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Kevin Jenness
12-28-2015, 7:29 AM
For those operating a solar kiln with electric fans on a thermostat, do you adjust your thermostat setting seasonally to a given point above average ambient temperature, or use some other strategy for kiln control? My understanding is that significant drying occurs with the kiln at least 20 degrees F above the outside air temp and have been setting my thermostat in that range. I currently have it at 65, as the early winter temps so far have been unusually high, but I expect them to go down to the typical 20 degrees. I don't want to waste power pushing air through the stack at temps too low to accomplish much. Any advice?

Cody Colston
12-28-2015, 8:51 AM
The fan in my solar kiln is on a 24 hr timer, not a thermostat. I set it to come on at 10:00 a.m. and to shut off at 8:00 P.M. Temperature, as well as drying rate, is adjusted via the vents. For a hard-to-dry species like Oak, I just open the four vents about 1 inch. That doesn't vent the moist air very fast and the moisture also keeps the inside temperature down. When the wood is dry, I close the vents entirely to raise the temperature as high as possible. It's always a compromise between the kiln temperature and venting moisture.

Most of the time, the lumber I place iin the kiln is already air-dried very close to EMC and the kiln is for bringing it down to 7% MC. However, on a hard-to-dry species like Oak, I'll go to the kiln immediately with it, especially in the summer, since air-drying is pretty much an uncontrolled process.

Kevin Jenness
12-28-2015, 11:23 AM
Up here in VT this is a pretty cloudy time of year, so there are many days when the internal temp of the kiln is very close to ambient.I'm not convinced that running the fan in those conditions gains me anything, but I'm open to rethinking it. The fan is on a timer as well so at night after a sunny day the load gets a chance to "relax", but that is the least of my problems in the winter season.

Cody Colston
12-28-2015, 4:33 PM
Ok, I understand now. The thermostat is to conserve the energy used to run the fans on days that the temperature is too cold for effective drying. I can see where that would be a concern in Vermont.

Someone will correct me if I'm wrong but I don't think any drying will take place below 40 degrees. The kiln vs ambient differential doesn't matter if it's 20 degrees ambient and 40 degrees inside the kiln. No drying will occur.

So, I think the 65 degrees you have chosen is a good temperature to have the fans cut off. Below that temperature not a lot of drying will occur nor is mold/mildew a concern. Those same conditions make winter (at least around here) a good time to start air-drying lumber. The process is slow and gentle while mold/mildew is not a concern.

Charles Randal Smith
12-29-2015, 1:05 AM
According to Dr Gene Wengert, who was a researcher at the US Forest Products Laboratory and has written several books of relevance, "As a general guideline, wood dries very, very slowly when under 50 F (10 C)." When using a solar kiln in wintertime, you are in a balancing act between adding sufficient heat and having sufficient insulation. He goes on to say, "Adding heat to a solar kiln when the sun is not shining or it is cold is very inefficient unless the collector has an insulation blanket on it. Of course, the walls and floor need to be insulated well too. Even on a sunny day, if the temperature is around 50 F, the solar input for eight daytime hours is less than the heat losses, so it is better in cold weather to go non-solar, if you want the kiln to work... If heat is not added, winter solar drying is very slow."
I hope that helps.
Charles

Ed McEowen
01-07-2016, 8:06 PM
Cody,
You say you go directly to the kiln with oak in the summer. What steps do you take to keep from drying too fast?

Scott T Smith
01-07-2016, 8:47 PM
Cody,
You say you go directly to the kiln with oak in the summer. What steps do you take to keep from drying too fast?


Ed, in answer to your question it all depends upon the design of your solar kiln. The most popular design is based upon the Virginia Tech unit, which is sized for 1000 board feet of green 4/4 oak. The key to keep it from drying too quickly is to load the kiln to the design limits (1000 bd ft), to have a deflector between the solar panels and the wood stack so that the sun can't hit the lumber directly, and to ensure that your kiln is built fairly tight so that there is not excessive moisture loss.

