Catalytic Combustor Question

[4-8-4 Northern]

I have a stove with a catalytic combustor (Woodstock Fireview), and as I understand it, the combustor will only ignite if the temperature inside the stove is above 500 degrees. Last winter was the first year we burned wood. We we would engage the catalytic combustor during the evening hours when the inside temperature hit 500 degrees, and then go to bed. By morning, the temperature inside the stove sometimes dropped below 500 degrees, but of course, the catalytic combustor was still engaged. My question is this: Is any damage being done to the combustor by the smoke or other particulates passing through it when the temperature is lower than 500 degrees? Should I be getting up in the middle of the night and checking the stove temperature, bypassing the combustor if the temp is below 500?

 

[Rhone]

No damage, the Cat is self-sustaining once it's started and there's gases to burn. As the cat works, it creates heat which keeps it hot and burning the secondary gases even if your stove drops to low temperatures.

You need to worry about start-up temperatures, and on reloading disengage the cat for a bit because as Mo Heat explains the thermal shock to the cat going from burning dry high temperature gases to getting a mouth full of cool moisture rich gases from a fresh reload can be thermal shock and cause the cat to pop.

 

[DavidV]

AS I understand it the temperature must be that hight to start the smoke burning, but it continues to burn long after the tempurature comes down below that level. I havn't had any problems with mine as a result of operating it just as you have described. And for the most part any residue build up on the cats will get "burned off when you get the temps up to firing range. IF you are burning wet wood, etc you may end up gumming up the cats and have to clean them. That's a pain but really no big deal, just a dirty job that takes several hours.

 

[elkimmeg]

This is a good question We engage the cat at 500 surface temp but the interior fire box is a lot hotter. What happens is the combuster ignites and it continues to light till the interior fire box temp drops below 500 degrees. Usually all the volitals are burned off and the final stages of the burn cycle (coals) emits few if any particulates that would clogg the combuster.
However this is the time of year that combustor should be taken out and cleanned then you can determine if you opperated the stove correctly using dry wood. Combustors work effeciently when clean I clean them at the start of the season and at mid season

 

[cbrodsky]

We we would engage the catalytic combustor during the evening hours when the inside temperature hit 500 degrees, and then go to bed. By morning, the temperature inside the stove sometimes dropped below 500 degrees, but of course, the catalytic combustor was still engaged.

That's why I love this stove - "sometimes" it's not still raging hot in the morning and is a balmy 350-400 degrees instead of 500 degrees.

You are perfectly fine - the surface temperature on a Fireview only needs to hit 350 to engage the cat initially - due to the damped heat transfer with the soapstone, 350 on the surface means it's much much hotter inside. (as opposed to steel/iron where you'd want to see a higher surface temperature to be sure you can engage the cat)

Towards the latter half of your burn cycle, all the volatile wood gases that were being burned by the catalyst are long gone so even if it cools down to room temperature with the cat engaged, no harm is done. Just bypass and get the surface temperature back up over 350 if it's not that hot when you reload - this will keep it running at peak efficiency and ensure the gases in the new wood reload will burn nicely.

-Colin

 

[wg_bent]

so when a CAT stove is running with the damper open...isn't it essentially working like an old tech wood stove and pretty inefficient/polluting? I'm not intending to be critical of CAT stoves...I'm truely wondering.

 

[cbrodsky]

so when a CAT stove is running with the damper open...isn't it essentially working like an old tech wood stove and pretty inefficient/polluting? I'm not intending to be critical of CAT stoves...I'm truely wondering.

Yes and no... you are burning it really really hot with max air for that initial phase, so much like a fireplace, you probably get better combustion than a damped down old stove. I would assume it's basically same as a non-cat trying to reach secondary burn. I'm sure they are now designed to go through that phase a little cleaner than the old technologies, but certainly you do see smoke when first starting, while once up to temp and in cat mode, there is nothing to see.

In general, this is one of the reasons I think wood burning is so much better for the environment when you run it 24x7 keeping it above cat or secondary burn temps. This was part of the reason we went towards soapstone / cat / large firebox combo - optimum for extending burn temps while keeping in a clean operation mode since we use it as a major heat source. (as opposed to steel/iron that spikes up faster for quick heat, but falls off a lot faster too, requiring more cold starts - that is probably much more appropriate if you are using it in a vacation cabin or somewhere that requires really fast heat output from cold starts)

-Colin

 

[Roospike]

#(*&$

 

[Rhone]

I think what NY Soapstone is saying is that the properties firebrick is designed not to have storage, rather designed to reflect the heat. Firebrick is significantly worse then even normal brick in terms of storing btu's and whose purpose is to reflect the heat back into the fire and into the living area ASAP. Soapstone is the contrary, holding 11.5x - 13x the btu's over firebrick it's like a battery.

