Strange CAT Problem

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Gridlock

Member
Feb 13, 2010
223
New Paltz, NY
I have strange CAT problem, where full loads of wood don't light-off the CAT very well, but small to mid loads do.

This has been my procedure for packing the stove for the night, or for the day when I leave to go to work:

Pack stove full over some hot coals left from last burn. I leave about 2-3 inches of space between the top of the packed wood, and the top (griddle) of the stove. I let the stove heat up for about 30 minutes at which point the stove-top thermometer reads about 600 degrees. I then engage the CAT.

The Condar digital CAT temp readout (probe is installed on exhaust side of CAT) shows a very slow rise in CAT temperature; it takes about 30 minutes just to reach 800 degrees, with lots of smoke coming out the chimney. If I repeat the same as above but with a much smaller load of wood, the CAT lights off much faster with temperatures rising to 1400 degrees or so within about 10 minutes, and with no smoke coming from the chimney at that point.

I did an experiment a couple of times which seems to work a little better: instead of packing the firebox full right away, load it half way of so, get it hot and engage the CAT for about the 10 minutes needed to get the CAT to the 1400 or so degree point, at which point add remaining wood to pack it full and re-engage the CAT. The CAT temp does drop a bit, but seems to remain high enough initially to burn most of the smoke, then will eventually rise more where it gets to a temp that burns all the smoke.

Anyone have had similar experience or can shed some light why the CAT doesn't light off well with a fully loaded firebox?

Some specifications:

Wood is dry (moisture readings are between 15 and 20%)
Flue is 8 inch insulated stainless running 23 feet; one short one foot horizontal run into a 90 degree T fitting
Draft is VERY strong
CAT was new at the beginning of this season and is the ceramic type
Stove is VC Defiant CAT model 1945
 
Wow I have the exact same issue. Never could figure it out.

On a full load I have to get the stovetop up much hotter than a partial load for the cat to light off. Just getting the entire load charred isn't always enough, I need to be 650+ to get the cat to catch reliable. However, on a 3-4 split load, 500F stovetop is usually enough.

My wood all measures 20% or less on a MM too.... cat on its second year.


Curious if you figure this out...
 

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Anyone have any ideas about this? C'mon all you seasoned wood-burners!
 
Does your operations manual have a trouble shooting section?
I tried to look for a op manual online, but couldn't find one.
When I was having some issues it was "operator error", I read the book,
again, & am slowly getting used to burning a cat stove. The learning curve
for me, is taking some time since I'm so used to the non-cats, but having better success now that i
basically am following the recommended procedures in the op manual.

Maybe your dealer has some suggestions.
 
Gridlock said:
I let the stove heat up for about 30 minutes at which point the stove-top thermometer reads about 600 degrees. I then engage the CAT.


Can't you engage the cat sooner than that?
 
BrowningBAR said:
Gridlock said:
I let the stove heat up for about 30 minutes at which point the stove-top thermometer reads about 600 degrees. I then engage the CAT.


Can't you engage the cat sooner than that?
That temperature seems to be the magic number for getting the CAT to light off quickly (with a partial load anyhow). At the beginning of the season when the CAT was brand spankin' new, the CAT seemed to light off more easily, but now seems to require a bit more heat.
 
Gridlock said:
BrowningBAR said:
Gridlock said:
I let the stove heat up for about 30 minutes at which point the stove-top thermometer reads about 600 degrees. I then engage the CAT.


Can't you engage the cat sooner than that?
That temperature seems to be the magic number for getting the CAT to light off quickly (with a partial load anyhow). At the beginning of the season when the CAT was brand spankin' new, the CAT seemed to light off more easily, but now seems to require a bit more heat.


I have some doubts that a few months would effect that cat's performance. Have you noticed any other differences in your burns? Is the fire taking longer to get going? Anything?
 
The Condar digital CAT temp readout (probe is installed on exhaust side of CAT) shows a very slow rise in CAT temperature; it takes about 30 minutes just to reach 800 degrees, with lots of smoke coming out the chimney. If I repeat the same as above but with a much smaller load of wood, the CAT lights off much faster with temperatures rising to 1400 degrees or so within about 10 minutes, and with no smoke coming from the chimney at that point.

