BTUs v. Heat Output

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nola mike

nola mike

Minister of Fire
Sep 13, 2010
928
Richmond/Montross, Virginia
Just curious if there's a difference in heat output between wood species. Obviously, a higher BTU wood is ultimately going to put out more heat over time, but do I get more actual BTUs/hr (needing to reload more often) from say, pine or poplar than oak? I think that moisture is probably the most important factor, and size of split may be next, but if that's the same...
 
I know there can be a big difference between species. Other people here will have more accurate info but...my short experience has shown that a stove full of Oak at bedtime lasts a bunch longer than Silver maple. All things being equal such as outside temps, wind, split sizes, moisture content, etc.
 
Yeah, but that's not what I'm talking about. Oak has a lot of BTUs and lasts a long time (thus giving off BTUs over time). So something that didn't burn as long but had equivalent BTUs would theoretically give off more heat. This would be of more interest to those of us with undersized stoves (I have 2 of them) or that need to get a house up to temp quickly from a very cold temperature (I have one of those).
 
Throw a full load of dry pine in and you better have good gaskets as it will go up quick with some air. The less dense stuff is a nice way to get quick heat first thing in the morning, when you first get home, etc.
 
From the science geek angle...


Combustion of wood is a chemical reaction that only occurs at the surface of the wood.

Very simplified equation: Wood + Oxygen --> CO2 + H2O + heat

The faster the wood reacts with oxygen, the more rapidly heat is produced (greater BTU/hr). "BTU/hr" is an expression of the reaction rate.

Things that commonly decrease this reaction rate:

1. Add less oxygen (reduce air intake)
2. Provide less surface area for this reaction to occur (stack wood closely in a less open build)
3. Decrease surface-to-volume ratio of wood (use larger wood splits)
4. Limit removal of reaction products (reduce exhaust)
 
I think good BTU wood will get the stove hot nearly as fast as a pine/poplar would. You won't have to reload as often or cut as much wood. Reduce split sizes for times when quicker heat is desired (not neccasarily attained!) and the fire will get hot more quickly. Wood stoves just don't heat fast no matter what kind of wood is in them. I would research blowers and focus on a variety of spit sizes in higher BTU species.
 
I'm not sure if this helps, but it is my understanding that all wood has the same BTU's per pound.(comparing similar quality wood) The density is where the difference is.
 
Hmmm...I'll take a stab at this. To the OP - if I am reading this correctly, your question is about the RATE of heat output, not the actual BTU's in a piece of wood.

To add to the geek post above, The opposite of many of the points made is true as well. More surface area will increase the RATE that the btu's are given. Species can also affect the RATE. Much of it has to do with density of the wood. Osage is going to burn slower (rate of output) than silver maple. So to answer your question (if I have it correct), yes - some of the lesser woods can actually release their trapped btu's faster than the hard stuff. Split size/oxygen/draft can also be players.
 
+1. Jags has it. So the answer to your question is Yes. With ALL else equal, if you feed pine or poplar to your stove as fast as it can burn it you'll be getting more btus than if you were feeding oak just because the less-dense wood can burn faster.
 
midwestcoast said:
+1. Jags has it. So the answer to your question is Yes. With ALL else equal, if you feed pine or poplar to your stove as fast as it can burn it you'll be getting more btus than if you were feeding oak just because the less-dense wood can burn faster.
Click to expand...

X2... If its based on volume (cord of pine vs cord of oak) then Oak has more BTU's...

But if its based on weight (100 lbs of pine vs 100 lbs of oak) there will be more Pine by volume and it will harbor more BTU's. . .

You will load the stove more with Pine and achieve higher temps more often than loading only twice a day and cruising at a steady temp.
 
I agree with what Jags is saying, but I think split size and shape is much more important than density when it comes to burn rates. For example, oak or hard maple pallet wood burns a heck of a lot faster per pound than a big old pine or doug fir log at equivalent moisture contents. Exposed surface area is the main thing governing maximum burn rates if there is sufficient air space surrounding each split.
 
