VC Defiant 2n1 Cat overtemp

[arnermd]

Hello all,

Looking for some suggestions on the following problem: End of last season I noticed the cat temp was running hotter than normal (I have a TC installed right at the cat exit) at one point it was tickling 2000 but the griddle was steady at 450, not a lot of flame in the primary chamber. I shut the primary air off and closed my pipe damper (I have a round damper in the exit stack) but the temp would not come down. Had to open the bypass damper to cool the cat down, but as soon as I closed it the temp would shoot right back up to 1800. This occurred several times but not every time.

Figured I must have an air leak so over the summer I replaced seals on doors, glass, ashpan and flue collar. This season the cat temps are driving up to 1800F, again griddle is at 350 - 450 with a little flame in the primary chamber. When I shut the primary air off the secondary does not want to cool down. Seems like the primary combustion is under control but the secondary is raging. Funny thing is it does not happen all the time, maybe 50% of the time. Some times it burns just fine, full load, no problem, cat temps 1000 - 1200, griddle at 400-450.

Other things I have tried.
- Tried running full open primary air, thinking the primary fire was too smokey and overwhelming the cat with smoke. No real change in behavior, Cat went to 1800 griddle was hotter at ~600.
- Removed the cat completely. Cat temps still went to 1400 with no cat installed, griddle at 450.
- Smaller loads seem to do better, less likely to over-temp.
- I have had some loads where it was purring along just fine, cat at 900 -1000 for a few hours then mid-load the secondary temp takes off again.
- Tried running with the pipe damper full or partially closed (thinking maybe too much draft) still got high secondary temps but it took longer to get there.

Today I tore into it and everything looked good to me except for a 3/16" gap between the fireback refractory and the stainless fireback plate (#35 and 42 on parts diagram). The fireback plate (35) was warped when I removed it. My thinking is that this could be the problem as it would allow extra secondary air into the stove.

All the gaskets looked fine, some turned white, most were jet black. Thermostat worked, tested it with a torch and it responded. I have not removed the front, back and sides off the base yet, debating whether or not I should, just to be thorough......

Some background:
- This is a Defiant 2n1 model 1975, I bought it new in 2012
- I just installed my third cat, getting 2-3 years out of a cat, seems typical I think.
- Wood is mostly red oak, some birch and ash. Aged under a tarp for 1.5 years.
- 8" flue pipe and chimney liner inside a masonry chimney, ~15' tall
- Primary air control does seem to be working, more air the air flames get bigger, less air the air flames get smaller.
- When the stove was working "correctly" I typically saw 1000 - 1200 cat temps, steady. Although the stove has never been very responsive, even when new. Seems to just do what it wants temperature wise, adjusting the air control has some, but not a lot, of control on the griddle temp.


Sorry for the long post and thanks for any ideas. I have enjoyed this forum a lot of the past few years a lot of good info here.

 

[defiant3]

Guessing that your chimney is really clean? Also, do you get all the heat you need or is the house still chilly in places? What kind of burn times/ how long is the max. between reloads? How much wood do you burn each season? Any damage to the refractory or other components that you know of?

 

[arnermd]

Guessing that your chimney is really clean? Also, do you get all the heat you need or is the house still chilly in places? What kind of burn times/ how long is the max. between reloads? How much wood do you burn each season? Any damage to the refractory or other components that you know of?

Just cleaned the chimney 3 weeks ago, I would say it was about the usual amount of buildup. Chimney is generally clean up until the top 2 feet or so where it does not stay as warm.

We do get all the heat we need, when operating properly I usually run the stove half throttle except on the coldest nights. Lately i have been burning smaller loads and throttling down the air because I am worried it is going to runaway on me.

Normal burn times are usually 8 hours, even after 12 hours I generally have enough coals to light off the next load. Lately I have been down to few coals after 8 hours due to the smaller load.

We do 4 cords per year

The refractory is spalled / peeling in the combustion gas path, other parts not in the gas path are fine, gray but smooth finish. I did notice the engine has a bow / wave in the upper left hand corner on the face that seals with the fireback, about 2 inches long. It created a big enough gap (1/16 - 1/8") where I am sure smoke was leaking to the flue collar area and bypassing the cat in that local area. Other than it seems sound, a few nicks and dings.....

The refractory in the corners of the primary combustion area look great.

 

[defiant3]

Might be interesting to see pic.s of affected areas. I'm guessing that there is a seal or something that is wearing, may be the things you mentioned.

These stoves have only been around for a limited time and so we're just finding out now what the likely wear items are. This is very interesting! Wonder what would happen if you partially obstruct the air feed hole in the bottom at the rear? Fiberglass insulation would limit the amount of air available...

