Adding Heat to a Solar Kiln in Winter

Supplemental heat is tricky, because there's a risk of over-drying and casehardening. May 13, 2009

Question
Would adding heat to a kiln in the winter time degrade wood? I was thinking of taking a 55 gallon drum and making a 55gal drum heater with sheet metal around it then blowing air over the drum and into the kiln.

Forum Responses
(Sawing and Drying Forum)
From contributor S:
I'm doing something similar right now although supplemental heat is from propane and electric used in tandem. I'm assuming you are talking solar kiln otherwise it would have heat all the time? If so I can tell you from experience that the solar collector will bleed off so much heat that you will need massive heat input to keep up with the losses. I found a calculator for greenhouse heat loss and my 10'x17' collector required about 70,000 BTU to heat to 150 at -10 outside. Adding 2" removable Styrofoam panels (the foil faced reflective type) to the roof allows my 5000w (about 19,000btu) heater to maintain 140-150 at -5 so far.

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From contributor L:
If you are bringing in outside air and running it over the heater into the kiln, it will over-dry. Not really a good idea. Never do it with wood over 20% if you decide to do it.

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From contributor K:
What about using an old walk-in freezer, and a dehumidifier? Would that work?

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From Gene Wengert, forum technical advisor:
For the common "semi-greenhouse" solar kiln design, you really need to insulate the collector very well before using supplemental heat. With heat, you can expect to generate some drying stresses (casehardening) as well. As mentioned, the heating process using outside air can easily over-dry and result in cracking of wetter wood and over-drying at the end of the cycle.

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From contributor S:
As long as the OP drew inside air from the kiln and re-circulated it around the heater and back to the kiln there would be no outside air introduced so less chance of severely drying the air correct? To run a propane heater in the kiln I need to duct in a small 3" pipe to just behind the burner or the burner quickly uses up the available O2 and dies. Will this small amount of combustion air present an over-drying problem? RH seems to get to around 17% in that environment and the lumber will already be dried to 12%.

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From Gene Wengert, forum technical advisor:
Yes, even with the small amount of air, there is a good risk. For this reason, a plenum system is suggested where the combustion air and the kiln air are not mixed. Then as you indicate, recirculation will avoid low RH’s.

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From contributor S:
I will only use propane with external combustion air for a short time for heat treat so say 6-8 hours. After that i can let it cool right off and the RH come back up. Will short exposures like this with already air dried lumber (12%) be a problem? I probably will adapt a plenum system in future similar to the OP as well.

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From Gene Wengert, forum technical advisor:
That should be just fine. One other advantage of a plenum system is the reduced risk of fire. With a direct heating system that you have, make sure that you use a smoke detector, that your insurance is appropriate (commercial insurance for fire if you are selling the wood, etc.), and that you clean the kiln very well after each load. Fine dust is very combustible. In case of a fire, always wait until the fire department arrives before opening the door. Avoid supplying oxygen before the department is there.