2023/24 VC Temperature discussion thread

[sargeott]

Something weird happening right now. I put a new cat in the stove last week and it’s been running a top. Today while I was at work the wife has been running the stove. She said good STT and cat in the 700-800’s all day. I get home and notice that the cat is about 600. 30 minutes or so goes by and now the cat is 520. I turn the air up and the cat jumps back to 600’s. I turn it down a little and now the cat is in the 400’s and the STT is 550.

We are going to let it go out and I’ll take a look at it in the morning. Feels like the bypass is a little loose, it is way to easy to close so I’m thinking that I don’t have a good seal.

Take pictures if you would on the inside of the engine where the cat sits. I am curious how the cat is oriented in that stove.
You didn't mention the ammount of wood in the box when you posted your temperatures. That is a factor in all of this.

 

[NewGuy132]

Take pictures if you would on the inside of the engine where the cat sits. I am curious how the cat is oriented in that stove.
You didn't mention the ammount of wood in the box when you posted your temperatures. That is a factor in all of this.

I’ll take pictures in the am. Looking at it it might have filled up with ashes. It’s a good 1 inch above the glass. The wife said that it was still hot this morning so she didn’t empty it.

It is a small stove so there wasn’t too much wood in it. It has been going on for a few hours since I was home. It definitely wasn’t at the end of the burn cycle.

 

[DullAxe]

Wow, had my first overfire situation tonight where the cat increased to over 1500 degrees and set the alarm on my Auber probe monitor off. The secondary air was all the way open, so I tried knocking it back a bit, but it climbed as one might expect it to as the load of wood in there hadn't released all of it's smoke yet. Had my wife open the bypass and I choked the air down - vacillated between that and closing it and opening the seconday air to try to drop the temp. It finally worked, but definitely has me a bit shaken tonight. I didn't notice anything at the chimney outside - but then again it was dark out.

I'm not sure what caused it as I've never had that issue before. Using the same wood that I have been using. I had loaded the stove a bit more than I normally do, a little over half full. Not sure if something shifted at some point, or what. I closed the secondary a bit earlier than I normally would as we were going to start watching a movie. I'll inspect everything tomorrow and possibly sweep the chimney - which I did about a month ago.

 

[Rusty18]

Must have been the night for it. My encore had been doing pretty good but i had it up to 1610 last night. Poor loading on my part though. Filled the box full of uglies and it all went off at once.
If you stove runs like mine the secondary combustion runs on fuel lean side of the curve so closing the secondary was probably taking away cooling air.
All that being said once I got my pile knocked down it ran good the rest of the night. Even had an ember or two left in the box this morning.

 

[arnermd]

Im very interested to see if either of you or preferably both, put in a device to keep space for the smoke and gasses to freely flow into the engine.

The theory regarding wood placement is that if you have wood burning right at the secondary inlet the flames and heat channel right up into the cat, instead of mixing in the firebox and then going to the cat. I do not think I have ever experienced secondary blockage due to wood placement.

@arnermd if your cat was dead, your engine/cat temps would be closer to your flu temps. The issue somehow surrounds the air fuel ratio in the firebox & or the cat area.

I am not sure about this... maybe. If you have secondary combustion without the cat it could look a lot like catalytic combustion temperature wise, but it would require more heat / higher temps to sustain itself. The cat just promotes the combustion at a lower temp so the secondary burn starts at a lower temp and goes for longer into the burn. I am thinking I need to pull the cat out and get some temp plots to compare, with cat and without cat.

We have seen others @JohnDaileyNH for example do the manual secondary air control where he uses fresh air into the cat to lower temps.

Agreed, unfortunately with the 2n1 designs there is no way to increase the secondary airflow unless you add a blower (or modify the ceramic refractory). Achieving lower secondary temps by adding air makes perfect sense assuming you are on the lean side of air/fuel ratio.

Cats don't need much fresh air or any to do their job. BUT, there still needs to be flow of gasses passing the precious metals inside the cat which creates the chemical reaction to happen.
Think about vehicle exhaust cats. Those don't get fresh air into them to work. To much unburned fuel plugs them up or kills the material.

Cats do not alter the required fuel / air ratio, that is determined by chemistry. They just promote the reaction to occur at a lower temperature. Vehicle cats do not require additional airflow because the primary combustion air fuel ration is carefully controlled such that excess oxygen remains in the cylinder exhaust for the cat to combust. This is the job of the upstream oxygen sensor.
Agreed that not enough oxygen entering the cat will plug them up.... pretty quick.

