Drying firewood too long drastically reduces Btu content?

virtual sharpening: in your dreams

 

I think that was a dad joke...

 

Yup

 

Belongs in the your thread. He has some good ones!

 

It’s a head scratcher to me that wood can lose energy by being over dry.

I learned that wood burning is a 3 stage process:
1) moisture is driven out of the wood -
2) volatiles (gasses) burn -
3) coals (non volatiles) burn

Accomplishing the first absorbs BTU.

I know from too much first hand experience that wood with higher moisture has poorer BTU output. It burns slow and too much energy is absorbed evaporating the water in it.

In addition to the above, the microbes that break down wood are moisture dependent. Wood that stays dry doesn’t deteriorate. Damp wood does, and rotted wood even dried out lacks BTU also.

I’ll take my firewood nice and dry please.

 

I was wrong when I said wet and dry is the same heat energy. This conversation got the ole grey matter twitching and this is something I touched on in my firewood app formulas. I found this page in my notes it is also listed on my apps methodology page: UNIFIED BIOENERGY TERMINOLOGY - UBET

"The more water per unit weight, the less fuelwood."

 

I tried to attach the Purdue research about lower Btu's for 10 yr verses 3yr dried wood, but this site did not like it, so I'll try a copy and paste below.
I'm probably not going to dump my 3 cords of lesser value Btu oak firewood in the woods cause it is "over-dried " .


As an expert in firewood dynamics, the short answer is no, they will not have the same BTUs. Even though both pieces of wood are the same size and have achieved an identical, ideal moisture content of ~15%, the 10-year-old piece will produce fewer BTUs than the 3-year-old piece.

The reasons for this difference come down to the chemistry of wood preservation, volatilization, and slow structural degradation over time. Here is the breakdown of why this happens:

1. Volatilization of Organic Compounds
Wood is not just made of cellulose and water; it also contains volatile organic compounds (VOCs), resins, saps, and oils (especially in softwoods like pine or aromatic hardwoods like cherry).

• These natural compounds have a incredibly high energy density and burn hot and fast.
  
  
• Over a span of 10 years, even when sheltered from rain, these volatile compounds slowly evaporate (volatilize) into the atmosphere.
  
  
• By losing these high-energy resins, the 10-year-old log loses a portion of its potential chemical energy, resulting in a lower overall BTU output.
  

2. Slow Micro-Decomposition (The "Punky" Effect)
Firewood is organic matter, and air-drying environment ambient relative humidity fluctuates. Even if the wood is kept under cover at 15% moisture content, it is still exposed to oxygen and microscopic fungal spores.

• Over 3 years, a hard wood like oak or hickory reaches peak seasoning—fully dry but structurally sound.
  
  
• Over 10 years, slow, microscopic oxidation and fungal activity break down the lignin and cellulose bonds within the wood.
  
  
• This causes a loss of wood mass and density without changing the external dimensions. The wood becomes slightly "punky" or brittle. Since BTUs are directly correlated with the weight of the dry wood substance, a drop in density means a drop in total BTUs for that size log.
  
  
  

3. The Equilibrium Moisture Content (EMC) Trap
Wood is hygroscopic, meaning it continuously absorbs and releases moisture to stay in equilibrium with the surrounding air.

• A piece of wood dried for 3 years has reached its stable Equilibrium Moisture Content (EMC).
  
  
• Leaving it out for an additional 7 years does not make it any "drier" or more efficient; it simply subjects it to hundreds of seasonal cycles of absorbing ambient humidity during wet months and drying out during hot months. This constant expansion and contraction weakens the cell walls, accelerating the physical breakdown of the fuel source.
  

Characteristic

3-Year Air-Dried Wood

10-Year Air-Dried Wood

Moisture Content

~15% (Ideal)

~15% (Ideal)

Wood Density

Maximized (Solid structural integrity)

Decreased (Micro-degraded/brittle)

Resin/Sap Volatiles

Mostly intact

Largely evaporated

Burn Profile

Long, hot, predictable burn

Faster, less intense burn with more ash

BTU Potential

Peak Energy Output

Noticeably Diminished



The Expert Verdict
While the 10-year-old piece is perfectly safe to burn and will light very easily, it has passed its prime. For maximum heating efficiency and longevity in your wood stove or fireplace, the 3-year-old piece is the superior fuel source. Wood generally hits its absolute peak performance between 1 to 3 years of proper seasoning, depending on the species.

