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

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That's true and it's cracking stuff.  We buy that now and again because our own supplies don't quite keep up with our use.  

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On 26/05/2020 at 12:47, aesmith said:

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.

I used 5.17kWh/kg for dry wood and my calculation  gave 4.29% loss in steam at 300C.for 20% and 5.36%

at 25%.

 

I would like to know if people using flue temperature gauges actually see temperatures as high as 300C as it seems a bit high to me. Flue temperatures have to be above 100C at the chimney exit  to avoid condensation  with wetter wood but it would take a bit of working out what the dew point would be for dry wood, it would be less than 100 because the partial pressure of the water vapour  would  remain above the partial pressures of all the other combustion gases down somewhere below 100C. Of cause the chimney would be absorbing some heat and depressing the temperature of the column of flue gases, more so if it is not insulated. I use this as a feature, so the chimneybreast's thermal mass keeps the house warm after the fire is out.

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10 hours ago, openspaceman said:

I used 5.17kWh/kg for dry wood and my calculation  gave 4.29% loss in steam at 300C.for 20% and 5.36%

at 25%.

Interesting, I wonder if our method was different. I calculated loss from a kg of using ..

Specific heat of water from 20 deg to 100 deg

Plus Latent heat of evaporation @100 deg

Plus specific heat of water vapour from 100 to 300 degrees

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8 hours ago, aesmith said:

Interesting, I wonder if our method was different. I calculated loss from a kg of using ..

Specific heat of water from 20 deg to 100 deg

Plus Latent heat of evaporation @100 deg

Plus specific heat of water vapour from 100 to 300 degrees

I just burn what I have . Split in the spring , stacked in the log stores and burnt in the winter . Got a meter somewhere but don't use it .  Never a problem . 

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10 minutes ago, Stubby said:

I just burn what I have . Split in the spring , stacked in the log stores and burnt in the winter . Got a meter somewhere but don't use it .  Never a problem . 

That’s exactly my practice too, I’ll sometimes knock a couple of logs together and listen to the note as dry wood gives a distinct higher note than wet wood. Lost my moisture meter ages ago and don’t miss it.

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12 hours ago, aesmith said:

Interesting, I wonder if our method was different. I calculated loss from a kg of using ..

Specific heat of water from 20 deg to 100 deg

Plus Latent heat of evaporation @100 deg

Plus specific heat of water vapour from 100 to 300 degrees

@Stubby avert your eyes

 

That's what I did too:

 

1kg log at 25% mc wwb contains 0.75kg oven dry wood with 3.875kWh thermal energy. The 0.25kg of water needs heating to 100C from 20C with 83.6kJ, turning to steam at 100C with 564.1kJ and then raising to 300C (I still think ~200 is more realistic) with a further 100kJ which adds up to 0.208kWh. This represents 5.36% of the energy in the wood.

 

Allow the same piece of wood to dry to 20% mc before putting it on the fire means you have the same thermal energy but now only need 66.88+451.28+80 kJ or 0.166kWh that is 4.29%

 

This is not a rigorous way of looking at it as that 5.17kWh not only varies between species, part of tree and season when felled but it is a Lower Heating Value, it already allows for the water formed by combustion of dry wood.

 

I put something here a couple of years ago

 

 

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On 27/05/2020 at 09:32, Big J said:

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. 

 

Yep, I concluded years ago I had no desire to clog my yard up with cubes of logs seasoning for picky customers. We only sell logs as a sideline from domestic arb work. I have a small number of longstanding well trained customers who buy in advance and season them themselves.

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11 hours ago, openspaceman said:

@Stubby avert your eyes

1kg log at 25% mc wwb contains 0.75kg oven dry wood with 3.875kWh thermal energy.

That's my mistake right there, my calculations were for M/C on dry basis, so when I said 25% I was assuming 1kg of wood plus 250g of water.  Re-running on wet basis I expect my figures would pretty much match yours.

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Back in the real world I don't need any sort of calculation to tell me that air dried softwood at £50/cube is miles better value than kiln dried hardwood at £140.   Does anyone find prices in their area sufficiently different so that kiln dried wins out?  

 

Also in the real world we have the luxury of enough storage space that I can dry stuff for at least two summers before use so even if I do need to buy in now and again it doesn't need to be ready for use.  

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On 27/05/2020 at 12:51, 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.

 

 

 

 

In this neck of the woods 100% hardwood Air dried is £ 120 and 100% hardwood Kiln dried is £ 140.

 

Up to now, in the majority,  the general public have been educated to burn Kiln dried firewood below 20% MC. 

A 5 minute search online today backs this statement up by various claims such as:

 

Other firewood will tar up your chimney, kiln dried gives double the heat of air dried logs, various graphs that show the expected kWh from kiln dried in comparison to other firewood, often shown on graphs as being at of 30 - 40 % MC !

 

Having spoken to many end users, several definitely think that kiln dried is best as it is very dry, its 20% MC and doesn't tar up your chimney. They rarely even think about value for money. They clearly believe just what they have read and been told by the woodburner installer and what it says in the manual that came with the stove. 

 

When we all have to comply with 'Ready to burn' legislation and all timber of whatever type hard or soft, or of whatever species has to be at 20% MC, all the marketing attached to retailing kiln dried as a premium product will be invalid.

 

Up to now, in the majority,  the general public have been educated to burn Kiln dried firewood below 20% MC. That 20% figure is ingrained in people's minds and unless government sees sense, takes stock and takes a step back to a rational MAX. 25% MC threshold -  that 20% figure will be there in perpetuity - period.

 

In addition, all the woodburner manufacturers would also have to fall in line to get it to stick and print Max. 25% MC naturally air dried or kiln dried softwood or hardwood or a mixture of both is suitable for burning in this appliance.

 

Is this likely to happen?  We shall see. I personally think that even though it will destroy thousands of smaller firewood retailers who will be unable to meet the threshold, due to their geographical location, Planning requirements, insufficient site space or lack of funds to put in place the infrastructure to naturally air dry / kiln dry the timber, it is inevitable that the proposed legislation is coming in its current form within 9 months and tightened further in the following 12 months.

 

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

 

Based on all the available data - there is clearly very little in it.

 

However the fuel burning process to force dry -  kiln dried firewood without doubt,  produces 100% more pollution, which is harmfull to the environment we live in and the air we all breath.

 

 

 

 

Edited by arboriculturist
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