Battenkiller said:
I agree with what Jags is saying,
Click to expand...

You made that WAY too easy. I can see that I need to arrange a group from the wood shed to go heist that pellet stove out of your house. Your getten' soft I tell ya. %-P
 
Jags said:
Battenkiller said:
I agree with what Jags is saying,
Click to expand...

You made that WAY too easy. I can see that I need to arrange a group from the wood shed to go heist that pellet stove out of your house. Your getten' soft I tell ya. %-P
Click to expand...

Ha, Ha! Soft and fat, I might add. I usually drop about 10 pounds over the winter. Not this year. I've always thought I went into "hibernation" mode in winter, but maybe it was all the wood handling. One forty-pound bag of pellets carried up the basement stairs every day ain't exactly a way to stay fit.

I think the pellet stove is a good analogy for what I was pointing out, though. Very high-density wood fiber (denser than just about any hardwood), but fed into the fire in little bite-size nuggets. Lots of surface area allows a very high rate of heat output with only a relatively (compared to a wood stove) small fire going in the fire pot. It'd be a pretty short-lived fire, though, if not for that little man inside shoveling pellets into the pot all day (that is how they work, isn't it?) %-P
 
Battenkiller said:
if not for that little man inside shoveling pellets into the pot all day (that is how they work, isn't it?) %-P
Click to expand...

I thought it was a mouse on a treadmill, but what the heck do I know about pellet stoves. :)
 
This is exactly what I just described in another post earlier today. All seasoned wood types have virtually the exact same btu/lb, most of what we see are differences in densities or moisture. Lower density usually correlates with faster hotter burning, probably because it is more porous to air, but there does seem to be some other factor, such as when I described the stunning difference in burn speed between hickory and oak with almost identical moisture. Both woods have similar densities and BTU/cord, but the hickory lit up like a flare.

Burning wood in an EPA stove is much, much more than just a surface reaction. Heating wood to 450F generates flammable gases that are burned at the surface or in the secondary air, so perhaps wood that either conducts heat inward better or is porous enough to release those gases quickly will generate heat faster. Maybe the cross-linked fibers of hickory that make it such a pain to split are helping this effect.

TE
 
nola mike said:
Just curious if there's a difference in heat output between wood species. Obviously, a higher BTU wood is ultimately going to put out more heat over time, but do I get more actual BTUs/hr (needing to reload more often) from say, pine or poplar than oak? I think that moisture is probably the most important factor, and size of split may be next, but if that's the same...
Click to expand...

That's strictly a function of how rich (less air) or lean (more air) your fire is.
 
TradEddie said:
This is exactly what I just described in another post earlier today. All seasoned wood types have virtually the exact same btu/lb, most of what we see are differences in densities or moisture. Lower density usually correlates with faster hotter burning, probably because it is more porous to air, but there does seem to be some other factor, such as when I described the stunning difference in burn speed between hickory and oak with almost identical moisture. Both woods have similar densities and BTU/cord, but the hickory lit up like a flare.

Burning wood in an EPA stove is much, much more than just a surface reaction. Heating wood to 450F generates flammable gases that are burned at the surface or in the secondary air, so perhaps wood that either conducts heat inward better or is porous enough to release those gases quickly will generate heat faster. Maybe the cross-linked fibers of hickory that make it such a pain to split are helping this effect.

TE
Click to expand...

This is more correct, I believe , than earlier posts. You don't burn wood, you burn wood gas. The wood gas is realeased and burned at many places in the firebox and not just at the surface of the wood. Folks with an EPA stove have likely seen this when you have no flame on the wood but the roof of the firebox is engulfed.

The variable then is how fast can you get the gas out of the wood. You increase gas production with more surface area, high firebox temps, and also lower density. It becomes no surpise then why we use kindling (from low density woods) to start fires.

Even better are evergreens that can have lots of pitch/sap on the surface. That stuff adds an extra kick to the fire.

A totally different question is how do you heat a stove quickly. See, if you provide too much air you will carry all the heat of combustion up the flue. Too little air and the fire will be slowed.
 
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