In my old job, i had worked very closely with Oregons only coal fired generation plant (RIP) before they demolished it. That plant was the cleanest burning coal plant on the west coast. They ran massive catalysts and amonia injection. They also monitored emissions in real time because OREGON is a big green state.
If the burners on the hot side the emissions were reduced and catalyst temps were lower, less amonia was injected and consumed. When we had to lower power output, the emissions were the same, but cat temps were very high and amonia consumption was higher.

That's all very interesting, thanks. I am going to have to do some research on ammonia injection, not my area of expertise.... Looks like it is used for reducing NOx emission.

 

[arnermd]

Something weird happening right now. I put a new cat in the stove last week and it’s been running a top. Today while I was at work the wife has been running the stove. She said good STT and cat in the 700-800’s all day. I get home and notice that the cat is about 600. 30 minutes or so goes by and now the cat is 520. I turn the air up and the cat jumps back to 600’s. I turn it down a little and now the cat is in the 400’s and the STT is 550.

We are going to let it go out and I’ll take a look at it in the morning. Feels like the bypass is a little loose, it is way to easy to close so I’m thinking that I don’t have a good seal.

This does not seem abnormal to me.... Sounds like you have a nice bed of hot coals. These give off a lot of heat with very little smoke so the cat has very little to combust. It just follows the flue gas temp when you move the air around.

 

[arnermd]

Wow, had my first overfire situation tonight where the cat increased to over 1500 degrees and set the alarm on my Auber probe monitor off. The secondary air was all the way open, so I tried knocking it back a bit, but it climbed as one might expect it to as the load of wood in there hadn't released all of it's smoke yet. Had my wife open the bypass and I choked the air down - vacillated between that and closing it and opening the seconday air to try to drop the temp. It finally worked, but definitely has me a bit shaken tonight. I didn't notice anything at the chimney outside - but then again it was dark out.

I'm not sure what caused it as I've never had that issue before. Using the same wood that I have been using. I had loaded the stove a bit more than I normally do, a little over half full. Not sure if something shifted at some point, or what. I closed the secondary a bit earlier than I normally would as we were going to start watching a movie. I'll inspect everything tomorrow and possibly sweep the chimney - which I did about a month ago.

Sounds very familiar, although 1500 is not that hot to me.... I routinely see cat temps at 1600+ for 30 - 60 min. My cat alarm is now set to 1650 cause I am tired of hearing it.....

Your attempts to bring it under control sound very familiar. 3 things I have found that work reliably when in this condition.

1. Open the damper, however this can drive stack temps through the roof and there is a potential risk of cat thermal shock.
2. Kill the draft. I installed a key damper in the exit pipe and even sealed around it so I can almost completely kill the airflow. This works every time immediately. But I use it sparingly and under supervision as you can end up with some in the house.....
3. Completely block the secondary air inlet with a plate and a magnet. This takes a while but I have found it to be effective.

 

[arnermd]

Last nights burn with the ceramic cat.

• I did a preburn with a small load to get things heated up again
• As cat temps rolled back down to 600 I loaded full, enagged the cat, 100% air.
• At 1200 started dialing down the air, worked it down to 20% open and went to bed.
• Cat temps peaked at 1500 and then just crashed
• In the morning I had a very thick bed of coals, opened the air up to heat things back up and got a lot more heat out of the coal bed.

My initial impression from the temp plot was this was a really bad burn, but.... The glass was pretty clean, a little smoked and my flue gas temp probe is clean. If I have a bad burn the probe gets coated with black creosote, it was very clean this morning.

So.... I am not sure what to think of this burn. If ignore the cat temps crashing it seems good.

It does appear the cermic cat behaves a lot like my metal cat, cat temps crash quickly with low air flows.

 

[Eman85]

Cats don't need much fresh air or any to do their job. BUT, there still needs to be flow of gasses passing the precious metals inside the cat which creates the chemical reaction to happen.
Think about vehicle exhaust cats. Those don't get fresh air into them to work. To much unburned fuel plugs them up or kills the material.

Automotive cats do use fresh air injected into them The older ones used a mechanical air injection pump that was belt drive. Now they use an electric one.

@arnermd Surprised to see how low your STT is even when the cat is at it's peak, not much heat coming off of the stove after that either.

I just drop my wood in, no special placing of it. My splits are on the small side which isn't what this stove wants but it's what it's getting, much easier to load and carry the wood in especially for my wife

 

[arnermd]

Automotive cats do use fresh air injected into them The older ones used a mechanical air injection pump that was belt drive. Now they use an electric one.

