2023/24 VC Temperature discussion thread

[arnermd]

Are you leaving the secondary blocked for the rest of burn when you’ve done this? I can’t understand how it is breathing at all.

Normally I do not leave it blocked, I will wait for Cat < 1100 or so and then remove it. But for this burn I did leave it in place.
The glass was a mess in the morning so it was not breathing well at all...... Probably created a bunch of creosote in the stack.

Now that you’ve found a control input that is actually effective, any chance the watlows can run a small servo to meter the secondary air?

I have thought about this over the years.... It could be done with the Watlows, just need to work out the details. But... I really do not want to go that route as there are some nasty failure modes and it is just more complicated than I want my stove to be....

The other thought I have had is, I am not sure blocking the secondary air is the "right" solution. I do it as an emergency measure but I am not sure it is the right thing to do. It is possible the correct solution is to increase secondary air to dilute the exhaust and bring temps down that way.... but if secondary air is feeding the primary combustion then you can get into a runaway situation if you force feed secondary air with a blower. This is where I would love to hear from VC about what the real design intent is.

Whenever this happens my first though is always I have an air leak.... but if I had an air leak then the STT would be running hot and uncontrollable, right? My STT have never been a problem and if I close the air down the STT does drop, although it can take a while. Problem is although STT drops Cat temps go to the moon.....

 

[U235]

Normally I do not leave it blocked, I will wait for Cat < 1100 or so and then remove it. But for this burn I did leave it in place.
The glass was a mess in the morning so it was not breathing well at all...... Probably created a bunch of creosote in the stack.


I have thought about this over the years.... It could be done with the Watlows, just need to work out the details. But... I really do not want to go that route as there are some nasty failure modes and it is just more complicated than I want my stove to be....

The other thought I have had is, I am not sure blocking the secondary air is the "right" solution. I do it as an emergency measure but I am not sure it is the right thing to do. It is possible the correct solution is to increase secondary air to dilute the exhaust and bring temps down that way.... but if secondary air is feeding the primary combustion then you can get into a runaway situation if you force feed secondary air with a blower. This is where I would love to hear from VC about what the real design intent is.

Whenever this happens my first though is always I have an air leak.... but if I had an air leak then the STT would be running hot and uncontrollable, right? My STT have never been a problem and if I close the air down the STT does drop, although it can take a while. Problem is although STT drops Cat temps go to the moon.....

From a simple model I think you have the heat generated by combustion balanced by heat lost. Some of that goes out the chimney. Some of that is extracted from the stove body by radiation and convection. The stove and STT will equilibrate at whatever temperature is required to get heat of combustion that isn’t going up the stack into the room.
I agree with you that an air leak will likely increase the heat produced. However I could see how maybe the heat going up the chimney could increase more (especially with a catalyst-dominant burn). In the limit of an open bypass, open doors, I could see you have a roaring fire, but all the heat goes up the chimney, and STT are low. As we’ve discussed before, draft delta P isnt flow and flue temp isn’t energy. If you have a massive leakage path, you’ll have more flow and more loss up the chimney at a constant flue temp and draft.

I’m sure you’ve checked your gaskets (bypass, front doors, ash tray, top door) probably repeatedly. Maybe the set screw has backed out or the gaskets have decayed. Personally, I do not possess such a firm understanding of design intent operation that I could exclude an air leak as a possibility based on low STT.

 

[Eman85]

This is where I would love to hear from VC about what the real design intent is.

You and me both! I've tried VC, dealers and stove repairs. Haven't had anyone that knew anything about secondary air. My stove has a secondary air flap that is temperature controlled.

 

[skb31]

Do you have a damper @arnermd? When loading the stove how full is it? I have the same stove so you can use the internal components as references. Is ash the main issue?

I have read about people blocking some of the air inlets in bottom of the back of the stove in the firebox along the refractory wall. I am curious what effect that would have. That pulls air from the secondary air intake into both the firebox and the refractory.

