09 January 2011

Top-down fire

So you think you know how to light a fire - think again.

John T introduced me to the concept of the "top-down" or "upside" fire. I was a bit sceptical but after listening to his explanation and then watching a few youtube videos I decided to try it out.

The Theory
In a normal fire we start at the base with a ball of paper, then place kindling and finally, logs on top. The principal here is that since the heat and the flames rise, then they will ignite the overlying material. More stuff is then added to the top.

In a top-down fire the logs are placed on the bottom, packed as tightly together as possible. The kindling is put on top of the logs and the newspaper on top of the kindling. In this case the fire burns slowly downward. Proponents of this method suggest that it is more efficient, produces more heat, burns better, produces less smoke and takes less work. The only problem is that it is totally contrary to everything you have ever been taught about lighting fires.

The Practice
I packed the logs into the stove, getting them as tightly together as I could. Then I piled about 10 pieces of kindling on top. I was fairly sure that simple balls of newspaper would not burn long enough to ignite the kindling so I rolled a couple of sheets of paper into long thin sausages and then tied them in a knot. The idea was that these would burn far longer than a simple ball. I made 4 of them. I then put a ball of paper on top to light the rolls.

I lit the ball and this indeed got hot enough to light the rolls. After about 3 mins the kindling took and after another 7 the logs started to burn. I was surprised at how well it worked. The fire lasted for about 2 hours with no attention. There was lots of heat and virtually no ash. I left the door of the stove open as I wanted to watch and there was no smoke at all.

This was a complete success. I am a total convert.

Why does it work?
The theory is that in a "normal" fire, the kindling heats the logs above, this drives off any steam and also some of the volatiles. The steam and volatiles form the smoke. The smoke is lost energy as it is not burning or providing any heat.

In the "top-down" fire a limited amount of heat goes down, any volatiles that are generated from heating the unburnt wood are ignited as they rise into the fire. This means that the wood burns in a much more efficient manner, producing lots of heat and very little smoke.

Some videos
This guy does a controlled experiment where burns a similar amount of wood as a top-down and a tee-pee fire. Its good to see a controlled experiment and he gets the right result however, he did not pack the logs together tight enough. It would have worked better if he had not left the gaps between them. Also the vid is in three parts and most of it is a bloke in camo chopping wood with a large knife. Go straight to part three.

These guys build the ultimate top-down fire. Don't try at home. More here and here.

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