For Those Who Would Like to Dry Wood

[jteneyck]

I have a solar wood kiln that I use to dry rough sawn lumber.  It works well from May to October.  So what about the other 6 months of the year?  Well, it's pretty useless here in western NY.  Wood I've put in it in November has dried about 1%/month through the winter months.  Like I said, pretty useless.  So to add some much needed drying capacity I recently added a dehumidification kiln in the back of my wood storage shed.  I designed the shed for it when I built it 3 years ago but never added the kiln until I became desperate for more dry wood.  

A dehumidification kiln is a pretty simple affair.  All you need is a well insulated box to hold the stickered wood, a low wattage heater, a small dehumidifier, a fan to circulate the air, and a couple of simple controllers to regulate the RH and temperature.  My new kiln is about 11' long x 4' high x 4' wide inside.  I built it by first insulating two walls of the shed with fiberglass insulation, then covering it with 6 mil plastic to form a vapor barrier.  I added an insulated knee wall to give me the third wall and then added an insulated roof.  The roof has a layer of 2" foam on the inside and then 3-1/2" fiberglass in the stud cavities of the framing.  

 


The floor is insulated with more 2" foam over a layer of plastic.  There are sleepers every 2' to support the wood that will go on top.  I made the door in three sections.  More 2" foam backed up with a 2 x 4 frame.  BTW, I made all of the lumber from some fir logs I got for free last Fall and cut into 2X stock.  I had no idea what I would do with it at the time, but it came in very handy for this project and saved me some money.  Lumber prices right now are nuts.  Anyway, the doors seals against the rubber gasket around the perimeter of the box and give a pretty good seal.  I can easily maintain the 75% RH needed at the start of a drying cycle for air dried wood.  

Here's a shot of the kiln loaded.  




I should have made it a foot longer because I didn't have room at the left end for the dehumidifier and heater with the 9-1/2' boards I had.  With 8' lumber it will be fine.  So I had to get creative in how to load the kiln.  The fan is on the left and circulates air to the back and than across the back face of the lumber where it travels through the lumber stack to the front side and then repeats.  The heater is just a 1500 W oil filled room radiator set for 1000W.  

All buttoned up the kiln looks like this:



The RH and temp controllers sit on the outside and are easily adjusted to follow the EBAC drying schedules I use:




The kiln holds 400 - 500 BF of lumber.  Air dry wood at 20% or less should take 2 to 3 weeks to dry.  I'll let you know; this is the first load.  

John

[barryvabeach]

John,  very nice looking kiln.   Hope it works out.

[jteneyck]

John,  very nice looking kiln.   Hope it works out.

Thanks Barry.  I'm sure it will work as planned.  I had a dehumidification kiln in my basement for many years.  They are pretty dummy proof as long as you follow the recommended drying schedules and make sure you stay below the maximum rate loss per day for that species and thickness.  

I wrote the title of this post with the intent of showing that it's not hard to dry your own lumber, but then forgot to follow up on it.  Hopefully, you all see that the kiln itself is a simple affair.  If you have room for one in a heated space, like I had in my basement, a simple foam insulation panel box with the lightest of frames to hold it all together is more than adequate.  My new one is in an unheated shed, so I have more insulation.  The kiln in my basement only needed 300 W of heat to maintain 110F whiich is the maximum temperature most household dehumidifiers can operate at.  The new kiln has a 1500 W oil filled radiator heater but most of the time it won't be needed.  Right now the dehumidifier is supplying all the heat required. You only use much heater power if you want to sterilize the wood at the end of the drying cycle.  In that case, you turn off the dehumidifier and use the heater to raise the temperature to 135F for 24 hours.  I use an oil filled radiator because they do not get hot enough to cause a fire.

You can use any dehumidifier you want and don't need a large one because you are only removing a gallon or less of water per day if you start with AD lumber.  What about green lumber?  I don't do it because of all the water that has to be removed which means a lot more time and $'s to run the dehumidifier.  4 to 6 months of air drying will take most any 6/4 wood down to less than 25% moisture and 4/4 down to AD, or 12 - 14% in my area.  

I already had the dehumidifier and heater.  I built the new kiln for about $300 including the RH and temperature controllers.  

John

[iclark]

Very nice job and explanation. Thank you.

[ianab]

Seems to have all the bases covered. Once you get the wood "air dry", under 20% things are much more forgiving regarding schedules. 

Green wood tends to release acidic tannins as it dries, very corrosive, and domestic DH units wont last long. Commercial kilns tend to use a lot of stainless steel for that reason. But air dry wood isn't much of a problem, and you can get most wood air dry just over Summer. Then if you can get the temp up enough to kill bugs at the end of the cycle I think you are good to go.


Solar kilns do work, but yes in your climate that last load in Autumn might not be dry until Spring kicks in. 

[atgcpaul]

Hi John,

What are you using for stickers?  Is that a cut up cutting board?  I like that.

So that's about $3/day to run the space heater and another $1/day for the dehumidifier?  Not bad.

Do you have to worry about case hardening in the solar or dehumidification kiln?

Do you have to open up the kiln to drain the water from the DH?

Paul

[jteneyck]

Hi John,

What are you using for stickers?  Is that a cut up cutting board?  I like that.