If anyone knows, what material would be suitable to handle the heat? This is mostly just curiosity

 

[arnermd]

Do you have a damper @arnermd? When loading the stove how full is it? I have the same stove so you can use the internal components as references. Is ash the main issue?

I have read about people blocking some of the air inlets in bottom of the back of the stove in the firebox along the refractory wall. I am curious what effect that would have. That pulls air from the secondary air intake into both the firebox and the refractory.

If anyone knows, what material would be suitable to handle the heat? This is mostly just curiosity

I have a key damper in the exhaust stack about 18" above the stovetop. I went with a solid plate and even made some sheet metal seals that I riveted inside the pipe to seal around the edges. I have pics in a previous post from last year. I wanted something that would shut off as tight as I could make it as I found the typical plate and install had so much leakage it did not really help me during an over-fire. This of course increase the risk of smoke back in the house, but I have yet to have that happen to any great extent.

Load: I try to describe the load in my posts, they are all over the place this time of year. Lately I have been running 1/2 loads or even less. I have had the cat go to 1500 with 3 med splits of ash in the box..... In the heart of winter I am running full loads almost all the time, I hate relighting from a match, just annoying...

A half load to me is just above the andirons.

Ash: I am 100% convinced Ash is the primary culprit for my over fires. I have also had issues with red oak but not early as often and sporadically. With a 1/2 to full load of Ash I can almost guarantee the cat will go to 1600+ no matter what I do. I can slow it down by engaging the cat at lower temps and running lower air settings but it is going to get there eventually. Even closing my damper does not really seem to affect the ultimate peak cat temp.

Not sure what makes ash so different, I have check moisture in side by side pieces of oak and ash and they were pretty similar.

I have tried blocking every hole there is in various combinations over the years, it kind of helps but hurts the secondary burn, cat is slower to light off and faster to fall off. But I have not found any combo that consistently limits the peak cat temps.

What heat are you referring too? If you are thinking of blocking the secondary air inlet holes at the base of the refractory any pan head steel screw will work. I forget what size I used, but regular carbon steel is fine.

FYI... there are also secondary air inlet passages behind the removable refractory panel at the bottom corners. I have stuffed those with ceramic insulation in the past.

 

[arnermd]

From a simple model I think you have the heat generated by combustion balanced by heat lost. Some of that goes out the chimney. Some of that is extracted from the stove body by radiation and convection. The stove and STT will equilibrate at whatever temperature is required to get heat of combustion that isn’t going up the stack into the room.
I agree with you that an air leak will likely increase the heat produced. However I could see how maybe the heat going up the chimney could increase more (especially with a catalyst-dominant burn). In the limit of an open bypass, open doors, I could see you have a roaring fire, but all the heat goes up the chimney, and STT are low. As we’ve discussed before, draft delta P isnt flow and flue temp isn’t energy. If you have a massive leakage path, you’ll have more flow and more loss up the chimney at a constant flue temp and draft.

I’m sure you’ve checked your gaskets (bypass, front doors, ash tray, top door) probably repeatedly. Maybe the set screw has backed out or the gaskets have decayed. Personally, I do not possess such a firm understanding of design intent operation that I could exclude an air leak as a possibility based on low STT.

I can always count on you to make me think deeper and challenge my own assumptions.

Agree with all your statements. I guess my assumption is that the air fuel mixture is not super lean.... As you said a massive air leak could have cold STT and high combustion rate, all the extra air will drop the exhaust temp and the heat will go up the stack.

I guess another piece of evidence I use to prove no air leaks is that I can close the air down and kill the flames and the STT will drop. And then there is the back puffing, it seems unlikely to me a leaky stove would back puff. Of course it always a question of degree....

I routinely inspect gaskets and they all seem fine. I have used a lighter/candle flame over all the joints to check for leaks when the stove is good and hot. It is pretty obvious when there is one as long as you get the flame in the right spot. The only place is see the flame being sucked in is at the air flapper, even with it full closed. If the leak is real bad it will extinguish the lighter flame and that did not happen.