Do all modern cars inject air upstream of the cat? I am aware that some do for sure.... I know there is excess oxygen at the cat inlet but I was not sure if they were running the cylinders lean or injecting extra air.

@arnermd Surprised to see how low your STT is even when the cat is at it's peak, not much heat coming off of the stove after that either.

I rarely have issues with runaway STT, which is why I think my problems are not do to air leaks....
I really like running low air settings (thanks @Woodsplitter67) if I can keep it clean and the cat under 1500, much more even heating and longer burns. Even with last nights burn the house was not cold.... cooler than normal but comfortable.

Oh... and my STT readings are about 50F low... so add 50 to the plot readings.

 

[Eman85]

If you read my posts on running without the cat there isn't that much difference in burn times. I own a wood stove for heat just like my HVAC system. When I want it warmer I want control, with the cat I don't have as much control the cat ends up having control which you have experienced many times.

I've been out of auto repair for a few years and don't own a modern car so I can't comment on what they have now. The diesels use the DEF injection to clean up the exhaust which is like the ammonia injection.

My only concern with burning clean is my chimney. I don't think my burning without the cat makes much of a difference for the environment compared to the number of brush piles I burn every year or the wildfires that happen.

 

[arnermd]

f you read my posts on running without the cat there isn't that much difference in burn times.

Been a while since I have run without my cat, but as best I recall there was not a dramatic difference in burn time. So I agree with you.
However, my point was: Running with low air settings definitely gives me longer burn times and more even heating of the house. I would not consider running my stove at low air settings without a cat, too much creosote, which was your next point....

My only concern with burning clean is my chimney. I don't think my burning without the cat makes much of a difference for the environment compared to the number of brush piles I burn every year or the wildfires that happen.

Totally agree.... my understanding is that burning wood is carbon neutral. If you burn the wood it releases just as much carbon as it would rotting on the forest floor.... I recognize I may be treading on some sensitive/debatable topics here, but it makes sense to me. I am sure some government funded scientist somewhere has authored a paper that says otherwise.

That's not to say you can't have some temporary local issues with particulates / smoke as you are collecting the wood to one place and burning it much quicker than it would rot.

I would expect your brush fires are neutral for the environment as well and I would say good forest management.

 

[AsylumResident]

This is my run from yesterday. I have been doing cold starts everyday when I get home from work with the outside temps being a little warmer lately. The first part of the graph is when I engaged the cat on 3 medium splits. The cat peaked at about 850. Once the cat dropped under 750 I loaded up about a 3/4 load to burn. I have found that I need a decent amount of wood to get the cat over 1000; those three splits were not enough.
- 3/4 load of locust (almost out of locust)
- Engaged the cat with air 100%
- When cat reached 1050 I dialed air to about 50%
- After an hour and half I dialed the air down to about 10% for the rest of the burn. You can see the quick drop around 9:00pm when I dialed the air down then a small recovery followed by a steady drop off (typical).
- Cat peaked at 1389.
- Small bed of coals left at 6:00am (I slept in this morning).

I would take this burn every night!

 

[arnermd]

This is my run from yesterday. I have been doing cold starts everyday when I get home from work with the outside temps being a little warmer lately. The first part of the graph is when I engaged the cat on 3 medium splits. The cat peaked at about 850. Once the cat dropped under 750 I loaded up about a 3/4 load to burn. I have found that I need a decent amount of wood to get the cat over 1000; those three splits were not enough.
- 3/4 load of locust (almost out of locust)
- Engaged the cat with air 100%
- When cat reached 1050 I dialed air to about 50%
- After an hour and half I dialed the air down to about 10% for the rest of the burn. You can see the quick drop around 9:00pm when I dialed the air down then a small recovery followed by a steady drop off (typical).
- Cat peaked at 1389.
- Small bed of coals left at 6:00am (I slept in this morning).

I would take this burn every night!

View attachment 321225

Interesting, I see a lot of similarities with my burns.... Your cat dropped out (like mine) shortly after you cut the air. You only had about 2 hours of cat>1000. Not sure that's a bad thing..... 10 -11 hour burn on 3/4 load seems good.
I too have some small loads where the cat just wont get up and go.... not sure why.

Sleeping in till 6.... what a lazy bugger.

