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Author Topic: Question about radiant floor and kiln drying.  (Read 1167 times)

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Offline Crusarius

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Question about radiant floor and kiln drying.
« on: August 07, 2017, 12:53:49 pm »
So i had this very random thought while laying in bed the other night. I have a concrete floor in my garage that is plumbed for radiant heat.

I am wondering if that may work for drying lumber? It would be very easy to build a box over the pile of lumber I want dried and put fans in it for circulation.

Any thoughts or opinions?
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Offline scsmith42

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Re: Question about radiant floor and kiln drying.
« Reply #1 on: August 07, 2017, 09:45:57 pm »
I would think that it would not be very economical to operate unless the slab were incredibly well insulated underneath and on the perimeter.
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Offline Don P

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Re: Question about radiant floor and kiln drying.
« Reply #2 on: August 08, 2017, 07:19:22 am »
Even then, it's a very inefficient way to get the heat to the wood.

Offline Crusarius

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Re: Question about radiant floor and kiln drying.
« Reply #3 on: August 09, 2017, 08:07:03 am »
The biggest benefit would be steady consistent temperature. But, since I know nothing about drying wood, I am not sure if that really matters.
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Offline K-Guy

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Re: Question about radiant floor and kiln drying.
« Reply #4 on: August 09, 2017, 03:23:05 pm »
Radiant heat is good for maintaining temperature but is inefficient at heating things up. It takes it a long time to heat from 70 to 80 for example. Heating a load of wood, would take to long time and you could get mold in the meantime. :)

Offline GeneWengert-WoodDoc

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Re: Question about radiant floor and kiln drying.
« Reply #5 on: August 09, 2017, 06:01:28 pm »
Drying wood requires about 2 million BTUs for air dried (22% MC) hardwoods.  That is about 700 kWh.  At 11 cents per kWh, that would be $77 per 1000 BF for electric heat, including light bulbs, radiant heat, strip heaters, etc.  With a heat pump (dehumidifier), you need about half of that. 

To get this much energy (2 million BTUs) we usually use a much hotter heat source, as a cooler source like radiant heat would have to be really large to get enough heat.  Plus it is expensive.
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Re: Question about radiant floor and kiln drying.
« Reply #6 on: August 09, 2017, 08:09:53 pm »
Drying wood requires about 2 million BTUs for air dried hardwoods.  That is about 700 kW.  With a heat pump (dehumidifier), you need about half of that.  To get this much energy, we usually use a much hotter heat source, as a cooler source like radiant heat would have to be really large to get enough heat.  Plus it is expensive.

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Offline GeneWengert-WoodDoc

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Re: Question about radiant floor and kiln drying.
« Reply #7 on: August 12, 2017, 07:14:25 am »
My numbers in Reply #5 are 700 kWh and "per 1000 BF."

Evaporating water does indeed require a lot of energy.  For example, heating one pound (about a pint) of water from 70 F to 212 F requires 140 BTU.  Then to evaporate the water requires about 970 BTU.  So that is 1110 BTU total to evaporate a pound of water.  For oak, 32 pounds equal 1% MC loss per 1000 BTU.

Note then, if we then use a dehumidifier to convert the vapor back to liquid, we get this 970 BTU back.  Plus we get the 140 BTU back as the liquid cools to 70 F. This recovered heat shows up at the hot coils.  We then get this recovered "hot coil" heat into the air that we put back into the kiln to evaporate some more water.  Hence, a DH is very efficient and this is also why it is called a heat pump.  It does require energy to pump the heat, so this is why a DH is not 100% efficient.  It is also why we need vents in warm weather...the compressor heat will over-heat the kiln.  A good DH kiln with air dried hardwoods might require only 250 to 350 kW to dry 1000 BF.

