because having both the stove and flue thermometers have allowed me to better see what is happening while burning and see the relationship between the two temps.

As a side note, I was just thinking last night (as I was watching the temps on both thermometers) that I am very glad to have both thermometers -- not that I was burning incorrectly or out of control before with just the stove thermometer, but rather with both thermometers I feel as though I can better tell what is happening in the burn cycle and better control the heat output.

Let me give you some real life examples . . . when I start out with my fire the temp in the flue begins to rise pretty quickly compared to the stove temp which also begins to slowly rise (due no doubt to the large mass of the cast iron stove). At a key point the flue temp reaches the point where it is hot enough that creosote production in the flue should be reduced . . . and I begin to shut down the air flow to the stove which until now I've had wide open.

As I cut back the air to 3/4 or 1/2 the stove temp typically continues to rise (although it will sometimes stay around the same temp) . . . meanwhile the flue temp begins to decrease.

In answer to your second question . . . just like Ed . . . I like having both thermometers since I can adjust the air flow so that the flue temp is running high enough to minimize creosote production while at the same time my stove is pumping out plenty of heat at a slower pace.

And . . . just like Ed . . . while I used to frequently check the temps now I pretty much only do so on start up, after loading a fresh load of wood and if something seems a bit odd.

 

[oconnor]

I too agree with Ed, but

I've observed that in most stoves (steel), the only criteria often supplied from the manufacturer concerning temperatures is the glowing parts clause. That is the case with my Jotul Kennebec Insert, and I have seen it glow on top (woops). Enameled stoves and Soapstone stoves are obvious exceptions. If I could monitor flue temps, then I can't see how I would get into the problem of an overheated stove, as the flue will give a earlier indication of combustion rate, especially with a probe thermometer. It will respond earlier, and allow you to take action to set a burn using air control rate that won't permit an overheated stove.

I would assert that any overdraft/overfire that would result in a stove overtemp would be seen in the flue gas temp first. I'm thinking that is what a system designeer would measure in a process control situation, especially if clean efficient burning was the goal.

Firefighterjake - Is your description of flue temp a surface temp or flue gas temp - I thought flue gas temp should always be hotter than the stove, unless you just closed a keydamper and shut the air totally, which isn't an option is an EPA setup.

My temp monitoring is still quite active, as I am still finding wet wood and am finding that I can get the stove to cruise at different temps if I adjust the air at different temps, (sort of a max speed cruise @700 vs max range cuise @ 500, from my 4 engine airplane days) and therfore can modulate heat output into the room better.

 

[Jags]

I thought flue gas temp should always be hotter than the stove, unless you just closed a keydamper and shut the air totally, which isn't an option is an EPA setup.

This is not true with most of todays stoves. Also, the whole "system" can affect flue vs stove temps.

I have noticed on mine and in the many post on this site that when most stoves are at operating temps and damped down for secondary combustion that the stove temp will rise above the flue temp.

 

[PunKid8888]

I always had a flue temp on my moms VC aspen, then with my VC Vigilant I was reading online that most people did stove top, so thats what I did.

Now I have both, and it worth the extra 10 bucks. You learn so much about how the stove is burning. Also at a cruise I have never seen my flue gasses hotter then my stove other then initial start up and maybe when I reload. But I think part of this is due to my External block chimney with liner, I think its always very cold and pulling the heat out of the flue.

 

[oconnor]

Jags

I am at a loss to understand how the stuff that is heating the stove (combustion gasses) can be cooler than the stove itself in an equilibrium state? For short periods I can understand this, as once the primary air is restricted, the rate of BTU produced would be less than the rate of stove cooling, but on average, the stove has to be a lower temp than the equilibrium temp of the flue gasses. Otherwise, the steel in the stove is making it's own heat. Cat stoves might have higher cat temps than flue gas temps - but I don't understand how stove temp could be higher.

My guess is that flue temps mentioned are often actually pipe surface temps when folks post.

 

[Jags]

Jags

I am at a loss to understand how the stuff that is heating the stove (combustion gasses) can be cooler than the stove itself in an equilibrium state? For short periods I can understand this, as once the primary air is restricted, the rate of BTU produced would be less than the rate of stove cooling, but on average, the stove has to be a lower temp than the equilibrium temp of the flue gasses. Otherwise, the steel in the stove is making it's own heat. Cat stoves might have higher cat temps than flue gas temps - but I don't understand how stove temp could be higher.

My guess is that flue temps mentioned are often actually pipe surface temps when folks post.

I can understand your argument, but using that logic we would not have high efficiency forced air furnaces either.

I don't think the combustion gas is any cooler than the stove, I think it is a volume thing. If I allow less volume of the heated gasses to go up the stack, the stack temp will be cooler. Reducing primary air flow does this. By reducing primary air and reducing or slowing down the exhaust, I am also keeping the heated combustion gasses in the stove for a longer time, increasing the heat transfer from the body of the stove to the living area.

 

[oconnor]

I can understand your argument, but using that logic we would not have high efficiency forced air furnaces either.....

High efficiency furnaces use air to air heat exchangers to move the heat from the gases to the circulated air - the gases are still hotter than the exposed surfaces (i.e. the heat exchanger).

Even keeping the gases in the stove longer doesn't make sense - heat moves to the colder material. If the stove was hotter than the gases, then the stove would heat the gases until equilibrium is established. But since the heat source is inside the stove, the gases are going to change temp quicker relative to burn conditions, and once steady state is reached, they will be hotter than the stove.

