How I dry small pieces of lumber

This subject's come up a couple times in the year since I've been around, so I thought I'd take some pics of my little kiln-box and post them here. It's nothing fancy but it gets the job done, and fairly quickly too.


All it took to build was 2-1/2 sheets of 3/4" Douglas Fir plywood. It's about 8' long, 2' deep and 2' high. I lined the whole works with foil-clad bubble wrap insulation. It's very cheap and holds the heat in very well. My heat source is just a couple 250W heat lamps, though it's set up to hold 4 if I need the extra heat. The white disc visible on the top-right of the box is a meat thermometer. I just drilled a hole through the side and shoved the thermometer's probe skewer through.


Here you can also see the dehumidifier I sometimes use. For smaller pieces that are already fairly dry, it isn't necessary and actually can dry too fast. With the two heatlamps at full and the dehumidifier running, the thermometer shows about 140°F which is plenty hot for most applications. Just the lamps on their own can hit about 110°F which is still pretty good. At the bottom of the pic you can see the two dimmer switches to control the heat output of the lamps, and also the switch that controls the in-line duct fan. I have ducting running out this end, around the back, and into the other end, and have a 6" in-line duct fan to move the air. It works pretty well, but I might add another fan at the other end to boost it a bit. The wood blocks along the sides of the box are supports to rest the base stickers on, then the first boards go on top of those.


Here are some 3' pieces of Western Red Cedar I had drying overnight. They were already pretty dry but had picked up a bit of ambient moisture in the last couple weeks sitting outside. They're down to 9% now.


This is the left-hand end of the box. You can see the return of the ducting coming in the end, and also the lamp support base. This is made of two right-angle "bookend" type supports, with two 1" X 6" pieces of Douglas Fir on top. All surfaces underneath are insulated with the same foil stuff. The lamps are just screwed into basic ceramic lightbulb holders that are screwed horizontally to the upright piece where you can see the wire going through. I did this so that the lamps would only provide indirect heat to the wood, otherwise there would be two very hot spots above each lamp which would be a fire hazard if nothing else.


This is my heat control. It's just an old hot-water tank thermostat screwed to a piece of 2" Aluminum angle stock. The heatlamps are wired through this, and it lets me set the bulb shutoff temp anywhere between 90°F and 180°F. Works very well. Also visible is the probe from the meat thermometer.

So there it is, nothing fancy and very inexpensive. The dehumidifier cost much more than all the other components put together. Total cost was maybe $300.

Nice!

Looks really good Brad - when I get my new shed I will probably make one like that. Where I live in Australia I probably won't need a dehumidifier so I will just partially vent the box to get rid of the excess humidity because it never gets really cold here. It's late fall here but today it will still reach 83F! We have awesome falls here crystal clear warm days and mild nights and no rain for weeks at a time (we could use the rain though) Even in winter it reaches 65F most days.

Have you measured the differences between the vertical temps in the box? I wonder if the temps would be more even if the air was recirculated on a diagonal?

Very interesting Brad. Looks like you have thought things through. I would be interested in knowing if you ever calculated how many amps/watts you are using when it is on. It should be easy to calculate. Just curious how much $$ per hour/day electricity you are using for it.

Cool. And cheap.

Any issues with warping since the boards aren't weighted?

cool!

woodshop said:

Very interesting Brad. Looks like you have thought things through. I would be interested in knowing if you ever calculated how many amps/watts you are using when it is on. It should be easy to calculate. Just curious how much $$ per hour/day electricity you are using for it.

Click to expand...

Hi WS, youv'e been very quiet lately? Vacation or working or time out?

Woodshop - The bulbs are 250W X 2, and I'm not sure what the fan is exactly but most small ones like that are less than half an amp at most. The dehumidifier is the one that really draws the juice. I don't know exactly what it's rated for, but they (along with air conditioners) are generally in the 1000 - 1500W range. I know the basement lights dim a bit when it kicks on.

Wavefreak - I do have some big 18" square concrete patio blocks that are about 2" thick that I put on the top occasionally. Since these were Cedar (which doesn't move much at all anyway) that was nearly dry already, 1-3/4" thick, and quite short, I wasn't worried about warping at all with this load. If I'm doing 4/4 Douglas Fir or Birch (the most common woods I work with) then I put the weights on top. would ultimately like to have some way of using ratchet tie-down straps instead of weights though. I actually haven't used it much at all, maybe 10-12 loads through it, but it's been handy to have.


