Author Topic: low tech heat source for sterilizing lumber  (Read 1680 times)

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Offline mrFinpgh

  • Posts: 216
low tech heat source for sterilizing lumber
« on: August 19, 2017, 07:05 PM »
I've got a big batch of air dried walnut.   A couple of the boards have some pin holes in them, mostly in the heartwood. No dark rings around the holes, so I don't think it's ambrosia beetle damage.   I don't see any indications of an active infestation, but I think I'd prefer to play it safe and sterilize the lumber before I go putting it into my house.

A helpful sawyer provided me with some plans for making a foil faced polyisocyanurate box that you pump heat into in order to get the lumber up past 130 degrees.   I've got a thermostat-connected power supply to keep the temperature around 150-155.  This isn't meant for kiln drying, but just sterilizing to make sure it's critter-free.   The original article was about cooking the sap in pine, which actually takes a higher temperature.

Where I'm running into a challenge is that the little space heater I bought stops heating up and will switch to fan mode for a while before reheating.  So the temperature in the box goes to about 115 or so, and then it drops down to 90-94 before the heater kicks back on.  I'm trying to think of some other ways to generate the heat I need to get it up to temperature and looking for some suggestions that I could set up and break down relatively quickly, as this isn't in any way a permanent installation.


Thanks,
Adam





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Offline bkharman

  • Posts: 1899
Re: low tech heat source for sterilizing lumber
« Reply #1 on: August 19, 2017, 08:30 PM »
Can the board be submerged? 

Would that take care of any critters? 

Cheers. Bryan.


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Offline Gregor

  • Posts: 411
Re: low tech heat source for sterilizing lumber
« Reply #2 on: August 19, 2017, 08:35 PM »
Depending on the space heater you have you could replace the temperatur sensor (most likely a cheap bi-metal switch) with one for the target temperature range (preferably 5-10 ° more to have enough delta-T to get the heat from the circulating air in the box into the core of the material within a reasonable timespan).

In case you gave your target temperature in °F then this 'upgrade' shouldn't give a standard cheap space heater (and your intended box) any big problems (the fan of the heater might die prematurely though), in case it's in °C I wouldn't do it with a plastic one (as it could ignite itself).

Should you not have the training to work on electric items: better hand that project to someone that has, to avoid both killing yourself through electric shock and converting the heater into an igniter (that light up the lumber, the box and then your house).

Offline mrFinpgh

  • Posts: 216
Re: low tech heat source for sterilizing lumber
« Reply #3 on: August 20, 2017, 08:25 AM »
Quote
Can the board be submerged? 

My understanding is that prior to drying, you can saturate lumber with a boron-based solution and that this will prevent any infestations from happening, although it will not kill off anything existing inside the wood itself.   

Quote
Should you not have the training to work on electric items: better hand that project to someone that has, to avoid both killing yourself through electric shock and converting the heater into an igniter (that light up the lumber, the box and then your house).

That's probably out of my league for now.   I'm not sure that it would make sense to hire someone to modify a $50 heater.

Reading about all the variables in drying lumber has been an interesting education.   

I'm thinking that, for now, I might try to use an array of 150 watt incandescent bulbs to get the air temperature up.  That's well within my skills.  I probably would not do this for a permanent installation, but for something that will run for 4-5 hours, I think it will be okay.

Thanks,
Adam

Offline Gregor

  • Posts: 411
Re: low tech heat source for sterilizing lumber
« Reply #4 on: August 20, 2017, 10:08 AM »
I'm thinking that, for now, I might try to use an array of 150 watt incandescent bulbs to get the air temperature up.  That's well within my skills.  I probably would not do this for a permanent installation, but for something that will run for 4-5 hours, I think it will be okay.

Also a nice low-tech solution. Wire in something like this (<7$, just keep the amount of lamps within the 16A it is rated for) to get control of the temperature - mount the dial in an outdoor junction box like this (<$15 but just for reference, no idea if the dial part fits in this linked one - check dimensions before buying) to have the wiring protected from weather and touch, but the dial accessible without the need of tools and put the sensor part in the inside of your box.

Then you have peace of mind that the lamps won't be able to overheat the box, preventing warm accidents. You can still put the existing space heater inside the box on a non-switched cord to speed up initial warm-up and supply air circulation to get even temperature distribution inside the box.

Offline Bob D.

  • Posts: 577
Re: low tech heat source for sterilizing lumber
« Reply #5 on: August 20, 2017, 02:04 PM »
This might seem crazy but right now in the attic space over my garage it is close to 150°F today.
I am roughly the same latitude as you (near Atlantic City, NJ) and on a sunny day the temp up
there passes 140°F many a day in the Summer. I know it is over 140°F because I have an
indoor/outdoor thermometer in the shop and the outdoor sensing bulb is placed up in the attic
space above the shop. The instructions that came with the thermometer says that when the temp
exceeds 140°F that the display will start flashing HHH, which it has been doing since before Noon
today. If I am out in the shop late in the evening after the Sun has set the temp will still be above
100 at 9PM. By Morning it has dropped to near ambient.

