I built a 12.5' x 28' greenhouse to store and dry wood--working awesome.

solarguy2003 · Jul 19, 2014

We bought a new blaze king for this fall. I figure to get the most out of it, we should give it very dry wood. We have heated with wood for a long time. Up until now, we stored our cut/split wood under tarps that are on frames to keep it (mostly) off of the wood. That mostly worked, but it was hard to get the wood under 14-15% moisture content.

You will get long term reports of course, but the short term report is awesome!

This is a hoop house type greenhouse, made of 3/4" emt. It was relatively inexpensive to put up (well under a thousand dollars). It has plywood/framing walls for the ends, and 5 year greenhouse plastic for the roof/covering. You don't get much of a wood framed shed for a thousand bucks any more...

I think the key is that I put vapor barrier plastic down on the ground, because the ground can pump mass quantities of water vapor into a hot greenhouse. I put black tar paper over that, to give the sun something black to hit, and to protect the integrity of the vapor barrier somewhat. The second key is the 18 dollar box fan on a timer. There is a 21" window on each end, one holds the box fan and the other allows the hot humid air to leave. And man is it hot and humid.

I set the timer so the fan doesn't come on until 2 pm and the greenhouse is already quite hot. And it shuts off at 6:00, so I'm not spending a ton on electricity, and the fan runs when the moisture is removed most efficiently.

The greenhouse has a door on each end, so no old dry wood gets trapped by new/wet wood.

The wood is stacked on pallets.

I'll try to post a picture this week sometime.

Water can't get on the wood at all, and water (vapor) is being pumped out in a hurry by heat and forced circulation.

I know that more moisture = more wasted BTU's in my firewood. But how much? I thought I had seen a chart with moisture content on one axis and energy or efficiency lost on the other axis.

Any hard data laying around???

thanks!

troy

velvetfoot · Jul 19, 2014

The metallic conduit can bend so that the diameter is 12.5'? I've got no experience with hoop houses, only that my wife has said she'd like one.

solarguy2003 · Jul 19, 2014

Most EMT is made of soft annealed steel so it will bend nicely for wiring purposes. Wire never just runs in a straight line. So yeah, a 12 foot radius is super easy. Tons of youtube videos on how to make a hoop house. The special greenhouse plastic cost more than anything else.

troy

Woody Stover · Jul 19, 2014

solarguy2003 said:
I know that more moisture = more wasted BTU's in my firewood. But how much? I thought I had seen a chart with moisture content on one axis and energy or efficiency lost on the other axis.

Yeah, I thought there were some posts with some of that info. I don't think it's a huge amount of wasted BTU if it's not sopping wet, but keeping the creo at a minimum might be the even greater benefit.

DanCorcoran · Jul 19, 2014

velvetfoot said:
The metallic conduit can bend so that the diameter is 12.5"? I've got no experience with hoop houses, only that my wife has said she'd like one.

Presumably you meant 12.5 feet, not 12.5 inches.

velvetfoot · Jul 19, 2014

Yes. Sorry.

mustash29 · Jul 20, 2014

The weight of water varies with temperature, about 8.3 to 8.0 lb/gal from freezing to boiling. Lets use 8.0 lb/gal for arguments sake. So 1 lb for 16 ounces of fluid water.

http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html

BTU = amount of heat to raise 1 lb of water 1 deg F.

If the (wet) wood is frozen - You have to heat it up to 32, then change the state from ice @ 32 to water @ 32 (latent heat of fusion), then heat it up to 212, then change the state from water @ 212 to steam @ 212 (latent heat of vaporization).

Lets say our "wet" wood contains an extra pound (16 ounces) of water that is frozen to 0 deg F.

32 BTU to heat it up to 32 deg.
144 BTU to change it to water @ 32.
180 BTU to heat it up to 212 deg.
974 BTU to change it to steam @ 212.

= 1330 BTU wasted up the flue getting rid of that pound of water.

http://en.wikipedia.org/wiki/Latent_heat

http://www.onlineconversion.com/

solarguy2003 · Jul 20, 2014

My back of the napkin calculations based on the pure thermodynamic/phase change numbers above, suggest that if my previous wood storage method produced wood at 15% moisture content, and my new method produces wood of 10% moisture content, I will improve my BTU output by 1%.

Humphh. Dropping the moisture by a third only improves efficiency by 1%.

That seems low to me, but that's what the math says, unless I have dropped a decimal place somewhere.

This site suggests that dropping the moisture from 20% to 10% improves the BTU content from 6660 per pound to 7,300 per pound. That's a 10% improvement by moving the water content down 10 points (cutting it in half)

http://mha-net.org/docs/v8n2/docs/WDBASICS.pdf

So the truth must lie somewhere in between 2% and 10% more BTU's for cutting the water content by 10 points, (20% to 10%, or 15% to 5%).

troy

STIHLY DAN · Jul 20, 2014

I have done kind of the same thing with a shelter logic shed. Although its dark green and both ends are always open. Tarps on the ground for vapor barrier. This has worked very well, the red oak was ready in a year, its been two now and I know the wood is premo. Plus it looks as new as when stacked with no none bugs. That also gets very hot in there when the sun is out.

