Why do cat stoves have lower flue temperatures?

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pgmr

Feeling the Heat
Jan 14, 2006
403
Central Indiana
Perhaps this has been discussed before, but I could not find anything via search.

The question is basically the same as the title: Why, exactly, do catalytic stoves have a lower exhaust temperature than non catalytic stoves? The air coming out of the cat is as hot (or even much hotter) than that produced in the firebox of a non-cat. Is it simply that there is less air movement through the cat so that by the time it gets to the flue it has given up much of it's heat to the stove body? I've not drilled a hole in the flue to allow a thermocouple to be inserted for testing at various air settings. My guess is that at higher airflows, the flue gas temps would rise proportionately.


In the video below, the presenter actually sticks his hand into the demonstration section of a flue pipe to show how little heat is coming out of the BK stove. When I did my paint curing burns outside, I used a 5' piece of HVAC duct as a flue and was able to verify that this is not a special effect and the low exhaust temps are not limited to the BK stoves.

You'll find the "hand in the flue" part around the 2:00 mark.

 
I find this to be an interesting question and am looking forward to hearing the answer.

Looking at Blaze Kings performance chart, in their brochure, the flue temp stays fairly consistent at about 200* while the cat temp fluctuates considerably (300* - 1350*), but probably averages around 700*.

I have noticed my BK cat temp runs between 800* - 1200* most of the time. But, I have often wondered what the flue temp is...I'm not monitoring that.
 
Both stoves are burning smoke, but the cat allows it to burn at a lower temp. It would seem to follow if secondary combustion is occurring at lower temps in a cat, the flue gases could be at lower temps.
 
If i was to put my hand in my flue well it would be bad.
 
branchburner said:
Both stoves are burning smoke, but the cat allows it to burn at a lower temp. It would seem to follow if secondary combustion is occurring at lower temps in a cat, the flue gases could be at lower temps.

The cat allows the smoke to begin burning at 4-500°F, but for most of the burn cycle it is usually at least 1000°F (and often much higher). Even during these high cat temperatures, the exhaust temp is quite low, as evidenced by the "hand in the flue" demonstration". The question is, why does the cat exhaust cool off so quickly compared to the non-cat if they are approximately the same temp leaving the secondary combustion area? My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. But, I've been wrong before and welcome other ideas.

I don't currently have a flue probe hole, but do have a couple digital thermocouples to do some testing if needed.
 
pgmr said:
branchburner said:
Both stoves are burning smoke, but the cat allows it to burn at a lower temp. It would seem to follow if secondary combustion is occurring at lower temps in a cat, the flue gases could be at lower temps.

The cat allows the smoke to begin burning at 4-500°F, but for most of the burn cycle it is usually at least 1000°F (and often much higher). Even during these high cat temperatures, the exhaust temp is quite low, as evidenced by the "hand in the flue" demonstration". The question is, why does the cat exhaust cool off so quickly compared to the non-cat if they are approximately the same temp leaving the secondary combustion area? My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. But, I've been wrong before and welcome other ideas.

I don't currently have a flue probe hole, but do have a couple digital thermocouples to do some testing if needed.

Not to be redundant, but I agree. The cat thermometer for the BK is located on the leaving side of the cat...between the cat and the pipe connection. Why does that read 1000* and the flue temp read 200* (per BK's brochure)?
 
Ok, I just drilled a small hole in the flue pipe about 3" above the top of the stove. Stove is in the middle of a burn cycle, primary air controls are all closed, so the temps have stabilized. Cat thermometer is reading 1050°F and the thermocouple probe is showing 400°F. Not quite as cool as I'd imagined, though it is over 600°F difference. The cat itself is probably 15-18" away from the probe hole, with no meandering channels for the air to traverse. It does have to make 2 90° turns (one after leaving the cat and one to leave the stove).
 
I can't wait to hear a clear answer on this one too. It has seemed odd to me as well. My bets are on the reduced airflow theory allowing more heat to transfer as the gases exit the system...
 
Just a thought...

Could the cat temp be "artificially" impacted because it is so close to the cat?
 
Three hrs. into the burn. Probe located 18" above the stove. And the two are stovetop
 

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pgmr said:
branchburner said:
Both stoves are burning smoke, but the cat allows it to burn at a lower temp. It would seem to follow if secondary combustion is occurring at lower temps in a cat, the flue gases could be at lower temps.

The cat allows the smoke to begin burning at 4-500°F, but for most of the burn cycle it is usually at least 1000°F (and often much higher). Even during these high cat temperatures, the exhaust temp is quite low, as evidenced by the "hand in the flue" demonstration". The question is, why does the cat exhaust cool off so quickly compared to the non-cat if they are approximately the same temp leaving the secondary combustion area? My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. ... .

+1 pgmr

Temp coming off the Cat may be just as high as the temp coming off a burn tube/baffle system. Think about it - both are burning the same partially combusted gas. To get complete combustion, you need to get to at least 1000 - 1200 F. But the volume of combustion gas can be lower, much lower, in a Cat stove if the air is turned down. So the gas is moving slower through the flue - not as much heat going up the flue.

