Flue temp vs. Stove top temp, and cat light-off

[Machria]

You've clearly not burned a downdraft stove! If I engage too early, the stove stalls. Cat will hit somewhere between 350 - 500F, hang there a while, and then start to fall. In the absolute extreme case (not common), I will come home to a cold stove full of half-charred wood, and a smelly house. I've only done that once in two years, but other downdraft cat owners have reported similar problems.

No I have not. Not even sure what a "downdraft stove" is. But if a flame-out occurs that easily, sounds like a poor design to me. Are you speaking of the Firelight 12?

 

[Ashful]

No I have not. Not even sure what a "downdraft stove" is. But if a flame-out occurs that easily, sounds like a poor design to me. Are you speaking of the Firelight 12?

Yep... Firelight 12... claimed to be the "best stove I ever owned," by many long-time forum members, including this guy:

I'm a "Woodstock guy", too and you're spot on. I need to clean the ash from my Progress' pre-cat screen regularly. I let it go too long last month and it was almost completely clogged. OTOH, my 12 has never accumulated any ash in the cat. I only inspect it once a year, after the burning season, and have never had any clogging. The Jotul cat is also easier to access/change. The Jotul 12 is the best stove I've ever burned for many reasons - hopefully this one will last long enough for Jotul to get back into the cat stove business! I remember your search for your 2nd 12 and recall thinking, "I understand exactly why he'd go to the ends of the earth to find this stove."

Downdraft stoves may not be the easiest to operate, and surely a large portion of my troubles stem from burning wood that is not yet properly seasoned, but they do have their advantages, too.

 

[Gark]

Our Encore is a downdraft type also, cause the cat is in back of the firebox and smoke/gas/ flame has to go upside-down to get into the cat, then goes down through the cat. We find that waiting for both griddle temps and flue temps of 500 before closing the damper makes for good cat ignition. If we engage the cat before that, it is very likely to stall. Maybe cat stoves with the combustor above the firebox have the advantage of the heat rising to the cat.

 

[Ashful]

Our Encore is a downdraft type also, cause the cat is in back of the firebox and smoke/gas/ flame has to go upside-down to get into the cat, then goes down through the cat. We find that waiting for both griddle temps and flue temps of 500 before closing the damper makes for good cat ignition. If we engage the cat before that, it is very likely to stall. Maybe cat stoves with the combustor above the firebox have the advantage of the heat rising to the cat.

Thank you. This is exactly what I was saying. Ever watch your flue temps?

 

[Waulie]

I think you're probably right on the wood issue being part of it. Downdraft stoves seem to be finicky in general from what I've read. I've never burned one. Regardless, I personally wouldn't be real comfortable continuously running my stovepipe to 600 degrees, but that's just me. I'm sure you've played with the air settings a ton. Does the manual have any guidance?

In general, most people are going to report that their cats light off at 500 degrees. For most stoves, this will correspond to about twice the reading from the exterior of the stovepipe or stovetop (depending) near the cat. In your case, it doesn't correspond to either (and the cat probe is no help) so it's going to be hard for many of us to say too much about it.

In general, I definitely agree that stovepipe temps are a much better gauge than stovetop temps. I started by using stovetop temps, but I would use a mental shift depending on where I was in the burn cycle. Now I use stovepipe temps and it's much easier. If I run my stovepipe to 300F (external) right above the stove, I know the cat must be over 500F due to the exhaust path.

It sounds like in your case you basically need to run your pipe to the limit before you can get the cat to engage. Again, I wouldn't be thrilled with that but if your cats are good and you've played with the air settings when you engage, I guess it is what it is. Drier wood should definitely help too.

 

[rideau]

Our Encore is a downdraft type also, cause the cat is in back of the firebox and smoke/gas/ flame has to go upside-down to get into the cat, then goes down through the cat. We find that waiting for both griddle temps and flue temps of 500 before closing the damper makes for good cat ignition. If we engage the cat before that, it is very likely to stall. Maybe cat stoves with the combustor above the firebox have the advantage of the heat rising to the cat.

Are you talking 500 interior or exterior measured flue temp?

Joful, I'd be concerned if it were my stove getting the flue that hot all the time. Maybe especially in your case, with that wood in your chimney....I'd be interested in knowing next year if you are able to engage the cat with a lower flue temp once your wood is dryer. Hope so.

