Split size trade-offs

[firecracker_77]

This will come as no surprise to anyone that a few big chunks will burn much longer than a firebox stuffed with smaller thinner splits. I can leave a fire with large splits unattended for hours and come back to a nice coal bed which easily reignites anything laid on top of it. My question is...am I getting more heat out of a few large pieces burning for 6 hours or a dozen or more small splits burning hot and cleanly for a shorter period of time. In theory, equal wood mass yields the same BTU's when burned. However, I'm wondering do you get more realizable heat out of a few large splits burned for a longer period or some quicker fires with the same larger pieces split a number of times?

 

[SolarAndWood]

I find I can get a lot more mass in the firebox with big splits. Between the added mass and the more consistent burn rate, I would only burn the big stuff given the option. And, if that were an option, I would then only burn big splits of Hedge, Locust, Hickory, Oak, etc.

 

[Swedishchef]

I believe it comes down to the principle of combustion. The more splits you have, usually, the more air space as well. Therefore, the wood will tend to burn faster as the combustion can occur from various "sides" and places within the mass of wood. Kinda like burning the candle at both ends. Whereas large splits are much bigger and have less air gaps enabling a more controlled/slow burning process.

And I could be completely wrong...after all, I didn't sleep much last night.

Andrew

 

[leeave96]

With my Engander, I'm finding a new wrinkle on split size vs my Keystone (a smaller stove). The Keystone splits are 16 to 18 inches long and about 5 inches diameter max. They are easy to split (but require more wacks to get there with my Fiskers X27 - WHICH I LOVE!), easy for me, my Wife and boys to handle. I cut and split some longer splits and for the Englander and have a few very large diameter pieces to try out. Longer/larger diameter splits are harder to split and heavier to load onto the stove - but burn for a long time.

Bill

 

[DBoon]

I think the answer has the most to do with heat transfer rate of the stove. Though this would be impossible to calculate or estimate, my hunch is that the same mass of wood in smaller splits will burn less efficiently due to a hotter, faster burn. As the internal stove temp goes up, the efficiency of heat transfer to the outside of the stove will go down, and the extra heat will go up the flue pipe instead. You could compensate for this some, I would guess, by turning up the blower fan on the stove to move the heat away from the stove faster and increase the heat transfer rate of the stove.

This assumes that large splits have equal moisture content as the small splits.

I often burn smaller split loads in the morning when I want a quick burst of heat to warm up my house (and I turn the blower to maximum) and then I settle it down later in the morning with a load of bigger splits.

 

[firecracker_77]

I think the answer has the most to do with heat transfer rate of the stove. Though this would be impossible to calculate or estimate, my hunch is that the same mass of wood in smaller splits will burn less efficiently due to a hotter, faster burn. As the internal stove temp goes up, the efficiency of heat transfer to the outside of the stove will go down, and the extra heat will go up the flue pipe instead. You could compensate for this some, I would guess, by turning up the blower fan on the stove to move the heat away from the stove faster and increase the heat transfer rate of the stove.

This assumes that large splits have equal moisture content as the small splits.

I often burn smaller split loads in the morning when I want a quick burst of heat to warm up my house (and I turn the blower to maximum) and then I settle it down later in the morning with a load of bigger splits.

This makes sense to me. If the fire is burning real hot on small splits for a shorter time, it probably doesn't transfer as much heat as a slow burn. Either way, I agree...smaller pieces in the early part of the day to get a nice coal bed...then some larger chunks in the evening.

I would guess that this largely depends on the stove too. The forum users have so many different kinds...every answer will be somewhat different. I don't have a blower for example, but if you do...that could drastically impact your experience. It seems like Lopi, Blaze King, Jotul, Englander, VC, Woodstock, and Hearthstone are the most common. I wish I had a chance to live with each one of these for a season.

 

[SolarAndWood]

I wish I had a chance to live with each one of these for a season.

Yep in a passive solar house on the south side of a mountain in the middle of mature hardwoods with plenty of winter recreation available in the immediate vicinity and no financial responsibilities :lol:

 

[Wood Duck]

You have to think about the energy balance in the stove to figure out if big or small splits are more efficient, and there are several steps in the calculation. By energy balance I mean how muh energy goes in (in the form of wood) and where does that energy come out? First, assuming you have the same weight of wood and the same moisture content, the two loads (big splits vs. small splits) will have the same amount of stored energy that could be released as heat. If you burn both loads with enough oxygen you should have the same efficiency and you will convert that potential energy into the same amount of heat energy, although you may convert it faster with the small splits than with the larger splits. Where does the energy go? Some goes up the flue and the rest stays in the stove/house. With big splits you have a lower air flow rate up the flue and perhaps a lower gas temperature but the flow continues for a longer period of time. Multiply flue gas temperature, time, flow rate, and some constants like the heat capacity of the flue gases and you have the amount of BTUs lost up the flue. I am not sure if a short hot fire loses more or less than a long, less hot fire. A short fire may have higher temperature and higher flow rate, but maybe the shorter time offsets the higher temperature. Any heat generated in the fire that does not go up the flue is transferred to the house - there is nowhere else for it to go. Either the stove gets real hot and transfers heat to the house faster or it heats up less and transfers heat slower. In either case all the heat is retained in either the stove or the house, and since the stove is in the house it really doesn't matter which.

 

[woodchip]

I wish I had a chance to live with each one of these for a season.

Yep in a passive solar house on the south side of a mountain in the middle of mature hardwoods with plenty of winter recreation available in the immediate vicinity and no financial responsibilities :lol:

I'd settle for passive solar on the south side of a small hill at a push...... ;-)

But back to the question.

Which is similar to another question which I have been thinking about........

Different stoves are probably designed to burn different size splits at maximum efficiency.

And when stoves are rated at, say, 70% or 75%, or 77% efficiency, what burn rate are these ratings set at?

Big splits flat out? Small splits burned slowly? Or a mix specially made up for that firebox burned as carefully as possible.......

And my other thought on different split sizes has to be what type of wood the splits are, oak, ash, birch, cherry etc.

That could be worth an experiment in itself (bet someone somewhere has already done that)..........

 

[jimbom]

Jay Shelton in his Solid Fuels Encyclopedia, figure 14-5, page 217: Ten percent increase in overall efficiency of an air tight stove using 15% moisture content wood burning 6.5 pound splits vs 1.6 pound splits. This apparently was from lab tests conducted at his company. So big splits are more efficient in his set up.

At home a person could weigh 20 pounds of small splits and 20 of large splits from the same tree. Seasoned to the same moisture. Then regulate the stove to the same surface temperature for a cycle of each batch. Record time versus flue gas temperature and stove top temperature for identical exterior weather and indoor conditions. Might give an idea of how a stove works with certain wood. Might be hard to pick out ten percent in a home based experiment.

I fool around all the time with different procedures to see what works best for my stove, house, chimney, wood, etc. I am getting better at running the stove at lower and lower outputs and still keeping the cat going. That is my goal. I need low output on many days and want to get as close to the edge as possible.