Winter Operation of Solar Kilns

Solar drying kiln performance slows significantly in the winter. Here are tips on wintertime operation. July 15, 2012

In a couple of months there will be sub-zero temps in NW WI. How will that affect the lumber in my kiln? It's at 13 percent MC now.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
When the outside is below 50 F, you can expect very, very slow drying, if any. The sun angle is also low and the hours of sunlight are much shorter than summertime. Basically, you will get little to no drying from November through most of March in the area where you are located.

From the original questioner:
Thanks Gene. I have the fans on a timer. Would you recommend the fans run or should I shut them down for the winter?

From Gene Wengert, forum technical advisor:
Run them whenever the inside is over 50 F. Shut them off when it cools under 50 F.

From contributor W:
I use a solar kiln (greenhouse style with plastic under the floor) for most of my winter cutting, since it gives a nice slow start to the drying. Last year I was surprised to find the white oak sawn to 4/4 in December was between 20% and 30% by Spring. No fans, just stacked and stickered. So I'd agree to not run fans, unless the temperature climbs, but don't be surprised to get more drying than you think.

From Gene Wengert, forum technical advisor:
White oak has about four gallons of water for each 1% MC per MBF, so if you dried from green (65% MC) to around 25% MC, that is a 40% MC loss. So, a 1000 BF kiln would have to lose about 160 gallons of water. So, ask yourself how all that water is getting out of the kiln. In other words, the vents need to be open. The average humidity in most of the USA in the wintertime is 65% RH, which is 12% EMC, so the air will indeed encourage drying, but the cold temperature will mean any drying is really slow.

From contributor W:
Definitely venting. I guess I didn't mention that, but we do use a vent fan on days when we've had a lot of sun on a very wet pile. At times you will also get condensation dripping off the ceiling (or freezing to it), so we plan for that with a rain cover on the piles.

I should also explain that my modified greenhouse approach is not the classic solar kiln design. Itís intentionally more of a passive system - a 24'x72' hoop house with a double layer plastic pillow. At any point I have dried wood on one end and wet wood on the other, with just a tarp draped between them. That's why I don't run fans on the individual piles - I don't want the air to circulate too much.

Typically we try to keep the vents closed during the day to generate as much heat as possible, but then try to do a couple air changes of venting toward evening just to pull the wet air out before it starts condensing. The wood also dumps whatever heat it has left into this cold dry air, which in turn lowers the RH (and maybe donates a bit more moisture also). I'm in sunny southern Wisconsin, so a bit warmer, but when not actively venting I only drop below freezing inside my greenhouse on nights that are below 10 degrees.