 

[Gark]

That 500 f. for the singlewall connector pipe is measured by mag. condar thermo about 4" out of the collar. What it takes to burn without smoke from the flue is based on experience and a thousand runs outside to peek at the chimney. After the damper is closed, the flue pipe temp. drops to 300-350 where a long smokeless burn happens. Yes, I find that the flue temp. Is alot better an indicator for when to close the damper and when you're burning smokeless. With the Encore, you can have a cool flameless firebox (300) and HOT cat chamber (750) with no chimney smoke. BTW, the Encore does have a built-in cat probe of sorts- the bimetal secondary temp probe turns in proportion to the refractory box temp- just remove the cover to see it.

 

[Ashful]

Are you talking 500 interior or exterior measured flue temp?

Joful, I'd be concerned if it were my stove getting the flue that hot all the time. Maybe especially in your case, with that wood in your chimney....I'd be interested in knowing next year if you are able to engage the cat with a lower flue temp once your wood is dryer. Hope so.

500 - 600F on the exterior of the single wall, measured using an IR gun, about 6" above the collar.

No wood in this chimney! That was what one inspector/installer tried to claim, and tried to sell me a $9000 plan to fix the problem. I proved he was wrong, then made him come back, reinspect, and give me a new report confirming that what he said was wood, was really brick. I should really post those reports one day...

That 500 f. for the singlewall connector pipe is measured by mag. condar thermo about 4" out of the collar. What it takes to burn without smoke from the flue is based on experience and a thousand runs outside to peek at the chimney. After the damper is closed, the flue pipe temp. drops to 300-350 where a long smokeless burn happens. Yes, I find that the flue temp. Is alot better an indicator for when to close the damper and when you're burning smokeless. With the Encore, you can have a cool flameless firebox (300) and HOT cat chamber (750) with no chimney smoke. BTW, the Encore does have a built-in cat probe of sorts- the bimetal secondary temp probe turns in proportion to the refractory box temp- just remove the cover to see it.

Similar here. A few minutes after engaging the cat, my single wall drops down to perhaps 225 - 250F. Five or ten minutes after cat light-off, I can usually have the stove top down to 350F with the cat crusing at 1400F - 1500F. Cat usually peaks above 1700F an hour into the burn, still with stovetop closer to 400F. Ever since it got warmer last week, though... I've had nothing but trouble.

Two hours ago, I loaded both stoves, usual mix of 1 year oak and poplar. Both stoves were running 350'ish prior to loading. Took both to 500F stove top at wide open throttle, then closed throttle to 50%, and took both stoves to 550F on the stove top. Engaged both cats, and each cat probe was reading 550 - 600F within a few minutes. Left both at half throttle a half hour, hoping the cat temps would climb a little higher, and they did, but both stayed below 700. Started incrementally lowering the air after 30 minutes in cat mode, and watched the cat temps fall back down to 400'ish on both stoves. Stalled!

Went back to bypass, took the stoves back up to 550F stove top on wide open throttle, and the single wall pipes were reading 675F - 700F on the exterior, 6" above the collar. Re-engaged, and both cat probes went to 750F. Left at half throttle another 10 minutes, and one went to 900F, while the other held in the 700's. Started lowering air, and one cat is holding in the 700's, whereas the other fell quickly back down to 500F. Last night I came downstairs to check the stove an hour after seeing something similar, and found the cat probe reading only 325F!

I'm tempted to go back to bypass again to get that one back up, but I've been up way too late three nights in a row dicking around with these stupid stoves, and I need some sleep. Both stoves were running much better than this when it was cold out.

 

[Ashful]

Just checked again:

Stove 1: 450 top, 1050 cat (good)
Stove 2: 400 top, 450 cat (bad)

 

[rideau]

I guess I'm not remembering properly. I thought you posted pictures a while back with shots of wood storage boxes off the smoke shelf (maybe smoke shelf) as well as a large beam or two.....??Or maybe joist or two??

 

[ddddddden]

. . .Both stoves were running much better than this when it was cold out.

I think many of us could say the same, even if we're not having problems. Everything just gets better when it's "clear & cold" out. Draft is affected by barometric pressure as well as temp. Maybe you have draft issues as well as wood issues. A combination of the two will give any stove fits.

