Which is better value for money - Air dried @25% MC or Kiln dried Firewood @ 20%MC ?

[Stubby]

46 minutes ago, aesmith said:

Surely 20% is 20% whether air dried or kiln dried?  Or are you thinking that the kiln drying would have altered the properties of the wood as well as removing water?   I expect that's possible.  Some woodworkers prefer air dried, and I'll bet that kiln drying for firewood is a lot more brutal than for joinery timber.

Yep . I used to be of the opinion that 20% is 20%  ( so why pay extra for kiln dried ) but its how you get there  . I am convinced that 20% genuine kiln dried burns fine but should it get wet from poor stacking / storage it then is harder to dry again than air dried if the same thing happened .  Something to do with the cells in the wood after kilning .

Edited May 25, 2020 by Stubby

[openspaceman]

9 hours ago, aesmith said:

One cube of firewood will create the same amount of heat when it's burned whether it has 20% or 25% moisture content.  The difference will be whether that extra water stops it burning properly by reducing the temperature or in other ways. 

You've got it

Quote

 

Some heat will be lost turning the water into water vapour but the science of that is beyond me at the moment.  It's apparently not just as simple as each kg of water using 0.6 kWh of energy to turn into vapour, the actual value depend on the temperature at which it happens.

Actually the latent heat of water does vary with temperature but your 0.6kWh holds good for the approximation we are looking at.  When you burn wood it changes from a solid with associated moisture  to ash (a solid but very small part of the original mass) and gases/vapours that go up the chimney at whatever temperature it leaves the stove, this mix of gases carries away heat from the wood that never gives you benefit as heat in the room. If the moisture content of the wood is high then you lose that 0.6kWh for every kg of moisture that goes up the chimney plus the additional amount of sensible heat it contains at temperatures above 100C. I guess most stoves run a flue temperature of 150-350C so this sensible heat of the excess water is also considerable.

 

As you say wet wood is harder to burn cleanly, mostly because it quenches the burn, this shows up as smoke and particulates, Products of Incomplete Combustion, so some of the potential energy of the wood also leaves as PICs. Worse still is the business of excess air; because wood is a complex mix of substances that burn in stages we have to add more air than  is what is needed to combine with the fuel to ensure enough oxygen meets the fuel to burn it out. Good wood stoves will use 1.5-2 times the ideal (stoichiometric) amount of air but this all has to go up the chimney at the flue temperature. Thus the massflow of products up the chimney consists of excess air, excess moisture plus the products of combustion (ideally steam and carbon dioxide, even burning bone dry wood produces water)). Worse still is wet wood tends to need more excess air than dry wood, hence increased sensible heat losses from the high massflow as well as extra steam.

 

Bear in mind you need a stoichiometric mass of air about 6 times the mass of the burning dry wood because only 21% of the air is oxygen, the rest is nitrogen which has a free ride through the fire but still carries away heat up the flue, increase that with excess air and you see what a heat robber it can be.

Quote

 

  I am also not sure whether "bound" moisture when wood is below fibre saturation takes additional energy to separate it from the wood.  So although I intended to do a rough calculation, I've had to admit defeat.

 

I think it does but it is a tiny effect only of consequence when discussing how much moisture is regained as dry wood picks up moisture again as the humidity increases in the winter.

Edited May 25, 2020 by openspaceman

[difflock]

Anyway my air dried wood is coming out of the shed at 15%, so there is that, so why would I pay for wetter kiln dried stuff?

[aesmith]

14 hours ago, openspaceman said:

You've got it

Actually the latent heat of water does vary with temperature but your 0.6kWh holds good for the approximation we are looking at. 

Just for fun I did some rough calculations and assumed starting temperature for fuel is 20 Deg C, vaporisation at 100 and flue temperature 300.  And calculated the losses for each kg of water to be 0.83kWh.  That's actually not very much in the scheme of things.  Assuming you get 5kWh per kg of dry wood, the direct loss due to water is only around 3.3% for wood at 20% m/c, or just over 4% for 25.

