Why do cat stoves have lower flue temperatures?

[Renovation]

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.

Congrats on your great experience and wood savings!

More evidence for the dwell-time (low-airflow) theory. IIRC Beetle-Kill reported a higher flue temp at higher burn rates--he runs his stove harder to get more heat out of it. I recall his flue temp being closer to cat temp in that situation. Very cool. (sorry).

 

[Renovation]

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).

Neat. If you're willing to drill another hole, how about trying one in the flue 4' above the stovetop? That's where BK got their 200* reading. Regardless, that confirms a big temperature difference.

 

[pgmr]

Neat. If you're willing to drill another hole, how about trying one in the flue 4' above the stovetop? That's where BK got their 200* reading. Regardless, that confirms a big temperature difference.

Sorry, the stove is sitting inside my fireplace and 4' up is beyond my lower blockoff plate.

 

[HotCoals]

What's a SWAG?

Scientific Wild A** Guess.

LMAO..thanks.

 

[HotCoals]

George,

What's really strange is after having the old BK for so many years I knew how it ran in all diff weather and temps...now the new one looks just like the old one.
So I find myself walking over to the stove to feed it when I used to feed the old one...and this one is just not hungry at those times..I just walk away and shake my head...lol.
Also I have been now filling it up pretty full..something I was very nervous of doing with the old one.
I can set it on one or a little under and the stove top will not go over 500..the other one would see way higher temps a few hours into the burn.
The cat will get 3/4 or higher most times..though I have not done it yet I could run the blowers more...maybe all night on low would really help to keep the cat temp reasonable.
When it gets into the teens I will try that.
I loaded at 10 last night and this morning it was 69 in the house at 9:30..would have been closer to 62 with the old stove running.
At 9:30 today I just turned the temp up to 2.5 because I have wood left yet..nice flames now and I doubt I put wood in for another 3 or 4 hours.
Even then I could crank it up and burn the coals down for another hour or two.
The old Bk would have need wood for sure at 9:30 and it might not have caught on it's own.
Mid 20's here.

But you know I'm interested in that new woodstock stove with tubes and cat and you could be one of the first!
I know I would not change out but still that new one might be the the cats a--! lol

 

[Green Energy]

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.

I agree that a Cat can produce secondary combustion with flue gas between 4-500 F. However, once the Cat is doing its thing, the flue gas exiting the Cat, after the secondary combustion is complete, will be between 800 - 1200 F.

 

[jharkin]

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.

A non cat stove accomplishes the reburn solely by keeping the firebox temperatures above the natural unaided ignition temperature of smoke (Ive seen 1200F quoted). To maintain high heat requires burning up the fuel faster. Burning fuel faster means consuming more air. By EPA regs non-cat stoves are supposed to be designed so that you are not able to burn them low enough to kill off the secondaries completely so they have to allow a lot of air.

The catalyst allows secondary combustion to occur at much lower ignition temps (500F). Lower teams mean less fuel burned so you need less air. So the designers can get away with a lower minimum air input.

The result of this is that for practical purposes cat stoves can remain burning - and burning cleanly - at lower temperatures than non-cat. This is what makes cat stoves so useful in the shoulder season and in mild areas. When burning at high heat this advantage goes away and non-cat stoves are as efficient or maybe even more if designed right.



As an illustration I found an old thread someplace that linked to a pdf presentation on catalytic combustion made by the folks at blaze king. If I can remember where it was I will post the link... One part of it was a study of two 40k BTU stoves, on cat, the other non cat. They tested them at 4 burn rates. The result:

At low and medium burn, the cat stove produces as little as 1/4 the particulates of the non-cat and was much more efficient.
At high the stoves were equal.
At max burn the cat stove actually made slightly more particulates.

Graphed out it shows that the higher published efficiencies of cat stoves are achieved at low/medium burn rates. At max burn most EPA stoves are fairly close in efficiency regardless of technology.

 

[SolarAndWood]

The result of this is that for practical purposes cat stoves can remain burning - and burning cleanly - at lower temperatures than non-cat. This is what makes cat stoves so useful in the shoulder season and in mild areas.

It also means you can put a much bigger stove in the middle of your living space, load it full every time and not overheat the house.

 

[Todd]

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.

A non cat stove accomplishes the reburn solely by keeping the firebox temperatures above the natural unaided ignition temperature of smoke (Ive seen 1200F quoted). To maintain high heat requires burning up the fuel faster. Burning fuel faster means consuming more air. By EPA regs non-cat stoves are supposed to be designed so that you are not able to burn them low enough to kill off the secondaries completely so they have to allow a lot of air.

The catalyst allows secondary combustion to occur at much lower ignition temps (500F). Lower teams mean less fuel burned so you need less air. So the designers can get away with a lower minimum air input.

The result of this is that for practical purposes cat stoves can remain burning - and burning cleanly - at lower temperatures than non-cat. This is what makes cat stoves so useful in the shoulder season and in mild areas. When burning at high heat this advantage goes away and non-cat stoves are as efficient or maybe even more if designed right.



As an illustration I found an old thread someplace that linked to a pdf presentation on catalytic combustion made by the folks at blaze king. If I can remember where it was I will post the link... One part of it was a study of two 40k BTU stoves, on cat, the other non cat. They tested them at 4 burn rates. The result:

At low and medium burn, the cat stove produces as little as 1/4 the particulates of the non-cat and was much more efficient.
At high the stoves were equal.
At max burn the cat stove actually made slightly more particulates.