Bob - No, I haven't even bothered measuring temps in different parts of the box. I've never gotten very scientific about drying wood since I work with a very limited species range, and the wood properties are fairly similar across them. I'm not entirely sure what you mean about "diagonal" airflow though. Do you mean to pull out of the top of one end and go into the bottom of the other? If so that might not be too hard to rig up. I have a few misc. pipe elbows kicking around still. Like I said though I would like to get another fan in the duct line to get a bit more flow. One disadvantage (or advantage, in the case of this cedar) with keeping this setup indoors is that when I throw a fresh load in, the whole house smells like whatever type of wood is being dried. This is nice for Cedar & Douglas Fir, but other stuff like Birch and Poplar can stink the whole place up for a day or two. Good thing I'm single.

Regarding your temperatures - I think that your winter is about the only time I could handle living there. Don't get me wrong, we get our share of hot, dry weather here too but not nearly as much as you do. Anything over 30°C and I'm pretty much useless as I heat up and sweat profusely very easy. I'd be perfectly happy if it never ever got above 70°F.

Where exactly are you at anyway? I saw part of a TV program last week that told about a town on the West coast of Australia that recorded over 160 consecutive days over 100°F back in the 1920s. I can't remember the name, I think it was "Port" something, but that's INSANE. After just a month of that I'd say :censored: this and pack up and leave. I can't believe there's a permanent settlement there. Looked like a nice spot otherwise though.

Hey Brad......will 140° F 'set' the pitch in doug-fir?

I'll have to read up a bit on that, but I think it will. I think the higher temps (~190°F) required for lumber to be officially stamped as KD-HT (Kiln Dried - Heat Treated) are mainly to kill any pests in the wood.

Brmorgan said:

Bob - No, I haven't even bothered measuring temps in different parts of the box. I've never gotten very scientific about drying wood since I work with a very limited species range, and the wood properties are fairly similar across them. I'm not entirely sure what you mean about "diagonal" airflow though. Do you mean to pull out of the top of one end and go into the bottom of the other?

Click to expand...

More or less - out of the top corner of one end and into the opposite corner at the other. No biggie really it's the cross flow that is important.

If so that might not be too hard to rig up. I have a few misc. pipe elbows kicking around still. Like I said though I would like to get another fan in the duct line to get a bit more flow. One disadvantage (or advantage, in the case of this cedar) with keeping this setup indoors is that when I throw a fresh load in, the whole house smells like whatever type of wood is being dried. This is nice for Cedar & Douglas Fir, but other stuff like Birch and Poplar can stink the whole place up for a day or two. Good thing I'm single.

Click to expand...

There are very few wood smells I dislike although Cooktown Iron Wood from Queensland does smell like a well ripened doggy doo! I had a guy send me some through the mail at work and when I opened up the package everyone started looking at the soles of their shoes!

Regarding your temperatures - I think that your winter is about the only time I could handle living there. Don't get me wrong, we get our share of hot, dry weather here too but not nearly as much as you do. Anything over 30°C and I'm pretty much useless as I heat up and sweat profusely very easy. I'd be perfectly happy if it never ever got above 70°F.

Click to expand...

Summer time here is too hot for me too. I stop milling over 30ºC and I head south about 250 miles away on the south coast where it rarely reaches than 30º and my buddies Hud and Sandi have a 250 acre property with lotsa nice trees to mill!

Where exactly are you at anyway?

Click to expand...

Put "Perth WA" into Google Maps and that's where I am.

I saw part of a TV program last week that told about a town on the West coast of Australia that recorded over 160 consecutive days over 100°F back in the 1920s. I can't remember the name, I think it was "Port" something, but that's INSANE.

Click to expand...

It's called Marble Bar, 161 consecutive days above 100F, it also has an average daytime summer temperature of 104F. My MIL was born there and her father operated a gold mine there in the 1920s! He sold the mine to pay off debts following the great depression and now it still makes millions a year in profit!

Brmorgan said:

I'll have to read up a bit on that, but I think it will. I think the higher temps (~190°F) required for lumber to be officially stamped as KD-HT (Kiln Dried - Heat Treated) are mainly to kill any pests in the wood.

Click to expand...

