Experimental passive solar wood kilns

[Poindexter]

I am going for it. Have to start putting up wood for winter of 16-17 now so I can have everything C/S/S by March of 2016.

No more pallets. No more fungus. No more 18-22% MC.

I am going to build hoop style greenhouses, 42x96 inches. Same footprint as 2 common pallets.

2 squares under each iteration represent 6 cinderblocks under each segment, same as I have been running under my broken pallets successfully.

Floor on each will be ground contact rated pressure treated 2x4 with a layer of vapor barrier plastic and then a floor of common CDX plywood.

Verticals, common 2x4 stud wall.

The arc drawn on each represents the plastic roofing sheet, with no roof framing shown. One key is the gap at the bottom to let condensate drip outside the kiln.

Roof framing will be 3/4" PVC pipe on 24" centers.

Hope to begin construction of the first module Saturday. I am going to have green splits delivered this year so I can just build modules and stack delivered splits.

Discussion welcome.

 

[Poindexter]

Passive solar. I forgot to include "passive" in the title. Sorry.

 

[ElmBurner]

What moisture content are you shooting for, if 18-22% won't cut it?

 

[Poindexter]

I would like to see 12 to 16 (wet basis) as shown on a consumer grade meter with a battery and two metal pins.

 

[Frank625]

Here you go how about 11% on a fresh red oak split

 

[Jags]

Passive solar. I forgot to include "passive" in the title. Sorry.

Fixed it for you.

 

[iluvjazznjava]

I'm interested in your project, as I am considering a budget solar kiln a project to tackle in my near future. If I understand your drawings correctly, you are shooting for something 42" wide and then 96" tall? That seems quite narrow to me, but maybe you aren't trying to build a kiln you can really move around in - more of a stand outside and load it up type of concept. You probably want to give yourself a few extra inches on each dimension for a bit of leeway. I've heard of using PVC pipe for this sort of thing, but in my somewhat limited experience with it, it was semi-flexible, but not super flexible. I'm curious to see if you can get the flex you need to build somewhat narrow and tall, rather than vice-versa. Most of the plans I have seen for similar structures call for them to be about as wide as they are tall.

Have you considered how you are going to vent it? I seem to recall another thread where the builder used a simple box fan on one end with an opening on the other which seemed to work.

Good luck and keep us updated on your progress!

 

[iluvjazznjava]

So looking again at your description ... I assume by "passive" you mean no sort of mechanical ventilation? I can't quite envision how your condensate removal actually functions. Is the idea that the condensation on the inside of the plastic will run down and drain at the bottom on the sides? If so, my experience with greenhouses tells me that may not work, since condensate on the inside of the roof (if vents weren't used) just tended to drip right back down when things cooled off at night.

I hope it works though - would probably keep things simpler.

 

[DougA]

I think you are going to need a vent or fan along the top. It could be at the ends but a kiln works by eliminating the condensation and I agree with iluvjazz that the condensate will simply drip back down. The easiest would be to get a solar powered roof vent and put it at the top or put a roof vent strip like you see on roof peaks (8' long).

 

[Poindexter]

Sorry fellas, I was hurrying. 42x96" floor, then vertical framing 7-8 feet tall.

If I can fit two layers of 16" splits on the floor in side by stacks 7 feet tall and 8 feet long I'll have 1.2 green cords of wood in each module that after thorough seasoning (18% shrinkage for me) should come right in at one seasoned cord per module.

Load from the lawn, not walk around inside of kiln yes.

Thanks Jags.

Frank625 wasn't that wood stacked four years? I think I remember your thread. I don't have enough land to wait that long. What I have split and stacked in March is what I have to burn in September.

jazznlava, I built a hoop style greenhouse for my wife to garden in a couple or three years ago. I am probably not communicating my design clearly, but bending 3/4 white PVC pipe to my needs will/should work for what I mean even it is doesn't work for what I said.

I have a couple ideas about condensation, but I won't have any liquid water to observe for several months yet. For now I need to get my floors built and my cordwood stacked. I figure I'll tarp the green stuff as I go to keep the worst of the snow off it; then pipe and plastic cover in maybe early march or so.

 

[Andy S.]

I am going for it.

Good luck with it and good call. I ran a small-scale experiment this year and it was a success. Hope the same is true for you. Please post pics of your finished product.

 

[iluvjazznjava]

Yes, please post pics of your progress and finished project. It sounds promising - I just can't quite visualize it.

 

[paul bunion]

I would like to see 12 to 16 (wet basis) as shown on a consumer grade meter with a battery and two metal pins.

I thought that consumer grade moisture meters were all displaying moisture on a dry basis. Dry basis being the weight of the water/weight of the wood when bone dry. Am I mistaken?

 

[iluvjazznjava]

I thought that consumer grade moisture meters were all displaying moisture on a dry basis. Dry basis being the weight of the water/weight of the wood when bone dry. Am I mistaken?

That is my understanding as well - the meter is showing the moisture content of the wood (not the "dry" content).

 

[Poindexter]

Have you considered how you are going to vent it?

Can I get back to you on this? I agree that it is a key point, I have considered it, I am going to try several things and it might be a lot easier with pictures and brief captions rather than pages of text.

I seem to recall another thread where the builder used a simple box fan on one end with an opening on the other which seemed to work.

