Abstract

Portable sawmill owners quickly recognize the advantage to kiln drying lumber they 

produce. Having the ability to provide properly kiln-dried lumber opens new market 

opportunities and can increase profi t margins. However, the construction and 

operation of a dry kiln must be economical and simple. A small dehumidifi cation 

dry kiln constructed and tested in Princeton, WV indicated that costs can be 

much less than a package kiln, and electricity costs to run the kiln seem to be very 

reasonable. Th e kiln did a good job drying maple lumber.

INTRODUCTION

Portable sawmill owners quickly recognize the advantage to kiln drying lumber 

they produce. Having the ability to provide properly kiln-dried lumber opens new 

market opportunities and can increase profi t margins. However, the construction 

and operation of a small dry kiln must be economical and simple. A small 

dehumidifi cation kiln with an 800 board foot (bf) capacity (Fig. 1) was constructed 

by the U.S. Forest Service in Princeton, WV. Construction and initial use of the 

kiln were discussed in an article in “Independent Sawmill & Woodlot Management” 

(Bennett 2012). Th e cost of the dry kiln included $2500 for the dehumidifi cation 

unit and $500 for construction materials. Scrap materials were used where possible 

to keep construction costs down.

Drying Hard Maple 

(Acer saccharum L.) Lumber in 

a Small Dehumidification Kiln

Neal Bennett

United States Department of Agriculture

Forest 

Service

Research Note

NRS-196

Northern 

Research Station December 2013

NEAL BENNETT is a general engineer with the U.S Forest Service, Northern Research 

Station, 241 Mercer Springs Road, Princeton, WV 24740.
2

Th e kiln was initially tested with two loads of 4/4 white oak boards that had been air 

dried to less than 25 percent moisture content (MC). Yet, questions remained as to this 

kiln’s ability to dry fresh cut green lumber and the costs associated with drying a load of 

lumber. In November 2011, the U.S. Forest Service acquired 33 hard maple trees from 

the Fernow Experimental Forest in Parsons, WV for use in another research study. After 

completing the study, many of the butt logs were sawn into lumber and used to further 

test the dry kiln’s capabilities.

PROCEDURES

Hard maple (Acer saccharum L.) trees were felled in mid-November 2011, and 12 logs 

were sawn into 4/4 lumber with a portable sawmill on December 13, 2011. All logs 

were end coated prior to sawing. After jacket boards were edged, 441.4 bf of lumber was 

placed in the kiln as stack 1 on December 14, 2011. An average MC of 54 percent for the 

lumber stack was measured using fi ve oven dried samples collected as the stack was being 

put together.

Two sample boards created using standard procedures outlined in the “Dry Kiln 

Operator’s Manual” (Simpson 1991) were placed in the front of the lumber stack and 

were weighed approximately every other day to keep track of drying progress. A second 

load of 4/4 hard maple lumber was sawn between April 10-12, 2012, using 18 logs from 

the batch previously collected. Jacket boards were edged and a stack of 700 bf (stack 2) 

was placed in the dry kiln on April 13, 2012. Th is load had an average MC of 39 percent. 

Again, two sample boards were created using standard procedures and were placed in the 

front of the lumber stack to monitor drying progress.

Th is kiln dries lumber by dehumidifi cation, only adding heat once the lumber is dried 

to 40 percent MC. At this point the heating element is employed, but the temperature is 

never increased more than 5 °C per day. Th e dehumidifi er manufacturer provides a chart 

that outlines the maximum safe temperature that should be set according to the MC 

Figure 1. — Dry kiln. Photo by Neil Bennett, U.S. Forest Service.
3

(ranging from 40 percent down to 0 percent) of the lumber. Maximum 

temperature setting for the unit is 50 °C.

To measure the electricity used by the dry kiln, power was run from a 110v 

receptacle to an electric wattage meter and then to the dry kiln (Fig. 2). 

Th e meter required the entry of our facility’s electricity cost which was 7 

cents per kilowatt-hour (kWh).

