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Using heat dump to dry firewood?

LarryCLarryC Member Posts: 331
It occurred to me as I was driving back home yesterday, would it make sense with a non drainback solar heating setup that has excess collector during the summer, to harness the excess heat to dry firewood?  I am visualizing a bay window like structure with the firewood stacked on horizontal CI radiators, fresh air vents on the bottom, and a whirly ball exhaust on the roof.  I don't have a huge heatsink like a swimming pool and this would be doing productive work instead of just collecting the heat and then reradiating it.

With a PV DC powered circulator, I don't think I would be too concerned about power outages.



  • Brad WhiteBrad White Member Posts: 2,393
    As any bear in the woods can tell you,

    any place you can do that is a good thing!

    I like the CI radiator aspects, durable and cleanable. So long as you do not crack them, why not? I can see some lumber mills getting ideas too, at least for pre-heating or finishing kilns.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • GordanGordan Member Posts: 891
    Airflow? Overdrying? Pyrolysis?

    Seems like good airflow would be critical - have you considered forced convection, using a fan coil driven by a DC motor? That would have the advantage that you wouldn't have to stack wood on top of it, as long as a sufficient portion of the airflow passes through the wood, like it would if, say, you blew it under the wood grate. You'd need sufficient free area at the top to vent. Vapor diffusion would do the rest. Things are bound to get very hot and humid in there, so the shed should be constructed from materials that can cope with heat, humidity, and things that like heat and humidity.

    Overdrying the wood makes it burn too quickly and less efficiently. The good news is that the heat dumping aspect should still work even if there's no wood in the bin - perhaps it would even work better (less resistance to upward draft.)Maybe, when you're done with drying wood to an optimal moisture content, you could hang some clothes lines in there. :-)
  • Brad WhiteBrad White Member Posts: 2,393
    Wood overdrying

    Hey, Gordan-

    I have a question the wood over-drying and efficiency aspects and their effect on efficiency. I mean, short of an actual kiln drying, most wood dries to about 15% MC, depending on your area. Kiln drying for lumber might take that down to 8% give or take 2%, more likely give, to 10%.

    As a fuel, most firewood is rated at BTU's per pound at a stated moisture content. That is stated more as a reference point, to my thinking, a qualifier to compare BTU content.

    The core fuel value of the wood is in the embodied carbon. The wood moisture content, if higher than ambient allows, merely slows the combustion, until that moisture is driven off. Essentially a governor or drag on the process.

    If the moisture content is lower, there is less of that drag but the core heat value is still there. It just gets down to releasing it sooner.

    It may burn more rapidly, but it is the same heat value ultimately. If I understand your point, do you mean that too rapid a firing leads to shorter retention time (say in a wood stove or boiler) and higher flue gas temperatures hence lower efficiencies? Or would this be compensated for by using less of a fuel charge and tending it more frequently?

    Not trying to make too fine a point about it, I am more curious as to your line of thinking.  Anyway, your thoughts?


    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • GordanGordan Member Posts: 891
    I think your thinking and my thinking are along the same lines

    Wood burning appliance manufacturers tend to have recommendations on the optimal moisture content. Too dry, and you lose the ability to capture the energy created through combustion and transfer it to your living space which, as you suggested, leads to the excess energy being blown up the stack. Too wet, and you're using some of the energy to dry the wood, leading to excess energy being blown up the stack as latent heat of vaporization.

    In my line of thinking, if we had a good thermal storage strategy, a big heat exchanger and variable pumping through the HE, we wouldn't need to worry too much about fuel being too dry. If we had the ability to recapture latent heat of vaporization through condensing, we wouldn't need to worry too much about fuel being too wet. So, a well-buffered, modulating, condensing wood-burning boiler should be fairly agnostic to moisture content, within reason. Got one? :-)
  • GordanGordan Member Posts: 891
    And, admittedly...

    You'd have to have some pretty hot water circulating for a pretty long time in order for overdrying to become a concern, but I've never attempted anything like the above suggestion and summers are long... it's more of a theoretical thing to check for, than a proven issue.
  • Brad WhiteBrad White Member Posts: 2,393
    I think that our curious natures

    as exhibited in this thread, also means that our definition of, "Got Wood?" differs from the population at large, Gordan. 


    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • LarryCLarryC Member Posts: 331
    Airflow concerns

    Would using the whirlygig, or whatever those spinning ball ventilators are called, be necessary or an inpediment to getting sufficient air flow to keep the humid air exhausting?
  • LarryCLarryC Member Posts: 331
    Would using a different heat transfer fluid allow higher temps?

    On a related but different thought path, by substituting a higher temperature rated transfer fluid for water, could this idea be used for reducing maple syrup sap, desalinating water, or any other process that requires 200+ degree F energy?


    Would any of the silicone or mineral oils be viable heat transfer fluids?
  • Brad WhiteBrad White Member Posts: 2,393

    on a few variables, height and interior temperature vs. exterior temperature for starters. Wind too, of course, but absent wind, the stack effect can get you some good airflow. Naturally you will be taking care of the heat part, so even if you did not have one of those ventilators, (whirligigs, works for me!), a standard passive ventilator such as a shanty cap, ought to draw enough air.

