Mixed woods burn hotter than straight Oak

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rudysmallfry

Minister of Fire
Nov 29, 2005
601
Milford, CT
I had a very nice load of Red Oak delivered two seasons ago. Moisture content freshly split is around 12%. Logic suggested that stuff would burn very hot and longer burn times than other species. Yesterday, I brought in some different woods from other piles that had been stacked for shorter times. Mostly Maple with some Ash mixed in. Moisture content up to 18%. They are shorter, but fatter pieces than the Oak, so I figured it would be good for the bottom layer of a top down fire. I was surprised to find the mixed load burned both hotter and longer than the Oak alone. Has anyone had a similar experience? I'm glad I stumbled onto this before I blew through all the Oak, but I am scratching my head as to why this is. I'm thinking it has to do with the various species burning at peak temp and off gassing at different times, but it's just a guess. Would love any insight.
 

Kevin Weis

Minister of Fire
Mar 3, 2018
1,158
Union Bridge, Md
Maple is a closed grain wood. More dense than most Oaks. May account for the longer burn time. I'm not a big fan of Maple as it leaves significantly more ash behind than most other woods.
 

Poindexter

Minister of Fire
Jun 28, 2014
2,975
Fairbanks, Alaska
I am going to start by tagging @Ashful , as he probably has a meaningful contributions.

When I look at wood fuel, I have two questions. How many BTUs per cord is first, but my second question is how fast can I have those BTUs. If I can look at a fuel that has say 40 million BTUs per cord, but the burn lasts 40 hours because of long coaling stage, I am only getting, average, 1 million BTU/hr into my home.

When I look at white spruce I see 18M btu/cord, but I can burn that down in 4 hours, so 4.5M BTU/hr into my insulation envelope.

No, my stove doesn't actually hold an entire cord of wood, I was trying to make the math easy.

Hotter and longer would require an act of God to bend the laws of physics I think.

Fascinating topic though. I like to run my high BTU hardwoods in shoulder seasons for prolonged (but adequate) smolder, saving the lower BTU but faster burning softwoods for cold weather.

@Diabel ? @stoveliker ? Bueller? Bueller? Mrs. Bueller, your son is leading you down the primrose path...
 
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brenndatomu

Minister of Fire
Aug 21, 2013
7,785
NE Ohio
I also prefer mixed species loads...and it doesn't matter what species, they all seem to gain something by working together, just like people.
As far as the longer burn times, if the oak is really 12% then that's pretty dry and in many stoves will burn faster...it still makes great heat though, just not for as long...just as with most things in life, there is a perfect balance with wood burning too
 
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kborndale

Minister of Fire
Oct 9, 2008
519
LI
Maple is a closed grain wood. More dense than most Oaks. May account for the longer burn time. I'm not a big fan of Maple as it leaves significantly more ash behind than most other woods.

Maple is not more dense that oak. Especially silver or red maple. Sugar maple is pretty dense but still not as dense as oak.
 
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stoveliker

Minister of Fire
Nov 17, 2019
6,483
Long Island NY
I see "burned hotter and longer". I don't know what that means. What was hotter.
Was the draft exactly the same? Was the pounds of wood in the stove exactly the same? Was the heat.loss of your home exactly the same?

To me this is too qualitative to assess.

On that same qualitative level, I can see that (if all other parameters are indeed the same), one could get a more even output. With wood that goes hot first, wood that goes hot later and wood that goes hot last. And that then also depends on what is on top and on the bottom. And e.g. an oak coaling stage might be different depending on what else burns, and how much ash is covering it.

A more consistent heat output over the hours (rather than a peak and a longer low output decline, aka coaling stage) may feel like "hotter, longer".

All I can say is, your experience is what matters - after all you do this to experience (feel) the heat. If this makes you feel and enjoy the stove better, by all means go for it because your goal is achieved. Beyond that, there is not much to say about it, I think.
 

EbS-P

Minister of Fire
Jan 19, 2019
4,083
SE North Carolina
12% is as dry as wood will ever get in your climate. Want to double check your measurements. Weight small split on the kitchen scale bake at 240 degrees for several hours weigh again.