 

[defiant3]

I wouldn't go too deep into the weeds yet tearing it down though.

 

[arnermd]

I wouldn't go too deep into the weeds yet tearing it down though.

Haha... too late, took the front and all the sides off this morning, figured I went this far...... No signs of an obvious problem.

Bunch of pics attached. You can see where the engine refractory warped, the upper left hand corner curled towards the back of the stove. Pictures with the straight edge are that same corner, top down and side views. This is obviously not right but I struggle to understand how this could cause my problem. A leak in the spot would cause smoke or secondary air to bypass the cat and shoot up the chimney right? How does that give me high cat temps? Maybe too much secondary air is leaking past the cat? Maybe the secondary combustion is designed to run with excess air and that helps keep things cool?

I did try plugging the secondary air and not much seemed to happen, I did not run it that way for long though 5-10 mins. Might support my earlier theory, if the cat was not getting enough excess air due to the leak, plugging the secondary inlet might not have much of an an impact....?

Does the white gasket signify anything? Notice in the one pic the gasket goes from black to white, obviously it means it got hot, but does it mean it got too hot? Or is that normal?

A lot of the gaskets are in very good shape so I am thinking of reusing them. Probably should replace the "white" ones.....

Many thanks for your thoughts.

 

[arnermd]

An update on my stove rebuild. I did a complete rebuild, all new gaskets and replaced all the refractory yet continue to have cat overheating issues but not as severe. Any comments or suggestions would be appreciated.

- Did a complete teardown and rebuild with all new gaskets, all new refractory & seals (kudos to Vermont casting for honoring the warranty). Did not find any obvious issues other than the warped refractory flange noted above in previous post. Only gasket that was not replaced was the ashpan door and it does not appear to be leaking. Checked it several times with a lighter and smoke stick.

- Initial light off after rebuild showed cat temps quickly climbing to 1450F even with primary air completely closed. I have a damper in the exhaust pipe and I closed that as well with minimal effect. Only way to cool the cat back down is to bypass the cat but as soon as I re-engage the cat temps climb right back to where they were or even hotter.

- Tried a load of fresh wood and the stove burned much better, cat temps less than 1200F griddle nice and stable at 400F. The “fresh” wood was felled in spring of 2017 and then cut and split this month, December 2017. My seasoned wood has been under tarp for 18 months.

- Left to conclude either the wood is too dry or there are air leaks. I have a hard time believing the wood is to dry but I don’t have a moisture meter. After an exhaustive search for leaks I can’t find any.

- Completely plugged the secondary air inlet with insulation and that seems to work pretty well. I can burn a full load of seasoned wood with 25-50% primary air and cat temps stay below 1300F. The temps are much more controllable as well, when I turn the primary air down the cat temps respond immediately.

In general it seems like the secondary burn is just too vigorous. Griddle temps generally run 400 -500 F but the secondary is cranking at well over 1400F and uncontrollable, unless I plug the secondary air.

 

[defiant3]

OK so I wouldn't think of 1400 degrees as overfiring necessarily. Also, griddle temp. of 450 is actually kinda low end. Glad they came through on the warranty though. On;y thing I might offer is that the gasket around the top casting is like 5/16 or 3/8" from the factory. In older Encores and Defiants, the seal around the top has been a place where the cement tended to wear out, even though the rest of the stove body was fine, so I've been replacing the top gasket with 1/2" hoping for a better seal and greater longevity.

This is new and I guess it'll be a few years before I find out if it's fruitful or not but it's a thing I'm trying. Might help??

 

[arnermd]

Right.... so this is confusing me. What is an appropriate limit for the cat temp?

I found the Condar website says: "Catalytic reaction maxes out at about 1300° Fahrenheit through any combustor. Exceeding this temperature is destructive to the combustor and to the components of your stove."
http://www.woodstovecombustors.com/How_They_Work.html

But in other documents they indicate: "catalysts best operating temperatures are between 600° and 1500°F (316°-816°C)" and "A very active combustor can easily reach temperatures over 1800º F."
http://www.woodstovecombustors.com/Operation_Tips.html
http://www.woodstovecombustors.com/combustorcleaningmanual.pdf

So I set my alarm at 1350. Is that too low? Should the cat be running that hot? I seem to recall the first few years I had the stove the cat generally ran < 1200F but I could be mistaken. Sadly I don't see a recommended max cat temp from VC in the manual......

