Intrepid II cat too hot, had trouble getting the temp down

[hansenjw]

Background: I'm a new wood stove user and I bought a used Intrepid II 1990. The previous owner was apparently unaware that it didn't even have a cat installed in it, so I installed one. The refractory chamber is in fairly rough shape, but good enough to physically hold the cat in the right place, so I'm going with it for the first year. There aren't any obviously warped parts, and I replaced any missing gaskets. My wood is a mixture of whatever trees we cut down and had previously stacked under the eves of my house. I tested MC by bringing to room temperature, splitting, and testing, and it came in at 18%. This weekend I bought a Condar Watchman catalyst thermometer and installed it. It was very useful, but now I'm concerned that I was burning wrong and getting the cat temperature too high.

Problem: The very first time I fired up the stove with the new cat thermometer, I closed the damper when the cat hit 500 and left the primary air open. I was surprised to find out how quickly the cat temp rose after that. I came back a couple minutes later because the Watchman was beeping and had hit 1400f. I turned the primary air down to about 25% and the temp kept rising. After it passed 1500f I turned the air all the way down. Then the temp flirted with 1600f before slowly coming down to about 1200. I didn't want the cat temp to go too high again so I didn't re-open the primary air. Then a few hours later when I added wood again the same thing happened. Because of this, I didn't have a very hot fire and didn't get the cabin above 62f that night. The next morning when I re-loaded it worked much better. I was able to open the thermostat about 33% and the cat stayed down around 900, which I assume is correct.

I learned in the forums today that maybe I was operating it backwards. Maybe to cool the cat, I should have opened the primary air instead of closing it. I guess that by closing the primary air, I was creating a smokey fire, which was providing lots of extra fuel (smoke) to the cat, causing it to overheat. Is that correct? Alternatively, could this have been caused by poorly seasoned wood or a leaky stove?

PS, I also learned that I could open the bypass to cool the cat. That did work, and it seemed that doing that a couple times managed to reduced the cat temperature so that it didn't overheat again. Why would that be?

Thanks all! I've been reading a ton on these forums and it's all very helpful.

 

[begreen]

New catalysts are often lively for the first few fires, then settle down. Opening the bypass allowed the smoke and hot flue gases to go straight up the flue, bypassing the cat.

 

[hansenjw]

Opening the bypass allowed the smoke and hot flue gases to go straight up the flue, bypassing the cat.

Thanks begreen. I understand why it lowered the cat temp initially, but I don’t understand it had a sustained effect. Why didn’t the temperature skyrocket again after I re-closed the damper?

 

[begreen]

Just a guess, you may have released the majority of the wood gas bloom up the chimney. The Intrepid has a small firebox so not that much fuel in there to outgas. The cat should settle down and hold a lower temp as it breaks in.

 

[Highbeam]

Background: I'm a new wood stove user and I bought a used Intrepid II 1990. The previous owner was apparently unaware that it didn't even have a cat installed in it, so I installed one. The refractory chamber is in fairly rough shape, but good enough to physically hold the cat in the right place, so I'm going with it for the first year. There aren't any obviously warped parts, and I replaced any missing gaskets. My wood is a mixture of whatever trees we cut down and had previously stacked under the eves of my house. I tested MC by bringing to room temperature, splitting, and testing, and it came in at 18%. This weekend I bought a Condar Watchman catalyst thermometer and installed it. It was very useful, but now I'm concerned that I was burning wrong and getting the cat temperature too high.

Problem: The very first time I fired up the stove with the new cat thermometer, I closed the damper when the cat hit 500 and left the primary air open. I was surprised to find out how quickly the cat temp rose after that. I came back a couple minutes later because the Watchman was beeping and had hit 1400f. I turned the primary air down to about 25% and the temp kept rising. After it passed 1500f I turned the air all the way down. Then the temp flirted with 1600f before slowly coming down to about 1200. I didn't want the cat temp to go too high again so I didn't re-open the primary air. Then a few hours later when I added wood again the same thing happened. Because of this, I didn't have a very hot fire and didn't get the cabin above 62f that night. The next morning when I re-loaded it worked much better. I was able to open the thermostat about 33% and the cat stayed down around 900, which I assume is correct.

I learned in the forums today that maybe I was operating it backwards. Maybe to cool the cat, I should have opened the primary air instead of closing it. I guess that by closing the primary air, I was creating a smokey fire, which was providing lots of extra fuel (smoke) to the cat, causing it to overheat. Is that correct? Alternatively, could this have been caused by poorly seasoned wood or a leaky stove?