Certainly I would not expect the flapper to seal up tight, even closed and it did not strike me as excessive. But maybe this summer I will pull that assembly out and see if I can file it flat to get a better shutoff. I have also experimented with blocking part of the flapper opening to reduce the amount of leakage at shut off. I did not find it to make much difference but I want to try it again.

 

[arnermd]

Something else I have noticed.....

I have been running with the secondary blocked by a plate / magnet. Recently I have been cracking the plate open to let a little air in, right now I have a quarter wedged in there.

Since I have done this it seems like the STT is running hotter than typical. I am running about 600 on the STT with 50% air, at 100% air I am seeing 650 - 700. If I close the air off I can bring STT down to 350 so it is responding but my STT never ran this hot before, even with ash.

Loads recently are typically 1/2 or less, all ash.

I can't explain this....

I also noticed I am getting a lot more secondary burns (auroras) in the firebox when I turn the air down below 50%. I think this supports my theory that some of the secondary air is supporting primary combustion.

 

[skb31]

Such a thorough and detailed explanation, thanks. How much ash do you have left to burn?

 

[AsylumResident]

I have a key damper in the exhaust stack about 18" above the stovetop. I went with a solid plate and even made some sheet metal seals that I riveted inside the pipe to seal around the edges. I have pics in a previous post from last year. I wanted something that would shut off as tight as I could make it as I found the typical plate and install had so much leakage it did not really help me during an over-fire. This of course increase the risk of smoke back in the house, but I have yet to have that happen to any great extent.

Load: I try to describe the load in my posts, they are all over the place this time of year. Lately I have been running 1/2 loads or even less. I have had the cat go to 1500 with 3 med splits of ash in the box..... In the heart of winter I am running full loads almost all the time, I hate relighting from a match, just annoying...

A half load to me is just above the andirons.

Ash: I am 100% convinced Ash is the primary culprit for my over fires. I have also had issues with red oak but not early as often and sporadically. With a 1/2 to full load of Ash I can almost guarantee the cat will go to 1600+ no matter what I do. I can slow it down by engaging the cat at lower temps and running lower air settings but it is going to get there eventually. Even closing my damper does not really seem to affect the ultimate peak cat temp.

Not sure what makes ash so different, I have check moisture in side by side pieces of oak and ash and they were pretty similar.

I have tried blocking every hole there is in various combinations over the years, it kind of helps but hurts the secondary burn, cat is slower to light off and faster to fall off. But I have not found any combo that consistently limits the peak cat temps.

What heat are you referring too? If you are thinking of blocking the secondary air inlet holes at the base of the refractory any pan head steel screw will work. I forget what size I used, but regular carbon steel is fine.

FYI... there are also secondary air inlet passages behind the removable refractory panel at the bottom corners. I have stuffed those with ceramic insulation in the past.

When I started burning ash it was just like this. Now when I burn ash, it takes longer to get the cat up to temp and it definitely isn't running very hot at all. If I see 1300 cat temp I'm lucky, barely likes to get above 1100. This might be the first time we are not seeing the same (or close to the same) results in burning.

 

[arnermd]

Such a thorough and detailed explanation, thanks. How much ash do you have left to burn?

only about 8 cord..... and I have a lot of standing dead ones to drop this summer....

 

[arnermd]

When I started burning ash it was just like this. Now when I burn ash, it takes longer to get the cat up to temp and it definitely isn't running very hot at all. If I see 1300 cat temp I'm lucky, barely likes to get above 1100. This might be the first time we are not seeing the same (or close to the same) results in burning.

Interesting.... why is that? What changed?

I hit 1600 again last night with a 1/2 load, 50% air.

 

[U235]

Something else I have noticed.....

I have been running with the secondary blocked by a plate / magnet. Recently I have been cracking the plate open to let a little air in, right now I have a quarter wedged in there.

Since I have done this it seems like the STT is running hotter than typical. I am running about 600 on the STT with 50% air, at 100% air I am seeing 650 - 700. If I close the air off I can bring STT down to 350 so it is responding but my STT never ran this hot before, even with ash.

