solar kiln by bill stuewe

8/3/2019 Solar Kiln by Bill Stuewe

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Solar Kiln

byBill Stuewe

I reviewed close to 50 solar kiln plans on the Internet and settled upon this modification of my

own. Its basic dimensions are: 20' long x 8' wide x 7' high from the floor on the south side and 11'

8" high from the floor on the north side. Orientation is facing due south and the roof slope is 30

degrees (which is my latitude).

Originally, I had planned to pour a slab but found out that the concrete acts as a heat sink which

is not good for drying wood. Concrete can be insulated but the fumes from drying oak can

degrade the cement. Then, I was going to pour 5 concrete beams. The beams would have

worked just fine, but I came across a free 30 foot piece of railroad track and cut it into five 6'

sections to use as beams. I used dry sack-crete under each end to level them. I inset the first

and last 1' from each end and set the rest up on 4' 6" centers.

When I had all the beams perfectly level I poked holes in the sacks, watered them well, and let

them set up. I then put four evenly spaced 4" x 6" x 20' runners down (see Illus. 1) and then put a

deck of 3/8" exterior plywood on top of them. Next I put 2" x 4" x 8' floor joists down and put R-13

batt insulation between them. (I found out later that it would have been better to have used a

rigid foam type insulation instead of batts. Batts are not good to use because of the high

moisture problem associated with drying wood). Once the insulation was down, I put a layer of "

plywood decking (see Illus. 2) and painted it with aluminum asphalt roofing paint.

Illus.1 Illsu.2

I then framed the building on 16" centers leaving the square part of the East End unframed so I

could make it into a door. I covered the outside with corrugated sheet metal that I had on hand.


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Illus. 3 Illus. 4

Before I put the rafters on, I covered the entire attic floor with flat black 4'x10' flat sheet metal. I

set a few rafters in place to get the correct distance for sheet metal coverage on the south edge,

placing the tin against the "v" formed by the intersection of the rafter and ceiling joist (Illus. 6). Be

sure and cover the ends and north walls with the metal also. The entire attic area must be flat

black (Illus. 5). I put the rafters on 24" centers and made them 10 feet long. I ordered five 4'x10'

double wall polycarbonate panels for the roof (the roof needs to be double however you do it soas to prevent as much heat loss as possible-Illus. 4).

Illus. 5 Illus. 6

I capped the ends of the rafters top and bottom with 1"x6" cypress (cedar will do). The top soffit

hinges so that I can open it when the kiln is not in use and close it when the kiln is in use. (Illus. 7

& 8)

Illus. 7 Illus. 8


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The bottom soffit is doubled and two 5" holes were installed with a fly cutter between each rafter.

Attach screen wire on the top of the holes of the top board to keep bugs out, The bottom board

is fixed in place so that it will slide and regulate the intake of fresh air. (see Illus. 9 & 10)

Illus. 9 Illus. 10

I installed short 2"x4" spacers between the ceiling joists under the edge of the attic sheet metal

on the south side of the ceiling. I then ran the electrical wiring for the fans and lights. I put 2 attic

fans from the local home center about 5' from each end and right next to the north wall. They fitnicely between the 16" ceiling joist spacing and blow the hot air from the attic collector down into

the wood chamber along the north wall. (See fans in Illus. 13 & 15)

I then installed the insulation in the north, south, and west wall and the ceiling. Next, I put in a

vapor barrier of 6 mil plastic sheeting on the walls and ceiling and then I installed " plywood over

those areas (I used OSB but wish I had used plywood as it will hold up better in the moist

environment).

I then constructed the East End as one large door with a walk door built inside of the main door. I

only open the big door when I am loading/unloading the kiln. Two vents were then cut into the

bottom of the south wall about two feet from each end. I put a screened soffit vent cover from

the local building supply on the outside and a sliding " plywood cover on the inside to regulate

the amount of cooler moist air vented to the outside. (Illus. 11)

Illus. 11 Illus. 12The entire inside - walls, ceiling and floor were then painted with the aluminum asphalt roofing

paint to protect the construction from moisture damage. I then placed 2"x3"x4' boards on edge

on 16" centers to stack the wood on (see Illus. 12).

Once I had loaded the kiln with about 2,000bf of wood I attached a curtain of 6 mil plastic to the

ceiling next to the fans running the full length of the kiln from the ceiling to the top of the wood

stack and to the floor at each end. The curtain serves to force all of the hot air driven down from

the collector to have to go through the wood stack. The curtain is long enough to reach the floor.


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I leave it bunched up on top of the stack and it is slit from top to bottom in the center. That way if

I want to dry a half of a load on one end I can let that end go the floor. I use duct tape to tape

any seams that need taping.

Illus. 13 Illus. 14

Illus. 15 Illus. 16

I have a box mounted on the outside with switches that control the fans and an electronic

controller that a friend gave me. It monitors the temperature in the attic by means of a

thermocouple and turns the fans on at a preset temperature. I adjust it periodically so that the

fans come on when the temperature in the collector is about 30 degrees F hotter than the

morning low temperature for that time of year. Attic fans come with a thermostat that will do

about the same thing, but you must crawl up in the collector to adjust them.

Observations:

-- Normal kiln design is for 100 sq-ft of glazing area per 1,000bf of wood to be dried. (This kiln will

handle 2,000bf with its 200 sq-ft roof) This design could be easily modified to 8' wide x12' long

and would accommodate 8' long boards instead of 16'. This would be fine for most situations.

--- Temperatures will soar to over 200 degrees F rapidly in the summer time if the collector is

closed and the fans are left off.

-- I have reached 148 degrees F inside the fully loaded kiln in August and at the same time the

operating temperature in the collector with the fans on was 175 degrees F.

-- I have dried 8/4 loblolly pine in August from well over 22 % MC to 7%n MC in 2 weeks. (500+bf


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were stacked on one end and all the air from the collector was forced through that stack-see

Illus. 14)

-- Much information on kiln design and operation was gleaned from the "Sawing And Drying

Forum" at: woodweb.com

-- I may be contacted at:[email protected]

Or at my website: http://www.billstuewe.com/
mailto:[email protected]:[email protected]:[email protected]://www.billstuewe.com/http://www.billstuewe.com/http://www.billstuewe.com/mailto:[email protected]

solar kiln by bill stuewe

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