RMH powered lumber kiln

Greetings permies,

I'm curious if anyone has used an RMH to power the heating of a lumber kiln. Or if anyone has familiarity with a lumber kiln and would be willing to share insights that could be used to guide a concept, design, or operational procedure for powering one with a RMH, I would love to hear it. Experience with solar kilns would be great to hear about, too, because I'd make a solar/RMH hybrid kiln at the slightest encouragement.

I'm asking because I'm considering purchasing a small business that has a large (66" diameter) lumber mill. Current owner has a smaller kiln, but does most of his drying by air. There's plenty of space to put up a kiln, or a few kilns, on the property, but I have a deep desire to power it with small cut-offs from trees that don't have any lumber value.

Thoughts?

Greetings Benton!

I think a rmh powered kiln could work OK.
Are you thinking a hoop house with a rmh bench down the middle and lumber racks on top of that?

I think this is an awesome way to stack functions. What are the temperature and airflow requirements for the kiln you would design? I gather that it's better to have even, consistent heat as opposed to extreme peaks, so getting the mass of an RMH in the equation would certainly help. The difference between your application and heating a space with people in it is that you're probably more interested in hot air than radiative warmth, so I would look at a way to have a heat exchanger between the mass and a forced air system.

There was a great video posted this weekend of a lumber mill and he mentioned that there is a USDA guide for building and running wood kilns. I searched around a bit over the weekend and saw that as well as a few other PDF's released by different universities that also goes in to details of various types of kilns, including ones that are solar or powered by scraps from the lumber mill.  

There were a few noteworthy things for those who don't want to watch the whole video. They let the wood air dry for months first to let some of the moisture out before kiln drying to avoid case hardening. Once kiln dried, the cells of the wood on the outside collapse and will slow down moisture both from leaving or entering the wood. That means once properly dried it won't be as affected by seasonal change as air dried wood.

The solar kilns have an added benefit that because they don't run constantly, the down time between heating allows the moisture from deeper in the wood to equalize and reduces grain stress and less cracking or warping. I'd imagine the same would be true of the manually operated wood fired kilns. Depending on your goals, it might not be as desirable to have a large mass heated and keeping the temperature constant. I guess it depends on whether you want to dry wood more quickly to move it through and be able to load the kiln up again, or if you prefer the slower method which yields more consistent results. I haven't read through all of the materials available yet, but this video should help point you in the right direction:

I'm not sure about the function for a kiln but I do think a batch box would be much more fun to operate for this than a J tube.

Daniel Schmidt wrote:
The solar kilns have an added benefit that because they don't run constantly, the down time between heating allows the moisture from deeper in the wood to equalize and reduces grain stress and less cracking or warping. I'd imagine the same would be true of the manually operated wood fired kilns. Depending on your goals, it might not be as desirable to have a large mass heated and keeping the temperature constant. I guess it depends on whether you want to dry wood more quickly to move it through and be able to load the kiln up again, or if you prefer the slower method which yields more consistent results.

That's some really good info and shows how my intuitive assumption wasn't necessarily the best one. So a batch box RMH is even more ideally suited to the use case than I initially thought. This could fit into the workflow of a sawmill pretty easily, just load the thing up with offcuts a couple of times a day and let it do its thing.