The early freeze has caught me a little unprepared as we don’t have much spare around the cottage to store wood and I need to get into the woods to collect more firewood soon. My hopes of a swift thaw have been dashed. It looks like we’ll get freezing rain tonight instead and with the ground so cold it will add ice to the snow. Still it was pretty this evening as the sun set on the snowy scene – it’s been several degrees below zero all day!
Until I can get into the woods I’ve been eaking out our supply around the house by clearing up in the yard. It’s easy to get into a rut with and think of some species as ‘firewood’. There is a fair amount of Ash in my yard at the moment and I know plenty of people who’d regard it as seasoned – but as I never quite know where the next load of Ash for turning is coming from I’m loathe to burn it (yet)!
Ash is a prime example of this, far too valuable to burn in my view unless it’s no use for planking, turning or handles, but it makes the best firewood by far according to many. Why is this? What is it that causes one wood to have the right qualities and the next not? There is a lot of firewood lore about and what is it based upon?
Peter Jameson emailed me about my post on ‘logs to burn‘ to say ‘does the poem mean Horse or Sweet Chestnut? In my experience Sweet Chestnut burns well but spits. Horse not so well and is a swine to split the fibres crush and laugh at the splitting axe. ‘ I’d assumed it was referring to Sweet Chestnut as common lore is that Horse Chestnut is no good as firewood. But why is that?
I’ve just been burning well seasoned Sweet Chestnut on our stove. Some of it found it’s way onto our open fire which is a Jetmaster, a bit like a stove without a door, and I agree with Peter. Surprising how light well seasoned chestnut is, but it still pops and bangs, though not too much, great firewood but you wouldn’t want it in range of a new carpet (ours is not so new!).
I’ve also tried out some well seasoned Holly which seemed fine – I didn’t really notice to be honest. With plenty of dark hours to spare I turn to Google (and Wikipedia my favourite on-line encylopedia which has blown away the business models of the commercial ones) to learn a little about what it being said about firewood online.
Cue a photo of the pretty sunset on the commons this afternoon. First time the sun has been out for quite a while, get it while you can at this time of year.
Back to the firewood. Freshly felled wood can be upto 60% moisture. Wet (or green) wood is hard to burn, with so much moisture it’s hard to light and when it does burn much of the released energy is used to burn off the water as steam – which in turn is released into the flue or chimney. Without a strong draw fed by a hot fire the steam will likely condense on the sides of the chimney together with any unburnt volatile chemicals in the smoke. A bit like a garden bonfire greenwood will burn cool, smoky and smelly.
As well as being a poor heat source this is also a way of increasing your carbon footprint, not reducing it, as the incomplete combustion produces unnecessary greenhouse gases and toxic chemicals into the atmosphere. To reduce your carbon footprint the combustion needs to be as complete as possible reducing the exhaust gases. So burning green firewood is not Green at all if you seen what I mean, in terms of climate change at least!
The calorific value of burning greenwood at 60% is typically about 1.7KWh/Kg but as the moisture reduces to 25% this can increase to 4 KWh/Kg. Note to self – drying my firewood properly will mean I need less, or produce twice as much heat!
Interestingly the calorific value of all wood species is about the same in KWh/Kg. But of course, the weight or density of each type of wood varies a lot. Since firewood is traditionally sold by volume that can make a big difference to the heating power of a load. The densest (heaviest) woods yield almost twice as much heat from a log as the lightest woods. Typically fast growing softwoods are lightest and oak the densest with other woods spread between depending a little upon how fast grown they are – as fast growing tends to lighten the density of the wood.
That’s a long winded way of saying that you’ll get 4 times as much heat from the pickup load of seasoned oak as you will from a pickup load of unseasoned softwood. But it’s more complicated than that, of course.
Typically the densest woods have a structure that’s hard to burn efficiently and they tend to smoulder. Perhaps wood that tends to be knotty and twisted in grain has a similar problem with burning unless the fire is hot enough? This might help to explain why Elm and Horse Chestnut have such a poor reputation as firewood, combined of course with how hard they are too split.
It seems to me that the ultimate firewood will be the one that has the best balance of characteristics. It needs to be low in moisture content (so it’s easy to dry), medium density so it’s not too hard to burn but has reasonable lasting properties, low in volatile chemicals so it’s not too smokey or clogs up the chimney and last but not least easy to split. Not surprising that Ash wins the contest then. But if you don’t have plenty of spare ash, knowing how to select, split, season your firewood and which to feed onto the fire to start and to last will do you just as well if not better! Must go now – just off to pick up a load of Birch – great firewood burnt hot, but you do get through it.
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