190 That's a pretty high temp. I'd heard that if you heat the wood up to 150 for 6 hours it will kill all the bugs. Now I'm thinking I could take the parts off our old electric oven to build a kiln(now that the new kitchen is done)(-it's digital so precise temp settings would be easy-5 deg increments. I would use this for bug kill only not to speed drying process as we have low humidity levels in NM where I am.
Nice box Br!

BIG JAKE said:

190 That's a pretty high temp. I'd heard that if you heat the wood up to 150 for 6 hours it will kill all the bugs.

Click to expand...

What I've read over at woodweb.com says 130 degrees is sufficient to kill bugs. Now I'm confused.

BIG JAKE said:

190 That's a pretty high temp. I'd heard that if you heat the wood up to 150 for 6 hours it will kill all the bugs. Now I'm thinking I could take the parts off our old electric oven to build a kiln(now that the new kitchen is done)(-it's digital so precise temp settings would be easy-5 deg increments. I would use this for bug kill only not to speed drying process as we have low humidity levels in NM where I am.
Nice box Br!

Click to expand...

Just be aware the like car speedometers, domestic oven temp settings are not very accurate. Fortunately they generally read lower than they really are, Our oven says 250ºC (482F) but it's really only 238ºC (460F).

With the 190° I was referring to sawmills being able to use the KD-HT designation on commercial lumber stamps on finished lumber. This is in the big gas-fired kilns that can dry over 100 slings of lumber at a time (and in about 24 hours, give or take depending on weather conditions). Like I said I'm not exactly sure on the temp and will have to re-read my grading materials, and keep in mind that could also be more of a local requirement in British Columbia and may differ elsewhere.

Bob - I thought you were around Perth somewhere, just wasn't sure exactly where. When I was in elementary school, there were two girls at my school who were born there, though their family moved to Canada when they were still quite young and they didn't remember much of it.

Pretty cool!!! I was wondering about how to do small pieces...Great idea!!!!

Brmorgan said:

With the 190° I was referring to sawmills being able to use the KD-HT designation on commercial lumber stamps on finished lumber. This is in the big gas-fired kilns that can dry over 100 slings of lumber at a time (and in about 24 hours, give or take depending on weather conditions).

Click to expand...

Sort of like the difference between slow roasted and charbroiled.

To kill bugs the middle of the wood needs to get to 130 for a little while. I run 130-135 for about 24 hours. I think its 160 to set pitch.

I take my dehumidifier out when I up the temp to kill bugs, seems like I read a dehumidifier is only rated for temps around 110 ??

If I could put a theromostat on the great outdoors it would be 86 during the day and 68 at night

Brmorgan said:

Woodshop - The bulbs are 250W X 2, and I'm not sure what the fan is exactly but most small ones like that are less than half an amp at most. The dehumidifier is the one that really draws the juice. I don't know exactly what it's rated for, but they (along with air conditioners) are generally in the 1000 - 1500W range. I know the basement lights dim a bit when it kicks on.

Click to expand...

Thanks for the info, I was just curious. So, assuming that humidifier takes the middle of your estimate at 1250 watts, and adding 500 watts of bulbs, you're talking about 1700 watts/hr. You must have cheap electricity up there. Here on the east coast where I am it's 18 cents a kilowat hour last time I checked. That would mean 31 cents an hour to run your kiln, over $7 a day if I had that setup here. That would be some pretty expensive wood even if it dried it in only a week or so. Of course that humidifier doesn't run 24/7 so my numbers are not real world, but it still would be pretty expensive to run that here where I live. Figure how many board feet of wood it will dry and how long it takes to dry it and do the math... I'd be curious how much electricity it takes per bd ft.

Still a very interesting setup though, not knocking it.

Ya I don't know the exact numbers on electricity rates. They kinda have us over a barrel and we have to just pay whatever the rate is anyway. The gov't keeps giving them permission to hike the rates, and they also keep selling our rivers off to private power projects which will only up the rates even more as they have to siphon their precious "profit" off the top.

All I know is that my method HAS to be cheaper than the box my friend has, which is just heated using a 240V shop cube heater. I think those are something like 3000-4000W, not sure.

Backwood - 86°F is WAY too friggin' hot to be comfortable while working in IMO. Or anything else other than just standing around, for that matter. But then again, hot activities like going to the beach don't interest me in the least. It was only about 18C here today (close to your 68F night-time temp) and I was sweating like nobody's business pushing the lawn mower and dethatcher around all day.