Venting and Fans for a Solar Kiln

A quick expert explanation from the Wood Doctor of how and why vents and fans are used in a solar wood drying kiln. April 20, 2011

I got my solar kiln up and running last weekend. I loaded it up with some air dried pine at 15 - 20% MC and want to bring it down. Daytime highs in the kiln are 120F and 45% RH during the day. I am down to about 12 - 15% MC now and just wondering how much venting should happen? It is not airtight and I have some venting at the bottom and nothing at the top at this moment. Should I start venting at the top as well? I am also not sure I have a large enough fan in there but I am not in a hurry and no mold issues so time will tell.

Are there some guiding principles on how much/when to vent during the drying cycle? Would more venting lower RH in the unit? It seems logical. MC has not changed in the wood the last 24 - 48 hours.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
First, mold will only occur above 20% MC, probably above 30% MC for pine. Venting is done to lower the MC to achieve lower MC’s. Always open both top and bottom vents together, but only crack them open a small amount. Too much venting lowers the temperature and that means slower drying and higher RH values. But you do need some venting except for the last few days.

From the original questioner:
I will vent the top this eve and monitor and report back.

From Gene Wengert, forum technical advisor:
If you run the fans 24/7, you cannot go much under 12% MC. Getting 12% will take some time. Generally, run the fans only when there is solar heating. 10AM to 8 PM oftentimes. Avoid excessive venting. Just crack the vents a very small amount.

From the original questioner:
I am in Ottawa and we had a few dry sunny days last week. Did not turn the fan on till noon one day and it was up to 55C and 20% RH inside. Much of my wood is in the 10% EMC right now so I plan to take it out tomorrow and reload before I go on vacation. Most wood in my house is 10% EMC as well so when I get back I can finish my dining room table project. Though this idea of a solar kiln is something I should have built a few years ago! Once I take this load out I may get it a bit more airtight and add another fan.

From Gene Wengert, forum technical advisor:
The wood in your house in the wintertime will be under 6% MC (30% RH). If you’re wood inside is 10% MC now, that means that the house has been at an average of 65% RH, which is not possible in Ontario, as that is more humid on the average than outside. So, I suspect your moisture measurement is in error by 2% MC or so.

Regarding fans in a solar kiln: They have two jobs - circulate air and vent moist air to the outside. When circulating air, they have two jobs. Number one is to pick up heat from the black collector surface and convey this heat to the lumber and number two is to pick up moisture from the lumber. With this last item it is really important to use high velocity when the lumber is above 40% MC as higher velocity means faster drying. However, under about 25% MC, the velocity has very little effect on the drying rate, so increased velocity means more energy use, but little benefit in drying (unless you need more venting).