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Thread: Extending the Season of my Solar Kiln

  1. #1
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    Extending the Season of my Solar Kiln

    I built a solar kiln last Summer. It works as expected from late Spring to early Fall. But from mid October to mid April moisture loss is on the order of 1%/month when starting with wood below 28%; might better leave it outside. So in an effort to at least extend the seasons, if not through the entire Winter, I added a false ceiling inside the kiln, and added a heater, blower, and dehumidifier, effectively turning it into a dehumidification kiln. Here's what it looked like as built:



    It's 8 x 12 ft and can hold 5 - 700 bf. Fully AD 4/4 lumber will dry to 7% mc in less than 2 weeks, while nearly green 8/4 lumber takes almost 100 days in Summer. Here's what the inside now looks like with the new false ceiling:



    It's just a layer of 2" closed cell foam and a layer of R19 fiberglass on top (added today). It's held by ledgers around the walls and propped up in the middle on the wood stacked below. If you look closely you will see the back of the dehumidifier, too. On the other end is the fan and heater:




    The blower fits in a baffle to block off the end of the wood stack, forcing the air to the front of the kiln and back through the stack. The heater is a 1500 W oil filled heater, now set at 900 W. I loaded the kiln with 500 BF of 6/4 and 8/4 walnut at about 34% MC last week and in the first 5 days it has averaged a moisture loss of 2%/day with a temperature of 105 - 112F. This morning it was 30F and the temp. had dropped off to 101F (still a 70 deg delta which seemed amazing with only 2" of foam for a celing) so I added the R19 fiberglass on top. My calculations showed that at 20F it should take about 1200 W of heat to keep the inside of the kiln at 110F, assuming I add a layer of 2" foam to the walls and floor, which is yet to be done. If true, that should allow the kiln to operate through most of the Winter.

    So far I'm pleased.

    John

  2. #2
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    It looks great John but I'm concerned about your electric bill

  3. #3
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    Quote Originally Posted by Tom Bender View Post
    It looks great John but I'm concerned about your electric bill
    I think the added cost will be less than $0.50/bf. I'm OK with that. Logs are free and my sawing costs are less than $0.10/bf. Not many folks can buy hardwood lumber for $0.60/bf.

    The first load of wood has been in the kiln 2 weeks now. It went in at 32% MC and is now at 13%. It should be to 7% in another 10 - 14 days, pretty much what I had hoped for. If so, it will be less than $0.50/bf in added drying costs.

    John

  4. #4
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    The big issue with 'extending' the season of a solar kiln, is keeping the inside above freezing. It is a large heat load to warm up from below 40 degrees. I have seen solar kilns with folding insulated shutters to maintain as much temperature as possible.

  5. #5
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    OK, an update. The modified kiln reached 7% MC on day 29. I kept going until I got to 6% which required a total of 33 days. I was very pleased with that. I ended up adding a second heater for a combined total of 2500 watts, which gave me a 100 deg temperature rise over ambient, up to 140F where one of the heaters reached its cutout limit. After I unload the kiln I will add 2" foam board to the walls and floor. That will increase the insulation enough that I will be able to go back to one 1500 watt heater and maintain at least a 90 deg rise above ambient.

  6. #6
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    So school me here... You're transferring water from inside the lumber into the air inside the kiln. If you're running the kiln at 100 degrees above the outside temperature, I presume you've sealed the kiln to avoid trying to heat the whole world. So where does the water go?

  7. #7
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    Quote Originally Posted by Jamie Buxton View Post
    So school me here... You're transferring water from inside the lumber into the air inside the kiln. If you're running the kiln at 100 degrees above the outside temperature, I presume you've sealed the kiln to avoid trying to heat the whole world. So where does the water go?
    Ideally, the dehumidifier I mentioned would remove it. But since I didn't seal the unit up as well as I would have liked, the RH dropped below the lower control limit of the dehumidifier pretty quickly. So, yes, there's a little air leakage, not much, but it doesn't take much for the moisture to escape.