 

[Eman85]

Been a while since I have run without my cat, but as best I recall there was not a dramatic difference in burn time. So I agree with you.
However, my point was: Running with low air settings definitely gives me longer burn times and more even heating of the house. I would not consider running my stove at low air settings without a cat, too much creosote, which was your next point....

Totally agree.... my understanding is that burning wood is carbon neutral. If you burn the wood it releases just as much carbon as it would rotting on the forest floor.... I recognize I may be treading on some sensitive/debatable topics here, but it makes sense to me. I am sure some government funded scientist somewhere has authored a paper that says otherwise.

That's not to say you can't have some temporary local issues with particulates / smoke as you are collecting the wood to one place and burning it much quicker than it would rot.

I would expect your brush fires are neutral for the environment as well and I would say good forest management.

The dirty pipe is my only concern when I burn with no cat. My stove is old school and the first step in the catalytic stove world by VC. Not much difference in me burning with no cat than someone with a pre-cat VC Defiant burning. I burn both ways, just depends on if I want to be occupied by the stove or something else.

 

[Woodsplitter67]

Do all modern cars inject air upstream of the cat? I am aware that some do for sure.... I know there is excess oxygen at the cat inlet but I was not sure if they were running the cylinders lean or injecting extra air.

I rarely have issues with runaway STT, which is why I think my problems are not do to air leaks....
I really like running low air settings (thanks @Woodsplitter67) if I can keep it clean and the cat under 1500, much more even heating and longer burns. Even with last nights burn the house was not cold.... cooler than normal but comfortable.

Oh... and my STT readings are about 50F low... so add 50 to the plot readings.

Thats awesome.. thanks for the heads up, glad its working. It hasn't been super cold so alot of low for me also. Im still going through some wood..

 

[Woodsplitter67]

This is my run from yesterday. I have been doing cold starts everyday when I get home from work with the outside temps being a little warmer lately. The first part of the graph is when I engaged the cat on 3 medium splits. The cat peaked at about 850. Once the cat dropped under 750 I loaded up about a 3/4 load to burn. I have found that I need a decent amount of wood to get the cat over 1000; those three splits were not enough.
- 3/4 load of locust (almost out of locust)
- Engaged the cat with air 100%
- When cat reached 1050 I dialed air to about 50%
- After an hour and half I dialed the air down to about 10% for the rest of the burn. You can see the quick drop around 9:00pm when I dialed the air down then a small recovery followed by a steady drop off (typical).
- Cat peaked at 1389.
- Small bed of coals left at 6:00am (I slept in this morning).

I would take this burn every night!

View attachment 321225

Uour burns are looking good.. Id take this..

 

[arnermd]

Yesterday I had the cat shoot from 1200 to 1600 in a few minutes, I could hear the tell tale combustion noise. I opened the damper and the griddle to see what was going on in there.... what I saw was all the logs in the front and top were just smoldering and the only thing burning was right at the secondary smoke inlet. Flame was channeling right up into the refractory. Unfortunately the bottom splits got wedged in there and I could not move them without pulling about 4 other splits and sending them to the back yard..... I opted to just close my key damper and let it calm down. It took about 20 min but the cat came back down to 1000 when I opened up the key damper and all was fine.

Even after I opened the damper and griddle I could see a vigorous flame channeling right into the secondary smoke inlet..... interesting.

When I was loading I noticed the coal bed was almost entirely at the back of the stove, next time I will rake them around so the load burns more evenly across the bottom layer.

 

[Todd]

It almost sounds like these stoves run like a rocket stove heater if you don’t keep some good flames going in the firebox. Snuff out the flames and the cat is overwhelmed with smoke and rockets up to those higher temps. Yeah, maybe spreading out the coal bed evenly would help.

 

[Eman85]

Yesterday I had the cat shoot from 1200 to 1600 in a few minutes, I could hear the tell tale combustion noise. I opened the damper and the griddle to see what was going on in there.... what I saw was all the logs in the front and top were just smoldering and the only thing burning was right at the secondary smoke inlet. Flame was channeling right up into the refractory. Unfortunately the bottom splits got wedged in there and I could not move them without pulling about 4 other splits and sending them to the back yard..... I opted to just close my key damper and let it calm down. It took about 20 min but the cat came back down to 1000 when I opened up the key damper and all was fine.

Even after I opened the damper and griddle I could see a vigorous flame channeling right into the secondary smoke inlet..... interesting.

When I was loading I noticed the coal bed was almost entirely at the back of the stove, next time I will rake them around so the load burns more evenly across the bottom layer.