Loads recently are typically 1/2 or less, all ash.

I can't explain this....

I also noticed I am getting a lot more secondary burns (auroras) in the firebox when I turn the air down below 50%. I think this supports my theory that some of the secondary air is supporting primary combustion.

I’ll admit - I struggled with the back puffs when proposing the leak=low STT mechanism. I understand backpuffs occur at low primary air setting, but I don’t know why. Does the mixture go overly rich at the catalyst causing it to flame out? Not sure what else could create instant reversal of pressure in the fire box. (I have not had a back puff that I’m aware of).

However, I think the behavior you are observing with the secondary nearly blocked is exactly what I was describing for a low heat up the stack (ie desirable operation) scenario. More open primary generates more burn in the fire box, but if similar heat goes up the stack, STT increases. This also supports your theory that your secondary air circuit is polluting your primary burn (presumably in a non-intended way if this was logically designed)

 

[arnermd]

I’ll admit - I struggled with the back puffs when proposing the leak=low STT mechanism. I understand backpuffs occur at low primary air setting, but I don’t know why. Does the mixture go overly rich at the catalyst causing it to flame out? Not sure what else could create instant reversal of pressure in the fire box. (I have not had a back puff that I’m aware of).

Backpuffs are generated on the primary side not the secondary side. If you have never seen one, it looks like the whole fire box fills with flame and then goes out very rapidly. It lasts maybe 1 - 2 seconds from ignition to extinguished and repeats every 1- 2minutes or so. Intensity and timing can vary widely. I believe it is the same thing as the flash auroras we talk about but backpuffing is just more intense.

I believe the cause is an imbalance of heat and air in the primary burn, there is enough heat in combustion are to create volatile gas but not enough air to combust it all, so the oxygen builds up until the mixture reaches LFL (lower flammability limit) and then the whole mix goes up at once. Certainly local effects / mixing plays a role, oxygen is not evenly distributed and may be stratified. Zones may be above LFL but are not exposed to an ignition source. Once the LFL zone finds an ignition source and combusts, it induces mixing and the whole thing lights off until all the oxygen is consumed, rinse and repeat.

My observations that support this theory are:

• Backpuffing occurs more frequently when I have a hot bed of coals (lots of energy for off gassing) with fresh wood on top, and then I quickly turn down the air.
• Stepping the air down helps mitigate this because the coal bed is given time to cool as the air is reduced, so the air / heat balance remains favorable.
• I do not think I have ever had back puffing with significant active primary flame. I suspect the flames promote mixing so we end up with a more homogenous mix of oxygen and combustible gasses.
• Nor have I ever had backpuffs when the fire has reached the coaling stage, I suspect this is because all the off gassing has stopped and we are now converting the remaining carbon to CO2. So there are no volatile gasses to ignite.

This also supports your theory that your secondary air circuit is polluting your primary burn (presumably in a non-intended way if this was logically designed)

That was my thinking as well..... agreed.

Whole bunch of rambling and postulating here.... time to do something constructive.

 

[skb31]

I wonder if you could trade some of your ash for another type of wood so you could mix loads.

I’ve had several burns recently and tried a couple combos blocking those holes. Blocking just 2 seems to be the right one for me, giving me a noticeable increase in primary air control. I have to trim the air as the cat rises but recently I haven’t had to close 100%. I was 80-90% closed earlier with a 500 STT and the cat around 1175. Cruised in that range for a few hours and for what it’s worth these are smaller loads because it’s not all that cold.

 

[arnermd]

I need to start a local wood swap club.... My wife does cookie swaps at Christmas, kind of like that.

Keep us updated on how the hole blocking works for you. I have tried every combination there is and found it works for a while but then it doesn't. I may have been getting confused by ash vs oak in the past though. It was really only this season where I definitively realized Ash was a different animal in this stove, for some reason.....

