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2014 Heater Fuel Tests at Shawville

Test HK-M26
March 8, 2014

Red oak cordwood. Large wood
Side ignition.  Newspaper 3/4 up the kindling pile.
60.1 lbs including 5.2 lbs kindling
18.7% moisture.

Stack temperature corrected for height.
Pisla HTT 602 door, 22" wide
Austrian eco firebox, see
Custom air frame, setup "D"
Rear grate closed

Condar Spreadsheet Results
Condar spreadsheet results, calculated at 140 minutes.
This was a long burn. See photo sequence below, which goes to 180 minutes

Condar spreadsheet results, calculated at 180 minutes.
Efficiency goes down, as one would expect with the air wide open when there is only charcoal.
This highlights the important issue of defining the end of burn, which is needed to calculate average burn rate, since the heater is not on a scale.

In the Finnish system, underfire air is used to burn up the coals at the end, so that the chimney damper can be closed.
In the Austrian system, chimney dampers are not allowed. When the wood has burned down to charcoal, you simply close off the air.
Any test is then automatically over, since there is no more flow in the flue to get gas and temperature readings from.

This has important implications for how you define efficiency.
It also highlights the fact that any stack loss efficiency number is dependent on definitions, including burn rate and end of burn.
95% oxygen recovery is one way to define end of burn. In the Austrian system with a very large coal bed at the end and no bottom air at all, this could take a very long time indeed and give very low numbers.
Therefore, stack loss efficiency numbers can only be compared based on using the same end of burn and burn rate definitions.

This firebox design is intended for an airtight system that does not require a chimney flue.
When you close off the air, you can generate a significant amount of charcoal, or biochar.
Biochar is carbon negative, capable of carbon capture and storage when used as a soil amendment.

Condar Spreadsheet - 140 minutes (includes detailed fueling data)
Condar Spreadsheet - 180 minutes (includes detailed fueling data)

Animation,  5 minute intervals.  Newspaper is located 3/4 up the kindling stack. Sequence goes to 180 minutes.

See burn videos, below.

gas analysis
Flue gas graph to 140 minutes. Note the pile collapse at 90 minutes, which was unattended, and the poke at 125 minutes.
See burn sequence animation, above.

TESTO Excel data file

Testo graph
Testo gas graph, entire cyle.
Notice the large CO drop at 12:15 PM, when the collapsed pile is poked.

Opacity graph. 
Compare with opacity graph for HK-M25, where the kindling was more staged.
On this burn, there was some lower at the front, which resulted in the entire kindling pile catching faster.
Note the initial PM spike, reduction and then PM increase again as the kindling pile catches.


Red oak cordwood with bark left on.  Largest pieces were placed at the bottom, smallest pieces at the top.

Stacked in firebox

Burn sequence to 180 minutes


This page was updated on March 9, 2014
This page was created on March 8, 2014