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Eco Firebox Testing in France

This page was created on February 12, 2015

Below is information forwarded by Lopez Labs Co-Op member Boris Kukolj, on recent testing by Damien Lehman in France. Damien will be attending the 2015 MHA Meeting at Wildacres.

The data file is linked at the bottom. 
Of particular note are the later tests, which added an insulated baffled firebox ceiling, and yielded some very impressive numbers.

From: Boris Kukolj <>
Subject: Damien's testing results
Date: 20 January, 2015 10:41:54 PM PST

Each day's results are in a separate folder with photos and a spreadsheet (Test.xls). Please note that the spreadsheets from the first four days are incomplete.

The tabs in the spreadsheets show :
General = Condar page; look at "Averages from Testo Template" to see Stack Temp, O2% and ppm CO. Please note that fuel information is not relevant
Graph : 2 graphs + combustion & temperature data
USI Data : length of recording, CO2 max, CO adjusted @ 13% O2
Temperatures : data from the thermocouples

According to EN 15250 (the standard for manufactured masonry heaters) :
- recording is stopped at 25% CO2 recovery (from the point of max CO2). This invites the operator to seek the lowest O2 possible so recording is stopped as early as possible in the charcoal phase
- CO is adjusted at 13% O2; the acceptable limit is 3000 ppm
- 12 Pa draft is guaranteed
- LHV (dry) is used (for hardwood 18 MJ/kg = 7756 Btu/pound)
- wood must be birch, beech or hornbeam


• 2014 11-08
Firebox size : 54 x 54 x 79 cm
Configuration : firebox is capped by the bake oven floor slab; black bake oven.
Air supply : a 150 x 200 mm conduit
Air intake : air slits are starting between row 2 and 3 for a total of 4 rows (5th row is stuffed with ceramic wool); three 8 mm x 6 cm slits per wall; there is also air coming from the top of door (4 cm x width of door)
Picture PB080188.JPG shows Damien's German made ECOM EN2 combustion analyzer equipped with a water filter
Fuel : 200 year-old oak with the logs stacked vertically
Overall : high emissions, fuel doesn't burn hot enough
lowest CO recorded : 500 ppm, lowest O2 recorded : 13%
Thermo couple data : no

• 2014 11-10
23 kg of oak and chestnut
vertical kindling in the front
lowest CO : 500 ppm, lowest O2 : 11%
thermo couple data : no

• 2014 11-13
a first baffle (30 mm thick ceramic board) is added
oak + chestnut
lowest CO : 300 ppm, lowest O2 : 10%
thermo couple data : yes

• 2014 11-14
a second baffle is added (same material, 80 mm between baffles)
the air slit on top of the door is closed
lowest CO : 250 ppm, lowest O2 : 10%

• 2014 11-17
oak stacked vertically
vertical kindling in the front (practical but poor results)
very dirty start up (the analyzer goes in protection mode above 4000 ppm)

• 2014 11-18 :
short logs, half load, oak upright, front kindling
85 min. burn, 1432 ppm CO, 15.7% O2

2014 11-19 :
short logs, oak upright, red cedar kindling
1312 ppm CO, 12.6% 02
lowest O2 : 8.2%
still a poor burn

2014 11-24
switch to softwood : has a major effect on the quality of the burn
685 ppm CO, 12.7% O2,
lowest O2 : 6.8%
picture PB240096.JPG shows the heater with its heated wall
very quick start up

2014 11-26 :
2/3 load (3 rows) of softwood
a gap is left in the centre of the pile as an experiment
117 min. burn, 1186 ppm CO, 13.7% O2
start up too fast (multiple CO peaks)

2014 11-28 :
2/3 load (3 rows) of softwood
large chunks on the bottom, no medium size, top row is made of very small wood
72 min. burn, 1420 ppm CO, 13.1% O2
start up too fast (shows the impact of the operator)

2014 11-29 :
2/3 load (3 rows) of softwood
100 min. burn, 907 ppm CO, 14% O2
too much kindling, poor start up

2014 12-01 :
three 4 cm dia. holes are cut between course 1 and 2 (one in each wall) and back-filled by 50%
19 kg softwood
96 min. burn (TTL), 832 ppm CO, 13.2% O2
progressive start up and nicely staged burn
lowest CO happens when O2 is at its lowest
air is reduced at 65th minute (from 1.1 m/s to 0.45 m/s) causing the CO to rise; efficiency rises by 8% as less heat exits in the chimney

2014 12-02 :
20 kg softwood
67 min. stop (25% CO2 recovery), 396 ppm CO, 11.7% O2
lowest O2 5.9%
as the air untouched, efficiency drops at the end

2014 12-03 :
21 kg softwood
78 min. stop : 233 ppm CO, 9.4% O2
not meaningful because the first 20 minutes are missing

2014 12-04 :
21.5 kg softwood
76 min. stop : 521 ppm CO, 10.6% O2
100 min. stop : 765 ppm CO, 12.3% O2
lowest 02 : 5.1% (CO peak)
too much very dry kindling (14%) : start up is too fast

2014 12-05 :
19 kg softwood
71 min. stop : 379 ppm CO, 12% O2
98 min. stop : 978 ppm CO, 13.7% O2
lowest O2 : 7.1%
fast start up

Some observations :

• compared to a firebox with grates, a firebox with a solid bottom requires a different approach to firing : the air supply is closed in the early charcoal phase and the coals are left in firebox.
This method gives low emissions, high efficiency and plenty of bio-char.

• the twin baffle seems to create a high turbulence/high temperature zone where combustion air is plentiful.

• the main variables seem to be :
- the type of wood (ancient oak doesn't burn well)
- the arrangement of the load, including kindling
- the amount of kindling

• in order to break a burn down into its various stages, it would be good to measure the absolute CO (g/kg/T) : how much CO per kg of wood at a given time.

• a testing protocol is very much needed :
- when to stop measurements ?
- can the air intake be reduced and when ? When the test heater is also the house heater, there is pressure to reduce the air intake at coal stage in order to increase efficiency.
- can the door be opened to rearrange the ashes and when ?

Thanks Damien !

Data file:   (41 Mb)

This page was updated on  February 12, 2015
This page was created on February 12, 2015