Insulated chimneys?

How 'bout avoiding the oak for a bit and trying a few loads of poplar?

Stove 1: 450 top, 1050 cat (good)
Stove 2: 400 top, 450 cat (bad)

Both new cats?

 

[Ashful]

Some pissed off late-night reflection brought me to a conclusion: over the course of several weeks, I've gone from burning all poplar or cherry with a piece or three of poorly seasoned oak on top, to burning all oak with a piece of poplar or cherry on the bottom. My ratio has shifted dramatically.

Because I get such great coal beds from the oak, I've been able to load hotter each morning, and I've (mostly) gotten away with this. I have definitely seen massive amounts of steam exiting the chimney a few mornings.

I'm going to start adding more poplar and cherry back to the mix, and see if that puts things back in line.

The cats are both new Condar SteelCats, both clean and holding up well, as of my last inspection. Both chimneys have blanket insulated smooth wall 6" stainless liners.

 

[ddddddden]

. . .I've gone from burning all poplar or cherry with a piece or three of poorly seasoned oak on top, to burning all oak with a piece of poplar or cherry on the bottom. My ratio has shifted dramatically.

. . .I'm going to start adding more poplar and cherry back to the mix, and see if that puts things back in line.

Sounds like a good idea to me.

With my stove, it works better to put the smaller, drier wood toward the top and front of the firebox. It will get the cat going, regardless of what wet stuff may be at the bottom/rear. My usual 12+ hour reload procedure is to scoop the coals to the middle and put a couple of shorties N-S on either side of the coals. This lets air get to the coals and all around the load.

When I started doing this, I was thinking Tunnel of Love®, but apparently that's very different. I think I will call this technique the Love Mound™. I basically always have a mound of ash & coals going in the middle of the stove. The longer I wait to reload, the more I have to dig the mound to bring the coals to the surface.
If it's really c0ld out (maybe 20% of the season here) and I'm reloading more frequently, then I have large chunky coals that will serve the same function as the N-S shorties, so I leave 'em out.

 

[Todd]

Never owned a down draft type stove but if your exterior flue temps are up around 700 that means your internal temps are about double that and your cats should easily take off at those high temps unless your burning sopping wet wood which I don't think would be able to reach those high internal temps.

Even though you have new steel cats I would be leaning more towards a cat problem than anything else. I don't trust the steel cats just yet after all the problems Woodstock had last year with them. Maybe it's time to call in the warranty on them and replace with ceramic?

 

[Ashful]

Well, this morning's reloads were very telling.

In stove 1 (short flue), I used one medium split of poplar, a semi-punky split of something soft, and a shorter half-round of 4" cherry in the bottom of the stove on the coals, followed by a few medium splits of 1 year oak, and a massive split of 1-year walnut to top it off. Sort of the opposite of the way I'd load if I had good wood, but I find I need to put the stuff that will light the easiest on the bottom, given the condition of my wood supply. It took off well, and I was able to engage the cat at 550F stove top, and the cat shot up close to 800F.

In stove 2 (tall flue), I used two big splits of poplar on the bottom, followed by a little cherry, and then a few smaller splits of my 1-year oak. It also took off nicely, going pretty quickly to 550F stovetop. I engaged the cat, watched the cat probe shoot to 1400F, and the stove top settled back down at 450F.

I gradually lowered the air on both, and while the cat probe on stove 1 did eventually fall back down to 550F before I left the house, stove 2 was holding 1450F on the cat probe. Both stovetops were in the 400's. I think I've found my problem. I was just getting too aggressive with my 1-year oak to softwood ratio. Next year will be better.

I still want to explore the flue probe vs. stovetop thermometer, though. I do think there's some merit to the thought that the flue probe may be the better indicator of when the cat is ready to light off. I'll need to get some more thermocouple probes for my stovepipes, and make time to install them.

 

[Ashful]

Two more loads tonight, being careful to maintain more than 20% softwoods under my poorly seasoned hardwoods, and it's working like a charm. Got relatively easy light-off with both cat's hitting 1000 - 1250F at stove top temps of 550 - 600F. It appears my entire problem was entirely to blame on wet wood, without enough dry stuff to compensate.