Graphed out it shows that the higher published efficiencies of cat stoves are achieved at low/medium burn rates. At max burn most EPA stoves are fairly close in efficiency regardless of technology.

Was this the link? It could be a little bias since it was done by a cat stove manufacture but it sure makes sense to me.

http://www.chc-hpba.org/chc_news.htm

 

[jharkin]

Was this the link? It could be a little bias since it was done by a cat stove manufacture but it sure makes sense to me.

http://www.chc-hpba.org/chc_news.htm

Yea! It was the 3rd link on that page. Didn't see the first 2, I'll have to read those.

And yup I agree that the blaze king folks are not unbiased, but it makes a lot of practical sense to me as well that the catalyst mainly is useful for folks who do a lot of low slow burns, but if you burn hot all the time the noncat probably winds out over time with less maintenance.

 

[Slow1]

Was this the link? It could be a little bias since it was done by a cat stove manufacture but it sure makes sense to me.

http://www.chc-hpba.org/chc_news.htm

Yea! It was the 3rd link on that page. Didn't see the first 2, I'll have to read those.

And yup I agree that the blaze king folks are not unbiased, but it makes a lot of practical sense to me as well that the catalyst mainly is useful for folks who do a lot of low slow burns, but if you burn hot all the time the noncat probably winds out over time with less maintenance.

And thus the excitement over the burn tube + cat stove pending from Woodstock.

 

[Rockey]

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.

A non cat stove accomplishes the reburn solely by keeping the firebox temperatures above the natural unaided ignition temperature of smoke (Ive seen 1200F quoted). To maintain high heat requires burning up the fuel faster. Burning fuel faster means consuming more air. By EPA regs non-cat stoves are supposed to be designed so that you are not able to burn them low enough to kill off the secondaries completely so they have to allow a lot of air.

The catalyst allows secondary combustion to occur at much lower ignition temps (500F). Lower teams mean less fuel burned so you need less air. So the designers can get away with a lower minimum air input.

The result of this is that for practical purposes cat stoves can remain burning - and burning cleanly - at lower temperatures than non-cat. This is what makes cat stoves so useful in the shoulder season and in mild areas. When burning at high heat this advantage goes away and non-cat stoves are as efficient or maybe even more if designed right.



As an illustration I found an old thread someplace that linked to a pdf presentation on catalytic combustion made by the folks at blaze king. If I can remember where it was I will post the link... One part of it was a study of two 40k BTU stoves, on cat, the other non cat. They tested them at 4 burn rates. The result:

At low and medium burn, the cat stove produces as little as 1/4 the particulates of the non-cat and was much more efficient.
At high the stoves were equal.
At max burn the cat stove actually made slightly more particulates.

Graphed out it shows that the higher published efficiencies of cat stoves are achieved at low/medium burn rates. At max burn most EPA stoves are fairly close in efficiency regardless of technology.

Everything you wrote seems to agree with my real world experience in comparing both my Englander and catalytic insert. I always suspected that the cat was much more effiicient at lower burns. I hate/hated throwing wood into the Englander because I think about how much more useful heat I would get in the cat insert upstairs. I say useful heat because not a lot of the heat produced by the Englander makes its way upstairs.

 

[fire_man]

Here is what the Woodstock guys said in their blog for their new Hybrid (cat and tube) stove:

"The inside design is completely different from what we have done before as a result of one of our primary goals: MAXIMIZE efficiency and MINIMIZE emissions.
The new stove is truly a "hybrid" in the sense that it has a fully developed "secondary" burn plan and a catalytic combustor. Why both? Non-catalytic designs tend not to work very well (1) with wet wood, (2) for very long burns at LOW firing rates, (3) with inexperienced operators (i.e. those who don't get the secondaries lit after a cold start or after re-fueling), or (4) over a very broad range of operating conditions. They are not so good at low burns, very effective at moderate burns, and reasonably effective (to a point) at high burns. The hybrid has a greater range of output, greater efficiency, and is cleaner. Plus it cleans up carbon monoxide, methane and other combustion by-products - not things we are being graded on by the EPA (yet) but things that matter because they INCREASE efficiency and DECREASE pollution, which we do care about."

It really sounds like an interesting stove they are building. I wonder if the cat(s?) will last longer since there are burn tubes before them?

 

[Rockey]

Here is what the Woodstock guy's said in their blog for their new Hybrid (cat and tube) stove:

"The inside design is completely different from what we have done before as a result of one of our primary goals: MAXIMIZE efficiency and MINIMIZE emissions.
The new stove is truly a "hybrid" in the sense that it has a fully developed "secondary" burn plan and a catalytic combustor. Why both? Non-catalytic designs tend not to work very well (1) with wet wood, (2) for very long burns at LOW firing rates, (3) with inexperienced operators (i.e. those who don't get the secondaries lit after a cold start or after re-fueling), or (4) over a very broad range of operating conditions. They are not so good at low burns, very effective at moderate burns, and reasonably effective (to a point) at high burns. The hybrid has a greater range of output, greater efficiency, and is cleaner. Plus it cleans up carbon monoxide, methane and other combustion by-products - not things we are being graded on by the EPA (yet) but things that matter because they INCREASE efficiency and DECREASE pollution, which we do care about."

It really sounds like an interesting stove they are building. I wonder if the cat(s?) will last longer since there are burn tubes before them?

I know that it would be a misnomer but if this stove comes to fruition I'm calling it the "tertiary stove"