Auzzie Gumtree · Jul 20, 2014

solarguy2003 said:
Humphh. Dropping the moisture by a third only improves efficiency by 1%.

All i know is that there is a sweet spot regards moisture levels. Once you get there (in my case ~ 18-19%) there is not as much gain once you go lower but there is a massive difference if you go higher. ie its not a linear benefit.

Jutt77 · Jul 24, 2014

solarguy2003 said:
We bought a new blaze king for this fall. I figure to get the most out of it, we should give it very dry wood. We have heated with wood for a long time. Up until now, we stored our cut/split wood under tarps that are on frames to keep it (mostly) off of the wood. That mostly worked, but it was hard to get the wood under 14-15% moisture content.

You will get long term reports of course, but the short term report is awesome!

This is a hoop house type greenhouse, made of 3/4" emt. It was relatively inexpensive to put up (well under a thousand dollars). It has plywood/framing walls for the ends, and 5 year greenhouse plastic for the roof/covering. You don't get much of a wood framed shed for a thousand bucks any more...

I think the key is that I put vapor barrier plastic down on the ground, because the ground can pump mass quantities of water vapor into a hot greenhouse. I put black tar paper over that, to give the sun something black to hit, and to protect the integrity of the vapor barrier somewhat. The second key is the 18 dollar box fan on a timer. There is a 21" window on each end, one holds the box fan and the other allows the hot humid air to leave. And man is it hot and humid.

I set the timer so the fan doesn't come on until 2 pm and the greenhouse is already quite hot. And it shuts off at 6:00, so I'm not spending a ton on electricity, and the fan runs when the moisture is removed most efficiently.

The greenhouse has a door on each end, so no old dry wood gets trapped by new/wet wood.

The wood is stacked on pallets.

I'll try to post a picture this week sometime.

Water can't get on the wood at all, and water (vapor) is being pumped out in a hurry by heat and forced circulation.

I know that more moisture = more wasted BTU's in my firewood. But how much? I thought I had seen a chart with moisture content on one axis and energy or efficiency lost on the other axis.

Any hard data laying around???

thanks!

troy

Here's an interesting study. Seems to suggest that without proper ventilation and/or a large volume of drying area to wood ratio, there are no gains using a solar kiln or greenhouse vs traditional split stacks. Good ventilation seems sort of like a no-brainer so good call on the fans.

http://www.familyforests.org/research/documents/DryingFirewoodinKiln.pdf

Also, 15-20%MC is pretty darn good and that's what I shoot for. Although its nice if it gets a bit lower but really it's somewhat unnecessary to strive for under 15%.

One other thing, I couldn't find the study I read before but I thought MC bottoms out somewhere around average relative humidity?

Edit: Found the chart: http://www.woodworkerssource.com/moisture.php

solarguy2003 · Jul 24, 2014

When I was researching the idea, I ran across that paper. My conclusions were:

1. Has to be sealed from the ground, their's was not.

2. Needs some active ventilation, aka: a fan, theirs did not include a fan.

3. Split your wood, they didn't.

Jutt77 · Jul 25, 2014

solarguy2003 said:
When I was researching the idea, I ran across that paper. My conclusions were:

1. Has to be sealed from the ground, their's was not.

2. Needs some active ventilation, aka: a fan, theirs did not include a fan.

3. Split your wood, they didn't.

Yep, splitting wood is a no-brainer but regarding the test: both uncovered and covered stacks were unsplit similar sized rounds in order to compare deltas between the two.

Higher humidity levels in the greenhouse relative to humidity levels outside will offset even a significant rise in temperature. This is pretty easy to extrapolate from the below chart, but if its 80F/50%RH outside then the EMC is 9.1. At 120F/60%RH then the EMC is 9.7, the wood would actually be taking on moisture even though its 40 degrees warmer due to the bump in relative humidity. Even with good ventilation to at least match outside RH (a large fan(s) running 24/7) it seems you would need some sort of additional dehumidification process in the greenhouse to actually see a benefit.

This is an interesting long-term experiment though. Could you measure the temp/humidity levels in the greenhouse at various times and compare them to outside temp/humidity levels?

EDIT: As PB pointed out below, I forgot to account for the drop in relative humidity as temp rises.

solarguy2003 · Jul 25, 2014

Yes. I have a quality hygrometer and thermometer, so long term data will follow.

paul bunion · Jul 25, 2014

Jutt77 said:
Yep, splitting wood is a no-brainer but regarding the test: both uncovered and covered stacks were unsplit similar sized rounds in order to compare deltas between the two.

Higher humidity levels in the greenhouse relative to humidity levels outside will offset even a significant rise in temperature. This is pretty easy to extrapolate from the below chart, but if its 80F/50%RH outside then the EMC is 9.1. At 120F/60%RH then the EMC is 9.7, the wood would actually be taking on moisture even though its 40 degrees warmer due to the bump in relative humidity. Even with good ventilation to at least match outside RH (a large fan(s) running 24/7) it seems you would need some sort of additional dehumidification process in the greenhouse to actually see a benefit.