I did an experiment a couple of times which seems to work a little better: instead of packing the firebox full right away, load it half way of so, get it hot and engage the CAT for about the 10 minutes needed to get the CAT to the 1400 or so degree point, at which point add remaining wood to pack it full and re-engage the CAT. The CAT temp does drop a bit, but seems to remain high enough initially to burn most of the smoke, then will eventually rise more where it gets to a temp that burns all the smoke.


I also run my Vermont Intrepid II the exact same way, except I never overload the stove.
My cat usually runs in the 1200 to 1400 hundred range all the time. I have had it reach
the 2000 mark a few times with extreemly dry wood.
 
Gridlock said:
I have strange CAT problem, where full loads of wood don't light-off the CAT very well, but small to mid loads do. >>>>>>>>>>

So the question is: "why does it take longer to heat up a large amount of wood than a smaller amount?"

Well, initially the wood will not heat up itself but will need to be heated up by some external heat source (hot coals, stove walls, fire starters, etc). The more wood, the longer this endothermic phase (i.e. when the wood consumes more energy than it produces) will take.

At some point the wood will start gasifying well enough for the whole process to become exothermic (i.e. the wood is producing more energy than it consumes) and then the larger load of wood will overtake the smaller load fairly soon (when viewed along the same time line).

Henk
 
BrowningBAR said:
I have some doubts that a few months would effect that cat's performance. Have you noticed any other differences in your burns? Is the fire taking longer to get going? Anything?
I had a lot of problems (back-puffing mainly) with the stove (due to draft) that was corrected when I switched from a 6 inch to an 8 inch flue. The draft is now VERY strong (maybe a bit too strong!). Ever since then (beginning of this season), the stove runs very well, except for this issue. The stove lights and gets going VERY easily with no issues, which has remained consistent for the whole season. The ONLY difference I've noticed, is a bit longer for the CAT to achieve light-off, but that seems to only be the case with a large load.

I do wonder if the problem I'm having is with the amount of wood, or that the wood partially sits against the CAT throat when the box is loaded fully. I should try another experiment loading the box fully except for anything against the CAT throat.
 
bmwbj said:
I also run my Vermont Intrepid II the exact same way, except I never overload the stove.
My cat usually runs in the 1200 to 1400 hundred range all the time. I have had it reach
the 2000 mark a few times with extreemly dry wood.
What do you mean by 'never overload'? How much room to you leave between the load and the top of the stove? Does any of the wood cover the CAT throat?
 
Gridlock, I only burn 2 to 3 small splits at a time, and I never load wood against the "Cat hood".
 
PyMS said:
Gridlock said:
I have strange CAT problem, where full loads of wood don't light-off the CAT very well, but small to mid loads do. >>>>>>>>>>

So the question is: "why does it take longer to heat up a large amount of wood than a smaller amount?"

Well, initially the wood will not heat up itself but will need to be heated up by some external heat source (hot coals, stove walls, fire starters, etc). The more wood, the longer this endothermic phase (i.e. when the wood consumes more energy than it produces) will take.

At some point the wood will start gasifying well enough for the whole process to become exothermic (i.e. the wood is producing more energy than it consumes) and then the larger load of wood will overtake the smaller load fairly soon (when viewed along the same time line).

Henk
Not sure I understand how this applies directly to the question...

In either case, whether I have a small or large load, I wait until the stove-top temperature is hot enough (about 600 °F). I imagine that there is more gasifying with a large load than a smaller one, which would lead me to believe that the larger load would achieve CAT light-off more easily, but the opposite is true in my case.
 
bmwbj said:
Gridlock, I only burn 2 to 3 small splits at a time, and I never load wood against the "Cat hood".
Ah Ok, but how can you achieve an overnight burn with only 2 to 3 small splits? I have a large open space to heat, and need long burns for overnights and during the day when I'm at work, so need to load the box with as much wood as I can get in there.
 
Gridlock said:
PyMS said:
Gridlock said:
I have strange CAT problem, where full loads of wood don't light-off the CAT very well, but small to mid loads do. >>>>>>>>>>

So the question is: "why does it take longer to heat up a large amount of wood than a smaller amount?"

