Secondary combustion entering the cat. Should I be worried about flame impingement damage?

[WoodpileOCD]

I read this quote in another discussion on cats and now I'm worried that I'm killing my cat (the important cat. The other one I'd like to kill sometimes. >:-( . )

Here is the situation.

In the first hour or two of a full reload my cat probe jumps to 1500 regularly and with everything shut down as far as it will go, I get no flame in the firebox but plenty of smoke being generated from the smoldering. My cat is glowing like nobody's business above the flame impingement plate and I can see the smoke pouring into it. I then get the pre-cat combustion referred to below and it will dance between the plate and the cat. If I open the primary just a bit I can get some flame in the box and that extinguishes the flame below the cat. I don't like to open it up as it starts to go through the wood and I get plenty of heat with it shut down.

My question is whether or not I should be concerned about flame impingement damage to the cat with this going on. It doesn't happen all the time and haven't figured out yet the circumstances that start it because sometimes I get a good set of secondaries dancing under the impingement plate for a couple of hours.

I have to watch carefully when I have a full load and don't have the blowers on as it WILL try to run up to 1800-2000 on occasion which I know is also very bad. As I learn more about this stove I'm realizing that I don't have the low end air control that I need even with evrything closed down all the way. I'm going to have to start thinking about some mods to the primary air plates underneath. This is installed as an insert so a flue damper won't work.

Thanks in advance.


[quote author="jharkin" date="1326405008"]Ive learned to watch it like a hawk anytime I see the temp go over 1600. From experience Ive found that 1650-1700 sets up the right conditions to trigger spontaneous secondary combustion before the cat, resulting in the upper fireback glowing cherry red. very bad. Also Condars manual says anything over 1700 will degrade the cat.[/quote]

EDIT: Pic added. The lower black area is the impingement plate below the cat.

 

[BrotherBart]

Yeah that Florida Bungalow Syndrome isn't reserved just for non-cat stoves. A hard pulling pipe can make any stove zoom.

 

[Todd]

I've been told not to worry unless you see a steady stream of flame being pulled into and through the cat which is almost impossible with the newer designed cat stoves. I think you would really have to open her up to have this happen, a little bit of flame floating around or brushing the cat once in awhile won't hurt it.

 

[greythorn3]

heck ive seen flames touching the cat before, i didnt know that was a bad thing. seems dumb they would put them inside a woodstove if they didnt want flames touching them.

 

[pgmr]

As I learn more about this stove I'm realizing that I don't have the low end air control that I need even with everything closed down all the way. I'm going to have to start thinking about some mods to the primary air plates underneath.

You can probably tighten up the slots that the primary control tabs slide in with a few light hammer taps. Don't forget that there are secondary air inlets up high behind the cat housing. When the firebox is hot, with the primary controls closed, you'll occasionally see jets of flame coming out of the secondary inlet holes. If you have excessive draft, you might need to reduce the size of the opening of the secondary inlets. On our model 80, those inlets begin in the bottom left and right corners in the front of the stove. There are removable corner pieces that will reveal the channel iron that runs up the side of the firebox to the secondary inlet angle iron.

The cellular structure in your photo is not the bottom of the catalyst. It is a stainless steel grid which is designed to breakup flame before it gets to the cat.

 

[HotCoals]

Interesting..
I didn't know that some cat stoves have secondary air.
My BK doesn't.

 

[pgmr]

Interesting..
I didn't know that some cat stoves have secondary air.
My BK doesn't.

I'd be surprised if all cat stoves don't have some dedicated way of getting air that hasn't gone through the primary inlets introduced near the catalyst. If there isn't enough air from the primary supply to support flames, then there probably wouldn't be enough to support the combustion within the cat.

 

[pgmr]

My question is whether or not I should be concerned about flame impingement damage to the cat with this going on. It doesn't happen all the time and haven't figured out yet the circumstances that start it because sometimes I get a good set of secondaries dancing under the impingement plate for a couple of hours.

I've seen this before as well. My guess is that this isn't really considered flame impingement. What appears to be happening is that unburnt gases are igniting when they hit the ss grid that hangs below the cat. Kind of like the Bunsen burner shown in the image below or in the old time miners safety lamps. Gas can pass through the grid, but flame cannot. Modern gas water heaters have the same type of flame stop mechanism.

 

[WoodpileOCD]

As I learn more about this stove I'm realizing that I don't have the low end air control that I need even with everything closed down all the way. I'm going to have to start thinking about some mods to the primary air plates underneath.

You can probably tighten up the slots that the primary control tabs slide in with a few light hammer taps. Don't forget that there are secondary air inlets up high behind the cat housing. When the firebox is hot, with the primary controls closed, you'll occasionally see jets of flame coming out of the secondary inlet holes. If you have excessive draft, you might need to reduce the size of the opening of the secondary inlets. On our model 80, those inlets begin in the bottom left and right corners in the front of the stove. There are removable corner pieces that will reveal the channel iron that runs up the side of the firebox to the secondary inlet angle iron.

The cellular structure in your photo is not the bottom of the catalyst. It is a stainless steel grid which is designed to breakup flame before it gets to the cat.

Thanks. I had no idea there were any secondary air inlets on this stove. The manual is pretty poor in any description of the air flow in the stove and I've not been able to find any other source that describes the stove in any detail. So if I take the edge panels off I should be able to see the secondary inlets? I've seen posted here where people talk about their primary openings staying 1/4" or more open even when in the fully closed position but I've looked at the primary plates and they seem to close completely so maybe the secondaries are where I need to direct my attention. Thanks again.