With the bypass damper closed the draft is drawing through the cat chamber so it pulls the flames in. Just like it does with the bypass open and it pulls the flames up towards the pipe.
I've never removed burning wood from the stove.

 

[U235]

@arnermd - when you’re cautioning against flame impingement, are you talking about flames hitting the engine or like maybe the BK guys talk about direct catalyst contact? With the path flame has to take in our stoves - four right angles (up back right down), doesn’t seem to me that the flames can get to the catalyst even with very strong draft.
I’m asking because flames in the engine inlet is my primary method for lighting the catalyst, especially with these warmer temps. Seeing as you’re the expert in catalyst and refractory life (or at least most experienced with finding the end of both) thought you’d probably have some thoughts.

 

[AsylumResident]

Yesterday I had the cat shoot from 1200 to 1600 in a few minutes, I could hear the tell tale combustion noise. I opened the damper and the griddle to see what was going on in there.... what I saw was all the logs in the front and top were just smoldering and the only thing burning was right at the secondary smoke inlet. Flame was channeling right up into the refractory. Unfortunately the bottom splits got wedged in there and I could not move them without pulling about 4 other splits and sending them to the back yard..... I opted to just close my key damper and let it calm down. It took about 20 min but the cat came back down to 1000 when I opened up the key damper and all was fine.

Even after I opened the damper and griddle I could see a vigorous flame channeling right into the secondary smoke inlet..... interesting.

When I was loading I noticed the coal bed was almost entirely at the back of the stove, next time I will rake them around so the load burns more evenly across the bottom layer.

That is exactly what happened to me about a month ago when my cat shot up to the upper 1700s and my flue temp got over 1200. There was a slightly punky piece that fully lit right at the intake. Since then I have done my best to organize the wood to avoid that as much as possible.

 

[arnermd]

@arnermd - when you’re cautioning against flame impingement, are you talking about flames hitting the engine or like maybe the BK guys talk about direct catalyst contact? With the path flame has to take in our stoves - four right angles (up back right down), doesn’t seem to me that the flames can get to the catalyst even with very strong draft.
I’m asking because flames in the engine inlet is my primary method for lighting the catalyst, especially with these warmer temps. Seeing as you’re the expert in catalyst and refractory life (or at least most experienced with finding the end of both) thought you’d probably have some thoughts.

When I talk about flame impingement I am usually referring to flame hitting the catalyst grid itself. I would guess the refractory is much more tolerant of flame impingement due to its heft. My ceramic catalysts always fail by crumbling and I have read flame impingement can cause this. I also see spalling in the gas path on my refractory, this could be caused by direct flame exposure as well.

Again I think it is a question of severity and time of exposure. These materials are all good for flame exposure to some extent and for some period of time.

No doubt all the turns in the secondary path help to mix the exhaust gas and hold the flame back, but there are limits. I do not think I am getting flame impingement all the time, I would not consider that to be the normal operating mode by any means. Again i suspect it is a cumulative time and frequency thing, the longer and more frequently it occurs the quicker the catalyst will crumble.

Interesting my metal cat has shown very little stress, the grid has certainly deformed a bit and it does seem more brittle now but it is all still in one piece and the frame itself has not deformed at all, unlike my ceramic cat which warps so bad I can only reinstall it one way.

 

[Woodsplitter67]

When I talk about flame impingement I am usually referring to flame hitting the catalyst grid itself. I would guess the refractory is much more tolerant of flame impingement due to its heft. My ceramic catalysts always fail by crumbling and I have read flame impingement can cause this. I also see spalling in the gas path on my refractory, this could be caused by direct flame exposure as well.

Again I think it is a question of severity and time of exposure. These materials are all good for flame exposure to some extent and for some period of time.

No doubt all the turns in the secondary path help to mix the exhaust gas and hold the flame back, but there are limits. I do not think I am getting flame impingement all the time, I would not consider that to be the normal operating mode by any means. Again i suspect it is a cumulative time and frequency thing, the longer and more frequently it occurs the quicker the catalyst will crumble.

Interesting my metal cat has shown very little stress, the grid has certainly deformed a bit and it does seem more brittle now but it is all still in one piece and the frame itself has not deformed at all, unlike my ceramic cat which warps so bad I can only reinstall it one way.

I put a ceramic back in the other week I can tell the difference between the metal and the ceramic for sure

 

[AsylumResident]

I put a ceramic back in the other week I can tell the difference between the metal and the ceramic for sure

What is the difference? Which one would you rather run?