# solar lumber kiln



## tn.wood.turner (Aug 4, 2013)

i am about to construct my solar kiln . I have researched and researched and still have a few ?s . 
1) why cant i use whirly birds like i have on my roof instead of fans to move the air in the kiln . I planned on having 2 or 3 whirly birds and running conduit to the bottom/side of the kiln . I figured sucking the air thru the lumber will have the same results as a fan in the top blowing the air . I figured it will save the electricity that way , save me from running wires thru my yard to the kiln .
2) why the slopped roof to catch the sunlight . I understand the need for water run off on a roof , i understand facing the right way to catch the most sunlight , why not have an almost flat roof ? would that not catch more sunlight ? maybe the slopped roof helps move the air in the right direction also but i believe the air will move just fine in a properly built box/kiln . any advice will help ty .


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## TomC (Oct 27, 2008)

First let me say I'm no expert on solar kilns. However, my understanding of the angled top is to allow more sun light for the kiln. The angle is based on your latitude for the best angle of the sun. I believe if you use whirrly gigs you will get too much air flow thru the kiln and not reach the necessary temperature. Also, they would run at night when you have no heat source.
Tom


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## tn.wood.turner (Aug 4, 2013)

i can always use less whirly birds , i was afraid i would have to little air flow . winter time maybe i can burn waste lumber and run a metal pipe thru the fire and then into the mill . the hot metal pipe may warm the air in the kiln enough to assist the drying ? So building a sloped roof according to my latitude increases the amount of heat ?


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## mike1950 (Aug 29, 2010)

tn.wood.turner said:


> i can always use less whirly birds , i was afraid i would have to little air flow . winter time maybe i can burn waste lumber and run a metal pipe thru the fire and then into the mill . the hot metal pipe may warm the air in the kiln enough to assist the drying ? So building a sloped roof according to my latitude increases the amount of heat ?


Answer to last question is YES. Ideally the suns rays hit roof perpendicular. That grabs the most sun thus heat.


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## Tom the Sawyer (Sep 4, 2012)

tn.wood.turner

A slanted roof will also give you significantly more area to gather solar energy. If you had a flat roof on a kiln that was 4' wide and 12' long, you would have 48 sq. ft. of surface to gather solar energy. If you had that surface at a 45 degree angle, the surface area available would be 67.9 sq. ft. (a 41% increase - especially helpful on less than optimum heating days). In the winter it will shed show and absorb solar energy whereas a flat roof may hold the snow for quite awhile.

The roof angle, based on your latitude, also decreases the shading effect during the winter months. A flat roof would maximize solar gain in mid summer and, as the sun transits at a lower angle in the fall and winter, the side of the kiln would shade most of the wood inside the kiln.


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## Trav (May 30, 2011)

It's my understanding that Kiln drying wood is not just about drying the wood. It is about drying wood at a controlled rate. The electric fans can be wired thermostat or timer to maintain a smaller range of temperature within the kiln.


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## ETWW (Mar 27, 2011)

As already stated, the roof is slanted to the kiln's latitude to maximize the amount of solar energy available. You need one square foot of solar collector for every 10 board feet of capacity. That is based on safely drying Red Oak. For less than full loads, it's easy to simply cover part of the collector to maintain that ratio...not as easy to expand if needed. While not practical, an optimum design would also have the kiln rotating to track the sun's path across the sky.

Whirly birds would not only be difficult to control but they would also not provide enough air flow through the stack for efficient drying. On light-colored woods, you would likely have mold and mildew develop which will stain the lumber. You would also have to cover the whirlybirds at night. Otherwise, you might as well just air dry.

You want to shoot for shoot for approximately 150 feet per minute (fpm) air velocity through the stack. To calculate the size of fan needed (in cfm), multiply the number of sticker layers by the length in feet of the wood stack times the thickness in feet of the stickers (3/4 in. equals 1/16 ft.) times 150 fpm. 