PS, I also learned that I could open the bypass to cool the cat. That did work, and it seemed that doing that a couple times managed to reduced the cat temperature so that it didn't overheat again. Why would that be?

Thanks all! I've been reading a ton on these forums and it's all very helpful.

What you describe is totally normal for a new cat and good fuel in a proper stove. My cat sits there all day at 1400 at the lowest setting I can use without cat stall. Normal cat temperatures are 12-1500 and peaks up to 1600 are okay, especially at the beginning.

Adjusting the air on a cat stove does some odd things and there is also a delay. I don't ever open the bypass to cool the cat, just keep the intake air lower to slow the production of smoke down but it takes a few minutes for the load of fuel to slow down its smolder.

Attached photo of my cat meter right now when I'm cruising along at low. It'll sit right there for 15-20 hours.

You really have very little control over cat temperature. All you can control is fire temperature and to some extent short term smoke production. A small visible flame usually keeps cat temperatures a little cooler since you are flaring off some of the voltatile fuels with primary combustion before it gets to the catalyst.

When someone tries to run a cat cooler it usually means flirting with a cat stall. As in, you're right at the absolute lowest level of smoke production to even keep the whole system working. Not a place I want to be.

 

[gthomas785]

I have an Intrepid II that I just got this year (new, old stock) and it behaves very similarly to what you described. The cat will go to very high temp and hang out there regardless of what you do with the primary air, until about halfway through the load then it starts to cool down.

I have found that this stove is much easier to over-fire than my Encore in the kitchen. That's partly because I have it on a taller chimney, but I think there's also something about the stove design that makes it really heat up fast after reloading. I added key damper which did help a little, and I'm finding that I keep it shut most of the time.

Did you have flames in the firebox after closing the primary air all the way? That will tell you if you have a leaky stove.

 

[Rickb]

Mine sits around 600-900 but I have had it hot many times not shutting it down cause I was cold or forgot.

 

[hansenjw]

Did you have flames in the firebox after closing the primary air all the way? That will tell you if you have a leaky stove.

If I have it at a high burn and then shut down the primary air, some small flames will remain. Later on when the fire is settled down more, then I can get it down to no flame. Does that sound leaky to you?

I kind of assume it’s leaky because I bought it used and then moved it. I was a bit surprised that it worked as well as it does.

 

[hansenjw]

What you describe is totally normal for a new cat and good fuel in a proper stove. My cat sits there all day at 1400 at the lowest setting I can use without cat stall. Normal cat temperatures are 12-1500 and peaks up to 1600 are okay, especially at the beginning.

Adjusting the air on a cat stove does some odd things and there is also a delay. I don't ever open the bypass to cool the cat, just keep the intake air lower to slow the production of smoke down but it takes a few minutes for the load of fuel to slow down its smolder.

That’s good to hear! I didn’t know that new cats ran hot. I‘m starting to think that I may have made the problem worse by letting the temperature shoot up too high and too fast before closing the primary air. Next time I’ll try to begin turning the primary air down a little at a time as soon as the cat hits 600 so that I don’t overshoot. Does that sound right?

 

[RandyBoBandy]

“Problem: The very first time I fired up the stove with the new cat thermometer, I closed the damper when the cat hit 500“

I think you are waiting to long to close the damper. I don’t pay any mind to the cat temps prior to closing the damper. I watch my pipe temps first. When the stack hits 600 internal temp I close my damper. Then when the cat hits 700-800 I turn the air down about 1/4. If it’s a hot reload I shut the air 1/4 as soon as I close the damper. Then again when the cat hits around 1300-1400 I shut the air some more. Then for final air closing I wait until the stove top is above 450*. This seems to help keep the cat from screaming up to 1500 and beyond. One thing I’ve noticed is if you let it it get to hot before closing the damper you will be fighting a screaming cat. Also if you shut the air down to fast after you’ve let the cat take off you typically end up snuffing the flames in the firebox which dumps all the work load on the cat and guess what? She wants to keep climbing up to 1600 and beyond. The best remedy is to open the damper and let the cat cool off a bit followed by starting the process all over.
1) start fire with primary air full open
2) pipe hits 600* internal temp close damper
3) close primary 1/4 when cat hits 700-800
4) close primary 1/4 when cat hits 1300-1400
5) close primary fully when stovetop is at desired temp or cat is going past 1500-1525.
Every setup is different just like every fire is different so you will have to adjust your procedure accordingly.
Good luck.