Top Gasket: I used 3/8" per the factory manual with no cement, I used a spray adhesive to hold the gasket in place as I set the top on. The spray adhesive worked great in place of the self adhesive gasket VC calls for. Only place I used cement was on the damper door seal. Checked for leaks (again!) with a lighter and I don't see any drafts around the top.

Tried something new this morning: Full load of seasoned wood, opened up the secondary air inlet I had plugged earlier but I put screws in the lower fireback holes (all 8 of them) that feed air into the primary combustion fire box. Primary air wide open, about 3" of hot coals. Started off real nice, cat climbed to about 1200F in an hour then very quickly ramped up to 1500F in about 10 minutes. Closed the primary air completely but temp would not come down so I plugged the secondary air inlet again and temps slowly came down to 1200 again in 1/2 an hour. Griddle temp never exceeded 450F.

Again cat seems seem to be very active but griddle temps are relatively low.

 

[RandyBoBandy]

My cats typically cruise around 1250-1450. I don't worry a bit about it. I start to get concerned around 1500-1550.

 

[Rickb]

If your burn times are not shorter then normal I would not think you are leaking extra air into the combustion. If you were your burn times would be shorter. Is it possible your wood is dryer then you have used in previous years? I know my cat seems to run higher with good wood then bad wood.

 

[John Kuhn]

My question is how the average person would even know what the cat temp is, and whether it's a problem. And then what would happen if you just let it do its thing. I have a stove top thermometer and one for the stove pipe like most people, and I know what is normal for those but not anything else.

 

[Diabel]

My question is how the average person would even know what the cat temp is, and whether it's a problem. And then what would happen if you just let it do its thing. I have a stove top thermometer and one for the stove pipe like most people, and I know what is normal for those but not anything else.

A lot of us have installed the electronic cat probe (auber at100). At this point, I feel that I could not run this stove properly without that probe.

 

[RandyBoBandy]

A lot of us have installed the electronic cat probe (auber at100). At this point, I feel that I could not run this stove properly without that probe.

I'm so use to looking at the monitor that when we loose power I feel lost without my AT100 giving me real time temps on the cat.

 

[John Kuhn]

A lot of us have installed the electronic cat probe (auber at100). At this point, I feel that I could not run this stove properly without that probe.

Hmm, hadn't heard of it, guess I should check it out.

 

[trispad]

Has there been any resolution to this? I find I am having the exact same issues. The catalyst (> 1650) will take off as will my flue temps (> 1000 @ 18" above the flue colar). Over the summer I did nearly the same thing. Took off the top of my Defiant and replaced S/S firebrick plate and gasket (part number 35 and 41 in the diagram in the manual) as the plate was warped. I re-gasketed everything over the summer as well, doors, glass, griddle, top plate, damper, stove collar, ash door, etc... I did not do a full take down (i.e. I did not take off the sides and back and re-gasket).

My stove setup is pretty straight forward. Stove pipe (10') + chimney (6') is 16' straight up. I run a 8" oval connector to 6" pipe.

 

[RandyBoBandy]

Has there been any resolution to this? I find I am having the exact same issues. The catalyst (> 1650) will take off as will my flue temps (> 1000 @ 18" above the flue colar). Over the summer I did nearly the same thing. Took off the top of my Defiant and replaced S/S firebrick plate and gasket (part number 35 and 41 in the diagram in the manual) as the plate was warped. I re-gasketed everything over the summer as well, doors, glass, griddle, top plate, damper, stove collar, ash door, etc... I did not do a full take down (i.e. I did not take off the sides and back and re-gasket).

My stove setup is pretty straight forward. Stove pipe (10') + chimney (6') is 16' straight up. I run a 8" oval connector to 6" pipe.

Can you give us a quick step by step on how you run your stove?

 

[trispad]

Can you give us a quick step by step on how you run your stove?

Sure.

From a cold start, I will generally start with 5 or so splits of pine. I will get these going to warm up the stove. Once the stove top is between 400-450, I will fill up the stove. Depending on the time of year and how much heat is desired, it can be any combination of pine, maple, and oak. It has been cold for the area, so most oak and maple. After loading, I will allow the wood to burn in, somewhere between 5-10 minuets. I close the damper when the griddle is around 450. I leave the air wide open until the cat gets to ~950 and then start to turn the air down. I will continue to close the air while keeping the cat temp stable, generally trying to get the air closed down to 1/4-1/8 open before the cat gets to 1100. From there I let the stove do its thing and I will turn on the fan to low or medium. Thing will seem be be stable, the cat will climb to ~1500 and level off after an 30 minuets to an hour. Of note, I always seem to have very high flue temps, anywhere from 500-800.