I have had some very nice burns the last few days, secondary blocked but a wedged open with a Quarter, ceramic cat. All Ash but only 1/2 load or less. 40% - 10% air settings. Cat has been peaking around 1400 and then slowly decaying back to 1300 - 1000. Seems perfect, we shall see if it continues. STT is hotter than before for same air settings but controllable with the air.

 

[Eman85]

Stove was cold for quite a while with warm temps finally fired it up yesterday. Wife had it going without the cat engaged when I got in so I let it burn like that for a while. I wanted to load it once for the night at 9:30. Good coals STT over 500 I loaded the box to the top and engaged the cat once it was burning good. Got distracted and let the cat run to 1250 before I shut the air down. Cat behaved nicely dropping to about 1000 and then rising to 1200's when I went to bed at midnite. Plenty in the box at 8 this morning, cat dropped into the 300's opened the air and let it burn cat went to the 800's. Still burning and warm at 11 but it's getting warm out so I'll let it die out and fire it up later. Went to the 20's last night should be 50's today and cold again tonight.

 

[Hbob]

My new Encore has been burning great the first week. Everything seems to be controlled and for the first time in many years (I had an 1990 Encore that had catalyst removed) my chimney cap is expelling smokeless exhaust. It is amazing to watch the stack when the catalyst engages and things go clear. I have a question about the temperature probe. I am using the factory probe and it is working quite well except for one thing. I build a nice starter fire and after a half hour or so the SST is at about 700 F and I have a nice set of coals. The factory probe does not move to the "Engage Cat" level at all when stove is hot but the damper is still open to the stack. If I go ahead and engage the catalyst then the probe moves slowly to the "Engage Catalyst" level and stays there nicely in the mid-range until my fire is dying out when it starts to drop to the lower level of the "Engage Catalyst" range till I feed the stove again...pushing the coals to the back and loading. The probe has never elevated to the upper end, so the new catalyst and my setup seem to be behaving.
I do not have electricity nearby and we have a clean center of the room/attention installation and do not want cords running to the stove for the fancy probe. I like the analog probe and would upgrade if there are nicer ones, but this one is working for now. I did consider a battery operated thermometer clipped or magneted on the heatsheild, but now that I am running the stove, the shield is too hot for that.

I have read that the probe is measuring the air above the cat and I have read that the probe reads gas entering the cat. Which is it? I am using the factory mounting position.

 

[arnermd]

I do not know your model specifically but if the hole in the back is in the same place as mine you are actually measuring just downstream of the cat, the hot side.

Assuming your probe is the same as where I measure mine.....

• With the damper open the cat temp will never heat up. All the heat is going up the stack.
• I would suggest you use STT to know when to throw the damper, Mine works most times around 400 - 450.
• STT of 700 is getting pretty warm, I try not to run that hot. I like to see it under 600, but I have seen 700 lots of times but that's where I will turn the air down.

Everything you described seems like normal behavior to me, you just need to engage the damper a lot sooner. Try 400 on the STT and see if the cat lights off and climbs to mid range on your indicator, if it stalls open the damper and try again at 450 STT

If the stove is hot already you can engage the cat at much lower STT, I have had cat light off as low as 300 STT.

 

[Hbob]

Thank you that is very helpful. I was waiting a bit too long till I engaged the cat. The manual is not good on this subject.

 

[skb31]

Home now and read both hbob and arnermd’s post more carefully. I have an encore 2040 cat-c manufactured in the fall of 2023 so it’s the same stove as hbob I think.

Instead of saying “operate catalyst” the factory meter seems to show “catalyst operating.” You won’t get a temp spike in that chamber until you damper down which arnermd explained. He’s got the meter info for the stove correct as well. It sits below the catalyst.

 

[Eman85]

I used a thermocouple sensor from Auber with a battery powered meter. Many digital multimeters have a scale for temperature, and you just plug the sensor into it. VC must have had the temperature probes made in another country and something got lost in translation.

 

[skb31]

That's interesting your manual does not speak to running without the cat. Wonder if they changed something in the design or just decided not to allow it (and why?)

