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Author Topic: difference between firewood kiln and lumber kiln?  (Read 705 times)

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Offline BrianT5150

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difference between firewood kiln and lumber kiln?
« on: August 17, 2019, 10:51:26 AM »
Is there a difference between a firewood kiln and a kiln used for drying lumber (dimensional for wood-working projects)?  I have looked everywhere for this and all searches take me to differences between one of these and air drying.  I just joined this forum and did a search here as well and could not find it.  I just want to know if the same kiln can be used to dry both firewood and lumber (not at same time).  I know heat treating is all the rage and I will have questions about that later, but now I am curious about the drying process of the kiln.
thanks

Offline barbender

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Re: difference between firewood kiln and lumber kiln?
« Reply #1 on: August 17, 2019, 01:03:11 PM »
I think (I'm no expert, I've just looked into it a little bit) that a firewood kiln will likely have more btu's and airflow for a given size, because you are trying to push the moisture out as quickly as possible- not in a controlled manner as is the case with lumber. As far as heat treating, it is a standard of getting the core of the wood up to a certain temperature and maintaining it for a set amount of time. It could be done during the kiln drying process (I'm sure it usually is) but it is a seperate standard from drying. You can have firewood that is heat treated, that isn't dry.
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Offline BrianT5150

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Re: difference between firewood kiln and lumber kiln?
« Reply #2 on: August 17, 2019, 02:18:22 PM »
Thanks.  Been researching it all morning (and the last 3 months) and did find some stuff, but what you said makes good sense.  thanks again

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Re: difference between firewood kiln and lumber kiln?
« Reply #3 on: August 17, 2019, 03:41:35 PM »
A firewood kiln will turn good lumber into firewood.

A lumber kiln will turn good lumber into better lumber.

A firewood kiln is similar to a blast furnace for wood.  High temps, high moisture removal rate, short cycle times, cracked and split wood.

A lumber kiln is more of a slow dryer, or even a food dehydrator for some models.  Slower but very controlled drying rates, lower temps, relsult in much higher grade, defect free, boards.
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Offline Ianab

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Re: difference between firewood kiln and lumber kiln?
« Reply #4 on: August 17, 2019, 04:04:06 PM »
Doesn't matter if firewood gets a few cracks and warps a bit. So the priority is to get it dry as fast as possible to make space for the next load. You could dry your firewood in a lumber kiln, just it will take a longer time. Doing the opposite will usually also make firewood.
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Offline GeneWengert-WoodDoc

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Re: difference between firewood kiln and lumber kiln?
« Reply #5 on: August 17, 2019, 11:23:11 PM »
Because of the fast drying, a firewood kiln has a much larger heating system and also much larger vents to exhaust the moisture. Fans are smaller. 

In a lumber kiln, spacing between packs and the wall is critical, as well as spacing between packs. Air flow is also controlled. Although firewood needs some air, temperature should be 160 F minimum and humidity around 65% ideally.  With a lumber kiln we have strict final MC requirements so humidity is carefully controlled. Firewood is often dried to a bit under 20% MC, but I do not recall a universal standard.  Yet, drier means lower weight and faster starting.  As you imply, heat is important to kill insects and their eggs. 

Because you might get a complaint, keep records of heat and MC. Lumber kilns have recorders or computers, but firewood may have no permanent record except what you write down.

A lumber kiln can develop a gross profit of $25 per day per 1000 BF.  So, in a 5000 BF kiln running for 6 days, is the kiln load of firewood worth $25x5x6 = $750 more after drying?  If not, dry lumber in a lumber kiln. 

As firewood is dusty, sweep out the kiln after every load.  It reduces fire danger, etc. 
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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Re: difference between firewood kiln and lumber kiln?
« Reply #6 on: August 18, 2019, 09:59:43 AM »
I can understand a firewood kiln for sterilizing firewood.  I know some places require that and definitely if you're shipping out of state or sometimes county.  But why not air dry, especially in Arizona where the original poster is?  Everyone I know air dries firewood.  
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Offline Tin Horse

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Re: difference between firewood kiln and lumber kiln?
« Reply #7 on: August 18, 2019, 10:09:40 AM »
I always air dry for a year or more and covered. My question would be how can you make much of a profit after kiln drying.
The money wouldn't be in it around here.
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Re: difference between firewood kiln and lumber kiln?
« Reply #8 on: August 18, 2019, 07:02:18 PM »
Brian, you're in Parker? The weather there in the summer is almost like a kiln. :o

Offline GeneWengert-WoodDoc

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Re: difference between firewood kiln and lumber kiln?
« Reply #9 on: August 18, 2019, 08:07:42 PM »
I the old days, firewood was as dry as possible for using in summer cook stoves, especially in a house.  The reason was it burned faster, so you got the needed heat and then the fire went out and did not heat the kitchen too much.
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Offline K-Guy

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Re: difference between firewood kiln and lumber kiln?
« Reply #10 on: August 19, 2019, 08:13:51 AM »
@GeneWengert-WoodDoc 

Hi Gene
Our firewood kilns have different programs but do record the loads and have sensor that are placed into the wood to record internal temps. I believe most if not all of our competitors offer this as well.