 

[RandyBoBandy]

I should add that I have a short stack and I’ve blocked off my secondary air. So if you have extreme draft and or haven’t restricted your secondary air you might want to adjust your air adjustment intervals a little sooner.

 

[Diabel]

“Problem: The very first time I fired up the stove with the new cat thermometer, I closed the damper when the cat hit 500“

I think you are waiting to long to close the damper. I don’t pay any mind to the cat temps prior to closing the damper. I watch my pipe temps first. When the stack hits 600 internal temp I close my damper. Then when the cat hits 700-800 I turn the air down about 1/4. If it’s a hot reload I shut the air 1/4 as soon as I close the damper. Then again when the cat hits around 1300-1400 I shut the air some more. Then for final air closing I wait until the stove top is above 450*. This seems to help keep the cat from screaming up to 1500 and beyond. One thing I’ve noticed is if you let it it get to hot before closing the damper you will be fighting a screaming cat. Also if you shut the air down to fast after you’ve let the cat take off you typically end up snuffing the flames in the firebox which dumps all the work load on the cat and guess what? She wants to keep climbing up to 1600 and beyond. The best remedy is to open the damper and let the cat cool off a bit followed by starting the process all over.
1) start fire with primary air full open
2) pipe hits 600* internal temp close damper
3) close primary 1/4 when cat hits 700-800
4) close primary 1/4 when cat hits 1300-1400
5) close primary fully when stovetop is at desired temp or cat is going past 1500-1525.
Every setup is different just like every fire is different so you will have to adjust your procedure accordingly.
Good luck.

I follow pretty much the same process with very good success. Except, I close the damper when the pipe reaches 400. The pipe is about 23’. And here too the secondary air is sealed with foil tape. Every load is different and so is every setup. On occasion the cat will spike to 1600 but it will only stay there for few minutes before settling at around 1300

 

[gthomas785]

If I have it at a high burn and then shut down the primary air, some small flames will remain. Later on when the fire is settled down more, then I can get it down to no flame. Does that sound leaky to you?

I kind of assume it’s leaky because I bought it used and then moved it. I was a bit surprised that it worked as well as it does.

Based on my experience that sounds normal. If you had a serious air leak you'd see dancing flames in the box all the time, high stovetop temps, and a reduced burn duration.

Still it doesn't hurt to do a dollar bill test on the door and make sure your windows glass is seated properly -- mine was initially loose from 9 years of sitting around.

Edit: oh and check the gasket around the griddle, if you use it for top loading it's easy to have little bits of wood sit in there and compress the gasket ever so slightly creating tiny air leaks.

 

[hansenjw]

Great advice guys! I‘ll definitely shut the damper sooner next time, and easy down on the primary air in steps instead of all at once.

Regarding the secondary air, you are both blocking it entirely? You’re talking about the guillotine air shutter in the back bottom of the stove correct? I looked at that (as well as I could, given the location) when I was having my out of control cat temps, and it was closed, but it obviously doesn’t seal the air very well. The shutter itself appeared to be a little less than 1/16” away from the back of the stove, which would let air around the edges. Bad design? If you’re blocking it entirely, what are the downsides? Does blocking it just make it a little more difficult to get the cat going at first?

Thanks!

 

[RandyBoBandy]

I unhooked the the shutter from the rod and removed the tiny washer from the pivot bolt so it doesn’t move freely and just tightened the bolt snug enough to hold the shutter closed. Mine also had a a small pin that would prevent the shutter from closing all the way. I removed that as well.
I can’t speak for the intrepid but if you didn’t have the secondary probe timed correctly the coil would expand to much and start to open the shutter again and send the cat nuclear. I got tired of messing with it so thats why I said screw it and just disconnected it.
I don’t have any troubles getting cat going with the procedure I spoke about. Adds more control of the cat if you ask me.

 

[gthomas785]

Well last night I had the worst "thermo nuclear catalyst" incident so far with my intrepid. I had been burning all day because we were home (snow day!) and it had been just cruising along for hours without incident. I decided to do one last reload around 8pm. Stuffed it full, let it catch for a few mins closed the damper, and after about 10 mins the cat was nice and hot so I cut the primary air to about 1/2 or so and it looked like it was settling down as usual.

