I know, how complicated can it be? Bung a few old bits of wood in, and set fire to it. Simple. I agree, it is simple, but the number of corroded and sooted-up log-stoves that I’ve seen leads me to suspect that we’re not all getting it right… Essentially it’s all about avoiding a situation where un-burnt fuel isn’t being used (incomplete combustion). Incomplete combustion usually means the fuel you’ve paid for is heading off up the chimney as soot and tar, which corrodes your stove. You’re also running the risk of generating carbon monoxide; which is poisonous.
As far as log-stoves are concerned, air comes in two flavours: primary, which feeds the bed of the fire; and secondary, which feeds the flames above it. Flames are made of burning gas which is released as a proportion of the fuel evaporates on heating. This gaseous part of the wood forms around eighty percent of the calorific value of the fuel, so if you fail to burn these gasses, you’re losing most of the potential heat from your fuel up the chimney. The golden rule is, burning wood always needs secondary air. It doesn’t matter which species you’re burning, or whether you want a lot of heat from the stove or just a little. If you starve a wood fire of secondary air, you’re risking incomplete combustion. Normally, you’ll want to keep the draughts on the stove set relatively high to get a clean, bright burn. The only time you should really be closing anything is when the fire has died down to an ember bed. At this point, you can shut the primary air and reduce the secondary air to a low setting. Don’t be tempted to close the draughts completely to keep the stove ‘in’ overnight as; again, you’ll probably end up doing more harm than good.
The rule for fuel is, only burn wood. Sounds stupid doesn’t it, what else would you burn? Well, coal for a start, or paper, water, paint, nails, soil, the list is more or less endless. Growing wood is around 50 percent water depending on the species. You’d probably be able to wring it out if it wasn’t so rigid. Because of the heat needed to boil away this water before the wood can burn, green wood has about half the energy value of properly seasoned wood. This drop in energy reduces the efficiency of the combustion reaction, with a bunch of associated problems. If you can’t find a supplier who can reliably give you dry wood, try deliberately buying green and drying it yourself, it’s pretty easy, if you have the space. Other contaminants can be more serious. A lot of them either lead to heavy metals in the ash, or more toxic (and corrosive) flue gas. You really want to avoid anything with paint or preservative on it, as these will turn wood ash (a useful addition to the compost) into a waste that has to go to landfill. The same is also true of coal, aside from the problems associated with climate change.
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This isn’t a comprehensive guide, but following these two rules should keep everything working reasonably well. If you have any awkward questions, I’m around on Facebook or Twitter, why not contact me and let me know? Or buy your very own copy of The Log Book.
Will Rolls is a chartered forester with a strong interest in wood as a fuel. He is the author of The Log Book: Getting the best from your log burning stove. this article originally appeared in Country Smallholding Magazine in 2012.