Well, initially the wood will not heat up itself but will need to be heated up by some external heat source (hot coals, stove walls, fire starters, etc). The more wood, the longer this endothermic phase (i.e. when the wood consumes more energy than it produces) will take.

At some point the wood will start gasifying well enough for the whole process to become exothermic (i.e. the wood is producing more energy than it consumes) and then the larger load of wood will overtake the smaller load fairly soon (when viewed along the same time line).

Henk
Not sure I understand how this applies directly to the question...

In either case, whether I have a small or large load, I wait until the stove-top temperature is hot enough (about 600 °F). I imagine that there is more gasifying with a large load than a smaller one, which would lead me to believe that the larger load would achieve CAT light-off more easily, but the opposite is true in my case.


I humbly apologize for missing that detail and providing a half-baked response.

Let me try to make up with a more detailed response. Since I spent nearly thirty years of my life studying the pyrolysis and combustion behavior of biomass as well as fossil fuels as a function of temperature, heating rate, pressure, particle size and reaction environments (including catalytic) I have had plenty of time to make lots of stupid mistakes as well as learn from them ;)

If the gas temperatures at the intake side of the cat (is that indeed the same as what you call stove-top temperature??) are the same and so is the flow speed/volume of air plus gases and vapors through the cat then the conclusion has to be that there is a difference in gas/vapor composition between the two situations.

The mixture of vapors and gases supposed to light off the cat has of course a large component of water vapor. If you plot both the concentration of water vapor and that of the potentially oxidizable gases/vapors as a function of time, one would certainly expect both to have a different relationship with the amount of wood used since the evaporation/desorption process of water is a very different process than the gasification process producing the oxidizable components.

My guess would be that with the smaller load of wood the evaporation/desorption of water is completed faster so that by the time the catalyst intake side reaches 600 F there is a smaller proportion of water vapor and a larger proportion of oxidizable gases and vapors entering the catalyst than with the larger load (all other things being equal :) )

If water vapor is indeed the culprit, you should see less time difference between small and large loads with drier wood.

Alternatively, one could build a rather similar argument based on the differences in mass and energy transport conditions between the smaller and larger loads. If so, varying the size of the splits and the way they are packed together should affect the process more.

FWIW, my money is on the water vapor, though.

Success,

Henk
 
I wonder if during a reload you raked all the coals towards the back where the cat chamber is located and pile the wood on top if it would make a difference? Those VC's have that rear chamber cat design where I could see air flow getting restricted from solid wood up against it. If there was a good pile of coals the air might have a better chance getting up through there?
 
Todd said:
I wonder if during a reload you raked all the coals towards the back where the cat chamber is located and pile the wood on top if it would make a difference? Those VC's have that rear chamber cat design where I could see air flow getting restricted from solid wood up against it. If there was a good pile of coals the air might have a better chance getting up through there?

Todd's analysis makes a lot of sense (after all, you don't get to be a Fire God for nothing :) ) but also made me aware of the unusual internal geometry of your stove with the primary combustion occurring in updraft and the secondary catalytic flow first going though a "throat" in the stove's back wall and then entering the cat in downdraft mode.

This prompts two additional considerations:

(1) The stove top is so far away from the cat that the stove top temperature is not necessarily representative of the cat top temperature, especially during the heat-up phase; i.e. before the stove temperatures have reached equilibrium. With a big load starting to burn in updraft the stove top temperature might well rise faster and potentially run further ahead of the cat top temperature than with a small load.

(2) The large amounts of water/steam released by the large load at relatively low temperatures might more strongly adsorb on the cold cat surface (perhaps, initially even condense a bit there); temporarily making the cat less responsive until it is all desorbed again at much higher temperatures. Water is one of the stickiest common substances known in surface science and can be very hard to desorb from activated catalytic surfaces.

Finally, I could not help but notice that the manufacturer's directions for your beautiful stove suggest to start the cat (i.e. close the bypass flow) at 450 F. The fact that you typically need to go to a stove top temperature of 600 F either indicates that the cat temperature does indeed lag behind a lot or that the cat has started to lose activity. Perhaps it is possible to place a temperature probe directly above the cat in order to see what is really happening. On the other hand, you might as well try to reactivate the cat, following the manufacturer's instructions, and see if everything starts working better.