So, why doesn't everyone use DH kilns?  A key reason is that electricity can cost two or three times more than wood heat or even natural gas heat, especially in the large units.  But small kilns and small operations find that when the capital cost and operating cost of a smaller wood or natural gas heating unit is considered, DH is cheaper.  The breakeven point is over 1 million BF dried annually for many operations...DH is better under 1 million BF annually.
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Offline DDW_OR

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Re: Question about radiant floor and kiln drying.
« Reply #8 on: August 12, 2017, 03:19:42 pm »
GeneWengert-WoodDoc

how would an outdoor wood furnace like a Central Boiler work as a heat source?
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Offline GeneWengert-WoodDoc

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Re: Question about radiant floor and kiln drying.
« Reply #9 on: August 12, 2017, 06:15:10 pm »
For most hardwoods and some softwoods, we need to control both temperature and humidity.  So, a furnace alone may not be all that is required.  But such a heater would be a very good idea.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Crusarius

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Re: Question about radiant floor and kiln drying.
« Reply #10 on: August 14, 2017, 07:57:59 am »
I have a 750 gallon wood / oil burning boiler that feeds my radiant floor heat. that is why I was wondering if I could just build a box with vents and set it over a pile of wood.

The theory in my mind was sound. But it is quite noisy in there :)
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Offline 50 Acre Jim

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Re: Question about radiant floor and kiln drying.
« Reply #11 on: September 02, 2017, 08:12:47 am »
Found this thread while researching the idea of using a Central Boiler 6048 to dry my wood.  My boiler runs most of the year but during the summer it's mostly idling as its only job is to heat domestic hot water.  I've been planning a pole barn about 60' from the boiler and it would be fairly simple to set up a water to air heat exchanger in part of the barn, thus blowing hot air through the stickered wood. 

I was going to build a solar kiln but around here it would be all but useless during the winter months.  The Central Boiler kiln would be much more predictable, allowing for more manageable temps and could be used 24/7/365.  Anyone already doing this and if so, is it as easy as it seems to be? 
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Offline Crusarius

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Re: Question about radiant floor and kiln drying.
« Reply #12 on: September 02, 2017, 08:48:03 am »
I started this thread so I really have no idea but in my head theory is sound.

if you are doing a new build you could do a zone just for the drying area. depending on the temperature of your water you could get pretty good warm spot.

I would actually think that if setup properly from the start you could get the equivalency of a solar kiln out of it. I may be totally wrong but  I thought it was a great idea since I am in the same boat you are.
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Offline 50 Acre Jim

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Re: Question about radiant floor and kiln drying.
« Reply #13 on: September 02, 2017, 09:04:03 am »
if you are doing a new build you could do a zone just for the drying area. depending on the temperature of your water you could get pretty good warm spot.
The water temp is 185 (+-) and the air coming out of the HE is around 135*.  That doesn't mean the air will be 135* going through the stickered wood but I would think it would be hot enough to do the job.  Especially if it's running 24/7. 
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Re: Question about radiant floor and kiln drying.
« Reply #14 on: September 02, 2017, 09:06:08 am »
it also would not take that large of a heater to increase the temperature that little bit more. So maybe the radiant floor will start the process then you would need a "sterilization heater" to finish it?
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Re: Question about radiant floor and kiln drying.
« Reply #15 on: September 02, 2017, 09:43:48 am »
Wouldn't be using a radiant floor Crusarius, just a water/air heat exchanger. 

The hot water (185*) would come from the boiler and pass through the water to air heat exchanger.  A Water/Air HE heats the air being blown through the HE, resulting in 135* air that is blown directly through the stacked/stickered wood.   The exact same process that is used in a home heating system where a squirrel fan blows air through the plenum and into the rooms of the house.  But in this application, the air is blowing directly into the stacked wood and out the other side to be vented either into the other room of the barn or to the outside of the building.

There might be too much moisture to reuse the air in another room but that would remain to be seen.   
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Offline Don P

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Re: Question about radiant floor and kiln drying.
« Reply #16 on: September 02, 2017, 03:31:20 pm »
That would work fine, as Crusarius mentioned you might want to bump it up to set pitch or sterilize but it would dry wood. Where you are passing air through basically a radiator to heat the air and blow through the wood to heat it and whisk the moisture from its' surface, trying to do the same thing with a radiant floor is not going to work very well. Blowing air across a concrete floor in an attempt to pick up enough heat in a timely manner to blow through the stack and remove moisture just isn't going to work as well.

