Home-Built Dehumidifying Kilns

Practical issues and concerns with building a drying kiln that uses an off-the-shelf dehumidifier or a salvaged commercial air conditioner instead of air venting. July 13, 2006

Has anyone ever made or tried to make a DH kiln from a heat pump, air conditioning system, or from scratch? I've been toying with the idea but I really don't know where to start and I'm having problems finding any information on this. A Nyle L-50 would meet my needs, but for now I cannot come up with the money.

Forum Responses
(Sawing and Drying Forum)
From contributor A:
Have you considered a solar kiln? Get a copy of "Drying Hardwood Lumber". This free government publication will tell you everything you need to know. You can download it from the net.

From the original questioner:
I have thought about a solar and that is most likely what I will end up with. I am just looking for a possible better way to dry my lumber. Space is an issue here so one small to medium sized DH kiln would be better for me than two or three solar kilns.

From contributor B:
I made one from plans from American Woodworker magazine in which you use a home DH, attic fan for air flow, and lights to heat the kiln. I have dried only white pine in mine so far but it seems to work really well. My lumber has all been air dried for at least two years.

From Gene Wengert, Sawing and Drying Forum technical advisor:
A homemade DH kiln will work to dry lumber, but the capacity is limited and compressor life is limited. Plus, with the home-type DH unit, you will not get above 115 F. There are advantages in going hotter to set the pitch and kill insects. If you have only a small amount of wood, why not use your attic? If you have well air-dried lumber, you do not need a DH unit, but could go with heating alone.

From contributor B:
I have some red oak that has air dried for approximately 4 years. Would it be okay to dry with just heat alone in a homemade kiln or should I put a DH in the box as well? My house is a two story so the attic is out of the question. By the way, I did get the white pine I dried to 120 degrees to set the pitch. Is that warm enough to kill any insects that may be present?

From the original questioner:
I was hoping for a refrigerant guru who was able to make a DH from old residential cooling systems. Take an AC unit and blow the fan over the exchanger first then over the condenser then back through the lumber. In theory, in a sealed compartment you would have a dehumidifier and a heater. Has anyone ever tried that? I know something like this could be made and could be reliable, but I have no idea where to start.

From contributor D:
One of the magazine plans simply had you place a window air conditioner totally into the kiln. Use a drip pan to catch the condensate and pipe it outside. There are a couple issues with this. First, as Gene already mentioned, the temps won't get where you would like. Also the acids coming off the wood will eat up the condenser. Finally, consider how you will dispose of the condensate. It will be acidic and there will be more of it than you may expect. In some areas it may be considered hazardous. People with existing DH kilns may be able to add more information to this.

With all that said, it can still be a good idea if you only have a small amount of wood to dry. If you are going to run it year around you may be unhappy. If you only want to run two loads in the life of the kiln, you may be ecstatic.

From contributor E:
I take compressor units out of commercial refrigeration units quite often. I am building a kiln myself. I plan on using one of the compressors to pull water off the kiln. These are epoxy coated and shouldn't be affected by acids and heat (the way we are hooking it up). It will take a couple of months to get it up and running. In the meantime, I can sell these compressors for what they cost us. I think they are 1/2 hp Tecumseh compressors.

From contributor F:
I'm also in the process of building a homemade kiln. I tried the window AC unit to help condense the water, but as was mentioned previously, you just can't get enough heat from the AC units. Currently I am installing an LP furnace from a mobile home and wiring it with water heater thermostat to get the temperature where I need it. Also for water extraction I am planning to install a radiator (auto) and cycle cold water through the radiator with the above mentioned drip pan under it to drain condensation. I'll post results from this venture when I see how well it works. Does anyone else have any ideas on what problems I may run into with the proposed design? Any suggestions or recommendations would be greatly appreciated.

From contributor G:
I've tried the cold water condensing in the past. I now understand that in order to achieve much condensate flow, a large amount of cooling water will need to be pumped. The ratio of cooling power to air flow needs to be controlled. If a certain amount of airflow across the radiator causes its exterior temperature to rise towards the dew point you no longer have condensation occurring. In theory, water cooled condensation should be more energy efficient than simple air venting, but when the water pumping is factored in you've lost the advantage.