An anecdotal account to demonstrate this - last night I reloaded with a couple large splits from a pile that turned out to not be as seasoned as I thought. This required me to keep the side door opened much longer till the fire was burning hot. Even though my stove top was about normal, at 500 F, my flue surface temp, just above the T, 18" from the stove, was as about the same, 500 F. Normally, my flue runs between 260 - 320, when my stove top is running 400 - 600.
 
pgmr said:
My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. But, I've been wrong before and welcome other ideas.

That's my guess too. I'm guessing that less air flow is required for a cat burn than for thermal secondaries, and that increases the exhaust gas's dwell time in the stove, and lets it give up more of its heat and cool more before going up the chimney.

That's just a guess. It amazes me that a stove can have a cat burning at 1300*, and 200* gas temp 4' up the flue. Very counter-intuitive.
 
RenovationGeorge said:
pgmr said:
My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. But, I've been wrong before and welcome other ideas.

That's my guess too. I'm guessing that less air flow is required for a cat burn than for thermal secondaries, and that increases the exhaust gas's dwell time in the stove, and lets it give up more of its heat and cool more before going up the chimney.

That's just a guess. It amazes me that a stove can have a cat burning at 1300*, and 200* gas temp 4' up the flue. Very counter-intuitive.
I concur with you on the whole post.
It's really strange for me after having the same stove but without a cat and thermostat.
Should have bought way sooner..but the only real reason we updated was because of the rebate.
Otherwise I would have had a few more small cracks welded and used it for another 10 years or so.
So far I'm burning just over half the wood.
 
When he does the demo in the video, he sticks his hand into a very large opening in the flue. That big opening is allowing the flue gases to draw in a lot of cold air, diluting the mix. I'd bet that's how its cool enough for him to stick his hand in there.
 
jharkin said:
When he does the demo in the video, he sticks his hand into a very large opening in the flue. That big opening is allowing the flue gases to draw in a lot of cold air, diluting the mix. I'd bet that's how its cool enough for him to stick his hand in there.

Good point. One can stick a hand into a 400 degree oven with the door open for a number of seconds w/o getting burned.
 
Point is..it's still around 200° F...with stove top temps way over that.
 
So, we have a Buck 80 with 400°F flue gas temp at 3" above the stove top, a VC defiant at 300°F flue gas temp 18" above the stove top and BK's brochure chart (http://www.blazeking.com/Brochures-En/WoodProductPDF/KingWoodStove.pdf) which shows a 200°F flue gas temp at 48" above the stove top. Looks like a pretty linear relationship to me or maybe it's logarithmic...as a SWAG.

The next question is this: What are non-cat stoves temperatures showing in low fire mode at these approximate distances above the stove top? Is a cat stove REALLY lower or is that just hype?
 
What's a SWAG?
 
I've noticed in same chimney setup that my previous non cats burned about 100-150 higher flue temps than current cat stove. I think it's a sign of higher efficiency, more heat staying in the stove.
 
Green Energy said:
pgmr said:
branchburner said:
Both stoves are burning smoke, but the cat allows it to burn at a lower temp. It would seem to follow if secondary combustion is occurring at lower temps in a cat, the flue gases could be at lower temps.

The cat allows the smoke to begin burning at 4-500°F, but for most of the burn cycle it is usually at least 1000°F (and often much higher). Even during these high cat temperatures, the exhaust temp is quite low, as evidenced by the "hand in the flue" demonstration". The question is, why does the cat exhaust cool off so quickly compared to the non-cat if they are approximately the same temp leaving the secondary combustion area? My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. ... .

+1 pgmr

Temp coming off the Cat may be just as high as the temp coming off a burn tube/baffle system. Think about it - both are burning the same partially combusted gas. To get complete combustion, you need to get to at least 1000 - 1200 F....

Not quite true - with a cat you CAN get complete combustion at temps much lower - about 1/2 that. Although that point may be moot to this discussion as folks have been measuring flue temps while the cats have been running at significantly higher temps than that. However, in case anyone who reads this misunderstands - that is the whole reason that cat stoves can run clean for such long periods of time, by burning cooler (still clean) less fuel is consumed/time period and thus the fuel lasts longer (with corresponding reduced heat output) while still being smoke and creosote free.
 
RenovationGeorge said:
pgmr said:
My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. But, I've been wrong before and welcome other ideas.

That's my guess too. I'm guessing that less air flow is required for a cat burn than for thermal secondaries, and that increases the exhaust gas's dwell time in the stove, and lets it give up more of its heat and cool more before going up the chimney.

This makes a lot of sense. I would imagine to meet EPA numbers it is common for a non-cat stove to be designed with a much greater volume of non-adjustable incoming secondary air. You can't dial it down like a cat stove. Since the EPA goal is lower emissions, not lower heat loss, some efficiency is certainly sacrificed by design in the (overly?) leaky non-cats.
 
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