Hi Again - In the end I've constructed a solar kiln to reduce the MC in my old growth blue stain ponderosa pine boards, prior to cutting them to final stair tread length. I'm a newbie trying to put his best foot forward, so any suggestions are appreciated.

The kiln is 12' by 4' (outside walls)

The ponderosa boards are 5 1/2" thick, 11 1/2" wide. I have three 10' long boards and six 8' long boards. All ends painted with aluminum paint.

The MC varies within individual boards, between 22% and 14%.

My goal is to reduce the MC to as close to 6-8% as I can, within my time constraint.

My questions:

1) Is there an optimal temperature for drying thick ponderosa boards?

2) What temperature is the maximum without drying out too quickly? I've read softwoods are not as "touchy" as hardwoods.

3) I have two serious gable vent fans installed in the baffle, each capable of moving 1,450 cfm. I wanted this beefy for use when drying firewood. Is this way too much air for the boards? Should I install a speed controller, or even remove one fan?

Appreciate your help! - Dan

Hello Dan,

What is the finished thickness, width and length of the stair treads?

Where is this lumber now? Do you have the lumber stickered outside or inside.

Do you have any other heat supplying the kiln other then solar?

Does your kiln have a dry and web bulb?

How high of a temperature does your solar kiln get to?

Charlie

Hi Dan,
I just looked back at an older post you put up and I see the finished size for the stair treads is 5 ½ x 11 ½ x 36.

If you keep the lumber stickered it will get down to 12 to 14%.

Have you built a solar kiln before. I read in your last post that you didn't think you would have the time to build one.
Is the thickness of the lumber now 5 ½ and width 11 ½.

Charlie

Hi Charlie - the boards are now 5 1/2 thick and 11 1/2 wide. The shrinkage calculator indicates I could lose 1/16 to 1/8 in thickness off the 5 1/2.

I can add a heater to kiln. No wet bulb dry bulb instruments.

Lumber is in back of pickup waiting to be stickered in kiln. Was milled a week and half ago. Just finished kiln yesterday.

I read in the kiln operators manual that ponderosa at 15 to 20 percent MC can be kilned at 130 degrees. We have highly variable relative humidity here. Yesterday was19 percent during day, and 65 at night.

Thanks for any input!

Ok Now the boards are stickered in the shade next to the kiln. My understanding of the solar kiln is that monitoring wet and dry bulb is not necessary as long as the kiln air is allowed to rehumidify overnight, but my understanding may be incorrect.

I live in high desert, but our season is about to change with first rain in months coming in at the end of this week. If the solar kiln vents are to be closed during rain events, maybe adding a heater will be key to accelerating the drying process. I have an oil filled radiant plug in heater, do you think that would be adequate? Thanks!

Do you have any experience drying lumber. A dry kiln is a controlled environment when done properly.
Putting solar panels in a room with a few fans does not make a dry kiln. You have to have control of the heat and humidity.

Since you have had the lumber milled has any of the boards started to crack?

Is the building where you are going to put the stair treads heated? If it is you might want to take the lumber and sticker it in this room and let it dry there.

Charlie - you asked me if I'd built a kiln before. No, this is the first. I modified the Virginia tech design to be a smaller kiln.

I really don't have time because my building permit is up in 2 weeks, but I also am putting too much good money toward the stairway to rush the treads. I'm going to talk to planning Dept tomorrow to see what they say. The stairs are the only thing preventing final inspection of our addition, and maybe they'll just want more money to extend permit.

There were a few cracks after milling, but they were the air dried log ends. I coated the board ends with aluminum paint.

If I sticker the boards in the room, won't they be there for an unreasonably long time?

In a solar kiln, the temp rises until late afternoon. At night the kiln cools and relative humidity soars and moisturises the lumber, thereby reliving drying stresses.

I am not that familiar with Solar Kilns. Perhaps someone else that has experience with solar kilns can help you out.
I have dried lumber for over 30 years in conventional Kilns and I found it was always important to know the relative humidity in the kiln at all times.

When you said "In a solar kiln, the temp rises until late afternoon. At night the kiln cools and relative humidity soars and moisturizes the lumber, thereby reliving drying stresses." It would be nice to know what the depression you get in the afternoon compared to what you get at night. The thing you are trying to do is to get the center of the board to around 7%. If the solar kiln in the afternoon is drying the lumber to 4% or so, what is the lumber going to in the evening.

Hopefully someone with solar kiln experience will jump in and give you better answers then I have.

My guess is it will take a couple of weeks to dry the 5 ½ thick lumber to 6 to 8% but that is just a guess.