Then a little while later I was doing some dishes in the other room and I start to hear a rumble like I've heard from this stove before, but this is the first time I heard it from another room over the sound of the sink running. I run over to the stove. Cat temp is around 1600, but the flue probe is at 1000 and climbing. I tried to shut the primary air but I just got instant backpuffing.

Thankfully I had just been reading this post from @RandyBoBandy so I knew something must be wrong with the secondary air. So I get down on the floor to look and sure enough, the flap is wide open. Grabbed some tin foil, folded it a few times and stuffed it over the secondary air slot. Instantly it felt like I had the stove under control again. The flue temp dropped and the rumbling stopped. And after taking a few minutes to let my heartbeat slow down, I was able to return to my dishes.

I'm thinking the secondary air flap just got completely stuck open. I'll have to take the back off after work and check the timing of the cat probe and make sure that flap isn't catching on anything. I'm also wondering if it was, as described, going past "zero" and reopening because I finally got the stove a little hotter than it had been earlier. But based on that experience I may rest easier if I just block off (partially) the secondary air so it can't happen again.

 

[hansenjw]

I haven’t heard an explanation for the purpose of the thermostatically controlled air shutter on this stove yet. The fact that you can block it off without affecting the operation of the stove makes me curious. here’s a theory.

The fact that these stoves can go thermonuclear due to too much secondary air indicates that during these times, there is excess fuel (smoke), and it’s when the cat gets sufficient oxygen to burn off all that fuel that it gets too hot. So normally, a cat stove must have a set limit for how much air can get to the cat to prevent this, knowing that some of the smoke won’t be burned, wasting energy and emitting more pollution, but protecting the cat. In the Intrepid’s case, that shutter can sometimes get stuck open and allow too much air in there, allowing the cat to completely burn the excess smoke.

So then the reason for the flap in the first place may be that when the stove is cooler, it doesn’t have as much draft, and the fire is smokier at the beginning anyways. So there’s lots of extra smoke being produced, but not enough air is being pulled through the primary to let the cat burn all the extra smoke. By opening the secondary air during cool conditions, it would create a lot less smoke on startup, and also heat up the stove more quickly. So disabling the flap probably makes it smokier on startup and slower to heat up. After the stove is hot, it wouldn’t make any difference, since the flap is closed at that point.

This is obviously a guess, and I’ll probably try covering the flap anyways because I don’t want a runaway cat. What do you all think of the theory?

 

[gthomas785]

Based on my experience last night, I think there is another issue at play which is that somehow the secondary air can end up getting into the firebox. I'm not sure exactly how or when this happens but I know that when my secondary shutter was stuck open, it was not just the cat running lean and achieving complete combustion. It also made the fire burn hotter creating more outgassing and a runaway situation. I didn't really believe it at first but that is definitely what I observed.

I know the secondary air joins the exhaust from the firebox before the catalyst. Could it be backing up through the throat plate into the firebox somehow? That would mean the bypass damper is leaky, which I don't think mine is. Or maybe it "puffs" air into the firebox which is what makes that rumbling noise. Either way, closing off the secondary air did not decrease my catalyst temp all that much, maybe 100 degrees, but brought the fire and flue temps under control. If I do end up blocking the secondary air I will for sure be watching the chimney sometime when it is light to see if I'm getting any smoke. I guess it's a tricky balance to get the right ATF ratio into the catalyst without adding excess air to the fire.

Edit (accidentally hit post) I also have a newer VC stove which has a fixed secondary air intake. And yes it is smoky on startup when the cat is heating up, but then no smoke. I assume this is because the secondary air intake size is calibrated to a certain level of draft that isn't there when cold. I find this setup much preferable to a variable intake that can potentially run away. But I'm not sure why it would run away since it is achieving complete combustion of the smoke even with less secondary air. The only explanation, as far as I can tell, is that the extra air is going somewhere it shouldn't.

 

[RandyBoBandy]

When I was still messing around with the flapper I put a magnet on the bottom of the the stove. This prevented the flapper from going past “zero” even when the rod kept going past zero.

 

[hansenjw]

I know the secondary air joins the exhaust from the firebox before the catalyst. Could it be backing up through the throat plate into the firebox somehow?

I don’t think it could back up that way. From looking at it, it appears that the secondary air comes in a channel below the refractory chamber, and then up in front of it to join with the air coming from the firebox that is being sucked into the cat. I don’t think the secondary air could back up through the throat plate unless you had negative pressure in the firebox. Maybe the reason cutting off the secondary air helped control the fire is that it reduced the excess draft caused by the out of control cat? Just a guess!