I sized my solar kiln for 300 bf and have a 1000 cfm fan to circulate air through the stack. The fan is on a timer (about $30 at Homeless Despot) and is set to come on at 10:00 a.m. (when the humidity starts to drop) and go off at 8:00 p.m. (when the humidity starts to rise). On rainy days, I leave the fan off the whole time.

Adjustable, sliding vents on the back wall control the amount of moisture loss and the internal kiln heat. For hard-to-dry species like Oak, I generally leave the vents closed or nearly closed. The kiln is not vapor tight so there is still moisture loss but the drying is slowed so that it stays well within the safe drying rate.

I'd recommend following the Virginia Tech solar kiln design. It's a proven design based on the science of lumber processing. There are literally hundreds of them in successful operation across the country. You can downsize it to dry smaller charges as long as you maintain the solar collector-to-capacity proportions noted above.


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## tn.wood.turner (Aug 4, 2013)

well thats alot of feedback , ty . I may need to rethink the whirly birds . I really overlooked the aspect of a controled enviorment in the kiln . Roof will definetly be sloped now . awsome advice i look forward to sharing my results and feel much better having found a forum to discuss the matter .


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## Tennessee Tim (Dec 15, 2010)

Here's the best setup and actually used daily I've seen and read about. IF I was going to do a solar kiln THIS IS THE ONE!!!
http://www.timbergreenforestry.com/Solar Cycle Kilns.html and it's up in wisconsin....so if anything can handle their weather it's good!!

This is worth the reading for sawyers , kilners, and other saw business ideas....the site is not only about a kiln.

Something not mentioned regarding slope is originally the slope created a passive solar aspect in which the rising heat up the slope creates an air flow and the cooler being heated sucked up the slope and drew the hot air down the back wall and across the floor and back up slope creating a flowing/balancing effect. In wood drying we desire faster air movement for MC removal and so the fans are needed. The above link takes solar to another level and more control and choices when using.

Whirlybirds don't circulate air!!! they require a opening to inhale air that they exhale out the top defeating the purpose of using solar to heat with or circulate...they're design to remove heat. The fans are used to circulate the warmed air down and through the lumber removing moisture as it blows through.


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## Da Aardvark (Oct 11, 2012)

Ok. 
I'm bothered by all of this.

a) How can you keep even temps when confronted with differing sun/temp standards from one day/week/month to the next?
b) How can you expect to do a bug kill (130deg + for a solid day).

Sorry, but the sun isn't a stable reliable source and needs backup systems.
.


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## Tom the Sawyer (Sep 4, 2012)

One of the advantages of a solar kiln is that it does not maintain stable temperatures. It heats the wood significantly during the day and the temps obviously drop at night. Not as much as the outside ambient air because of the solar heat storage in the wood. The wood gives up its heat slowly overnight and the cycling of the temperatures and humidity in the kiln helps condition the lumber which helps avoid problems like checking, honeycombing, etc. 

I would recommend reading up as much as you can on the principles of solar kilns before building one - especially if you are going to come up with your own design. There are several, very successful and scientifically validated designs out there - free for the asking in many cases.


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## Tennessee Tim (Dec 15, 2010)

Kiln drying (getting a certian MC) and debugging (heating to kill the bugs) are actually two differ operations that are commonly thought to be the same and one. Example..most log home suppliers claim to have "kiln dried" logs (and WE ASSUME that's MC corrected) BUT they put them in a kiln to "debug" them, NOT correct MC them as we associate a kiln is used for.
I don't see a solar kiln being able to debug for 24hrs or more as I think it should be stabilized at 130 deg + for that long IMO. BUT I think it's probably the best way to speed up the drying process safely as Tom mentioned by the stress relieving/ conditioning by the fluctuation . I've read where some use solar's to speed up the conditioning and lower MC to 20% to safely enter other kilns for final speed MCing and debugging.