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Re: Question about radiant floor and kiln drying.
« Reply #17 on: September 02, 2017, 08:57:01 pm »
Jim. that sounds like a much better idea than mine. I never thought of the heat exchanger.

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Re: Question about radiant floor and kiln drying.
« Reply #18 on: September 06, 2017, 08:01:00 am »
There is a fella not far from me that has a shipping container kiln that he heats with a water-to-air heat exchanger and a OWB.  This is a feasible way to do it but I am also positive that he supplements it with another heat source for the final sterilization and pitch setting at the end.  I am not certain what heat source he is using in addition to the HE.  He has dried and sterilized wood for me on several occasions with great results.

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Re: Question about radiant floor and kiln drying.
« Reply #19 on: September 15, 2017, 07:28:01 am »
For anyone who might be interested, this is one of the units I was considering for my kiln.  I'm not referring this company or their boilers, I just happened upon their support page while doing a search the other day. 



https://hawkensupport.com/products/unit-heater-garage-space-50-000-btu
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Re: Question about radiant floor and kiln drying.
« Reply #20 on: September 18, 2017, 02:01:34 pm »
Jim, (great avatar, by the way!),

One of our older members - Mark Strickland (GaBoy), used to heat his kiln with an outdoor wood boiler.  You might check the archives to see if there is any information about it.

As I recall this was back in the 2004 - 2005 timeframe.

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Re: Question about radiant floor and kiln drying.
« Reply #21 on: September 18, 2017, 02:20:45 pm »
that would make a ton of sense with the amount of wood we all have laying around :)
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Re: Question about radiant floor and kiln drying.
« Reply #22 on: September 18, 2017, 07:14:33 pm »
Jim, (great avatar, by the way!)

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Re: Question about radiant floor and kiln drying.
« Reply #23 on: September 18, 2017, 07:17:48 pm »
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Re: Question about radiant floor and kiln drying.
« Reply #24 on: September 20, 2017, 08:02:20 am »
Up at 4:00 am researching kilns and how to best set this one up.  Most of the (non-solar) kilns I've seen have a dehydrator somewhere in the mix.  I'm wondering how important that is if I'm pumping in a constant supply of new, fresh 135* heat?  Wouldn't a vent opposite the side I'm introducing the heated air from force the humidity out with the exiting air? 
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Offline GeneWengert-WoodDoc

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Re: Question about radiant floor and kiln drying.
« Reply #25 on: September 20, 2017, 11:52:50 pm »
Because of the heat losses in the garage, if the heating unit delivers 50,000 BTU, you will have about 25,000 BTU per hour for evaporation.  This will evaporate about 20 - 25 pounds of water, which is 0.75 to1 % MC loss from 1000 BF.  Note that 50,000 BTU is about 15 kWh
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Re: Question about radiant floor and kiln drying.
« Reply #26 on: September 21, 2017, 07:23:39 am »
Because of the heat losses in the garage, if the heating unit delivers 50,000 BTU, you will have about 25,000 BTU per hour for evaporation.  This will evaporate about 20 - 25 pounds of water, which is 0.75 to1 % MC loss from 1000 BF.  Note that 50,000 BTU is about 15 kWh
I'm sorry Doc but I don't know if that is good or bad?
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Re: Question about radiant floor and kiln drying.
« Reply #27 on: October 05, 2017, 03:47:05 pm »
Slow,
Any way I could contact the guy by you?
I am considering creating a kiln using a shipping container with an OWB and would love to pick his brain.
 

There is a fella not far from me that has a shipping container kiln that he heats with a water-to-air heat exchanger and a OWB.  This is a feasible way to do it but I am also positive that he supplements it with another heat source for the final sterilization and pitch setting at the end.  I am not certain what heat source he is using in addition to the HE.  He has dried and sterilized wood for me on several occasions with great results.
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Re: Question about radiant floor and kiln drying.
« Reply #28 on: October 06, 2017, 11:57:14 am »
I will send you a PM with his contact info.