In a DH kiln the condenser coils are colder than is possible with well water. In the process of cooling via refrigeration there is always more heat created than cooling. This is the reason a good DH kiln uses less total energy than any conventional kiln. If commercial kilns had only electricity as an energy source, all kilns would be DH today. However, with the economy of scale afforded by large kiln operations, wood waste generated energy heating conventional vented kilns achieve the lowest total drying cost. My radiators have all found other uses.

From contributor H:
The drain water from a DH kiln is not really a problem. This is water that was moving in the growing tree and it does tend to be acidic but no more than the soil in a healthy forest or rain in most places. But at higher temperatures it is corrosive and you need to deal with that. It is easy to correct the pH of the drain water if you have to or want to do that. Some regulators or inspectors think that neutral pH is natural. Unfortunately the idea that neutral acidity is natural has taken hold and people are sometimes required to treat the drain water to pH neutral, which is pretty unnatural. It is so easy and cheap to do, that people do it rather than fight.

As for the gas furnace, the heat exchanger will corrode rapidly especially when the burner is off. That is why only stainless steel heat exchangers should be used on kiln furnaces. Rigging a conventional furnace is pretty dangerous. Using cold water to condense has been done for many years but it does require a large flow of water and a lot of heat to make up the cooling effect of the large flow of water. Coils will corrode if the wrong material is used.

As far as which is the most economical system, there is no one answer for that. Electric rates vary widely, the value of residues vary widely, the cost of capital varies widely, the attitude of insurance companies toward wood burning varies widely, the value of time and attention to running a boiler varies widely, and the allocation of resources to putting in a way to save energy cost verses a way to increase quality or production varies widely, etc.

From contributor A:
An L50 doesn't cost that much. Are you in the woodworking business? Do you operate a sawmill? Are you a logger?

From the original questioner:
I operate a sawmill, part-time, as a hobby. I usually cut about a 1,000-2,000 bd ft a month. But I'm in the process of converting the saw to fully hydraulic and changing a few other things to get my per hr rate higher. I built this saw last summer. I just got done building a sharpener and setter, and in the next few weeks I'm going to start on a heavy gooseneck trailer so I can take my tractor out with me to handle logs and clean up. Once I get everything set up I'd like to turn this into a profitable part time business. I think a kiln would really help in the marketing of my lumber. Obviously I also like to build things so I though why not a DH kiln? We're thinking about getting an outdoor boiler to heat our house - could that be helpful for drying lumber?

From contributor A:
The reason for kilns is not just to dry wood faster, but to dry wood under controlled conditions. If you want to dry quality lumber with minimal degrade, then there are many things to consider. This is the reason why low cost units as the L50 are so popular. They help you maintain controlled drying conditions. Most often, you will end up spending less money in the long run, rather than trying to homebrew something up. Solar drying is another issue. There are some wonderful designs for homemade solar kilns that work great with minimal degrade.

From Gene Wengert, Sawing and Drying Forum technical advisor:
When drying wood, for most species, it is more than putting the heat to it. We use a specified temperature, RH and air flow; the settings change with MC. The standard settings are conservative, in order that they apply safely in all cases. It is possible to go faster and reduce the safety factor or conservativeness, especially when the lumber is of good quality. In fact, some kiln sales people talk about how their kiln can dry wood faster than the standard, but in reality all they have done is to remove the safety factors. If you are running a business to make money, I can guarantee that you will do much better (quality, longevity, maintenance, economically) with a commercial kiln and commercial controls than with a homemade unit. I have provided a lot of assistance to people getting started in drying and so have seen the full range of attempts. Two special words of advice - first, do not spend cash for a kiln, but rather borrow. You need cash to cover log costs and poor sales months. Second, when making initial plans (using a business plan for sure), always consider where you will put the second unit, as most kilns end up expanding their operation within a couple of years.

From the original questioner:
I'm thinking maybe I'd be better off paying someone else to dry the lumber I have and will be cutting until I get ahead and can buy my own unit.