Cat failure can be detected by lower cat temps. I found the smoke method to be hit or miss, hard to tell sometimes. Usually for me I can tell because the ceramic cat just falls apart and pieces are laying at the bottom of the stove.... easy to tell. The metal cat did not fall apart but over several loads I could not get the cat over 700-800. So I swapped back to my ceramic.

Instrumentation:

• I have dual Watlow digital controllers that I use for primary readouts and audible alarms. Cat and flue gas temps.
• I use the Perfect Prime TC0520 for data logging (Cat, flue gas and stove top). I dump the data every 1 - 2 weeks via a USB cable and import it into Excel where I make all the plots. They do make a wireless version but it is more expensive.
• There is a bunch of new stuff out there that is wifi compatible, but I have not looked into them in detail.

I do not own a cell phone so no need for that...

I was wrong about running the stove without the catalyst and wanted to correct my mistake. I read my physical copy of the manual cover to cover again this evening. It states clearly on page 10 it’s not required but you’re advised to use it.

There’s a few oddities in the manual that I noticed now that I have a better frame of reference. On the same page 10 it says to immediately damper down on a hot stove reload. Then several paragraphs later it says that closing the damper too early after reloading can cause thermal shock and damage the cat.

A couple places express worry that people won’t get it hot enough. Made me laugh.

 

[arnermd]

I was wrong about running the stove without the catalyst and wanted to correct my mistake. I read my physical copy of the manual cover to cover again this evening. It states clearly on page 10 it’s not required but you’re advised to use it.

That sounds more like what I would expect. And I think most folks here would also advise you use the cat.

On the same page 10 it says to immediately damper down on a hot stove reload. Then several paragraphs later it says that closing the damper too early after reloading can cause thermal shock and damage the cat.

Hmmmm... I see the dilemma there....on a hot reload I frequently close the damper right after loading. Unless you are really slow at reloading the cat does cot cool off that much. I always open the damper when reloading so I do not get smoke back in the house, this also prevents drawing lots of cooler air into the cat when reloading.

A couple places express worry that people won’t get it hot enough. Made me laugh.

You can engage the damper too soon and the cat will stall out and not light off which can then coats the cat with creosote and masks it preventing future light offs. When I have had this happen a good hot fire will burn the cat off clean. But yes.... I have far more occurrences of cat too hot rather than too cold.

 

[Hbob]

I am posting on this thread something I started on another but not hearing back from the VC Encore gurus. Is the factory probe location (the predrilled hole in the back of the stove) and sits below the catalyst on the down stream side of the catalyst? What is the actual path of air/gases starting in the main fire box and ending in the chimney? My guess is up out of the back of the firebox into the opening of the refractory wall and then down through the catalyst, past the probe, and then back up to the flue. The air holes in the bottom of the refractory wall, appear to be air to the main fire box only....no opening into the refractory or catalyst area. Am I seeing this right?
Before I got the stove, I assumed the air went up through the catalyst.

 

[JohnDaileyNH]

I am posting on this thread something I started on another but not hearing back from the VC Encore gurus. Is the factory probe location (the predrilled hole in the back of the stove) and sits below the catalyst on the down stream side of the catalyst? What is the actual path of air/gases starting in the main fire box and ending in the chimney? My guess is up out of the back of the firebox into the opening of the refractory wall and then down through the catalyst, past the probe, and then back up to the flue. The air holes in the bottom of the refractory wall, appear to be air to the main fire box only....no opening into the refractory or catalyst area. Am I seeing this right?
Before I got the stove, I assumed the air went up through the catalyst.

My probe sits below the catalyst and it measures exit gasses. I found a diagram I attached that shows the VC airflow. This is why the probe digital or bipedal, will have a low or innactive reading before the damper is closed. There is very minimal air flow down there until the closed damper forces the air down through the cat then back out the back of the stove. When you think about it, it's a complex and probably over-engineered design in my opinion. But it works if the setup is right.