RESULTS AND DISCUSSION

Stack 1 dried to an average moisture content of 6.5 percent in 39 days. 

Moisture content was measured on samples collected from each tier of 

boards, starting from the top:

Row Moisture Content (%)

1 4.8

2 5.8

3 6.3

4 5.4

5 6.4

6 6.2

7 6.2

8 9.4

9 7.5

10 7.3

11 6.2

12 7.1

No drying defects were discovered with prong tests. 

Methods for creating stress sections, including 

prong tests, are described in Chapter 6 of the “Dry 

Kiln Operator’s Manual” (Simpson 1991). Some 

of the boards cupped slightly (Fig. 3). Boards in 

this stack were “log run,” and most of the cupped 

boards appeared to be from near the center of the 

log from which they were sawn.

Boards sawn from near the log pith have a greater 

tendency to cup or split than boards sawn from 

the outer portions of a log. Also, the stack was 

only 12 tiers high, and cupping might be reduced 

by placing additional weight on top of the stack while in the dry kiln. In the future, solid 

concrete blocks could be added for top weight. It was observed that there were very few 

end splits on the kiln-dried lumber in stack 1. Electricity usage for drying stack 1 was 520 

kWh, resulting in a cost of $36.43 or 8.3 cents per bf.

Figure 2. — Electricity wattage meter. 

Photo by Neal Bennett, U.S. Forest Service.

Figure 3. — Stack 1 after drying. Photo by Neal Bennett, 

U.S. Forest Service.
4

Th e second stack of hard maple lumber was removed from the dry kiln in 45 days. Oven 

dried samples were collected from each tier of boards, and moisture content was measured: 

Row Moisture Content (%)

1 5.6

2 5.6

3 5.1

4 6.0

5 5.1

6 5.3

7 6.5

8 5.7

9 5.4

10 6.0

11 5.6

12 6.3

13 6.9

14 15.0

15 8.1

16 7.3

17 12.3

Boards from the top 13 tiers dried to acceptable levels, but the bottom 4 tiers had boards 

that still contained unacceptable moisture contents. Th ese four tiers of boards were placed 

back in the kiln for an additional 5 days until oven dried samples resulted in an average 

MC of 6 percent. Stack 2 only had a few boards near the top that cupped, but many of the 

wider boards had end splits. A slower initial drying rate might reduce shrinkage defects, 

but you also need to be concerned about preventing 

stain in the lumber. About 15 of the 131 total boards 

used developed a light mold on the surface after a 

couple of weeks in the kiln. Mold growth indicated 

drying was too slow or relative humidity in the kiln 

was too high. After removal from the kiln, these 

boards were lightly sanded to determine the extent 

the mold penetrated the board. Examination of the 

sanded boards revealed that the mold aff ected the 

surface only (Fig. 4). Th is particular dry kiln has a 

very low air fl ow rate (6 meters/min) when measured 

at the front of the lumber stack. A low air fl ow rate 

could increase the risk of staining of the lumber. 

Additional fans could be added to the dry kiln to 

alleviate this, but that would add to the construction 

and operating cost of the dry kiln. Electricity usage for drying the second stack of lumber 

was 566 kWh, resulting in a cost of $39.66 or 5.7 cents per bf.

Figure 4. — Board with mold on right, sanded view on left. 

Photo by Neal Bennett, U.S. Forest Service.
5

SUMMARY AND CONCLUSIONS

Th e goal of this study was to evaluate the limits of this dry kiln as built. I feel that I almost 

overwhelmed the dry kiln on the second larger load. Moisture removed from the lumber 

could not be removed from the dry kiln quite fast enough by the dehumidifi cation unit. 

Th e dehumidifi er was set to run continuously, and the temperature was raised to the 

maximum amount recommended by the dehumidifi er manufacturer. No harm was done 

to the boards, but if the lumber stack was to be sold, the few boards that molded on the 

surface could be an issue. Th e wetter boards on the bottom of the stack are also an issue 

that must be addressed. Both problems are likely due to the low air velocity. Air drying 

the lumber fi rst or drying amounts smaller than a full load should prevent these issues. 

