Glowing Combustor-BK's & Other Cat Stoves

[BKVP]

There are a great deal of personal observations with regard to combustor activity. There are two facts we know about combustors and their efficacy. First, they need not be glowing to be working and second, a glowing combustor is "glowing" because of a rich fuel environment. Many consumers report glowing more often than others, which is completely understandable given the varying nature of the way stoves are operated and the fuel that is burned.

Moist or wet wood will have water boiled out of the wood (steam) and cool both metal surfaces of the stove (think heat exchanger) and the combustor itself.

When the air fuel ratio is just right (meaning very rich) combustors will glow slightly, partially. or completely. Some times they glow so brightly you can see them doing so across the room.
We know from 33 years of combustor research, residence time also plays a role in combustor glowing. This again does not suggest working or not working, but glowing. Typically, when the thermostat on our stoves are reduced down in burn rate (lower) the gases flowing through the combustor are slowed down, increasing residence time...how long it takes for the gases to pass through the combustor.

I have attached some images to help relate this to real world use. The stove is my own; the images were taken yesterday on Christmas Day. I burn NIEL's and had a very hot fire going. Image one shows no glowing whatsoever, yet the combustor was 915 degrees (thermocouple reading taken on exit temps)
The second image is after 10-15 minutes of having the thermostat moved from a high burn rate to a medium burn rate. After 15 minutes there is slight glowing taking place. Combustor temps increase to 1090.
The third image is only 5 minutes later and the combustor is glowing quite brightly. The combustor is now 1175, yet the burn rate is now at medium low.

The fourth and final image is 10 minutes later after adding two half rounds of western larch, 19% m.c. Two things are true, opening the door cooled the combustor and second, moisture in fuel is being released. As you can see, there is little glowing taking place.

There is also a slight amount of opacity eminating from the chimney, which disappears in 14 minutes. The combustor hovers around 875-945 degrees during this time.

So can you stall a combustor, you bet! It can appear you have sufficient draft when burning on higher burn rates, but if you reduce the burn rate below a unique level for the individual application, you can not have sufficient draft to keep the stove operating correctly. DRAFT IS NOT CONSTANT.

So while a catalytic equipped stove can operate just fine and have a strong amount of draft as measured in w.c. inches, you can operate a stove at too low a burn rate to keep enough hot gases in the flue and therefore keep the stove drafting and the end result is the combustor drops below 550 degrees and is no longer active.

We belong to a collective group of catalyst equipped stove companies and these observations are in line with observations made by these companies for their products.

Lastly, I hope you all had a wonderful Christmas and now go enjoy the New Year!

Chris

 

[Highbeam]

Looks good Chris, and on Christmas day too!

"slight amount of opacity eminating from the chimney" we call that smoke.

 

[tcassavaugh]

i'd love to get a bk. i just wish the king came in a 6 in flue opening. the princess is inviting, but i like the idea of burning for a COUPLE OF DAYS! would be so cool to just fill it and fo-get-a-bout-it until the next day.....or two.

 

[BKVP]

Begreen,

I stand corrected...smoke. Or as my hunting buddies call it, "jerky makin". What can I say?

 

[BKVP]

i'd love to get a bk. i just wish the king came in a 6 in flue opening. the princess is inviting, but i like the idea of burning for a COUPLE OF DAYS! would be so cool to just fill it and fo-get-a-bout-it until the next day.....or two.

The really long burn times are usually only in the shoulder seasons. Some areas where winter can be more mild, say Florida, long burns are forever! Conversely, in the interior of AK, fuel quality rarely allows for maximum burn times.

NIEL's sell for $600 a ton in Fairbanks! And sell they do because they are to the folks there what oak is to us soft wood burners. Douglas fir in a giant pellet, 6%m.c and easy to transport.

 

[Lundrunner]

Chris

Having just had a Princess installed a month ago (fabulous piece) what information can I glean from the non numbered (thermocouple ?) other than it reading "active"....would I be better off getting a numbered unit? and would it help the stove become more efficient?

thanks for the help
Dan

 

[BKVP]

Chris

Having just had a Princess installed a month ago (fabulous piece) what information can I glean from the non numbered (thermocouple ?) other than it reading "active"....would I be better off getting a numbered unit? and would it help the stove become more efficient?

thanks for the help
Dan

We did at one time provide numbered cat thermometers with numbers. The downside, we would get calls when the combustor dropped 100 degrees! The active line is about 550 degrees. The cat thermometers are not all that precise, so for anyone that wants to be a combustion engineer, thermocouples with digital read outs are way more precise.

My wife often says our house (stove) looks like a science experiment.

To answer your question, the most efficient way to operate your stove is to burn good quality, dry fuel and avoid "playing" with the thermostat knob. And of course, be safe.

Chris

 

[BKVP]

One additional observation, if you have fans on your Princess, take readings when the fans have been off for about 10 minutes. The air movement across the top of the stove can make the needle read a little off.

 

[Highbeam]

One additional observation, if you have fans on your Princess, take readings when the fans have been off for about 10 minutes. The air movement across the top of the stove can make the needle read a little off.

Do the fans actually lower the cat temperature? As measured by your thermocouple? I know the meter reading changes but I don't want to stall the cat and I often want to fire up the fans right after cat engagement.

Those fans really make a big difference.