 

[hansenjw]

When I was still messing around with the flapper I put a magnet on the bottom of the the stove. This prevented the flapper from going past “zero” even when the rod kept going past zero.

I got to play with my stove today and I took a close look at the secondary air flapper several times throughout the fire. Fortunately in my case, it appears to work correctly so far. When the cat got hot, the flapper was blocked from going too far, even though the rod dropped another quarter inch lower. Here’s the funny thing though. Later on when the cat was at its hottest (about 1500 today), I looked again and the shutter was still closed, but the rod wasn’t as low. I haven’t looked closely, but I wonder if the thermostat that the rod is attached to can rotate too far when it gets hot, and begin to lift the flap again, causing the runaway cat. It’s not the same problem as what you described, but it could be the same outcome. I don’t really want to create a runaway cat situation to test my theory, so I’ll just take the back panel off and examine the thermostat next time.

Speaking of which, how do you time the secondary? I’ve never run across a reference to that before this post.

Thanks!

 

[hansenjw]

By the way, by roughly following the instructions on this thread, I was able to have a nice fire without a runaway cat today! The one thing I did differently was because I don’t have a flue thermometer. So I waiting for the cat to hit 500, closed the damper, and then as soon as the cat hit 600 I closed primary 1/2, then at 700 another 1/4. That brought the cat temperature up nice and gradually to about 900, at which point I opened it back up to 1/2, and the cat settled in at around 1300. I’ll probably close a little more slowly next time, but I was being overly cautious this time. I’m really happy that I’m getting a handle on this stove now! Thanks everyone for their help!

 

[RandyBoBandy]

I got to play with my stove today and I took a close look at the secondary air flapper several times throughout the fire. Fortunately in my case, it appears to work correctly so far. When the cat got hot, the flapper was blocked from going too far, even though the rod dropped another quarter inch lower. Here’s the funny thing though. Later on when the cat was at its hottest (about 1500 today), I looked again and the shutter was still closed, but the rod wasn’t as low. I haven’t looked closely, but I wonder if the thermostat that the rod is attached to can rotate too far when it gets hot, and begin to lift the flap again, causing the runaway cat. It’s not the same problem as what you described, but it could be the same outcome. I don’t really want to create a runaway cat situation to test my theory, so I’ll just take the back panel off and examine the thermostat next time.

Speaking of which, how do you time the secondary? I’ve never run across a reference to that before this post.

Thanks!

On a cold stove the secondary probe should be somewhere around the 2 o'clock resting position. This is where the rod is connected to coil. There is a nut and set screw holding it all together. Loosen the nut and change the resting position to your desired setting. Make small increments in your testing and good luck. I have spent hours tinkering with it trying to get it perfect. Never could that’s why I decided to just disconnect the rod and leave it shut. I have started thinking about somehow making the shutter a manual control with some sort of lever attached to it. This way I can choose to open or shut it without having to reach behind a hot stove to make any adjustments. However, every time I start tinkering around I always end up just leaving it fully shut.

 

[gthomas785]

I don’t think it could back up that way. From looking at it, it appears that the secondary air comes in a channel below the refractory chamber, and then up in front of it to join with the air coming from the firebox that is being sucked into the cat. I don’t think the secondary air could back up through the throat plate unless you had negative pressure in the firebox. Maybe the reason cutting off the secondary air helped control the fire is that it reduced the excess draft caused by the out of control cat? Just a guess!

I'm still struggling to understand exactly what is going on. Seems to me, if you feed extra air into the cat when it's already running lean it should (a) cool off the cat, and (b) reduce the amount of draft sucking on the primary intake, which would calm the fire down. Maybe my stove has a leak that lets the secondary air into the firebox, or maybe your original theory was right that the cat actually is getting excess fuel and runs too hot unless you damp it down. In that case I'd expect to see smoke from the chimney when the secondary flap is shut, but I never have.

Whatever the reason is, it's clear that this flap needs to close when the cat gets hot and many peoples stoves are not doing that reliably. So closing it off permanently is the most "fail safe" approach.

 

[Rickb]

Not sure. I know typical combustion requires fuel and air. But a cat is partly a chemical reaction to the gasses the wood produces when it smolders. When you cut air to the burning fire you create more smoke/gasses. Im sure air is required for the cat process but I would guess less is required then for normal combustion?