Adding Supplemental Heat to a Solar Kiln

Extra heat for a solar kiln could help in some circumstances, but hurt in others. February 12, 2007

Question
I am trying to come up with a way to supplementally heat my solar kiln. We have a small solar kiln (1200bd') which is located on a piece of property with no grid power. We'd like to add some thermostatically controlled supplemental heat with propane. I was hoping for some sort of thermostat, as I would have to be able to leave it unattended during the day.

I thought I would throw it out here and see if anyone else had seen or come up with some sort of homebrew setup. The kiln is out in a field, so while safety/fire would be a concern, as I don't want to build another kiln, this wouldn't have to be some off-the-shelf UL listed setup either. Our fans are controlled by a small battery bank and a single 12v solar panel, so a small amount of power for the thermo/valve would be available.

Our goal is to be able to speed up the process a bit in less than optimal weather and also be able to get the heat up to the kill zone if needed.

Forum Responses
(Sawing and Drying Forum)
From contributor W:
One of those glow heaters that fasten on top of a propane tank might work - no power needed. We use them in our goose blinds. They need to be away from flammables and you would want to vent the kiln before entering.

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From Professor Gene Wengert, forum technical advisor:
If you add heat, you will likely caseharden the lumber, which you probably do not want to do. Check the archives for casehardening info. The advantage of a solar kiln is that at night, it cools off and the humidity increases and that relieves the casehardening stresses. (Nothing is harder than anything else; the word refers to stresses in the wood.) I discourage the use of supplemental heat in solar kilns. In fact, if you add heat, then maybe a non-solar kiln would be best.

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From contributor B:
When you say "discourage the use of supplemental heat," I will take you at your word. The bottom line problem I had with solar heat in a kiln was holding high enough heat for a long enough time to set the pitch in pine. Still haven't resolved that issue, so the solar kiln only gets hardwood.

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From contributor N:
What is wrong with adding heat, say maintaining the temperature at 120 degrees F for 10 hours or so during the day and turning it off for the night when the weather is cloudy day after day? I am thinking particularly of a kiln with a collector (not greenhouse type). I do this all the time and with good results.

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From Professor Gene Wengert, forum technical advisor:
It is okay to add heat if it is not too much and if the wood does not dry too much and if the MC is low. It is okay to set the pitch in pine the last day or two by adding a lot of heat. Casehardening stress only develops at high MCs.

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From the original questioner:
Thanks to all. Gene, my thoughts were somewhat towards contributor N's post in that I was thinking of adding the heat during the day and still allowing the kiln to cool overnight. I guess in hindsight I am more thinking of using the heat to keep the process going during bad/less than optimal weather rather than to achieve higher temps.

I thought that it may be very difficult to get sustained high temps in a solar kiln due to the loss through the glazing. I guess perhaps I should think about building some sort of collector kiln in the future which could be multi-fuel (solar, wood, propane) if needed.

To echo contributor N's post, what would be the problem with heating supplementally during cloudy times and still cooling at night? Waste of time?

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From contributor P:
In other words… Is the conclusion here that, once the wood in a solar kiln is pretty much dry, you could close the vents completely, supplement the heat so as to safely sterilize or set pitch, according to what your goal was for the load?

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From Professor Gene Wengert, forum technical advisor:
If you add heat during the day, you may develop more stress than can be relieved at night, especially if it is a cold night. Cold takes longer.

Contributor P, yes, that can be done.