To have an efficient firewood kiln, you need to engineer and design the kiln to be efficient. So, take your time when you design your kiln and seek good advice. Sadly, we have seen many companies who have constructed their own firewood kilns only to realize that the kiln does not work efficiently. We will review both poor as well as sound kiln designs.
|Poor design specifications|
|Container or van-body kiln.
These seem like a reasonable choice, but they really do not do the job well. It is hard to get the right air circulation, fairly difficult to load and the ones we have seen on the market are under powered on air flow and undersized for the heat system.
|Main Fans not located overhead.
Regular kiln designs nearly always have the main fans overhead becuase they simply create the best air flow.
|Main fans under sized.
For our 6 cord unit, we install 8 HP main fans with a total of 20000 CFM (cubic ft per minute) at 1/2in static pressure. (Most companies will only tell you the air volume in free air = 0 static pressure). Many firewood kilns on the market seem to have less than this capability.
|Heat system undersized.
A firewood kiln needs to have about 3-5 times as much heating capacity as a regular hardwood kiln. We have doubled our heating system for firewood kilns plus we have added heat recovery on the venting to recapture the heat- indirectly boosting heat efficiency.
|Only hot water heating.
We have yet to find an efficient firewood kiln that relies on only hot water heating to meet the 160F inside wood temperature required by USDA.
|Venting, undersized (and no heat recovery system).
This is only required if you are drying firewood (heat treatment requires no venting). If you are drying, then you need to be sure that you have a sufficiently large vent and (hopefully) an exhaust fan. No heat recovery system on the vents will make the drying process longer- as more heat is needed to keep the chamber hot.
|Poorly insulated chamber.
The kiln needs to be a minimum of R-20 and preferable R-24+ in the walls. This is one of the reasons a van-body (R10 to R16) is a poor idea.
|Insulated using the wrong insulation.
There are many insulation types on the market. We would not recommend using open cell foam, or mineral/glass-fiber insulation as this can get wet. We use a close-cell, Class 1 Fire Rated foam that cannot absorb moisture.
|Control system only relying on chamber temperature.
Actually, for drying firewood this is not so important. However, if you are only heat treating firewood, not using internal wood temperature sensors may extend your treatment time and gas consumption (or wood usage).
|Good design specifications|
|True kiln design.
The kiln should have been specifically designed and engineered to dry firewood (not just a modified lumber kiln). In 2009, Kiln-direct launched our new firewood kiln which has been designed from the "ground up" for firewood. This design is called our Quick firewood kiln design and has bottom-up air flow.
|Main fans location and performance.
For our 6 cord unit, we install 8 HP main fans with a total of 20000 CFM (cubic ft per minute) at 1/2in static pressure. For our 12 cord unit, we install 16 HP main fans with a total of 40000 CFM at 1/2in static pressure.
|Heat system (General).
Hot water from wood waste can be used to dry firewood, but you probably need something else to get the kiln temperatures up to 180-200F to reach 160F inside the firewood. We have found that a direct gas system does this efficiently. Some type of wood waste heating with air-to-air heat exchange can also get the job done
|Heating with gas.
A small heater will not do the job. Your kiln will need between 80000 to 100000 btu/hr per cord heat output capacity. 6 cord kiln has 600000 btu/hr heating output and 12 cord kiln has 1200000 btu/hr heating output.
|Heating with wood waste / hot water.
Normally, the hot water stove is the limiting factor and we suggest getting the biggest commercial hot water stove available from your local dealer. You must insure that you install a sufficient amount of hot water fin tubing inside the kiln.
|Heating with wood waste / air-to-air heat exchanger.
New Kiln-direct design allows much higher chamber temperatures for quicker turn-around and is able to match the gas heating in cycle time of 1-2 days.
|Heating with wood waste / direct flue gasses.
We do not offer this design- as we are certain it is not safe.
|Good venting, preferable with heat recovery system if you use gas heating.
We would strongly suggest that you install a heat recovery system on the vent to reduce the overall heat requirements and speed up the drying process. Naturally, if you are using gas a heat recovery system will reduce heating costs.
|Well insulated chamber.
The kiln needs to be a minimum of R-20 (preferably R-30+) in the walls. We would not recommend using open cell foam or a mineral/glass-fiber insulation. We use a Class 1 Fire rated, close-cell foam that cannot absorb moisture.
|Control system designed and optimized for heat treating with internal wood temp sensors.
You should have a computerized control system-preferably with a link to an office computer for recording the temperatures and to print HT reports for certification. Kilns can also make use chart recorders. Internal wood temperature sensors that are integrated into the control parameters insure certification and keep the HT process to a minimum of hassle.