 

Makes sense, wood is organic, and organics degrade over time, even if dry. Once EMC is reached, the wood is quite resistant to moisture as well, bonus.

 

I think this is old knowledge from the era when many stoves were leaky (before the 70s or so)

 

Exactly. Well said.
And I'm so glad my too dry firewood is "safe" to burn.

 

But many new stoves are engineered to be "leaky" though too. (secondary air, etc)
I know my little Drolet 1400i goes ballistic with a load of super dry stuff in it, even on a cold start sometimes...you can't turn it down enough with the factory controls to "control it" at that point.
If I do a load like that overnight the "heating" is pretty much over in 4 hours or so...which is fine if its not that cold out, or the house didn't get overheated too much.
But on the other hand, a load of say 18%MC oak will leave nice coals, and the stove still feels warm in the morning, unlike the previous scenario. (the heat is more "low n slow", drawn out over a longer time with good coaling...a pile of ashes doesn't do much for heating)
Even on my computer controlled Kuuma furnace sometimes I have to shut it down early to preserve the coals when burning super dry stuff...otherwise there is no coals, just ashes, and it goes cold, blower shuts down a lot earlier than would be otherwise.
So in my experience, I have kinda come around to believing that there is a degree of truth to this whole "too dry" idea...not that "too dry" wood is bad, just that there is a sweet spot (say 15-18%)
for the best heating value.
Different stoves may have slightly different results...

 

That article was not talking about stoves that I read even cited article said firePLACE tips

 

You are probabaly right on the sweet spot mc %. I just split 3 pieces of my 9 year old firewood and checked with moisture meter. Red oak - 10.2 %, Chestnut oak 10.2 and 9.8%. I'm going to take those pieces of wood to camp tomorrow and see how they perform in the campfire....pretty well I'm guessing.

 

No problem.

 

Sure a little VOC's have left the building in that time, but it's negligible.

Hoard on and burn whenever you want to!

 

Lots to think about in this thread. I have lots of wood stored inside for 6 or more years and sometimes pick up a stick which feels too light, especially sugar maple and sometimes red oak. And so, I can not dismiss wood losing some heat content from storing too long. Possible causes already noted cover almost every possibility. My favorite for causing lower heat content with age would be fungus (dry rot?) with wood absorbing moisture during the humid months. Volatile's weathering off is possible and proven by the drop in ignition temperature for house structural wood, especially near heater ducts, but most house framing is conifer. Meanwhile, specified wood moisture for a heating appliance is highly variable based on: coaling goals, desired heat flux, stack emissions, temperature limitations... (while no one keeps track of when a number is dry basis versus wet basis, 25% dry basis is 20% wet basis). I try to burn my oldest wood but do not struggle with the issue (I am always to busy HOARDING!!!).

 

I think too dry needs a qualifier.
Too dry ?
Too dry for my stove maybe. If the stove doesn't have the control range to control the burn rate of wood that is "too dry" , then I guess you could say the wood is too dry.
Some of these new stoves that are built to government testing standards rather then consumer use standards might have issues with wood that is "toodry" because the controls don't go to "zero". There's always some air coming in and some don't have controls on the secondary burn tubes and/or catalytic converter.

Plus, good luck getting firewood to "toodry" around here. If you dry wood outside you certain ly won't get below 10% even on a good day.
So, I doubt I'll ever know what "too dry" is.


and old wood loses BTU for sure. It doesn't keep forever. Maybe in a museum in a controolled environment.

Edit: Sorry about the typos. New keyboard and I struggle to teach these old fingers new tricks. Even with treats.

 

Exactly.
Equilibrium moisture content varies widely based on location and time of year too.