That said, I measure the stove top temp, as I have an insert and haven't rigged up a flue gas thermometer yet.

 

[PunKid8888]

It also depends on where you consider your flue starting point is and where your thermometer is. Does it start at the junction to the stove or where the flame stops inside the stove.

For instance on my Vigilant the stove top is basically an inch before from what I would consider the official start of the flue. but only during up burn mode. During horizontal burn the gasses are forced to zig zag up the back wall of the stove. the manual says something like 50inchs of flue length in the back of the stove. So now my flue temperature (surface mount) is at 18inchs above the stove, but in horizontal mode its more like 68 inches. obviosuly you can not expect the same flue temperature in both modes of bruning.

 

[firefighterjake]

I too agree with Ed, but

I've observed that in most stoves (steel), the only criteria often supplied from the manufacturer concerning temperatures is the glowing parts clause. That is the case with my Jotul Kennebec Insert, and I have seen it glow on top (woops). Enameled stoves and Soapstone stoves are obvious exceptions. If I could monitor flue temps, then I can't see how I would get into the problem of an overheated stove, as the flue will give a earlier indication of combustion rate, especially with a probe thermometer. It will respond earlier, and allow you to take action to set a burn using air control rate that won't permit an overheated stove.

I would assert that any overdraft/overfire that would result in a stove overtemp would be seen in the flue gas temp first. I'm thinking that is what a system designeer would measure in a process control situation, especially if clean efficient burning was the goal.

Firefighterjake - Is your description of flue temp a surface temp or flue gas temp - I thought flue gas temp should always be hotter than the stove, unless you just closed a keydamper and shut the air totally, which isn't an option is an EPA setup.

My temp monitoring is still quite active, as I am still finding wet wood and am finding that I can get the stove to cruise at different temps if I adjust the air at different temps, (sort of a max speed cruise @700 vs max range cuise @ 500, from my 4 engine airplane days) and therfore can modulate heat output into the room better.

Flue gas temp . . . I have a probe thermometer due to my double wall stove pipe.

In general my flue gas temp is usually around 450-600 degrees F while my stove top temp is 500-600 . . . of course these temps vary somewhat during the course of the burn . . . and I figure the stove top temp probably doesn't react as quickly to temp changes as the flue gas thermometer due to the mass.

 

[Peter B.]

Here, internal flue temps in a range of 350-500* (at a height of 12" from stovetop) generally correspond to internal stovetop temps of 700-1000* degrees... with a catalyst in use (just below the stovetop).

But even without the catalyst, stovetop internal is nearly always a margin higher than flue internal.

FWIW.

Peter B.

-----

 

[WES999]

Here is a pic of my temp probes about 1 hr after closing the air down. Left probe is stove top temp, right is flue gas temp.
As you can see stove top temp is about 30* higher than flue temp. So yes is quite possible for the stove top temp to be higher than the flue temp.

 

[oconnor]

Quite interesting guys, thanks for the details.

Now if only I could get a unit like WES999's that my wife would let me hang in the living room.

 

[firefighterjake]

Quite interesting guys, thanks for the details.

Now if only I could get a unit like WES999's that my wife would let me hang in the living room.

Just glue on some frilly lace around the units and your wife will think they will make a fine decoration for the living room.

 

[Jags]

Quite interesting guys, thanks for the details.

Now if only I could get a unit like WES999's that my wife would let me hang in the living room.

Brent, just another thought, if your side of the discussion held true, it would be impossible to get the re-burners (tubes or cat), up to the 1100F temp it takes to really make them work, unless the exhaust gasses were also at 1100F (using the equilibrium argument). I am pretty sure that is not the case.

 

[oconnor]

The truth is, at some point in the process the gasses have to be at 1100F, as the heat is coming from somewhere. For the tubes or Cat to get to any temp combustion needs to occur, and that combustion temp is reflected by the temp of the items exposed to the combustion process. You can't heat an object beyond the temp of the source heat - We don't get free heat. The gasses are a byproduct of that source heat. So they are certainly able to reach 1100F. In fact, liners are tested to 1000*F in the US and 1200*F in Canada for just that reason. (up to 2100F for 60 mins for chimney fires)

What I've learned from this thread is that a high degree of cooling occurs that will result in flue gas temps that are lower than stove temps. Somewhere in the stove box, gasses are going to reach 1100*F likely higher. My web search tells me that candles burn a flame of 1400*C (2552 *F), so I'm thinking that somewhere in the firebox is a temp beyond 1100F.

However the cooling is enough that once measured 18" up the flue, they aren't that hot anymore, and that they will likely be a bit less than stovetop temps.

 

[Jags]

What I've learned from this thread is that a high degree of cooling occurs that will result in flue gas temps that are lower than stove temps. Somewhere in the stove box, gasses are going to reach 1100*F likely higher.

However the cooling is enough that once measured 18" up the flue, they aren't that hot anymore....

Yes, absolutely, but that cooling did not happen in 18" of flue. The majority happened at the stove body. Ain't nothin but a heat exchanger and combustion chamber.

 

[oconnor]

I just realised something when I was looking at some great charts BeGreen posted in a different thread - when the air is open and reburn hasn't kicked in, there is likely still some combustion occuring as the gasses leave the stove, so flue temps would be higher. But once air is down and reburn kicks in, combustion is completed in the firebox (that is the goal of an EPA Stove), and cooling happens in the gasses after that.

Thanks for the feedback folks.