From a hot start, I will just load up the stove and let the wood burn in, then same process from there.

After another maybe hour, sometimes even hour and a half, things can start to get out of control, the cat will will go above 1650 (alarm is set to this temp) and the flue will start to push 1000 (alarm is also set to this temp). Various amounts of letting in more air and opining the damper will eventually get things to calm down. The griddle top never gets very hot, never goes over 500, when the stove is going nuclear it is generally around 300 or less.

Things I have tried: replacing gaskets, plugging the secondary holes inside the stove, covering the secondary air intake at the back bottom of the stove, closing the air more, letting more air in, etc... I have had the best luck with completely covering the secondary air intake. This seems to reduce the frequency of the stove getting out of control, but not solve it.

I am getting to the point of giving up on the stove, but my better half is not ready for fork out more money on a stove and wants to get this one working better. Let me know if I can provide more info.

 

[John Kuhn]

Sure.

From a cold start, I will generally start with 5 or so splits of pine. I will get these going to warm up the stove. Once the stove top is between 400-450, I will fill up the stove. Depending on the time of year and how much heat is desired, it can be any combination of pine, maple, and oak. It has been cold for the area, so most oak and maple. After loading, I will allow the wood to burn in, somewhere between 5-10 minuets. I close the damper when the griddle is around 450. I leave the air wide open until the cat gets to ~950 and then start to turn the air down. I will continue to close the air while keeping the cat temp stable, generally trying to get the air closed down to 1/4-1/8 open before the cat gets to 1100. From there I let the stove do its thing and I will turn on the fan to low or medium. Thing will seem be be stable, the cat will climb to ~1500 and level off after an 30 minuets to an hour. Of note, I always seem to have very high flue temps, anywhere from 500-800.

From a hot start, I will just load up the stove and let the wood burn in, then same process from there.

After another maybe hour, sometimes even hour and a half, things can start to get out of control, the cat will will go above 1650 (alarm is set to this temp) and the flue will start to push 1000 (alarm is also set to this temp). Various amounts of letting in more air and opining the damper will eventually get things to calm down. The griddle top never gets very hot, never goes over 500, when the stove is going nuclear it is generally around 300 or less.

Things I have tried: replacing gaskets, plugging the secondary holes inside the stove, covering the secondary air intake at the back bottom of the stove, closing the air more, letting more air in, etc... I have had the best luck with completely covering the secondary air intake. This seems to reduce the frequency of the stove getting out of control, but not solve it.

I am getting to the point of giving up on the stove, but my better half is not ready for fork out more money on a stove and wants to get this one working better. Let me know if I can provide more info.

Not sure how the support is these days, but have you tried asking the dealer or factory?

 

[RandyBoBandy]

if your GT is only 300 I'm guessing there is no flame in the firebox and the cat is doing all the work. Couple things to check. Timing of your secondary air shutter. It will start to open back up if the coil is ar the wrong timing. I have mine set at 1:30-2:00. I don't remember what model you have and you might not even have a secondary air shutter. If that's the case I would just foil tape it closed. Check the MC of your wood as well. I ran into a situation where I had some questionable wood in one of my stacks. How I don't know but it was there. I had a hard time keeping flame in the box but at the same time dealing with high cat temps. My guess is the wood was dry enough to burn in bypass and wet enough to smoke and smolder with the damper closed. The last is a just a wild guess. At 16 I think you are at the minimum chimney height so draft could be an issue. Is your cap higher than your ridge?

 

[trispad]

if your GT is only 300 I'm guessing there is no flame in the firebox and the cat is doing all the work. Couple things to check. Timing of your secondary air shutter. It will start to open back up if the coil is ar the wrong timing. I have mine set at 1:30-2:00. I don't remember what model you have and you might not even have a secondary air shutter. If that's the case I would just foil tape it closed. Check the MC of your wood as well. I ran into a situation where I had some questionable wood in one of my stacks. How I don't know but it was there. I had a hard time keeping flame in the box but at the same time dealing with high cat temps. My guess is the wood was dry enough to burn in bypass and wet enough to smoke and smolder with the damper closed. The last is a just a wild guess. At 16 I think you are at the minimum chimney height so draft could be an issue. Is your cap higher than your ridge?

You are correct, there is generally no flame once the air is shutdown. There is flame when that damper is closed, but before closing the air. There is no secondary air shutter (Defiant 2n1). I do cover it with a metal sheet, but it is not 100% air tight. As far as dry wood.... I think it is dry and no one likes to admit that they have wet wood, but I brought a few splits inside last night to warm up. I will split and check them with a moisture meter tonight and report back. My wife suggested that I tighten the front doors which I did last night.

