Hot Box

caveman · December 06, 2019, 10:11:46 PM

These are what I have for now. The inside was lined with 4" thick Styrofoam and the top initially had 4" also but a week or so ago, we removed the tin and added 4" more just because we had more foam. With temperatures in the low 50's outside it was no trouble to get to 160° with three lights to set the pitch on some heart pine recently. We may make another similar to this but next time we will probably cut the foam to fit between the studs, now the foam is just pushed up to them. The box is not air tight but it does the job. The slabs on the bench are some maple slabs that we were sterilizing.

YellowHammer · December 06, 2019, 11:43:53 PM

I'm glad the lights worked out for you, I have tried many different things but these just seem to work best with the least amount of fuss.

So 3 lights, or 1,500 Watts is all that's required for you. You didn't even have to use any chickens. Nice.

dean herring · December 07, 2019, 04:53:16 PM

Very nice 👍

WDH · December 08, 2019, 07:23:11 AM

Oh, he better hold on to the chickens. He will surely need them.

dean herring · December 08, 2019, 06:39:53 PM

How dry was the lumber you used?

caveman · December 08, 2019, 09:46:45 PM

The pine we just pulled out of it was air dried as low as it would go here and was at 9% a week after we pulled it out. It probably gained a little moisture over this week in the shop.

YellowHammer · December 08, 2019, 10:39:53 PM

That's a good point, the hot box can be used like a conventional kiln by venting the moisture laden heat periodically. I do it post sterilization as a way to fine tune my finished moisture levels.

GeneWengert-WoodDoc · December 09, 2019, 06:42:20 AM

So, it is all about money.

1500 watts of electricity is 5000 BTU per hour. In an insulated box with vents (or a kiln), about 1/2 the energy input goes into evaporations for water, 1/4 into building heat losses, and 1/4 into vent losses. (A dehumidifier will eliminate or greatly reduce vent losses, adding the savings to the amount of evaporation.) A typical batch of lumber of 500 board feet will lose about 14 pounds of water for a 1% MC loss. (Oak is more and soft maple less, etc.) Each pound requires 1000 BTU for evaporation.

So, 1500 watts of energy (lights or strip heat, etc.) or 5000 BTU per hour of gas or oil heat (not counting losses in the heating device, such as a gas furnace) will in a kiln or typical hot box evaporate 2.5 pounds of water per hour or 60 pounds per day or about 4% MC loss from 500 BF per day. Note that 1500 watts might cost 15 cents, which is $3.60 a day 90 cents per 1% MC loss. For air-dried lumber coming in at 27% MC and ending at 7% MC, that would be $18 for 500 BF. (A dehumidifier would reduce this to $10 but a DH unit has higher capital cost than bulbs or strip heaters perhaps.)

doc henderson · December 09, 2019, 08:21:26 AM

@GeneWengert-WoodDoc I really appreciate how you are able to speak in practical terms as well as scientific specs. thanks for all the knowledge you bring to the forum. And Merry Christmas to you and your family! :christmas:. how about solar with fans, is there a ballpark on 500 bf, and cost to run the fans?

GeneWengert-WoodDoc · December 09, 2019, 09:23:40 AM

Are you thinking of squirrel cage fans? With one or two gerbil power? Thanks for your comments

doc henderson · December 09, 2019, 09:28:19 AM

:D maybe a dog chasing a cat, or cat chasing a mouse. Tom and Jerry comes to mind. I was really thinking of the standard VT kiln with box fans x 3, and such. I was surprised at how cheap even the dehydration kiln was to operate. It suddenly was clear, and I was hoping to add the comparison of electric cost to operate the solar kiln. I assume it is cheaper than a DH kiln. Best regards Sir.

GeneWengert-WoodDoc · December 09, 2019, 02:18:45 PM

Generally, the fans for a given amount of wood will be the same whether a hot box, DH kiln or solar kiln. In the VT 1200 BF solar kiln, a 20" box fan, high quality, will use perhaps 70 watts, so running 12 hours a day and using two fans, would be 17 cents a day (or 6 cents per day per 500 BF as the previous posting was for 500 BF) or three fans, 25 cents (8 cents per day per 500 BF), assuming 10 cents per kWh. In a DH kiln, the fans run 24 hours a day, so double. Of course, the DH kiln dries every day, while a solar only works when outside is above 50 F and the sun is shinning.

dean herring · December 10, 2019, 05:35:07 PM

The lumber you used to build your kiln was air dried down to 9%?

dean herring · December 10, 2019, 07:05:41 PM

Is it a must to put foam board insulation over fiberglass?

