Apparently logs can be too dry

[Woodworks]

Has anyone ever actually done any tests to show this is actually the case?

 

It take a lot longer than a few weeks for timber to become dry, is the reverse procedure really so much quicker?

 

Could probably have go at that. Only way I can see to do it accurately would be to oven dry a sample to 0% and the measure it's weight gain over the coming weeks sitting in the log shed

[gdh]

Not a scientific test by any means but we dried a load down to 6-12% surface moisture and after a month or so in a normal shed a lot was back up to 16 in December although that seemed to be the highest it got.

 

The flip side of that argument is after the dry and very windy week 2 weeks ago, despite a wet month, our uncut wood was 14-20 in the piles (but 28-32 inside the logs).

[Stubby]

Is that a reading from the middle of a freshly split log?

 

I would expect the outer inch or so to change relatively quickly with the ambient humidity, but find it hard to believe that the moister goes in very far, very quickly.

 

I also think deferent woods will behave very differently.

 

Our home was very, very damp when we first moved in, it had never be properly heated. I fitted a huge out door boiler and many extra radiators. I heated the house 24/7 for three and half years, before it became dry and no longer needed heating in the summer.

 

We have solid walls, with no cavity. I believe I had to completely dry the walls from the inside out, when the walls were damp capillary attraction brought rain water into the walls, once dry any rain only penetrates into the walls a little and then finds its way back out the same way when the rain stops. Water normally follows the path of least resistance.

 

Measured on the end and the middle of a split log that was 3 years old in the round before I split it . Not measured when I split it but put in my log store in early spring . Measured in mid summer , then measured again in late November . Still burned a treat though .

[openspaceman]

From the table you posted logs at 25% V ones at 15% is a drop of 14% not 40%.

 

3700kwh per ton to about 4500 kwh per ton, my error, point taken.

 

A

 

Bur even that is misleading because we seldom sell split logs by tonnage (although you may well by roundwood for converting by the tonne.

 

So take your tonne of timber at 25%mc wwb and 3700kWh/tonne, it contains .75 tonne of oven dry wood which has a calorific value of about 3975kWh, season it to 15% mc and it now weighs 0.88 tonnes but has the same calorific value.

 

Burn the batch at 25% mc and you yield about 3700 (I make it 3787)kWh

 

Burn the same batch at 15% and I make that 3862 kWh

Edited February 5, 2017 by openspaceman

[Woodworks]

 

Bur even that is misleading because we seldom sell split logs by tonnage (although you may well by roundwood for converting by the tonne.

 

So take your tonne of timber at 25%mc wwb and 3700kWh/tonne, it contains .75 tonne of oven dry wood which has a calorific value of about 3975kWh, season it to 15% mc and it now weighs 0.88 tonnes but has the same calorific value.

 

Burn the batch at 25% mc and you yield about 3700 (I make it 3787)kWh

 

Burn the same batch at 15% and I make that 3862 kWh

 

Ah never thought of it like that

 

Is that right just 2% in it?

[openspaceman]

 

Ah never thought of it like that

 

Is that right just 2% in it?

 

Yes but you need to see this is not the full story.

 

If you burn wood at 50% mc wwb the energy cost of the water is only 14% of the available energy but we all know burning wood this wet is difficult because the fire never gets up to temperature and the flue gases are so cold that water and associated tars condense in the chimney and run back down.

 

The thing to realise is that the energy in the wood all ends up in the combustion products which then lose heat through the stove walls before they are vented. If the wood is wet then with any given stove the temperature may not get high enough to burn the wood cleanly. It looks like you need secondary combustion temperatures of around 800C to burn wood cleanly and there may not be enough energy in the wood to maintain this temperature if the firebox is not well insulated. Also small stoves have more heat loss for their volume than big ones. Which is why big industrial burners with refractory linings and some pre heated combustion air can burn whole tree chip at 45% mc but my little Jotul cannot.

[Stubby]

My Burley is well insulated with thick vermiculite board in the fire box and a single opening double glazed door for the above reasons . Cant see smoke from the stack when its up to temp .

[Woodworks]

 

Yes but you need to see this is not the full story.

 

If you burn wood at 50% mc wwb the energy cost of the water is only 14% of the available energy but we all know burning wood this wet is difficult because the fire never gets up to temperature and the flue gases are so cold that water and associated tars condense in the chimney and run back down.

 

The thing to realise is that the energy in the wood all ends up in the combustion products which then lose heat through the stove walls before they are vented. If the wood is wet then with any given stove the temperature may not get high enough to burn the wood cleanly. It looks like you need secondary combustion temperatures of around 800C to burn wood cleanly and there may not be enough energy in the wood to maintain this temperature if the firebox is not well insulated. Also small stoves have more heat loss for their volume than big ones. Which is why big industrial burners with refractory linings and some pre heated combustion air can burn whole tree chip at 45% mc but my little Jotul cannot.

 

Great stuff thanks

 

Lined the fire box of our cast iron stove with insulated sheet to up the temperatures. Appears to work a treat with much cleaner burning now.

[openspaceman]

 

Great stuff thanks

 

Lined the fire box of our cast iron stove with insulated sheet to up the temperatures. Appears to work a treat with much cleaner burning now.

 

Yes and as Stubby says this is one of the techniques a modern stove uses, essentially you move the heat exchange surfaces till after the combustion is completed, so they don't quench the secondary flame.

[cornish wood burner]

 

Yes and as Stubby says this is one of the techniques a modern stove uses, essentially you move the heat exchange surfaces till after the combustion is completed, so they don't quench the secondary flame.

 

Perhaps that idea might have been of use with those smokey glen farrow boilers talked about on here a while back, I know they wouldn't believe it but I think we both thought some of their problem was too cold a return/chamber cooling the burn.