My cap is probably in line with my ridge (which runs E/W), but does meet the 10-2-3 requirement. The roof is pretty flat (3/12 pitch?) Of note, winds are generally form the east.

I am going to try running up the temp on the griddle more and keeping it up to keep flame in the fire box and see if that helps. When running in the past I have generally focused on cat temp, and so long as it was up to temp not worried about the griddle temp so much.

 

[trispad]

Not sure how the support is these days, but have you tried asking the dealer or factory?

I have talked to them before, but without much luck. Generally the advice is don't burn pine. I will start recording my temps and see if I can get some hard data to bring to them.

 

[RandyBoBandy]

I generally close my damper between 350-400 degrees on the flue thermo. Maybe don't worry about trying to have primary complete shut down by 1100 cat temp. Focus more on smaller primary adjustments in attempt to keep at least one little flame in the fire box. And probably put foil tape over secondary air inlet.

 

[trispad]

Things seemed to work out better by keeping the griddle temp up. I did check the moisture content on a few white oak splits. I let them sit inside for ~24hrs and re-split. The measurements were coming in around 14%. But I don't split and check every piece of wood that goes into the stove, so there may be some wet ones that get it. It did seem to take some doing to get the griddle temp up once the damper was closed. Running wide open to get flames on top of the wood and then not closing down the air very much to keep it there (maybe 2/3 closed?).

 

[arnermd]

A follow up post for posterity, hopefully somebody will find this of use in the future.

I was able to resolve my cat over firing issue by plugging several air inlet holes.

1. There is a ~3/8" hole in the bottom of the stove, just behind the secondary air inlet. it allows air to flow unrestricted up into the ashpan area, I plugged it completely with some ceramic insulation.

2. There are 8 small holes in the lower fireback, ~1/8" dia, you can see them if you open the front doors and look into the firebox. I plugged 6 of these holes with machine screws, sorry forgot what size. These holes allow air from the secondary inlet to enter the firebox. I played around with plugging all of them and fewer, I found plugging 6 gives the best result for my setup.

After this modification the stove is much more controllable, with cat engaged if I close down primary air the cat temps will go down, open it up and cat temps rise, perfect. With a full load of wood and air full open cat will climb to 1200-1400 and hang there for hours. Griddle will run about 600F with full air, about 400 with air full closed. I usually run about 50% open on the air and full wood loads, griddle runs 450-500F cat hangs around 800 - 1000F.

One thing that I have seen happen: If you have a big fire going and you close down the air too fast the cat will climb up to 1400 - 1500 range. Choking the air off that quickly gives you a lot of smoke for the cat to chew on. This is a well known "issue" but is easily solved by bringing the air down in steps. I usually bring it from 100% air, to 50%, wait a half hour or so and then throttle down as desired. Cat will cruise at ~700 - 800F for many hours. If you do have a smokey fire going (because your wife doesn't really understand this concept....), and the cat climbs up to 1500F, opening the primary air up will cause the cat temp to drop back to normal ranges.

Back when I had the cat overfiring I could hear the air being sucked through the secondary path, near the back of the stove. It was not a roar but definitely a noticeable "sucking sound" (to quote Ross Perot). Since I plugged up the holes I have not heard this sound, not even once.

Final thoughts:
- I consider this problem to be solved, stove seems to be operating "normally" and under control for several months now.
- A complete tear down and rebuild was not required to resolve the issue, I never did find a smoking gun or any significant air leaks.
- I don't understand what VC was thinking when they designed this stove. As many of you know the newer 2n1 models do not have a thermostatic control on secondary air flow, only the primary has a thermostat. An uncontrolled secondary air inlet seems like a bad design to me, it might work under certain conditions but when those conditions change it can run away. Trying to manage the balance of primary and secondary air over all possible operating conditions seems like a losing battle to me with one air source uncontrolled.
- By plugging the secondary air holes in my stove I have essentially "tuned" the stove my typical operating conditions.
- Not sure how effective it is to plug the hole in the bottom of the stove (#1 above) I left mine plugged but one of these days I would like to open it back up and see what the effect is. I suspect the fireback holes (#2 above) are the critical ones.
- The stove did not so this for the first 4 years of its life. So the question remains, what changed? I simply do not know, there is nothing I can find. The installation / chimney / controls have not changed one bit, only thing I can think of is different wood or there is still some mystery air leak I have not found. I have given up trying to figure it out....

Nice to have a stove I can trust again, and sleep through the night without the cat alarm going off.

I hope this adventure will be of use to someone.