    I will try to seal the unit up tighter before the next run to have better control of the RH.

    John

  8. #8
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    A further update. I finished drying that load of wood in 34 days, going from 32% to 5.8% based on the sample weight. I was adding 2500 W of power at the end and the drier temp. was 142. After I emptied the drier I added 2" foam board insulation to the walls, then refilled it with a load that had an average moisture content of 21%. With 500 watts of temp. controlled heat it's running at 122F, even when the ambient temp. is 25F. I have no dehumidifier running. Because it's better sealed, the RH is running higher than the prior run, which is good, but it still slowly leaks out moisture. Fortunately, the leakage is just about right for it to stay in the correct range vs. MC per EBAC's drying schedule. In 7 days the MC has dropped to 11.9%. When it drops below 10% I will increase the temp to 130F and then 140F when it gets to 8% to kill any bugs. I think it will be done in another 7 - 10 days, and then a slow cool to ambient + 40F before I unload it. This is turning out to be a very easy and economical way to keep processing lumber during the Winter months. Selling just two or three slabs has paid for the cost of the insulation. The power cost will be less than $50 for a 20 day run. I'm very pleased.

    John

  9. #9
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    Man that sounds like it’s working perfect! I wish I had power out to mine, I’d give it a try.

  10. #10
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    Quote Originally Posted by Jebediah Eckert View Post
    Man that sounds like its working perfect! I wish I had power out to mine, Id give it a try.
    There's no permanent power to mine either. I ran two extension cords out to it, 200 ft from the house. 12/3 extension cords aren't outrageously expensive and are more than adequate for up to 1000W over that distance. Being able to dry lumber and slabs all year is a huge benefit and was well worth the $500 it took to make it happen. Next year I'll have to look into some permanent power.

    John

  11. #11
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    A further update. I have now processed 3 loads of lumber in my modified kiln. One load started at 32% MC and took 33 days to dry to 6%. The next two started at 21 and 24% MC; both took 13 days to dry to just under 7%. My power bill is about $50 higher per month when running the kiln, so that means about $0.10/bf for the 500 bf/load. Besides being able to dry wood all winter now, I also can sterilize it. At the end of the drying cycle I increase the temp. to 140F for 24 hours. Even at 20F outside 1000 W of power has been more than enough to do so.

    The drying equipment is cheap and easily controlled. I use a household dehumidifer and an oil filled radiator heater. Each is controlled by a digital Inkbird controller I bought on Amazon. The used dehumidifier cost $40, the heater $50, and the two controllers about $80. The largest expense was about $300 for the 2" foam board insulation. Call it $500 total. I generally follow EBAC's drying schedule, except I spend more time at 100F as that is the approx. maximum operating temperature of the dehumidifier. When the drying rate falls off below 30% RH I turn off the dehumidifier, raise the temperature gradually to 130F and let the remaining moisture leak out of the kiln on its own. When I get to 8% MC I raise the temp. to 140F for 24 hours, then gradually cool down. It's a simple, straightforward process and the lumber quality has been good.

    It is possible to dry lumber in the Winter in a northerly climate without spending a lot.

    John

  12. #12
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    That sure sounds like it is working great!

  13. #13
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    That's great, I'd like to try that someday. I didn't know it could be done so cheaply. I've always just air dried my wood so far.

  14. #14
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    Same, this is far away in the future, but these are really great results as far as time and money. Too bad im all out of room to build something as nice or large as John. I have plenty of lumber to dry however, and would love to churn out 500bdft of it every month.

  15. #15
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    Quote Originally Posted by Patrick Kane View Post
    Same, this is far away in the future, but these are really great results as far as time and money. Too bad im all out of room to build something as nice or large as John. I have plenty of lumber to dry however, and would love to churn out 500bdft of it every month.
    Gotta a garage or shed roof? All you need is some place out of the weather for a 12' x 8 ft x 4 ft box of foam panels. Maybe the boss will let you steal a few square feet in the warehouse at work. It doesn't have to be pretty or permanent. You'll come up with a viable option if you think about it a little.

    John

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