    If I had to size one based on heat output, I would use a ten degree differential on airflow. For example, say your solar dump temperature in those radiators gave you, oh, 5,000 BTUs per hour. At a ten degree rise, that would be about 460 cfm.

    A 12" diameter throat ought to handle that and the intake should be about two square feet, low resistance. Just a thought.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • Brad WhiteBrad White Member Posts: 2,393
    I've never used

    silicones nor mineral oil as heat transfer fluids, just glycol. To me, glycol is a necessary evil, ok maybe that is too strong. Necessary but not particularly pleasant especially with high temperature breakdown.  I am open to learning about the other fluids, just never have used them.

    But if you want to gain higher temperatures, pressure is your friend.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • hot rodhot rod Member Posts: 7,009
    You might consider a heat transfer fluid

    That Larry W used in his solar collectors. Vodka! :)

    I do have about 60 gallons of used Dow Slytherm silicone transfer fluid if you want to consider high temperatures.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Brad WhiteBrad White Member Posts: 2,393
    Oh, that's right!

    I forgot about Larry's heat transfer fluid. Maybe we should brand it, "StoliTherm"?
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • Larry WeingartenLarry Weingarten Member Posts: 1,173

    Are you using my vain in name?  Glad to "see" you anyway!  Actually, cheeep Lucky vodka on sale is the right thing to use.  Just get used to the fact that the checker will not look you in the eye when you roll up with a shopping cart full of cheeep vodka.  /~P

    Yours,  Larry
  • Brad WhiteBrad White Member Posts: 2,393

    Great to see you too!

    We only met once in person at Wetstock Denver 2008 and immediately you and The Lovely Suzanne were amongst my very favorite people! Also, I enjoyed the series of interview articles about you in Home Energy Magazine, to which I subscribe... "Hey, I know him!"

    Very gratifying, as is this.

    Re: Heat Transfer Fluids on the cheeep...

    I will have to research the various properties of vodka, specific heat, vapor pressure at temperature, flash point, etc. Maybe add a little orange juice to make it look respectable. But your ingenuity and resourcefulness inspires.

    Please give my best to The Lovely Suzanne and the hummingbirds on the hill.


    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • Larry WeingartenLarry Weingarten Member Posts: 1,173
    I'll let her know...

    ... you send greetings.  About that vodka and flash point; wouldn't that require the addition of oxygen?  And wouldn't O be a bit restricted in a closed system?  Orange juice would prolly be OK as long as the pulp was removed as pulp pumping could be precarious.  OJ certainly would help to keep the insides of the copper pipes clean! 

    I only copy what the Day and Night Solar Water Heating Company did in the 1920s with alcohol in the collectors.  It worked then... and I used to think history was boring!

    Yours,  Larry
  • Mark EathertonMark Eatherton Member Posts: 5,582
    One can only imagine the looks you were getting....

    Pushing grocery carts loaded with Vodka down the liquor store aisles. I can hear the comments in the back ground.

    i.e. "Must be some rich alcoholic doing his monthly shopping.." or, "Must be Russian, and planning on a large reunion party with his fellow countrymen..." :-)

    Ingenious! And if and when the world starts coming to an end, you won't have to make a quick trip to the liquor store. (I assume you like Vodka :-)) Just head down to the mechanical room, pull a bar stool up to the charging ports, and let er drip!

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Brad WhiteBrad White Member Posts: 2,393
    Flash Point of Vodka

    Hi Larry!

    No argument on O2, I was just thinking of pinhole leaks in a boiler or near an ignition source, not really a concern.  Just my curiosity.

    But I love how the past is preamble!

    I came across a trap capsule from a 1915 Webster vapor system on Beacon Hill in Boston. Still had the alcohol charge inside it, splashing around.  I found it in a radiator enclosure where it was obvious the traps had not been serviced since 1915, but still apparently worked. I gave that capsule to Dan for his collection last March. Seems before prohibition, it was the fluid of choice! Also seems during prohibition, it still was the fluid of choice!

    Funny about pumping OJ. The Nevada Department of Corrections finds it funny too, I bet. :)
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • Karl_NorthwindKarl_Northwind Member Posts: 140
    air kiln

    sorry to digress back to the original thread (and vodka (100 proof) costs us more than glycol properly mixed, has higher specific heat, and provides better freeze protection. )

    I did a system a while back that we put on a shed that later became a wood kiln.

    the system had 12  21 square foot collectors, and we used a pair of recycled fan convectors to heat the space, and a dehumidistat to control operation on a fan to change the air over at specific points.  I just set up the solar portion of the system, and the customer set up the control for the drying. there were controls to prevent overheating of the kiln area and the humidity of the kiln area.

    the system was designed to prioritize the DHW supply in the summer and Space heating in the winter.

    as far as I know it's been working fine.  but he's not just drying firewood, he's doing custom sawing and kiln drying.

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