I am absolutely stepping forward from the work done by Solarguy2003 in this thread: https://www.hearth.com/talk/threads...to-store-and-dry-wood-working-awesome.129149/

The one thing he did that really got my attention was the vapor barrier in the floor between ground water and his woodpile. I guess it isn't necessary in some areas, but I got to do this.

A few years ago I had about a cord at 18% per handheld meter, it was my best cord. I moved it to right by the man door to the garage, put it on pallets on the ground, then covered top and all sides with plastic. Within a week of good sunshine with nightly freezes and daily thaws it was up to 25% per handheld meter, I checked again because it wasn't burning as good as it had before.

Frank625 has a shed that belongs in the Taj Mahal section of the "show us your wood shed" sticky at the top of this page. I just don't have room to wait that long. https://www.hearth.com/talk/threads/opening-up-the-wood-shed-after-4-years.148952/

 

[Poindexter]

I think (and I stand to be corrected) the hand held meters under $100 read the resistance between the two pins - the water and the wood together- and express the result in wet basis. I googled this a little bit, begreen probably knows for sure and has maybe links to back it up.

At the end of the summer, I want to see 12-16% on my handheld $35 meter.

 

[DougA]

I want to see 12-16% on my handheld $35 meter.

That's very low.

I was looking at building a solar kiln for a lot of oak, cherry & walnut planks I had cut at a sawmill. The one that appeared to most simple was the Oregon one. http://owic.oregonstate.edu/solarkiln/plans.htm If you have some spare windows, plywood and tarpaper, it's not that difficult. Not sure if you get enough sun in the winter to have a kiln work well.
In the end, I decided that I don't have an area that gets good sun that I want to build one on, so I stacked the wood separated by stickers in the south facing garage. In this case, slow drying over the winter and into next summer is better for the boards than fast drying in a kiln. Not the same as your firewood though.

 

[Poindexter]

That's very low. I was looking at building a solar kiln for a lot of oak, cherry & walnut planks... slow drying over the winter and into next summer is better for the boards than fast drying in a kiln. Not the same as your firewood though.

Good points. I seem to be out on my own here with what I am trying to do. I will be experimenting with ventilation strategies. I am concerned about pulling out too much moisture too fast to end up with case hardened splits that are dry on the outside and a bunch of water trapped within.

But I got all summer. I am reading up on case hardening as a negative outcome of the kiln drying process now.

 

[Knots]

I think you are going to need a vent or fan along the top. It could be at the ends but a kiln works by eliminating the condensation and I agree with iluvjazz that the condensate will simply drip back down. The easiest would be to get a solar powered roof vent and put it at the top or put a roof vent strip like you see on roof peaks (8' long).

This. My last house came with a green house that I used for wood storage. It did not have good venting. It was much better than outdoor storage, but for the most part the sun would extract the moisture from the wood during the day, and then when the glass cooled at night the moisture would condense and it would "rain" inside.

 

[MikeK]

I think (and I stand to be corrected) the hand held meters under $100 read the resistance between the two pins - the water and the wood together- and express the result in wet basis. I googled this a little bit, begreen probably knows for sure and has maybe links to back it up.

At the end of the summer, I want to see 12-16% on my handheld $35 meter.

I just bookmarked this link for my own use, according to which it appears our meters are dry basis, I had no idea before reading.

https://www.hearth.com/talk/wiki/determine-moisture-content-of-wood/

I am super excited to see pics and read about your results!

 

[solarguy2003]

Go for it!

I'm sure you have 7 ideas for how to vent each enclosure. The first thing that pops in my head is 4" aluminum duct, like for dryers. A 5' piece is 6 bucks at menards. Cut it up into 4 or 5 pieces. Pop rivet or screw a sheet metal plate over the end at a 45* angle or so to keep most of the rain out. A slightly more expensive option is to stick an elbow on the top so the prevailing winds can't shove rain in there sideways.

I'm still tickled pink with the solar greenhouse.

I explored the option of a solar fan, and I was totally underwhelmed by the bang/buck ratio. I love solar, but not if it's stupidly expensive...

Any vented, plastic covered stack that stops moisture migration up from the ground, keeps all the rain and snow off, and has some solar heat, is going to work well. Exactly how fast, and how well, I look forward to your data.

solarguy

 

[Poindexter]

Been reading up on case hardening specifically and drying effects in general.

US Forestry Service _Dry Kiln Operator's Manual_ index here:

(broken link removed)

Chapter One: (broken link removed)
Chapter Eight: (broken link removed)
Chapter Nine: (broken link removed)

The short version seems to be to avoid case hardening (moisture trapped inside each split) I need to dry my stacks gently down to and just below the Fiber Saturation Point (FSP) before I really crank up the heat to force the last of the water out. This I can do.

There is a really interesting section in the wikipedia article about kiln drying that uses red oak as a specific example. I can't get any red oak here, but if you can get to a good quantity of it and know the Greek alphabet: https://en.wikipedia.org/wiki/Wood_drying#Model might be an interesting read.

 

[edge-of-the-woods]

Is that hooped-over 3/4" PVC going to be able to stand up under the weight of any snow you're going to get over it? Or are you moving it someplace else during snowfall season?

 

[DougA]

might be an interesting read.

I clicked the link, my eyes went blurry, my brain went fuzzy and I remembered how much I hated that stuff in high school. Now I need a drink.

 

[kversch]

I'll be watching to see how this works out. Keep us up to date