What are your thoughts on the flames you see just under the cat/screen as far as burning any length of time? I have always thought of flame impingement as being the primary flames going directly into the cat.

Edit: posted this before I saw your last post.

 

[HotCoals]

Interesting..
I didn't know that some cat stoves have secondary air.
My BK doesn't.

I'd be surprised if all cat stoves don't have some dedicated way of getting air that hasn't gone through the primary inlets introduced near the catalyst. If there isn't enough air from the primary supply to support flames, then there probably wouldn't be enough to support the combustion within the cat.

Mine gets it from the primary only,which is at the top corners of the door opening...right near the cat.
Maybe that's part of the magic,I dunno.

 

[pgmr]

So if I take the edge panels off I should be able to see the secondary inlets? I've seen posted here where people talk about their primary openings staying 1/4" or more open even when in the fully closed position but I've looked at the primary plates and they seem to close completely so maybe the secondaries are where I need to direct my attention.

On our stove, there is quite a bit of slop in the primary control tabs that slide over the inlets. Even though they are completely covering the hole, a good bit of air can get around the edges of the tab. I'm sure some of that is engineered into the design and if you are not getting huge amounts of flame during a low burn, then I'd say you're probably not really over drafting. You might be cutting the primary air too late in the cycle (firebox too hot) and too quickly (snuffing flames and getting huge amounts of smoke, causing cat to overheat).

After thinking about it a bit more, if you reduce the amount of air entering the secondary, you risk not giving the cat enough air to completely burn the smoke passing through it. You might get reduced temps in the cat, but more smoke pass through. It certainly wouldn't hurt to experiment a bit. You could try a bit of foil tape on the inlet openings of the secondaries and just see what happens. They are probably in a similar spot on your 91 if you've got removable panels in the front corners.

I'd start by cutting back the primary air sooner, but not as quickly and see if that reduces your cat temps.

 

[pgmr]

Interesting..
I didn't know that some cat stoves have secondary air.
My BK doesn't.

I'd be surprised if all cat stoves don't have some dedicated way of getting air that hasn't gone through the primary inlets introduced near the catalyst. If there isn't enough air from the primary supply to support flames, then there probably wouldn't be enough to support the combustion within the cat.

Mine gets it from the primary only,which is at the top corners of the door opening...right near the cat.
Maybe that's part of the magic,I dunno.

Could be, though it sounds like they are supplying air that's not part of the airwash primary supply directly to the cats. Maybe BKVP will see this and chime in.

 

[HotCoals]

Interesting..
I didn't know that some cat stoves have secondary air.
My BK doesn't.

I'd be surprised if all cat stoves don't have some dedicated way of getting air that hasn't gone through the primary inlets introduced near the catalyst. If there isn't enough air from the primary supply to support flames, then there probably wouldn't be enough to support the combustion within the cat.

Mine gets it from the primary only,which is at the top corners of the door opening...right near the cat.
Maybe that's part of the magic,I dunno.

Could be, though it sounds like they are supplying air that's not part of the airwash primary supply directly to the cats. Maybe BKVP will see this and chime in.

Well..I just see the two primary inlets ..and I have looked and looked before..and like I said..they are really close to the cat.
Hope he see this.

 

[jharkin]

Wow I don't think I ever inspired a thread before

Anyway, the idea of the pre-cat secondary burn was not mine, another member pointed that out when I described my symptoms. I wishI could recall who and try to get them in here.... Anyway, I'm not sure about your exact stove, but I know in my case this is exactly what I see in these overfire situations. The Encore, like all VC has the cat in the back of the stove in a ceramic refractory chamber. Flue gasses enter through a cast iron hood above the fireback. When this overfire happens you will see that hood glowing red, flames shooting out from INSIDE the hood (not flames from the box going in) and a load roaring noise.

VCs are complicated stoves. Sure they have a bad rep from the years of quality problems, but even when you get one working right they are complicated and that I think puts people off them. Ive been accused of being an engineer (guilty) so I like the challenge but its certainly a challenge. Anyway, these stoves actually have 3 air intakes:

- The primary airwash, which is bimetallic thermostatically controlled like a BK. This will close completely when you turn the air control to low
- An auxiliary air in the ashpan (2 1/4in holes). This is fixed and provides your minimum epa required air when the primary is shut. It also helps to feed air below the bed for burning down the coals.
- A secondary air that feeds the cat. This is also bimetallic thermostatically controlled by a probe inside the catalyst chamber, and feeds air through preheat channels around the cat box and empties directly into the cat intake hood.

I've had 2 challenges with this stove - both of which are happening a lot less often now but not gone completely. the first is theis overfire we are discussing, and the other is occasional sluggish light offs on big loads. Ive learned to avoid both most of the time buit not always, which tells me I still don't know exactly what causes them. This overfire is not strictly limited to a big smokey load, as for me it actually is more likely to happen on a medium load of say 4 splits. But not always. The other day I put in a load of chunks and uglies on a very cold day which you would expect to burn hot and fast.. cat ran up to 1700 but then stabilized and came down no problem. But one evening a few weeks back in mild weather on a 3 split load I had this issue of the shooting flames and glowing metal start up at only 1650 cat temp. Note that Im burning 2-3 year old wood here...

I almost wonder if something else is going on. Maybe somehow a combination of a lot of air in the main box (small load) and a sluggish draft maybe causes some build up that overwhelms the cat? Or as others suspect its not the load or the draft but rather closing down the primary too fast that is setting up this conditions. That might explain the behavior above as I'm running hotter and not turning the stove down as fast on cold days.