Then, we get into the specialties. NY Soaptone, Todd, other fireview owners are sort of in a unique situation with their soapstone. Most Soapstone stoves are single layers of half-brick, the Woodstock Fireview is a double layer. Now, I can't really fathom what kind of heat storage they have but it's a lot. If you figure a Fireview vs. a summit in just the first layer the fireview has probably 2x the soapstone than the summit has firebrick and theirs holds 11.5x-13x of the heat. That first layer I'm guessing puts it at 22-26x the heat storage over the summit. Then, we need to go into the second layer of the fireview's soaptone doubling that number at least to 44-52x the heat storage. Anyway, not sure how accurate that is but I'm taking a guess a fireview's heat storage is at a minimum 44x that of the summit.

There's penalties to having that as well, and NY soapstone mentions. There's so much heat storage it's not possible for it to quickly heat a place and soapstone units particularly problematic in spring & fall to remove the chill out of the morning, or coming home to a cold house the term quickly heating is certainly not part of their specialty.

 

[Roospike]

$&#*

 

[Rhone]

I'm saying that as well.

When you said "and can burn a load 8-10 hours with a few more hours of heat after that." where is this few more hours of heat after that coming from?

 

[elkimmeg]

Roo One comment Hot Flame did the physics / math last year and calculated the amount of heat per stove temp rise. A summary was a stove emits 100% more heat at 600
degree surface temp, than 450. I know it's not peak heating season, But I think you are running your stove at lo low temps. effeciency is not really achieved at 350

 

[Roospike]

%&#*

 

[Roospike]

&$*@

 

[Roospike]

%&#^

 

[cbrodsky]

O' ................. Rhonemas , Elk ..............

I think the confusion is coming from the stove ( Summit ) itself.
When some says "steel plate stove" and put them all in one group it changes the facts.
Not all steel plate stoves are the same.
The common steel plate stove is 3/16 thick with 1/4" steel top.
Some are 1/4" with a 1/4" steel top.

The Summit is 1/4" steel body with a massive 3/8" thick steel top.

Theres your difference.

Anybody that know about heat transfer / mass and thickness of steel knows that at a set BTU to a 3/16" plate of steel vs a 3/8" thick plate steel is going to heat different , get to a different temp and store heat different.

Its the same thing with Soap stone as when you have XX amount of soapstone vs a larger amount of soapstone your going to get different heat and different heat holding temperatures.

You guys have any idea how much heat i would be putting into my house if i ran the stove top temp at 550° !?!?!!?
That why the stove is rated at 97,000 BTUs at a 3.0 cf fire box and there are other stoves with a 3.0 cf fire box that are rated at 55,000 - 70,000 BTUs . The MASS of the STEEL is the missing key with the comparison of steel stoves.

When you said "and can burn a load 8-10 hours with a few more hours of heat after that." where is this few more hours of heat after that coming from?

When the fire stops burning the mass of this steel stove stays hot of many hours and thats where the extra heat is coming from.
Also note Rhonemas when your were stating storing heat in the fire bricks is what threw me off , i had no idea what you were talking about because thats not where the heat is stored like soapstone.

I think it's well established on hearth.com that a manufacturer's "BTU rating" is fairly useless. That being said, I absolutely can believe that a steel stove with the same firebox as a stone stove will indeed have a higher max BTU rating. This is because the thermal conductivity of the steel stove is so much higher. Hey, I want to keep my house hot - sounds great, so what's the catch?

That high thermal conductivity means you have to burn your wood at a faster rate to achieve the same tempearture inside the stove at the fire, all other attributes of the stove being equal. Steel/iron stoves readily conduct heat out of the fire, forcing a higher burn rate. But as noted above, this mode of operation, when oversized, can quickly overpower you and drive the user out of the room. So you damper down trying to force a longer burn rate, but when your surface temperature drops, the interior temperature drops right along with it, and before you know it, you have soot formation on the glass and your burning is not as clean.

Stone stoves greatly reduce this rate of heat transfer, lengthening the burn cycle, due to the lower thermal conductivity of the stone. Particularly when applied in an air-gap configuration as Woodstock does. (I assume others do this too?)