This is an interesting long-term experiment though. Could you measure the temp/humidity levels in the greenhouse at various times and compare them to outside temp/humidity levels?

View attachment 135773

I believe you are missing one thing. A rise in temperature with air pressure and actual moisture content held constant causes the relative humidity to fall quite significantly. If you take 80 degree air at 50% RH and raise it to 120 degrees the RH falls to 15%. Then you let some moisture evaporate from your wood to raise the humidity. Your wood just got drier, that's where the additional humidity came from. Vent the humidity and the moisture is gone, permanently.

If you can get your greenhouse to gain 40 degrees along with a gain of 10% RH you have gassed off a good deal of water out of your wood. You are pretty much going to have dry wood wood in a matter of months. As long as you can jack up the temperature significantly I don't think that rapid ventilation is that important, and it becomes progressively less important as the wood gets drier because the will be less moisture to vent.

Richie · Jul 27, 2014

I have a green house that was professionally made. It is contructed from wood and has about 12 windows and has a clear plastic roof. I have six of the the windows cracked. I recently cut a red oak fresh. After about a month it is down to 27%. I would say after another two months it should be ready. I would guess it gets about 130-140 degrees in there.

solarguy2003 · Jul 30, 2014

Ok, got around to collecting some data. Today is wednesday, july 30. It's 12:30 (11:30 not counting DST)

Outside temperature and humidity are 76* F and 62% RH.

The box fan has not kicked in yet. The greenhouse has had less than 2 hours direct sunlight. It's shaded by trees on the east.

Inside temp and RH are: 98*F and 6% RH!

I'll post the numbers once the fan kicks on and the sun heats things up.

Jutt77 · Jul 30, 2014

paul bunion said:
I believe you are missing one thing. A rise in temperature with air pressure and actual moisture content held constant causes the relative humidity to fall quite significantly. If you take 80 degree air at 50% RH and raise it to 120 degrees the RH falls to 15%. Then you let some moisture evaporate from your wood to raise the humidity. Your wood just got drier, that's where the additional humidity came from. Vent the humidity and the moisture is gone, permanently.

If you can get your greenhouse to gain 40 degrees along with a gain of 10% RH you have gassed off a good deal of water out of your wood. You are pretty much going to have dry wood wood in a matter of months. As long as you can jack up the temperature significantly I don't think that rapid ventilation is that important, and it becomes progressively less important as the wood gets drier because the will be less moisture to vent.

Yep, I forgot to account for that, good point.

Jutt77 · Jul 30, 2014

solarguy2003 said:
Ok, got around to collecting some data. Today is wednesday, july 30. It's 12:30 (11:30 not counting DST)

Outside temperature and humidity are 76* F and 62% RH.

The box fan has not kicked in yet. The greenhouse has had less than 2 hours direct sunlight. It's shaded by trees on the east.

Inside temp and RH are: 98*F and 6% RH!

I'll post the numbers once the fan kicks on and the sun heats things up.

Very nice! I might have to try something similar.

solarguy2003 · Jul 30, 2014

It's now 3:40 in the afternoon (2:40 without DST).

Ambient temps are 84F, RH 59%

Greenhouse, the tar paper floor measures 155F. Air temperature 141F.

Wood on the surface of the pile varies between 123 and 145, depending on how black it is.

RH measures 2%. Fan has been on since 2:00.

Totally enthused with the performance and cost of my new wood "shed".

troy

JCrean · Jul 30, 2014

You've got a heck of an idea there! You're gonna have all the "kiln dried" wood you need

. 141 degree air temp is ferocious!

Auzzie Gumtree · Jul 30, 2014

How much wood do you have in the greenhouse kiln? is there going to be enough for a winter?

Lakeside · Jul 30, 2014

solarguy2003 said:
I'll try to post a picture this week sometime.

Solar,
Any chance of getting those picture's posted

paul bunion · Jul 30, 2014

With humidity that low you probably don't need to worry about venting at all when the sun is on it. Increasing the temperature gain will significantly reduce your drying time. See http://www.firewoodkiln.com/pdf/fplrn254.pdf Dry wood in 2/5 the time by raising the temp from 140 to 180 in a closed (but not sealed) kiln. You obviously can't maintain that temp 24 hours so venting the moisture as greenhouse cools off in the evening might be the thing to do. Note that no where in that report do they claim the oak to have been cut-the-day-before-green, starting at 52% it certainly wasn't.

Hoozie · Jul 31, 2014

It would be very interesting to take some splits from the same round, weigh each one, and throw some in the greenhouse and some outside. Weigh them every week or 6 and note the difference, hopefully showing the kiln drying them much faster (and see if the outside ones catch up by Fall).

I was going to do this when I got some wood delivered, but got too busy stacking ;lol

I built a 12.5' x 28' greenhouse to store and dry wood--working awesome.

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