Air can only absorb a given amount of moisture based upon it's temperature. With 1000 bd ft of oak in a VT design solar kiln, the lumber will only give off enough moisture to saturate the amount of cubic feet of air inside the kiln, and then the lumber wont release any additional moisture.

If you load too little lumber into a solar kiln, then you risk drying too fast because there is more air to absorb moisture (since it is not occupied by lumber), and less lumber to give off moisture, which allows the smaller amount of lumber to release too much moisture.

Because 8/4 oak dries 60% slower than 4/4 oak, to keep from drying too quickly you would need to cover up approximately 60% of the solar collector on a VT based solar kiln in order to keep it from drying too quickly during the summer.

Clear as mud? (lol).

Ed McEowen
01-08-2016, 1:05 PM
Thanks for your very explicit response (it is clear). I have a V.T. kiln designed to dry 12' length lumber, but have never cut enough 12' boards to fill the kiln to its 1000 sq ft capacity. Would covering up a proportionate area of glass be all that is needed to dry a smaller than full load of 1" green oak in the summer without damage? (I assume you close the vents down until the moisture in the wood diminishes to a certain point). So far, I've always air-dried first, but do sometimes partially cover the glass on smaller loads.

Cody Colston
01-08-2016, 5:20 PM
Cody,
You say you go directly to the kiln with oak in the summer. What steps do you take to keep from drying too fast?

I've dried mostly 4/4 Red Oak in my kiln with a few planks of 8/4 mixed in and had very good results. It's a downsized VT Solar kiln design for 300 bf capacity of 8.5' boards. I use a piece of black-painted 1/2" plywood on top of the lumber to block direct sunlight. A black tarp stapled to the fan plenum and draping onto the plywood directs air flow through the lumber stack. Concrete blocks are placed on top to keep the tarp in place and add weight to the stack for flatter drying.

To prevent too-rapid drying, I start off with the vents only cracked open about 1/4". After a couple of weeks, I'll open them about 3/4" and leave them there until the lumber is at 7% MC. That time varies depending on time of year. I then close them all the way for a day or two to try to bring the heat up enough to sterilize the lumber. I don't think it completely sterilizes it but it makes me feel better.

I began going directly to the kiln with Oak after I tried air-drying some Post Oak in the late spring/early summer. It did not turn out very well. One day of 90+ degree temps with a hot, dry wind blowing can ruin Oak and I think that is what happened to the Post Oak. Now I go to the kiln with it so that I can better control the process. Like Scott wrote, the air inside the kiln can only hold so much moisture and then drying virtually stops. The moisture removed is regulated by the vents. I have always filled the kiln to capacity so I've not yet had to partially block the solar collector.

Remember, too, that at night, after the fan shuts off, the RH inside the kiln goes to near 100%. Drying stresses are relieved by that high humidity.

Cody Colston
01-08-2016, 5:27 PM
Ed,

The VT kiln design specifies 1 sq ft of solar collector for every 10 bf of 4/4 lumber being dried. In your case, I'm assuming you have 100 sq ft of collector. So, if your kiln was loaded to half capacity (500 bf) you would need to cover half of the solar collector or 50 sq ft.

Ed McEowen
01-08-2016, 6:13 PM
Thanks, someday I'll get brave and try a load of green oak in the kiln. When drying 18 per cent or lower, I haven't noticed much quality difference in (less than full) loads partially blocked or the ones completely open, so I quit covering. (But I'm usually at least 75 per cent capacity).

Scott T Smith
01-08-2016, 8:55 PM
Thanks, someday I'll get brave and try a load of green oak in the kiln. When drying 18 per cent or lower, I haven't noticed much quality difference in (less than full) loads partially blocked or the ones completely open, so I quit covering. (But I'm usually at least 75 per cent capacity).

Ed, once lumber dries below 25% MC, it is almost impossible to dry it too quickly. The most critical part of the drying process is from green down to 35% MC.

Yes, blocking off part of your collector should allow you to dry a partial load of green 4/4 oak. Additionally, shutting more vents so that the RH% stays higher in the kiln will also help.