Vents on VTech solar kiln

[RussMaGuss]

So I finished my kiln this fall and got my first load of 500bf of pine in there. I'm in IL, SW of chicago. The MC was around 18% and after about 10 days (maybe 2-3 cloudy days) It was down to 15% so I figure I get it down to 11 or so and it'll be fine for my purposes for it. 

My question however is more about solar kiln vent operation because I haven't seen much on here about what people do with the vents.

Do you guys ever open the vents at night to let the humid air out or will it just slowly trickle out by itself? I sealed my kiln pretty well. Currently, I have the vents closed at all times, day and night. First day I had the vents all open, fans on for 24 hrs, then closed them all with fans on when the sun was out. Saw a good amount of humidity built up on the polycarbonate roof panels during the day. Any advice about the vents on these solar kilns is appreciated. I also have a load of around 400bf of elm at 50%MC waiting to go in right after. 

One more question too: What kind of readers would you guys suggest to keep in the kiln to measure temp/humidity? Thanks,

-Russ

[GeneWengert-WoodDoc]

The kiln vents are the major, perhaps only, way to remove water from the kiln.  If they are not opened, then the kiln humidity will rise until drying stops and you get 100% RH or close to 100% RH.

When the vents are open, they also exhaust hear.  So, we do not want to open the vents too far.  With a slow drying wood, they are opened just an inch or two; with fast drying perhaps 50% to 65% open.

We only run the fans when there is enuf sunshine to heat the kiln.  When there is no sun, the fans should be off, as without sun energy input the kiln temperature will drop which increases humidity and slows or stops drying.

The vents are opened an appropriate amount and left open all day. And night too.  With the fans off at night, you do not need to close them.

I have seen almost all of the VT solar kilns run without measuring temperature or humidity.  Fans are run with timers...turning on perhaps at 9AM and off at 6PM, but these vary depending on the time of year...turn them on when there will be energy or heat input, and off when the cover no longer has direct sunlight.

[OlJarhead]

I was thinking solar attic fans would work since they would only come on when the sun was beating down on them.

Thoughts?

[Matrixxdg]

Hi Gentlemen,
I am at the same process. I am in East Texas and building my Solar Kiln in March. I am looking for solar fan recommendations. Some that would come on in the morning and shut off at night... any thoughts? I see allot of posts on fans but most post are quite old. Also,
I have no power where the kiln will be located.

Thanks for your time in advance.

Mike G

[btulloh]

Good luck with your kiln build.  The VT solar kiln design is a great addition and they work well.  Adding a kiln really expands the uses for your sawn lumber.  Hopefully you're making out well with your HM130 and your sawdust addiction.

One reason you don't see a lot of posts, and recent posts, about solar fans is that there's no easy, inexpensive solution.  Some people have talked about the inexpensive solar attic fans, but they really don't move enough air.  To get maximum performance from your kiln you need to achieve enough air flow through the stack and this means having enough CFM for the open area in the stack, plus losses around the ends where it's hard to block the air flow.  For my kiln (which was shortened to accept 12' max length stacks) that worked out to be about 3500 CFM from the fans.  

It all comes to down to power.  A certain amount of power is necessary to achieve the CFM necessary, and your solar panels need to be sized appropriately.  There are inherent inefficiencies in any power system, and these need to accommodated in the design.  The cost of a truly effective solar fan system can easily equal or exceed the cost of the kiln itself.  Of course if you have no grid power available, then solar is your only option.  

Perhaps a scaled down approach such as the solar powered attic fans will work with a reduced air flow, and it just means longer drying times in the kiln.  

I know this is probably not the answer you're looking for, but hopefully it will help you research some solutions.  Plus I'm sure there will be plenty of people chiming in on this.  Keep us posted on your progress.  You'll certainly be glad to have a kiln in your arsenal.

[GeneWengert-WoodDoc]

Appreciate that fans move the hot air in the kiln through the lumber and also control venting.   The fans also control how efficiently the air scrubs the moisture off the surface...this effect is most critical above 25% MC.  This means the amount of air movement influences drying speed above 30% MC.

There are solar powered fans, but to get the required cfm you need a lot of cells.  Plus, you need this power early in the morning and late in the afternoon, which again adds to the square footage of cells.  This means you have more power than needed between 10AM until 2PM (solar time) April through September.  An alternative is to reduce the square footage of cells, is to put them at an angle equal to your latitude (or 10 F flatter) and charge batteries, with the 12 v. battery output going through an inverter to create 110v that would be available throughout daylight hours.  The use of cells to charge batteries and then an inverter is common solar technology.