If that was the only affect of differing water content then it looks like kiln dried at 15% m/c would give something less than 1% more usable heat than air dried at 20%.

[Steven P]

I like the calculations - I'll come back here and read this properly another time, but they are all missing out one part of the equation, which is better value for money.

 

I am sure you will have noticed there is no straight answer to this. There is a premium added to kiln dried logs of course, and different suppliers will give you a slightly different volume of logs in a 'ton', builders bag or whatever (also the same supplier, depends how they are feeling). Hardwood isn't just hardwood, softwood isn't just softwood and if you look at the tables different woods have different amounts of energy in a volume (though by kg, and at the same water content they are all pretty similar). We buy logs by volume and not weight, a quick check for an example (ignore units) - Ash was "30" weight, oak was "45" for the same volume... but clearly there is more wood in the oak.

 

So got to really compare like for like., If you get the same wood at the same moisture content, air dried will be better value, however... a supplier will tend to sell kiln dried, or air dried as far as I can tell and so making a direct comparison of value becomes even harder.

 

So yup, we can tell you which will give you the best energy output by kg, but you are going to need to take these numbers and apply them to your local prices to work out which is best value.

[arboriculturist]

Some interesting replies coming in. 

 

As I mentioned, assumed values will have to be used, such as assuming we are comparing quantities like for like, species like for like etc.

 

Maybe if someone is really keen to stick their head above the parapet, they will set out the maths behind the science.

 

The general trend is leaning towards Air dried to 25% MC is better value than kiln dried.

However the public have been educated for so long that kiln dried circa. 20% MC is what they should burn, that they actually believe that kiln dried is better.

 

Nevertheless I have managed to persuade 100's and 100's of customers that this is actually not the case, so it is possible to get people to change their thinking over time.

Edited May 26, 2020 by arboriculturist

[gdh]

Isn't the bigger question which moisture content is the best value? Kiln dried/air dried is more of a processing issue than one for the customer to worry about even if it is used as a marketing tool.

 

If the maths are correct in some replies it looks like damper wood is better value, I'm not sure that's accurate, but either way there's also emissions and stove/chimney life to take into account so the initial p/KW might not be the only measure of value.

[Big J]

My opinion is pretty simple. Balls to selling moisture specified firewood. Sell it fresh, let the bloody customers dry it themselves. Most people have space to do so, and a tidy, nicely stacked woodshed is an aesthetic asset to any garden. 

 

Specifying hardwood or softwood? Pointless. Specifying moisture content? Ridiculous. Selling firewood? You've got more patience than me!!

 

I fear I am not suited to dealing with the public. ?

[aesmith]

18 hours ago, Steven P said:

I like the calculations - I'll come back here and read this properly another time, but they are all missing out one part of the equation, which is better value for money.

As you say you can't comment on value without some numbers.  So let's kick off with what's available locally to me, which is air dried softwood at £50/cu.m or hardwood at £90 both claimed to be sub 20% m/c.  The only local supplier of kiln dried is quoting £140 for hardwood, unspecified species, unspecified m/c.  Let's just compare hardwood.  Obviously if both are 20% m/c then kiln dried is 55% more expensive for the same end result.

Even if the kiln dried is only 15%, and the air dried is actually 25% then kiln dried is still 53% more expensive.

 

 

 

Edited May 27, 2020 by aesmith

[difflock]

Just now, aesmith said:

As you say you can't comment on value without some numbers.  So let's kick off with what's available locally to me, which is air dried softwood at £50/cu.m or hardwood at £90 both claimed to be sub 20% m/c.  The only local supplier of kiln dried is quoting £140 for hardwood, unspecified species, unspecified m/c.  Let's just compare hardwood.  Obviously if both are 20% m/c then kiln dried is 55% more expensive for the same end result.

Even if the kiln dried is only 15%, and the air dried is actually 25% then kiln dried is still 53% more expensive.

 

 

 

Bloody 'ell, never mind the air dried vis-a-vis kiln dried conundrum,

look at the astounding value the softwood is in comparison to the hardwood!

ah fink leastwise.