- MC- Moisture content
- BTU- British Thermal Unit
| Common Name | ft3/lbs | ft3/lbs, MC 20% | 1 cord of stacked green wood (lbs) | 1 cord stacked wood MC 20% (lbs) | 1/2 cord stacked firewood MC 20% (lbs) | 1/4 cord stacked wood MC 20% (lbs) | BTUs per cord MC 20% | BTUs per cord 70% fireplace efficency & MC 20% |
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Please note that the table describes the average volume of one net cord’s weight, which in this case is 85 ft3 (a stacked cord of wood)
Calculating the correct number of BTUs for each tree is a tricky process because each tree produces a slightly different amount of heat due to sap and resin content.
However, the amount of heat generated per pound of firewood is very similar. In ideal conditions one pound of wood, with 0% moisture content can produce 8600 to 9000 BTUs per pound depending on the resing content. Wood with higher resin burns slightly hotter.
Furthermore, the combustion process itself can reduce the amount of potential BTU amount about 6%. If we take that into account, we will get two numbers:
8084 BTUs per pound for non-resinous woods
8460 BTUs per pound for high resinous woods
Since we are doing calculations on bulk to get a general ballpark, let’s take an average of that number, which would be 8272
Now all that’s left to do is to calculate how the moisture content inside the wood affects its efficiency.
The results are as follows:
At 65% Moisture content the wood would output BTUs 5013 per pound
At 20% moisture content, the average heat output would be 6893 BTUs per pound.
Another factor that affects the heat output of the wood is the efficiency of your fireplace. For example, if your fireplace efficiency rating is at 70% it will reduce the amount of BTUs received by 30%.
Elaborating on BTU and Net Cord
BTU – this unit of measurement is most commonly used for evaluating the thermal output of timber. 1 BTU is the amount of energy it takes to heat 1 lb of water by 1 F. Cord of firewood measures 4x4x8 ft equaling 128 cubic feet in dimensions.
However, the net cord is the actual amount of wood in a cord. It is calculated by subtracting the bark and air space from the cord of wood. Usually, in one cord of stacked firewood, around 75 to 100 cubic feet of is the actual amount of wood, or ‘net cord’
To elaborate more, 1 BTU is the amount of energy that is released when lighting a match. In contrast, 1 Million BTU is equal to the energy generated by 179 gallons of oil or 7,283 kW of electricity.
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