I agree with everything you said here. What I’m not understanding is I’m burning from the exact same stock as last year, birch, a bit of popple, hard and silver maple, burr oak, but mostly oak and birch, and getting significantly less time in similar weather conditions. I feel I’m doing something different but can’t figure it out.
2021-2022 BK everything thread
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Go outside during the long burn while the meter indicates active and if the chimney is not smoking then the cat is working. Is your house overheating? The stove is just a fuel tank to feed the cat. If your fuel is dry but not very dense then it’s hard to pack enough fuel in for a 24 hour burn. It takes tightly loaded large splits.
Alpine1
Feeling the Heat
Large splits do make a difference in burn times when “riding the hole”. I burn mostly Norway spruce, but with big chunks and some tetris skills I can get 24 hrs out of a full load. Note that with 5 or 6 loaded N/S splits my Ashford is filled to the gills.
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I have been puzzled about the occasional remarks about the effect of the split size.
IF the firebox is a fuel tank, then the split size should *not* matter - it would only matter to the effect that they have on the "filling fraction" of the firebox. I.e. how much airgaps remain filling with large splits (fewer but larger gaps) versus smaller splits (more but smaller gaps).
The burn rate of a large split should *not* matter because the burn rate is governed by the thermostat: how much heat is produced is what determines the air flow. Meaning that for large splits the rate at which the burn- (or smolder)-front progresses into the wood will be larger because the gases produced are being produced at less surface area, while for smaller splits that burn-front progression will be smaller if more surface area is smoldering simultaneously.
I've been wondering about this for a bit - I can split large and have it dry longer. Or I can split small and have it dry quicker. As long as I get the same number of pounds of wood in the firebox it won't make a difference in burn time.
The large split ->longer burn time DOES make sense for non-Tstat (and non-cat?) stoves where controlling the burn rate is harder. Having less surface area accessible to combustion in the firebox will slow the burn, making it longer. But the Tstat in our BKs overrules all that.
The Tstat sets the "pounds per hour weight loss" rate (b/c each pound gives off a certain amount of heat when running at a certain efficiency), and it does not give a chit about the physical shape of that fuel...
So, is your experience truly that large split give you a longer burn time *at the same Tstat setting*?? If so, can some of the wizards here explain to me what the mechanism is there? Because to me it does not add up.
IF the firebox is a fuel tank, then the split size should *not* matter - it would only matter to the effect that they have on the "filling fraction" of the firebox. I.e. how much airgaps remain filling with large splits (fewer but larger gaps) versus smaller splits (more but smaller gaps).
The burn rate of a large split should *not* matter because the burn rate is governed by the thermostat: how much heat is produced is what determines the air flow. Meaning that for large splits the rate at which the burn- (or smolder)-front progresses into the wood will be larger because the gases produced are being produced at less surface area, while for smaller splits that burn-front progression will be smaller if more surface area is smoldering simultaneously.
I've been wondering about this for a bit - I can split large and have it dry longer. Or I can split small and have it dry quicker. As long as I get the same number of pounds of wood in the firebox it won't make a difference in burn time.
The large split ->longer burn time DOES make sense for non-Tstat (and non-cat?) stoves where controlling the burn rate is harder. Having less surface area accessible to combustion in the firebox will slow the burn, making it longer. But the Tstat in our BKs overrules all that.
The Tstat sets the "pounds per hour weight loss" rate (b/c each pound gives off a certain amount of heat when running at a certain efficiency), and it does not give a chit about the physical shape of that fuel...
So, is your experience truly that large split give you a longer burn time *at the same Tstat setting*?? If so, can some of the wizards here explain to me what the mechanism is there? Because to me it does not add up.
You're entirely correct, and that here is exactly the point. You can weigh it out if you want, but with smaller splits the sum of the air gaps is definitively larger than with large splits. It may not look like it, but it it is.
I always notice that when I bring firewood in. I have a plastic bucket for that, which holds roughly the amount that goes into one stove load. With bigger splits, the bucket is noticeably heavier than with small ones.
My thoughts... though maybe I'm nuts!
In addition to the lbs per fuel charge being higher without the air gaps, the other big reason is surface area. Wood gasses out from all surfaces and there's a lot more surface with small splits. Isn't that why we grind coffee beans?
Imagine filling the firebox with kindling stacked to allow each split to be exposed to firebox air, the mixture becomes very fuel rich from all of the rapid outgassing. The cat does its best to combust it all with low oxygen levels since the thermostat will quickly shut down but remember, the stat doesn't ever close. The hole in the intake flapper passes more or less air depending on the pressure in the fuel tank which will be very low during the rapid outgassing of the kindling load. The same number of lbs of fuel in four splits won't fill the firebox and will be slower to release fuel for slower combustion.
In real life, I have a hard time with just huge splits and need a mixture of some smaller ones to really get a heavy load of fuel in.
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After using both, I'm very happy with the ceramic. Not just money savings but useful life and resistance to clogs. I think metal cats mostly help dealers by not cracking, breaking, crumbling so are less likely to generate complaints. I may have had a crack or two in my first ceramic cat but my last couple have provided their full life with no physical degradation.
The good news is that you have a choice.
I think my cat was a bit plugged on the back side as high beam suggested, can’t tell for sure, but got a lot of big puffs of ash as I moved the air thru the cat. It did perform better last night after blowing it out, it was in the active zone in the morning at the 3:00 position, it had not been active the previous mornings with similar amount and type of wood. Stuffed the stove quite well at 9, if it performs like last year I’ll make it till morning. I’m optimistic.