Maintaining a bright appearance is an important attribute in drying hard maple lumber. 

Figures 5 and 6 illustrate the color of the hard maple lumber before and after sanding, 

respectively.

Electricity usage and drying costs were much less than anticipated. Our research lab is 

located in southern West Virginia, and 7 cents per kWh for electrical usage seems to be 

on the low side nationwide. To calculate drying costs for your location, multiply the kWh 

used above by your local power rate. Divide this amount by the lumber volume in each 

stack to get drying cost per bf. To calculate your savings, check the diff erence between the 

cost of green and kiln-dried lumber prices at your local sawmill or consult the “Hardwood 

Market Report” (Hardwood Market Report 2013). It must be noted that drying costs 

are for this particular dry kiln. Power usage is dependent upon the size of the dry kiln, 

quantity of insulation, moisture content of the lumber, and ambient temperatures outside 

of the dry kiln. Any reported operating costs for this kiln would not be accurate for another 

dehumidifi cation dry kiln. Air drying of lumber before kiln drying will reduce kiln drying 

costs. Stack 1 was dried for 13 days at a rate of 48 cents per day before the heating unit was 

turned on. Stack 2 was dried 6 days before the heating unit was turned on. As the dry kiln 

temperature was gradually increased, drying costs increased to about $1.50 per day.

One remaining concern is the eff ect of outside temperatures on drying quality and 

electricity costs. Our dry kiln was located inside a wood shop that was heated during 

offi ce hours. After offi ce hours and on weekends, shop temperatures were allowed to fall 

to 12.8 °C (55 °F). During the winter months when the shop temperatures did drop, 

the heating element in the dry kiln could not maintain the set temperature. Drying still 

occurred, but at a slower rate and obviously at a higher cost. Quality of the lumber was not 

aff ected. Of more concern is if this dry kiln was placed outside and exposed to the summer 

sun. It is likely the dry kiln temperature could get too hot too fast and could damage the 

lumber. We were fortunate to possess heavy equipment that allowed us to move our dry 

kiln inside or outside, giving us more fl exibility in its use.

Th is small dehumidifi cation kiln has done a good job drying hardwood lumber to this 

point. Any challenges of drying green lumber can be lessened by carefully air drying fi rst or 

drying smaller amounts of lumber in each load. I consider this particular dry kiln a “niche” 

kiln. Annual drying volume would probably be less than 10 thousand board feet (mbf) 
6

unless most of the user’s lumber is well air dried beforehand. One may have to consider 

this an “inside” kiln in order to use the kiln year round if local temperatures reach extremes 

in summer or winter. Construction costs for this dehumidifi cation dry kiln can be much 

less than a package kiln, and electricity costs to run the kiln seem to be very reasonable.

LITERATURE CITED

Bennett, N. 2012. Th e construction and use of a small-scale dehumidifi cation kiln. 

Independent Sawmill & Woodlot Management. March 2012: 22-27.

Hardwood Market Report. 2013. Kiln dried prices Available at www.hmr.com. 

(Accessed May 24, 2013).

Simpson, W.T., ed. 1991. Dry kiln operators manual. Agric. Handb. 188. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 274 p.

Figure 5. — Furniture blank from kiln-dried maple. 

Photo by Neal Bennett, U.S. Forest Service.

Figure 6. — Furniture blank after sanding. 

Photo by Neal Bennett, U.S. Forest Service.
Printed on Recycled Paper

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Bennett, Neal. 2013. Drying hard maple (Acer saccharum L.) lumber in 

a small dehumidification kiln. Res. Note NRS-196. Newtown Square, 

PA: U.S. Department of Agriculture, Forest Service, Northern Research 

Station. 6 p.

KEY WORDS: dehumidification kiln, portable sawmill, maple lumber

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