Burning Wood Efficiently

Patrick Whitefield
Tuesday, 11th February 2014

Patrick Whitefield explores how different stoves can burn wood with different efficiency.

The first point to make about burning wood is that the most efficient way to burn it, is at a very high temperature.

The traditional log burner, which burns large pieces of wood slowly over a long period of time, is very inefficient. All the tar and soot and most of the smoke which go up the chimney represent unburnt fuel. A stove which burns small pieces of wood achieves an intimate mixture of fuel and oxygen, so it burns much hotter and everything is burned. The only output is CO2 and water vapour, which may appear as a bit of white smoke at the beginning of the burn.

Of course this kind of burn uses the fuel up very quickly so if you want to heat your house for a long period you need to store the heat. One way to do this is with water, in a central heating system, which is very complicated and expensive. Two simple ways are the masonry stove and the rocket mass heater. Both burn small sticks for a short time and store the heat in a solid mass of concrete, brick or dried mud.

Masonry stoves, aka ceramic stoves, kachelofens or finovens, are quite high-tech and incredibly efficient. There's one in the bunkhouse at Ragmans Farm, shown below.

Coppiced willow or other small wood is burnt in the firebox, the lower, horizontal part of the stove. The tall, vertical part is made of concrete. The flue goes up through it, down and up again. We burn a hot fire of small sticks for an hour a day. This warms up the concrete, which radiates heat for 24 hours. Only in the coldest weather do we fire it twice in a day.

The disadvantage of a masonry stove is that it's not easy to make and to get one professionally made is very expensive. I tried to make one myself once and it didn't work. In retrospect I can see some specific things I got wrong, but I don't think I'm going to try again!

The rocket stove, by contrast, is much easier to make. The internet is full of instructions for how to make them. They're based on two principles: burning small sticks, as above; and that the air inlet is the same cross-sectional size as the outlet, i.e. chimney. They were originally developed as cooking stoves and have been adapted as space heaters by adding a long, almost horizontal chimney, which is covered in cob, which is basically dried mud. Just as with the masonry stove, one short burn heats up the cob, which then radiates heat over a long period of time.

They're not without their problems either. I have heard of people having troubles with rocket stoves they've made themselves. But both these models share three important features:

- they're very efficient, so you use less wood
- smoke, tar and soot become fuel rather than pollutants
- they make use of small diameter wood, which is hard to find a use for otherwise.

We have some brilliant videos below showing how to build a variety of stoves!

Patrick Whitefield is an experienced permaculture teacher and author of five books, including the permaculture classic The Earth Care Manual. For infomation about his residential and online courses please see www.patrickwhitefield.co.uk

Further resources

How to make a masonry stove from recycled paving slabs

How to make a rocket stove water heater

Rocket stove kitchen

How to make an outdoor brick oven from recyled materials

The Simple Art of Making an Earth Oven

Building a Traditional Wood Fired Clay Oven

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