Burning is all about thermal decomposition more dense wood takes more energy to reach the decomposition temp. So if a load of all oak is slower to heat up because it takes more energy and if you are relying on the burning of the oak itself I think it will just take longer and burn slower. All the whole you will need to keep the air setting more open to during the start of the fire.

I like a mixed load that lights my secondaries off right away. Then close the air down sooner and rely on the secondary combustion heat to offgas the load to feed the secondary flames.

I find my air setting for oak are higher. More air in = more hot air out = less efficient burn (more heat up the flue)
 

Hoytman

Minister of Fire
Jan 6, 2020
545
Ohio
I am going to start by tagging @Ashful , as he probably has a meaningful contributions.

When I look at wood fuel, I have two questions. How many BTUs per cord is first, but my second question is how fast can I have those BTUs. If I can look at a fuel that has say 40 million BTUs per cord, but the burn lasts 40 hours because of long coaling stage, I am only getting, average, 1 million BTU/hr into my home.

When I look at white spruce I see 18M btu/cord, but I can burn that down in 4 hours, so 4.5M BTU/hr into my insulation envelope.

No, my stove doesn't actually hold an entire cord of wood, I was trying to make the math easy.

Hotter and longer would require an act of God to bend the laws of physics I think.

Fascinating topic though. I like to run my high BTU hardwoods in shoulder seasons for prolonged (but adequate) smolder, saving the lower BTU but faster burning softwoods for cold weather.

@Diabel ? @stoveliker ? Bueller? Bueller? Mrs. Bueller, your son is leading you down the primrose path...
Did you mean 100,000 btu output, not 1 million? There are a few stoves rated for 100-110,000 btu and even then it is my personal belief that those numbers are greatly exaggerated and the stoves are only capable of slightly more than half of that in output. There are many people that believe the same, that output numbers are marketing and real world numbers don’t add up.
 
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stoveliker

Minister of Fire
Nov 17, 2019
6,483
Long Island NY
It was an example with fictional but nicely rounded numbers.
 
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Eman85

Minister of Fire
Oct 10, 2022
520
E TN
I think it's like fast fun and easy or fast cheap and good you don't get all of them. Hot and longer usually doesn't go together. Talk to anyone that's ever run a wood cookstove and what their assortment of wood is and what it does to suit the heat they require for what they are cooking/baking.
 

rudysmallfry

Minister of Fire
Nov 29, 2005
601
Milford, CT
I see "burned hotter and longer". I don't know what that means. What was hotter.
Was the draft exactly the same? Was the pounds of wood in the stove exactly the same? Was the heat.loss of your home exactly the same?

To me this is too qualitative to assess.

On that same qualitative level, I can see that (if all other parameters are indeed the same), one could get a more even output. With wood that goes hot first, wood that goes hot later and wood that goes hot last. And that then also depends on what is on top and on the bottom. And e.g. an oak coaling stage might be different depending on what else burns, and how much ash is covering it.

A more consistent heat output over the hours (rather than a peak and a longer low output decline, aka coaling stage) may feel like "hotter, longer".

All I can say is, your experience is what matters - after all you do this to experience (feel) the heat. If this makes you feel and enjoy the stove better, by all means go for it because your goal is achieved. Beyond that, there is not much to say about it, I think.
By "hotter and longer", here's the baseline that is based off of. I burn an older Hearthstone Heritage. While I love the stove, it has horrible burn times in terms of "active flame". Pretty much anything I put in there is down to coals within the hour. I like my house on the cooler side, so this is still pretty much shoulder season burning for me. Firebox only a little over 1/2 full using smaller splits of 12% moisture wood. It looks to be red oak, but I could be completely wrong on that. Regardless, it lights fast and doesn't burn forever, so it keeps the oil boiler from kicking on without blowing me out of the house.

