My quest of getting most out of the wood.

[Corniit]

Hi All,

New member here. I’ve got 2.8m3 / 100 cubic feet size Woodstove made from firebrick in my house. It is high efficiency stove where flue gases travel first to the top of the stove and then down through the side cheeks to maximize the heat transfer to the mass. I’m bit obsessed to get the maximum heat from the wood and tested different kind of setups. Even built some secondary air device to the hearth, but discarded it due to that causing more troubles than benefits. Stove has three adjustable air intakes: fire grate and door bottom and top vents.

For the most heat transfer to the bricks, I’ve tried to control the amount of intake air and from where to conduct the air in which phase. Without any real data it would be just guessing. So, I decided to build sensor to quantify the combustion process. I added K -type temperature sensor and lambda probe designed for wood burning (NGK OZA685-WW1) where the flue gases exit the stove. Then built and programmed device which has display to show lambda and temperature -value as well as plot both to chart. This latest version also added memory and USB to be able to download the data to PC for creating chart for better analyzing. Data is recorded every two seconds so the built in display can show bit over four minutes of history.

I found some Danish article regarding automatic control of stove and from that chart interpreted that the optimal lambda value would be between 11 and 12%. If someone knows better what’s the most efficient value, please let me know?

I’ve just taken this system into use so only three burns experience at the moment. Attached is one chart from today. Blue line is temperature in Celsius and orange line is O2 value. X axis values are seconds, 0 is the ignition. From the chart can be seen that it took around 15 minutes for the fire to properly start burning. I started the fire from the top of wood stack and start phase only intake from grate is open. Then closed grate intake and opened door lower intake (forgot to write down the time). At around 47 minutes/2858s, closed the door intake and opened the grate to burn effectively the embers as the proper burning started dying. Those peaks around 63min/3800s were due to burning some tissue papers. Then the next O2 peaks @4030s & 4495s were due to opening the door and moving the embers to the grate for burning them completely.

Thanks for reading my long story

 

[Todd]

Sounds like you have some kind of masonry heater, can you post some pictures? Masonry heaters should be burned once or twice per day with a full load wide open air. This will give you extremely hot flue gasses to heat up all that thermal mass. Smaller splits stacked with good air gaps between will give you a hotter fire.

 

[qwee]

100 ft3 = less than a cord of wood which is 128 ft3. So a little smaller than a cord of firewood stacked vertically. Let's say 4' wide x 8'+ tall x 3' deep, that gets us to around 100ft3. Your design is known as a Finnish contraflow - vertical channels (up and down). Other types of MH can have horizontal channels.

I've never heard anyone caring about air on a masonry heater - it is usually just a wide open burn with wide open air. Grate air is opened at the very end to get rid of the coals. There should be a damper somewhere in the stove's pipe/flue system. It is opened on burning and closed once the fire is out - this holds in the heat which radiates out for several hours. Thus a fast hot fire is favored. Very different way to burn compared to a wood stove.

Dry wood is best (< 20% moisture content). Since you want a short intense fire some say it is better to have up to medium sized splits. You don't want one large split left in the firebox taking an extra 1/2 hour to burn down. Because you want to close the flue ASAP to trap the heat in the mass. This Aussie, Alan Burdon, explains things. Some debate, but I believe top-down burns are less efficient than side or bottom startups with a lot of kindling. Alan likes the top-down burn.

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[EbS-P]

Firebox temps and flue gas exits determine efficiency. Can you burn all the fuel completely in the firebox. That’s why firebox temps are important. To extract the most heat you want low flue gas’s temps but not so low that you get creosote condensation.

I don’t see how lambda is useful IMO.