In numbers:

Soapstone: 45 Btu-in/hr-ft^2-degF
Cast iron: 520 BTU-in/hr-ft^2-degF

This is common sense... metal heats up faster, and conducts heat faster, so it's instant gratification as the fire heats up. But it all balances out in the end - to keep that steel stove hot, you have to burn the fire faster to keep up with the heat loss. Adding a fraction of an inch to the plate thickness doesn't appreciably change conductivity. On the other hand, the stone stove will burn longer, and will maintain a higher BTU rate later in the cycle as the burn cycle is stretched over a longer period.

In some applications, that is in fact the goal - in others, it may not be consistent with the goal.

Much is said about the thermal storage, but soapstone really only holds about 50% more heat than cast iron on a lb-for-lb basis. I also see soapstone frequently mis-identified as having "high thermal conductivity." Higher than some stones, yes, but much much lower than metal.

The biggest factor for long heat delivery time with soapstone is the lower conduction rate lets you run a clean fire much at a lower wood burn rate than a steel stove. The drawback is that you cannot feel instant heat from a cold start, which is why for spot heating, soapstone is not a great choice. Same goes for applications where reload frequency is not a concern and you require maximum heat output - in those cases, steel/iron would be better. But for 24x7 burners who want intermediate heat output with very long load intervals, it is an optimal solution.

Roospike is biased to the point of absurdity - a PE summit is not at all the right choice for a significant number of woodstove applications, yet I see nothing but defensiveness when shortcomings are discussed and rampant cheerleading behind it as the holy grail of all stoves. Great stove for some situations, but far from ideal for all.

Hopefully this discussion illustrates the pros/cons in a relatively unbiased fashion - I have no problem being forthcoming with the limitations of a soapstone application, and wouldn't recommend it for a casual woodburner.

-Colin

ps - for those highly technical types, I realize I did not get into the last part of the energy balance in regards to radiation efficiencies off the respective surfaces, but I think the point still stands...

 

[Roospike]

$*&#

 

[Roospike]

%*#&

 

[Roospike]

note the glass.

 

[tradergordo]

Couldn't ANY stove become a "soapstone" stove just by placing soapstone on (or around) it? Maybe this is an option for someone that wants the qualities of iron/steel sometimes and soapstone other times?

 

[Roospike]

%*#

 

[BrotherBart]

Couldn't ANY stove become a "soapstone" stove just by placing soapstone on (or around) it? Maybe this is an option for someone that wants the qualities of iron/steel sometimes and soapstone other times?

Essentially that is what Hearthstone does with the Morgan and Clydesdale inserts. They are metal stoves with the firebox lined with soapstone bricks. I think ya could do that with any stove. Some will say that other stoves weren't certified with soapstone in them but I contend that they weren't certified with red oak burning in them either.

 

[suematteva]

The phoenix from hearthstone is freestanding stove half cast/half soap...believe woodstock has a model or two with both materials also.

 

[MrGriz]

They are metal stoves with the firebox lined with soapstone bricks. I think ya could do that with any stove. Some will say that other stoves weren’t certified with soapstone in them but I contend that they weren’t certified with red oak burning in them either.

Does that mean that you could replace the firebrick in an insert with soapstone so it will hold more residual heat?

 

[cbrodsky]

Couldn't ANY stove become a "soapstone" stove just by placing soapstone on (or around) it? Maybe this is an option for someone that wants the qualities of iron/steel sometimes and soapstone other times?

Absolutely - the marketing behind soapstone sometimes overbills its "magical" qualities - just basic sound engineering principles at work

That being said, I personally wouldn't try any such modification that would seal up the stove enough to significantly alter the heat transfer rate unless I could monitor the internal firebox temperatures to ensure you weren't taking the concept too far and causing a meltdown... not exactly something for a homeowner to try as I'm sure Elk and others will chime in!

In another thread, we discussed the idea of trying to achieve similar results by having a large non-combustible thermal mass relatively close to the stove to basically soak up heat from a really high temperature burn. In that case, you don't have to slow the burn down to stay comfortable - you can avoid dumping all that heat right into your living space immediately by letting some of it soak into a massive hearth and stove surround, and then that mass would radiate to smooth out your overall heat cycle. Main downside in that solution is you have to build a new fire each time vs. stretching out the burn cycle itself.

-Colin