 

[Highbeam]

Chris

Having just had a Princess installed a month ago (fabulous piece) what information can I glean from the non numbered (thermocouple ?) other than it reading "active"....would I be better off getting a numbered unit? and would it help the stove become more efficient?

thanks for the help
Dan

Also note that there are little white tick marks around the perimeter of the cat meter. After looking at condars labeled version of this same meter I think you can make some assumptions about what each tick mark represents.

I follow the theory that the only information of value from that meter is the transition point from inactive to active. Well, one more thing is after a reload I want the meter to read active and be rising before closing the bypass.

 

[naptime]

Thanks a bunch for this post, Chris. I think this is the single most confusing issue for a first-time BK owner. I'm on my fourth winter with a Princess and even still, I learned a lot from this. I had a full load of wood started yesterday with the cat thermometer past 12 o'clock and no glow! I thought maybe getting to be time for a new cat, but I looked a couple of hours later and it was glowing away. I think it would be a good idea to include this info in the owner's manual, or at least I don't remember seeing it before.

 

[_CY_]

so much mis-information on how cats work floating around ...

 

[Lundrunner]

]Thanks a bunch for this post, Chris. I think this is the single most confusing issue for a first-time BK owner. I'm on my fourth winter with a Princess and even still, I learned a lot from this. I had a full load of wood started yesterday with the cat thermometer past 12 o'clock and no glow! I thought maybe getting to be time for a new cat, but I looked a couple of hours later and it was glowing away. I think it would be a good idea to include this info in the owner's manual, or at least I don't remember seeing it before.[/quote]

Naptime

I agree....I had so many questions I finally called BK directly, safe stove top temps etc and they called me right back and answered all of my questions, but it would be great if BK would put it all on their site. Fantastic company...fantastic product.

Dan

 

[coach0519]

Chris,
Thank you for the valuable info.

 

[ddddddden]

All I see is evidence of Alien Technology.®

We belong to a collective. . .

AKA the Borg

 

[AppalachianStan]

Chris, Do you know if the smaller cats had a problem with stalling? I have an old 1978 Appalachian 52. I have problems with it back puffing when the cats are glowing and keeping the temps above 1000*. How do I fix this? It will run good at 800* (cats not glowing) for hours but when it gets to 1000* it will start back puffing.

 

[Todd]

Good info, thanks for posting this Chris. What's the max temps you have seen on your cat thermocouple probe? I've read avoid temps over 1600.

 

[BKVP]

Chris, Do you know if the smaller cats had a problem with stalling? I have an old 1978 Appalachian 52. I have problems with it back puffing when the cats are glowing and keeping the temps above 1000*. How do I fix this? It will run good at 800* (cats not glowing) for hours but when it gets to 1000* it will start back puffing.

I am not an expert on your stove, but early cat designs were problematic for multiple reasons. First flame impingement and the potassium manganite attack. I can make my stove back puff if I drive the temps way up and shut off the air. The total cell count (cells per square inch) and total surface area have to be optimized for gas flow and residence time.

You might try a stainless substrate if you currently have ceramic. The stainless have thinner wall thickness but greater surface area and tolerate higher operating temperatures.

They heat up in half the time and can. Deal with modest levels of moisture at a rate higher than ceramic combustors.

Hope you can find a solution.

 

[BKVP]

Good info, thanks for posting this Chris. What's the max temps you have seen on your cat thermocouple probe? I've read avoid temps over 1600.

At home with ceramic substrate the highest I have observed was with NIEL's and a high burn...the cat was 1400 degrees. Now at work, in the lab we have seen much higher numbers. Part of the success of cat survival is designing the stove around limits we discover. We routinely see stainless go to 1800 and survive. Much over 2000 and precious metals peel off with the wash coat.

If the consumer keeps a nice tight door seal, the can last longer than a decade in lower 48 and less in parts of AK and NWT where heating is 9 months a year.

 

[HotCoals]

Chris do you agree that a hot cat pulls on the draft more?

 

[AppalachianStan]

Hope you can find a solution.

Yeah a new stove. Thanks for your info on cat stove.

 

[BKVP]

Chris do you agree that a hot cat pulls on the draft more?

Yes it can. However, we can see significant reduction in w.c." when single wall pipe is used, especially in ultra low burn rates. That's why we recommend the double wall pipe. The benefit of the double wall is most obvious during shoulder season heating. It's not unusual to see flue temps of 220+ in low burns.

The brilliant Paul Tiegs who founded OMNI TEST once was reported that someday some one will design a stove with 85% thermal efficiency but it would not draft unassisted.

A stoves' performance is tied to venting, fuel, draft but most vital is an understanding of how all these work with one another....at various burn rates.

 

[AppalachianStan]

Chris, Can we get some photos of your set up?
So double wall stove pipe stays hotter then single? would this help with back puffing?
Tonight I got my stove up to 600* on the cat probe closed the bypass down. got temps up to 800* closed the prime air down. It cruised there for about 2 hour then the temp started to climb to 1000* and it started to back puff. With the temps I just mentioned stove pipe temps at 200*. I just don't get it. I have 18' of class A chimney and stove pipe.
Now for me I am in a mild climate, but never thought a wood stove would be rocket science.

 

[HotCoals]

Stan,
Is your back puffing from spontaneous combustion?
You know,like no flame in the box then all of a sudden a big poof of flame then smoke coming out around the gaskets and the flue collar?

 

[AppalachianStan]

Stan,
Is your back puffing from spontaneous combustion?
You know,like no flame in the box then all of a sudden a big poof of flame then smoke coming out around the gaskets and the flue collar?

Yep but just around the gaskets.