Also to certified you must have regular inspections by organizations licensed by the USDA.
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Re: difference between firewood kiln and lumber kiln?
« Reply #11 on: August 21, 2019, 08:19:17 AM »
As brought up just a little bit.....Kiln drying for firewood is primarily for killing bugs....so 160 degrees for 60 minutes (other times and temp variants, but that is the easiest) of the center most stick core (heat treating) so not trying to get the moisture out just meet the standard....so you can use a more humid heat source and as long at it is below 20% humidity, you are also working to get your firewood "seasoned" with a less expensive heat source.  "kiln drying" while a selling point for customers is not a legal standard and will not allow you to transport and/or sell in areas that require "heat treating"  (you can run firewood through a kiln for 5 minutes and call it kiln dried....kinda like "all natural" versus "organic" in the food industry .  As explained before... "Heat Treated" is a USDA legal term and to market as such you have to get the USDA to inspect and sign off.   The Texas State park system is about to start limiting firewood brought into and sold at State parks to sourced 50 miles from the park or kind dried to the "heat treated" standard.  We asked for that as we build kilns and work toward the "Heat Treated" certification which is an arduous task. 

Offline Stephen1

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Re: difference between firewood kiln and lumber kiln?
« Reply #12 on: August 21, 2019, 08:27:46 AM »
Are the probes for inside a stick of firewood wireless? Are they hard wired in?
What probes or system are used in a lumber kiln?
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Offline GeneWengert-WoodDoc

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Re: difference between firewood kiln and lumber kiln?
« Reply #13 on: August 21, 2019, 11:28:43 AM »
We have found that insects are transported to a new area by campers bringing in firewood.  The first infestations are in camping areas.  Very hard to control, even with legislation...campers do not think there wood has insects.  Besides, they do not like to buy wood when they can get it free from their own home area.

We do not have a legal definition of what "kiln dried" means for hardwood lumber.  But we do have an industry-wide expectation, of 6 to 8% MC, and so this has the same effect as a legal requirement.  I do not think we have an industry-wide expectation for firewood yet.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline K-Guy

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Re: difference between firewood kiln and lumber kiln?
« Reply #14 on: August 21, 2019, 03:38:33 PM »
If you are talking about stopping the spread of bugs or disease. The APHIS requirements for firewood are here
https://www.aphis.usda.gov/aphis/ourfocus/planthealth/import-information/firewood/!ut/p/z1/04_iUlDg4tKPAFJABpSA0fpReYllmemJJZn5eYk5-hH6kVFm8X6Gzu4GFiaGPu6uLoYGjh6Wnt4e5mYGga6G-l76UfgVFGQHKgIAij1yng!!/

They are not just about kiln drying but Heat Treating for bugs or disease also. Basically it is 160F for 75 minutes using temperature probes placed in the wood. The probe location will be dictated to you by the group who certifies the unit, also records of each load must be kept on file or computer.

Yes. I am also the pallet and firewood guy at Nyle. ;D
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Offline doc henderson

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Re: difference between firewood kiln and lumber kiln?
« Reply #15 on: August 21, 2019, 03:56:16 PM »
I am not selling, but behind on putting up firewood so i am going to build a temp kiln for my wood.
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Offline GeneWengert-WoodDoc

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Re: difference between firewood kiln and lumber kiln?
« Reply #16 on: August 21, 2019, 10:52:56 PM »
The firewood recommendations dated March 2010 do not seem to be reasonable, as to have the center of a piece of wood reach 160 F means that you will need at least 180 F air temperature.  My recollection is that soon after this 2010 report was issued, they revised it to 140 F for 60 minutes.
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Offline GeneWengert-WoodDoc

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Re: difference between firewood kiln and lumber kiln?
« Reply #17 on: August 22, 2019, 02:42:01 PM »
Very confusing to me as the National Task Force Recommendation in 2010 were contradicted by the US Forest Service on March 2011, but it sounds like APHIS ignored them.  The US FS said 140 F for 60 minutes.

https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr200.pdf
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Re: difference between firewood kiln and lumber kiln?
« Reply #18 on: August 22, 2019, 03:23:36 PM »
It's also in section 4 of this document I just got from the USDA.

Gene is right.
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Offline BrianT5150

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Re: difference between firewood kiln and lumber kiln?
« Reply #19 on: August 23, 2019, 08:32:06 PM »
Brian, you're in Parker? The weather there in the summer is almost like a kiln. :o
Yes, and that's in the winter.  The summer is unbelievable and we treat summer like the rest of the country treats winter - go out only if you have to.  Someday I'll tell you all the things that got destroyed by being left in the yard, sometimes for just a day. Things that I went to pick up and they just turned to dust (or small pieces from being dried out - wonder how long it would take to dry firewood, especially in a solar type set up??)
This, of course, brings up a whole new set of questions, of which I will start a new post.  Thanks to all of you who replied.  Great information.  I will continue to use this forum to learn
take care everyone and thanks again!


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