GeneWengert-WoodDoc · December 10, 2019, 11:08:08 PM

Foam board (closed cell so it will not absorb water) that will withstand 160 F or hotter without "melting" is fine by itself. Or else fiberglass by itself is ok with an added vapor barrier to avoid any moisture in the insulation, destroying its R value and getting so heavy it will sag and decay is also likely. I have seen some fiberglass used, but it is put into a big, thick plastic sealed bag to avoid moisture issues.

caveman · December 10, 2019, 11:15:38 PM

Dean,

The wood we used to build the kiln was a combination of green and somewhat air dried. We just overlapped the boards for the exterior walls. If we do another we will probably do a proper lap siding or at least cut rabbets. There is no fiberglass in ours. We put some 2" chicken house foam between the floor joists and lined the inside with 4" Styrofoam. The top now has 8"-9" of Styrofoam. The front foam panels are just pulled out before the box is loaded and installed afterwards. It has leaks but I think that helps any air dried wood we put in there finish drying.

I think we paid $70-$80 for all of the Styrofoam as found on CL (8-9 4'x8' sheets). If it eventually breaks down we can probably get more. The guy who sold it evidently has it regularly after they use it for its intended one-time use at his job. It has small, melted, very precise lines cut about every 4" that are about 1/8" deep.

We have been talking about building another box but making it a solar kiln that we can heat sterilize lumber in. Our solar kiln is such a hassle to open and close that we don't really use it much even though it does a great job of drying wood.

Don Schmitt · December 29, 2019, 11:19:04 AM

Wood Doc:
Appreciate the hard data on drying costs ( 12-9-19). My question is: Is the drying of green lumber a linear or exponential function ?
Thanks for the help.

GeneWengert-WoodDoc · December 29, 2019, 10:47:39 PM

Drying time for lumber twice as thick takes about 2.5 times longer. Drying costs go up in that proportion as well, which is why we seldom kiln dry thicker species starting at a green MC, as the time is too long. A rough rule would be that the kiln dried lumber needs to be about $25 per day more valuable than green lumber...example, 10 days of kiln drying x $25 means the KD lumber should be at least $250 than if the same lumber was sold green. This same approach is what makes air drying so attractive...save two days by air drying the wettest piece of lumber that much drier will save $50 per mbf.

caveman · May 06, 2020, 08:58:26 PM

We had a pile of cedar slabs and cookies that were air dried down to around 15% but there were small, scorpion looking bugs between the bark and the wood so we gave them a few warm days. We were not quite able to get all of the cookies and slabs into the box but we pulled them out today and there was no sign of bug life.

YellowHammer · May 06, 2020, 11:07:40 PM

Cool. The oniy good bug is a dead bug. Cook em all.

Crusarius · May 07, 2020, 06:46:06 AM

now you gave me an idea. I have been paranoid to sell any wood because of the bug situation. Maybe I need to build me one of these hot boxes just to sterilize it. Then I will not feel bad letting some go.

Eciton · February 04, 2021, 12:43:57 PM

has anyone tried those livestock heaters in a similar system?

DON FRANK · February 15, 2021, 10:26:19 PM

There is a ton of information on this thread. As a longtime lurker on this site I appreciate the depth of experience that you all bring to the table.
Has anyone considered mounting a propane wall heater like you would put in a barn or workshop. They make them vented and unvented. They put out a ton of heat and require no wiring. They do have an open flame inside the cabinet but I wouldn't think this would be a huge safety hazard since they do have a thermostat so the unit comes on and off. Even though you are heating wood the environment is not dusty with a lot of airborne dust.
I would love to hear your thoughts.

WDH · February 16, 2021, 07:40:37 AM

Having an open flame in an enclosed box of dry wood would concern me to the point that I would locate it well away from anything else that could burn in the event that it does catch fire. That way you will not burn up your barn, workshop, yourself, and your loved ones.

KenMac · February 16, 2021, 09:47:39 AM

Quote from: WDH on February 16, 2021, 07:40:37 AM
Having an open flame in an enclosed box of dry wood would concern me to the point that I would locate it well away from anything else that could burn in the event that it does catch fire. That way you will not burn up your barn, workshop, yourself, and your loved ones.

As a commercial HVAC guy I agree completely. If the oxygen supply is not correct the heater won't work properly anyway. They require proper clearances and ventilation to be safe. Not worth the gamble.