Henk
 
(1) The stove top is so far away from the cat that the stove top temperature is not necessarily representative of the cat top temperature, especially during the heat-up phase; i.e. before the stove temperatures have reached equilibrium. With a big load starting to burn in updraft the stove top temperature might well rise faster and potentially run further ahead of the cat top temperature than with a small load.


Great insight here, if you use griddle top temps for cat engagement it can be tricky, on my intrepid if i reload at 400 griddle temp and cat is still at 900 the new load of wood will cause griddle temp to drop, but coals will continue to keep cat warm... i can reengage cat almost immedialty with dry wood and griddle temp will recover. i have waited before and engaged later and cat chamber was roaring and getting hotter.. guess it has a lot to do with stove design. i know there is no substitute for a cat probe thermometer, mine is immediate downstream of cat
 
if my cat is low temp before i reload i'll rake forward closer to wehere the cat is wait 5 or 10 min and let it get hotter. i notice on bigger loads it takes longer. just cause theres more volume of wood in the stove and the wood is packed tight so it takes longer than when a small load gets really hot realy fast.
 
It would be worth checking to be sure that the 2ndary air probe is alive and well and doing its job. Whenever you replace a cat. , replace the probe too. They often wear at about the same rate. Its job is to provide air to the cat. initially, the close the 2ndary air off. If it's working poorly, this may exacerbate the problem.,
 
Gridlock, I have the Defiant #1910 with an 8" pipe as well, which is the same as your #1945 except without the rear flue exit option. I noticed you said you have a very strong draft with the 8" pipe as I do as well.

Due to this stove design, with a fresh start up (with dry wood) the draft is pulling the flames straight up the flue causing very high flue/pipe temps long before my stove top temps start to climb. With a full load this problem is even worse.

I agree with the thought that the problem is with a larger load, the wood partially blocks of the baffle/cat chamber from the heat source, causing the cat temps to take longer to climb to operating temp.
 
PyMS said:
I humbly apologize for missing that detail and providing a half-baked response.

Let me try to make up with a more detailed response. Since I spent nearly thirty years of my life studying the pyrolysis and combustion behavior of biomass as well as fossil fuels as a function of temperature, heating rate, pressure, particle size and reaction environments (including catalytic) I have had plenty of time to make lots of stupid mistakes as well as learn from them ;)

If the gas temperatures at the intake side of the cat (is that indeed the same as what you call stove-top temperature??) are the same and so is the flow speed/volume of air plus gases and vapors through the cat then the conclusion has to be that there is a difference in gas/vapor composition between the two situations.

The mixture of vapors and gases supposed to light off the cat has of course a large component of water vapor. If you plot both the concentration of water vapor and that of the potentially oxidizable gases/vapors as a function of time, one would certainly expect both to have a different relationship with the amount of wood used since the evaporation/desorption process of water is a very different process than the gasification process producing the oxidizable components.

My guess would be that with the smaller load of wood the evaporation/desorption of water is completed faster so that by the time the catalyst intake side reaches 600 F there is a smaller proportion of water vapor and a larger proportion of oxidizable gases and vapors entering the catalyst than with the larger load (all other things being equal :) )

If water vapor is indeed the culprit, you should see less time difference between small and large loads with drier wood.

Alternatively, one could build a rather similar argument based on the differences in mass and energy transport conditions between the smaller and larger loads. If so, varying the size of the splits and the way they are packed together should affect the process more.

FWIW, my money is on the water vapor, though.

Success,

Henk
Hi Henk,

Thanks much for that expanation; that is a very interesting and plausible theory!
 
Todd said:
I wonder if during a reload you raked all the coals towards the back where the cat chamber is located and pile the wood on top if it would make a difference? Those VC's have that rear chamber cat design where I could see air flow getting restricted from solid wood up against it. If there was a good pile of coals the air might have a better chance getting up through there?
Thanks for the suggestion. I tried not blocking the cat chamber float last night and this morning, and it did help, though not sure if the reason it worked better was not blocking the throat, or because there was less wood in the firebox due to leaving the space clear next to the throat! It also still took substantially longer for the CAT temp to rise substantially, as compared to a somewhat smaller load. The more I think about it, Henk's theory above really makes a lot of sense.
 
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