Automatic vents for solar kiln

Can vents be left open at night or must they be controlled? (Sawing and Drying Forum) April 29, 2003

Has anyone fabricated automatic vents for a solar kiln? I was thinking about using the automatic vents you see on houses. They are fully closed at 35 degrees and fully open at 60. I was also pondering the idea of using an electrically operated actuator (like a hydraulic cylinder, but uses an electric motor to raise and lower the ram) to open them. Any comments?

Forum Responses
(Sawing and Drying Forum)
From contributor J:
A cheaper method would be to use a linear screw actuator. This way the hydraulic pump and ram are eliminated and you will get better proportional control with it. What do you plan on using for a controller?

How would you decide when to open and close the vents? Generally, we would never change the vent setting during the day anyway. Tell me more what you are thinking.

Gene Wengert, forum technical advisor

I am about to complete my solar kiln and have been thinking about the same thing. I figured I needed a way to open the vents when the fans came on and close them when the fans went off. I have a thermostatic controller to do the fans. I would manually set the exhaust vent maximum opening (depending on the type and dryness of the wood) and let the controller open and close it to that set point as the fans come on and off. I just have not figured out a cheap, easy way to open and close them.

From the original questioner:

If I have researched correctly, I'm supposed to close my vents at night to keep the moist air inside to help relieve drying stresses. There are some days I can't get to the kiln to shut the vents at night. Also, in the morning when the kiln heats back up and the thermostat turns the fans on, the vents need to open up and evacuate this moist air and start the drying process again.

This is the schedule of open/close I was thinking of: fans on in AM (timer controlled thermostat), vents open simultaneously; then in PM vents close (a timer) probably an hour or so before the fans turn off to build up some moist air for the night, then the fans cut off with my timer at sunset. This scenario would mainly be used in the latter part of the drying process, since I usually leave my vents closed until I get the charge to <28%.

I was thinking of using one of the linear screw actuators to open all my vents. I would have them all connected with levers. Hard to describe, but I think I could get it to work. I've read that some heat pumps or gas packs have dampers with actuators and they are controlled by a thermostat. I'm still researching the exact control devices, but am open to any ideas. I have my wet bulb working again, and I'm going to try just leaving my vents at one setting 24 hrs and see if I lose much moisture at night with them open. Since the fans cut off, I may not be losing as much moisture as I thought, but I know I'm losing heat leaving the vents open. When I check my temps in the AM, the interior is the same temp as the outside ambient air, which is why I'd like to close the vents. By doing that and adding my second layer of glazing, I think I can keep my heat in.

Contributor J, check with your local heating and cooling company. They have 24Vdc motors that work off a simple thermostat and relay. They use these in zone controlled heating and cooling systems. I have them in my house and they work great for opening and closing my vents.

The vents are open only a crack most of the time. They do not open wide, which would exhaust all the heat along with the moisture. They are just cracked open. You can leave them open at night without any problem. In short, you will probably adjust the vents three times during a 4 week run. So, you do not need automatic vents.

Gene Wengert, forum technical advisor

To the original questioner: Why would you let fans run after vents are closed?

Gene: If vents are only open a crack most of the time, it makes one wonder if the surface area of the vents are necessary (most of the time). What is this?

If you load a fast drying wood like southern pine or soft maple, you might want to run full blast with the vents open 100% to exhaust the surface moisture and prevent mold in these fast drying species. In essence, the kiln is used as a one-pass dryer - air comes in, picks up moisture, and is exhausted. Solar heating is not a big component initially. That is why the vents are large - some days you might need large vent capacity. But with oak and many other species, just a small vent opening. In fact, at the final stage, you want as much heat as possible in order to get low MCs.

Gene Wengert, forum technical advisor

From the original questioner:
I close the vents sometimes at night with the fans running for a brief time, just to circulate the humid air. Just a quirk of mine, makes me feel better about case-hardening. I think I am going towards Dr. Gene's advice about leaving them cracked ever so slightly all the time and not worry about closing them. I dry mostly oak, hickory and walnut, so I never open the vents wide open. The auto vent idea was something I've been thinking about for a while and just wondering out loud if it was a worthwhile venture. The only time I really open the vents up is from about 27% to 11%. I like to keep the moisture in > 27%. Evacuate the moist air from 27% - 11%, then with little moisture left in the wood at 11% build the heat up until finished. I think after this charge and I put my second layer of glaze on, I won't have to worry too much about the vents and heat loss issue.

My solar kiln

How about using gravity controlled vents? You know - the kind that open if the fan is on and drop closed when the fan is off? Now, the real trick would be to control fan speed relative to temperature.

That was my thought, too. To insure proper cfm, could the vents be "funneled" to the fans? Or which side of the fan does the air restrictive flap go? There is a strong feeling that a few trips to the kiln can be avoided. To clarify the above idea: seats are a couple degrees off plumb so that gravity rests the flap against them when there is no air flow. Naturally, intakes open in and exhausts open out. Isn't there also a butterfly type? And what about the option of putting this arrangement in the appropriate size door, so it can be opened for faster drying material?

How will you save trips to the kiln? You need to go to the kiln every day to check MCs. You will adjust the vents three times during the kiln cycle at most. With your gravity vents, you still need to adjust them to have them open only a little at the beginning with wet lumber, then a little bit more in the middle of the run, and then nearly fully closed at the end. I think you are making this too hard.

Gene Wengert, forum technical advisor

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
To vent my kiln, I had a flap of plastic about a foot square on the bottom of the door. A five buck timer turned on a small fan, which was aimed at the flap, every other 30 minutes, from morning til night. It took the charge of walnut from 18% to 6% in a bit under three weeks. That was late July/early August. I think I'll get a timer with finer time periods and alternate between 15 minute venting versus 45 minutes non-venting, for the next charge (oak).