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37
2012-11
Heating value of wood
SH Operation
Heating value of wood
Split wood is typically sold in stacks by the cubic
meter. The energy content depends not only on the
type of wood. The amount of heat a cubic metre
contains also depends on whether the wood is
unsplit round logs, metre-long split logs, or ready-
to-burn split logs, and on whether it is fresh from
the forest or dried for firewood.
The wood volume stays constant from when it is
freshly cut with a moisture content of 60% to the
fibre saturation point at 25%. The wood starts
to contract if dried below this point. It reduces
in size, and this is why air-dried wood (moisture
content of 15%) has more wood substance than
freshly cut wood per cubic metre. The difference is
typically 5% to 6% for softwood, and 6% to 9%
for hardwood.
Logs are not straight. The longer the logs are, the
less wood and the more air is present per cubic
metre. There is typically more air in "crooked"
hardwood than in "straight" softwood.
There is one further difference: round logs can
typically be stacked in a far more compact way
than metre-long split logs. If you split the wood
yourself, buying round logs will give you up to
15% more wood per cubic metre.
In practice, deviations on the order of +/-10%
compared with theoretical values are to be
expected. Up to 20% is possible in extreme cases:
not only if the wood is particularly straight or
crooked, but also because the density of the wood
mass itself can vary.
Incidentally, if you order a cubic metre of ready-to-
burn beech firewood as 50 cm split logs from your
wood dealer, the dealer is entitled under current
law to supply 0.85 m³ of half-metre split logs cut
from 1 m³ of 1 metre long split logs. To avoid any
unpleasant surprises, when ordering ask how your
dealer measures a cubic metre (if the price is good,
you may find 0.85 m³ acceptable).
Estimating your wood requirements
Each kilowatt of heat output requires 0.9 cubic
metres of half-metre split beech logs, or 1.3 m³ of
split spruce per year.
In other words, 8 m³ of half-metre split spruce
logs or 5.5 m³ of beech will replace 1000 litres of
heating oil.
Energy content of one cubic metre of wood in kilowatt hours
The table assumes that the firewood used is split and air-dry (after one year's storage) with a moisture content of around
15% when burned even if it was moist when bought.
Round logs 1 m
freshly cut
w=30-60%
Round logs 1 m
dry w=15%
Logs 1 m
freshly cut
w=30-60%
Logs 1 m
dry w=15%
Logs 50 cm
freshly cut
w=30-60%
Logs
50 cm
dry w=15%
Logs 33 cm
freshly cut
w=30-60%
Logs 33 cm
dry w=15%
Softwood
1 stere contains 0.65
m³
1 stere contains 0.56
m³
1 stere contains 0.62
m³
1 stere contains 0.64
m³
Fir
1,205 1,269 1,038 1,093 1,149
1,210
1,186 1,249
Spruce
1,299 1,373 1,119 1,183 1,239 1,310 1,279 1,352
Douglas fir
1,402 1,478 1,208 1,274 1,337
1,410
1,380 1,455
Pine
1,542 1,625 1,329 1,400 1,471
1,550
1,519 1,600
Larch
1,573 1,656 1,355 1,427 1,501
1,580
1,549 1,631
Hardwood
1 stere contains 0.59
m³
1 stere contains 0.50
m³
1 stere contains 0.59
m³
1 stere contains 0.62
m³
Poplar
958 1,020 812 864 958
1,020
1,007 1,072
Willow
1,107 1,200 938 1,017 1,107
1,200
1,163 1,261
Alder
1,191 1,270 1,009 1,076 1,191
1,270
1,252 1,335
Maple
1,472 1,550 1,247 1,314 1,472
1,550
1,547 1,629
Birch
1,475 1,570 1,250 1,331 1,475
1,570
1,550 1,650
Ash
1,658 1,760 1,405 1,492 1,658
1,760
1,742 1,849
Oak
1,664 1,760 1,410 1,492 1,664
1,760
1,749 1,849
Beech
1,655 1,800 1,403 1,525 1,655 1,800 1,739 1,892
Hornbeam
1,743 1,920 1,477 1,627 1,743
1,920
1,832 2,018
Black locust
1,743 1,920 1,477 1,627 1,743
1,920
1,832 2,018
Heating value of wood