So that's about $3/day to run the space heater and another $1/day for the dehumidifier?  Not bad.

Do you have to worry about case hardening in the solar or dehumidification kiln?

Do you have to open up the kiln to drain the water from the DH?

Paul

Hi Paul.  The stickers are PVC that a local millwork shop cuts from their scrap.  3/4 x 1" x 48" long for $0.50 each.  I must have bought 800 of them last Summer.  A hidden benefit of them is you can leave them out in the rain, snow, etc and they won't pick up moisture.  

You can still ruin wood in solar or dehumidification kiln.  As in all wood drying you have to control the drying rate.  In that regard, air drying is mostly an uncontrolled process and often has serious degrade.  Kilns not only speed up the drying process but by controlling the RH and temperature they control the drying rate and thereby minimize degrade. With the solar kiln you can't directly control RH or temperature, but there are vents you can adjust to control the drying rate, and you can even block off some of the glazing if the wood is in danger of drying too fast.  With the dehumidification kiln you have direct control of the RH and temperature to control the drying rate. 

Virginia Tech. has designs for a large and small solar kiln, and also has a chart that shows the maximum drying rate per day for many common hardwoods, which vary widely.  For example, the maximum safe drying rate for 4/4 white ash is 10.4%/day, while for white oak it's 2.5%.  For 8/4 white oak it's 1.0%/day. EBAC makes dehumidification kilns and publishes drying schedules for many woods.  For each % moisture content range there is a recommended minimum RH and maximum temperature. I use the RH guidelines of those kiln schedules for running my dehumidification kiln.  With both kilns I weigh a sample daily to determine the % moisture loss.  As long as I stay within the RH vs. % moisture content range and below the maximum rate loss per day I have had not have any problems with case hardening.  

Because I weigh a sample every day I have to open the dehumidification kiln.  While it's open I dump out the dehumidifier.  It's possible to infer the % moisture content of the wood by measuring how much water the dehumidifier removes per day, which you can duct out of the kiln into a pail, or by having a moisture meter with remote probes, but weighing a sample is foolproof and low cost.  
 

John

[jteneyck]

Hi John,

What are you using for stickers?  Is that a cut up cutting board?  I like that.

So that's about $3/day to run the space heater and another $1/day for the dehumidifier?  Not bad.

Do you have to worry about case hardening in the solar or dehumidification kiln?

Do you have to open up the kiln to drain the water from the DH?

Paul

The cost to run the kiln is not nearly as high as you suggest.  The heater is only needed to get the load up to 80F or so.  Once the water starts to migrate out of the wood, the dehumidifier comes on and supplies all of the heat needed until the end of the drying cycle, unless you want to sterilize the wood.  My kiln is now at 121F even though the heater is set at 90F.  The heat from the dehumidifier is heating the kiln above set point.  In the Winter the heater may be needed more, as it was in my solar kiln last Winter after I temporarily converted it to a dehumidification kiln.  But even with that leaky arrangement it only cost about $50/month to run it all Winter long.  With the new kiln in the Summer months the cost to run it is going to be maybe $20 - $30/month.  If I want/need to sterilize the load at the end of the drying cycle I'll turn off the dehumidifier and use the heater to take the temp. up to 135F for 24 hours.  That will cost less than $3.  

The key is a well sealed and insulated box. 

John

[Chad_Pearson]

I would love to hear about your setup for controlling the heater and dehumidifier. My understanding is that for the heater you need to disable the internal temp gauge so that it can reach sterilization temperatures. I have a “Johnson Controls” temp controller and have been meaning to set up a similar system to sterilize suspect lumber. Any help you can provide on how you bypassed the temp gauge on the oil heater would be greatly appreciated. An explanation of the whole controller setup would also be really helpful.

Thanks!

[jteneyck]

I would love to hear about your setup for controlling the heater and dehumidifier.  My understanding is that for the heater you need to disable the internal temp gauge so that it can reach sterilization temperatures.  I have a “Johnson Controls” temp controller and have been meaning to set up a similar system to sterilize suspect lumber.  Any help you can provide on how you bypassed the temp gauge on the oil heater would be greatly appreciated.  An explanation of the whole controller setup would also be really helpful.

Thanks!

Hi Chad.  I use two Inkbird digital controllers to control the heater and dehumidifier.  The heater control is this one:  Link.  The RH controller is this one:  Link 2.  I have two oil filled radiator heaters, both around 1500 W.  One is about 30 years old and has no internal temp. limit switch so I can heat the kiln up to 140F to sterilize wood.  The newer one does have an internal temp. limit switch.  I haven't bypassed it yet because I just use the other one if I want to go to higher temp., but it shouldn't be hard.  There must be a simple toaster type thermostat or a snap type one like in a woodstove blower.  I would just bypass it.  Oh, this brings up an important point.  You want an old school type heater with mechanical switches, no electronic controls.  Those won't survive going up to 140F.  

The controller set up is dead simple.  The dehumidifier is set to the max. dry setting and is controlled by the dehumidifier controller.  I set the heater to 900 or 1000W max. input and it gets switched on/off by the temperature controller.  The recirculation fan/blower runs continuously from start up until I open the kiln to unload it.  

John