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## ETWW (Mar 27, 2011)

As Tom said, the solar kiln makes use of the lower temps and higher humidities present at night/early morning to condition the lumber and relieve the stresses that are introduced by daytime drying. Commercial kilns do this at the end of the drying cycle but it occurs in solar kilns every night. The solar kiln mimics what happens when air drying but with more control of the process. 

The VT solar kiln was never touted as being able to sterilize the wood via heat. I doubt if any solar kiln can do that reliably because the internal temperature needed can't be sustained long enough except maybe in the desert Southwest. The last numbers I read were 132 degrees for four hours _at the wood's core_. Wood density and thickness would obviously affect the amount of time needed to get it heated throughout to a given temperature.

"Heat Sterilizing" wood is not an end-all process for eliminating bugs, either. PPB's, among others, can and will infest the wood after heat sterilization. Treating green lumber with a borate solution immediately after sawing is a more effective way to avoid bug infestation, specifically PPB's.

Solar kilns are just one way of drying lumber. I like mine because it's virtually a load-it-and-forget-it method. I don't have to be too concerned with safe drying rates, measuring moisture removal or conditioning at the end of the drying cycle. It may take slightly longer than other methods but lumber degrade is minimized and operating costs are minimal.


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## Da Aardvark (Oct 11, 2012)

Milwaukee Woodworks showed me a method of drying that was pretty good. He placed stacks of lumber in the attic of his garage, and the temps can be quite high. It's a slower process which is likely better in many respects.
That is about the cheapest solar kiln I've ever seen, and already on site.
That said, it still doesn't address the bug kill issue.

I do it a little different and just tarp over the stack and heat with a small electric space heater. It allows me the opportunity to fine tune the temps and adjust at will. A solar kiln doesn't allow that. I'll go for a number of days, at ~90deg, and when everything is near 6%mc, I'll kick the heat up for 24-36 hours to 130-140 deg and bug kill. After that, I'll gradually bring the temps down slowly to room temp over a days time. It's consistent.

With solar, there are too many variables that (personally) I don't like. Seasonal/night/day are the biggies.


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## Mountain (Aug 1, 2013)

When I worked at New Homestead Log Homes building log home kits, we would kiln dry the wood for three days. When the wood came out was usually still up around 25 percent moisture content, you could feel the moisture coming out of the logs as you worked on them.

As was previously posted, I think the kilning process there was about killing bugs rather than drying.

I have an old 250 gallon fuel tank that I converted into a stove, it will fit junk wood up to five feet long and ten inches square. I originally made it to help me more safely burn brush piles and whatnot, but my new intention is to provide winter heat for drying lumber. I can fit up to 1/5 of a cord of wood into it at a time and shut it down and it will burn for about 4 days.

I was never really concerned with drying my lumber, but with all this poplar/aspen hybrid I have been cutting, I am realizing it is basically the only way to dry this stuff.

I have a 20 x 50 greenhouse, that I may try to use for some summer drying.

Has anyone tried using a solar kiln in the dead of winter? The temps here in January are commonly hitting as low as -30, somehow I don't see solar doing me much good in winter time here.


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## ETWW (Mar 27, 2011)

Mountain said:


> Has anyone tried using a solar kiln in the dead of winter? The temps here in January are commonly hitting as low as -30, somehow I don't see solar doing me much good in winter time here.


At -30 F, little to no drying will take place in a solar kiln. The temperature inside will still be well below freezing. Below 40 degrees, very little drying occurs. Drying lumber is, after all, nothing more than evaporating off the moisture from the wood surface. Heat, air flow and low RH facilitate that evaporation process.

We rarely even get into the teens in my location and often see 60 - 70 degree daytime temps in the dead of winter. The solar kiln will usually raise the inside temp about 40 degrees over outside ambient so the drying rate is still acceptable. I actually prefer winter drying because it's a lot more pleasant handling the lumber in winter than it is during the 100+ degree days of summer heat.

In your case, a mechanically heated dry kiln is probably your best option. However, initial kiln cost and operating costs will be significantly higher than using solar energy.


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