When I was burning with all the one species, presumably red oak, the temp on the double wall worked to get above 325 even at full air and stove top was lucky if it reached 250. I had some shorter splits that were a PITA to stack, so I decided to burn them. They are sugar or silver maple. (Always get them confused) I put three of the fatter short splits under the faster burning oak. Still only about 1/2 full firebox. The stack at full air quickly climbed to 400, so I was able to close it down sooner and the secondaries took over. I still had some flame after 3 hours instead of 1. The stove top went to 350 and kept climbing up to 375 after the coaling stage. The only differences were the 3 pieces of Maple, the N/S versus E/W position of them since there were short, and the fact that the maple pieces were fatter.

Regardless I'm sold on mixed species loading. I'm thinking the maple was still coming up to speed as the oak was already peaking, hence the longer overall burn and more sustained heat, but open to more discussion.
 

PaulBunyun

Member
Oct 15, 2019
59
Michigan
Besides saving some big oak splints for the really cold days I throw whatever I have whenever It moves to the top of the stack. I don't separate my stacks by species, it's already enough work as is. Only hardwood though. 25f outside and 75 inside tonight.
 

rudysmallfry

Minister of Fire
Nov 29, 2005
601
Milford, CT
12% is as dry as wood will ever get in your climate. Want to double check your measurements. Weight small split on the kitchen scale bake at 240 degrees for several hours weigh again.

Burning is all about thermal decomposition more dense wood takes more energy to reach the decomposition temp. So if a load of all oak is slower to heat up because it takes more energy and if you are relying on the burning of the oak itself I think it will just take longer and burn slower. All the whole you will need to keep the air setting more open to during the start of the fire.

I like a mixed load that lights my secondaries off right away. Then close the air down sooner and rely on the secondary combustion heat to offgas the load to feed the secondary flames.

I find my air setting for oak are higher. More air in = more hot air out = less efficient burn (more heat up the flue)
Thanks for posting this. Your last two paragraphs exactly match my findings, especially about the secondaries lighting off right away. I've had this stove for 20 years. I can't believe I never played with this before.
 
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Kevin Weis

Minister of Fire
Mar 3, 2018
1,158
Union Bridge, Md
Maple is not more dense that oak. Especially silver or red maple. Sugar maple is pretty dense but still not as dense as oak.
Referring to mainly the hard Maples. Red, Silver, Mountain and the like in the soft Maple category. Try putting a nail through well seasoned Sugar Maple lumber.
 

alber197

New Member
Mar 5, 2022
55
Wisconsin
Maple is a closed grain wood. More dense than most Oaks. May account for the longer burn time. I'm not a big fan of Maple as it leaves significantly more ash behind than most other woods.
I agree with the hard maple leaving more ash and coals. I see this with the white ash also. In my short experience with newer epa stoves, I am having better time with mixed hardwood loads. No science behind it but burn times and the ability to reload when needed instead of messing with the coals is better. Seems to burn a little more "even"
 

kborndale

Minister of Fire
Oct 9, 2008
519
LI
Referring to mainly the hard Maples. Red, Silver, Mountain and the like in the soft Maple category. Try putting a nail through well seasoned Sugar Maple lumber.

Agreed that hard maples are a dense heavy wood, but oak is heavier and more dense .
 

Garbanzo62

Feeling the Heat
Aug 25, 2022
257
Connecticut
This is a great thread. New to burning and we are feeling out the stove (only had it a month). My wood sstock is not as good as I would like ti, so I've been trying to set for longer burns. However, the heat output is not what I expected. Full open damper throws more heat, but I go through more of my limited supply. I never thought that the mix of week in the stove would have a significant bearing for what I ahve available at the moment.. I'll give a mixed batch a try.
 

stoveliker

Minister of Fire
Nov 17, 2019
6,483
Long Island NY
It appears there is a new battle on these forums: the oak vs hard maple debates 🤪
 
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TradEddie

Minister of Fire
Jan 24, 2012
978
SE PA
I'm thinking it has to do with the various species burning at peak temp and off gassing at different times, but it's just a guess. Would love any insight.
I'd agree with this, and maybe also a difference in the proportions of what I think of as the three types of burning: primary flames, gases for secondary combustion and coals. Oak has weaker primary flames than some "lesser" woods, so it's not ideal for getting secondary tubes up to temperature, I often include a split of poplar in front to get up to cruising temperature fast without consuming too much of the oak I want for the long slow controlled burn.

TE