I’m going to order a ceramic cat as a backup. Midwest hearth has some for the princess. They have the expanding gasket included already on the cat, so I should be good to go??
Ok, reloaded at 9:00 with a full firebox. It’s 3.25 hours later and here is what’s coming out of my chimney and what the cat looks like.
![[Hearth.com] 2021-2022 BK everything thread](https://www.hearth.com/talk/data/attachments/284/284618-37dc2a80289b97fddbfdcb22b2d66bbc.jpg?hash=X4npQTFlAb "[Hearth.com] 2021-2022 BK everything thread")
Chimney
![[Hearth.com] 2021-2022 BK everything thread](https://www.hearth.com/talk/data/attachments/284/284619-048b5df4387f82885a776c37a8c5391f.jpg?hash=UtGoEKSGOl "[Hearth.com] 2021-2022 BK everything thread")
I feel like what is coming out of chimney is steam, maybe.
I have bought the last few from midwest hearth on amazon. They are the same cat that BK uses, come prewrapped with gasket, are cheap, and super easy to just pop right in. I have a spare cat on the shelf for when my current cat wears out. This is the first time I bought a cat well before needing it but seeing the national supply issues and witnessing some woodstock guys unable to get new cats I don't want to get caught with my pants down.
Your smoke/steam emission after 3 hours burning looks same as mine. I've been wondering if my cat is working up to snuff. I really can't tell if it's steam or smoke. But then I'm a worrier.
If one expects the cat to reach end-of-life within a year or so, that probably makes sense. My cat likely has less than 1000h on it by now, so for me it makes little sense to buy a replacement cat now.
To me this looks like steam. Though I can't truly see far enough away from the cap in this pic. The point is that steam dissolves (like your breath when it's cold). Smoke dilutes. Smoke also is (often, not always...) bluer than steam.
But the bottomline is that if your cat remains active, it's burning smoke and doing its job.
For cold starts I lean to smaller splits. Even with the loading door closed the stove comes up to temperature more quickly so I can engage the combustor more quickly and get to my regulated clean plume in the legal time frame.
For long burns on hot coals I put in the biggest splits I can find, and then fill in the gaps with smalls.
I _think_ I am getting more pounds of fuel in the firebox using the biggest possible splits. Imagine one enormous piece of wood carefully trimmed with wood working tools to just barely fit through the door and belly flop onto a bed of hot coals. How much would it weigh?
Someone with more free time than me and a scale to weigh their fuel charges is welcome to take this on.
For long burns on hot coals I put in the biggest splits I can find, and then fill in the gaps with smalls.
I _think_ I am getting more pounds of fuel in the firebox using the biggest possible splits. Imagine one enormous piece of wood carefully trimmed with wood working tools to just barely fit through the door and belly flop onto a bed of hot coals. How much would it weigh?
Someone with more free time than me and a scale to weigh their fuel charges is welcome to take this on.
I am not convinced by this argument. This may hold for the initial (half) hour of a load when easy (light, see oil refining) gases that are/form near the surface come out. But after that initial off gassing is done, the rest of the gases over the lifetime of the wood in the stove are produced by the heat produced in the firebox (wood needs heat to decompose, see gasification - also burning wood is decomposing wood -> gas -> flames). And it is precisely the temperature (needed for gasification) that is controlled by the Tstat.
So yes, "kindling" may provide a bit of a runaway in the first (half hour?), but once that initial episode is over, i.e. for the longest time of a normal stove load burn, it should not matter what the size of the splits is.
... Is my thinking ... And I've been told many a time that I'm nuts.
I agree, small splits -> quick start.
I agree small+big splits -> less gaps. That was my argument: split size does not matter (contrary to common belief), other than thru the volume (better: weight) one can stuff in the box.
Good news is that you can experiment with this issue. It’s easier to split and stack big as well so time savings at the fuel processing stage.
Yes, I have (split big). However, given this humid summer I've been wondering if that was smart. If things dry about 1" per season, and I have 3 seasons before burning, then I need to not be too big. I have one bay (for '23-'24, s-s-c this spring) of only red oak and black locust 5-6" splits. Wondering whether it will be dry enough, given that I have 3 seasons split red oak (that was dead standing) that is barely 20% now... Splitting smaller would help getting it ready.
My experimental feedback loop is rather long (3 years), so hence me asking here.
Not sure where you got that from, but to my knowledge wood mostly (!) dries from the cut sides. Meaning the thickness of the round is not that important. Makes sense to me as the fibers are likely to transport most of the water, and they run along the length of the round.
True. But (...), split a piece and measure in the middle of the length (not at the cut ends) and take measurements from the outside there to the center (i.e. crossing half of the short axis of the freshly split surface), and you'll see what I mean. Going from the edges in things dry out. I.e. measure along the "post" of the T in the "post reply" button below.
Yes at the end grain it dries out more, but for an 18" split, having a 2" path to the surface across the grain may be easier than having a lower-'resistance' path 9" to the end grain surface.
But this discussion is more "woodshed" than BK.
My question still remains: what have people experienced in a BK re: small splits versus large splits.
Any indication there is anything else than "better stuffing" with large splits? (tho for stuffing a volume with spheres there is zero difference between small and large spheres... )
But it's not. Just split your current dry wood down and see if it makes a difference.
This is a BK specific issue due to the thermostat and the catalyst that does not have its own air feed. I'm not sure there are any other cat stoves without an air feed to the cat. Even the woodstocks use either full hybrid or a smaller dedicated air hole for the cat.
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