So without any additional heat source, chemicals, etc you should be able to get what you need
in a similar location if you have access to one.

How many hours would you have to hold the temp above 130? I bet I will see at least 8 hours over
130 today, and I don't know what the top end is but it's >140.
« Last Edit: August 20, 2017, 02:07 PM by Bob D. »
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Offline mrFinpgh

  • Posts: 216
Re: low tech heat source for sterilizing lumber
« Reply #6 on: August 20, 2017, 02:40 PM »
I think that's actually a common way for people to dry small batches of lumber.  I think even Gene Wengert says that he does that for his personal stash.

Unfortunately, I don't have an attic or even a garage to do that in.  Otherwise, I'd probably be giving that some serious consideration.

I think the typical recommendation is to heat the air to around 150 and hold it there for about 4 hours.  There are variations, but essentially it's all about getting the core temperature of the wood hot enough to kill off any larvae or eggs in the wood.  Kind of like cooking an egg sous vide.

Thanks,
Adam

Offline Bob D.

  • Posts: 577
Re: low tech heat source for sterilizing lumber
« Reply #7 on: August 20, 2017, 02:58 PM »
Possibly you know someone who may have such a space and would let you sterilize your wood up there.

How much material and what size, maybe the trunk of your car is a big enough 'oven'.
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Offline mrFinpgh

  • Posts: 216
Re: low tech heat source for sterilizing lumber
« Reply #8 on: August 20, 2017, 05:57 PM »
Unfortunately, I don't.  At least not within a reasonable driving distance.

I thought about the car.  I'm not sure that it would get the job done.   In the worst case, there is a place about an hour from me that could kiln it for around 40 cents per bd ft.   Not sure how it would work in terms of getting in just for a sterilization cycle.    It's a big batch for me, but they probably throw away more material in a single day.  :-)

Today's experiments didn't pan out very well.   Four 150w incandescent bulbs barely moved the needle in terms of heat.  I tried running the space heater from a slightly different position, but I still run into the same issue where the temp. control on the space heater tops out around 120, and then it runs in fan mode for a bit before returning to heating.

After about 2 hours of this,  the temperature in this 13 cubic foot insulated box is still hovering at 120.

It seems as if I need some kind of heat source that won't shut itself off before the thermostat controller cuts power.   Having the fan from the space heater is probably better in terms of air flow and evenly distributing the heat.   

Of course, I don't know if any of this activity is current.   It could very well be old damage from when the lumber was freshly sawn.  Re-examining some of the roughsawn wood, I can see some black staining around some of the holes.    I still haven't found any significant galleries or tunneling, and haven't seen any evidence of activity besides some pin holes.  So it's completely possible I could be working myself up about nothing.  I don't have enough experience to know what's a big deal and what isn't.  :-\

Thanks,
Adam




Offline Bob D.

  • Posts: 577
Re: low tech heat source for sterilizing lumber
« Reply #9 on: August 20, 2017, 09:08 PM »
Is there some lower temperature that will kill them? It might be
easier to freeze them into oblivion than give them a sauna bath.

Update: This answers my question and I'll post a link here for anyone
who may come along wondering the same as I did.

Long answer short no, cold, even extreme cold (-28°C) over extended
periods will not kill 100% of the larvae according to the USDA.

https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5191794.pdf
« Last Edit: August 21, 2017, 12:16 PM by Bob D. »
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Offline TSO Products

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Re: low tech heat source for sterilizing lumber
« Reply #10 on: August 20, 2017, 09:16 PM »
if I were faced with this requirement I would take my lumber to a local wood pallet producer who has to produce export pallets which are guaranteed free of pests and have a hot stamp burned into the wood that says so.
This kind of business has to be plentiful in the greater PITTSBURGH area.

Hans
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Offline Gregor

  • Posts: 411
Re: low tech heat source for sterilizing lumber
« Reply #11 on: August 21, 2017, 12:03 PM »
Experience from my past adventures of building a chemical manufacture from scratch on a shoestring budget:
In case you box isn't completely airtight you'll get a chimney effect that'll cycle air through your box, counteracting the efforts to heat it up. You'll also have to replace what is constantly lost through the insulation, inserting 600W (4 bulbs) might work on a smaller box but with a high surface and thinner isolation (and being outside: wind) that wattage might not be enough to cover the losses.

Best is to test the box empty, as it'll show you if it works at all - testing it filled might give the impression that it dosn't when in reality it is working as intended but the matter in it absorbs the heat by simply being still colder than the air you're heating...