Firewood Thoughts

I get to thinking about firewood quite a lot at this time of year.  Our main heat source in the winter is wood and knowing how to get the most efficient burn makes quite a difference to the temperature in the cottage, and to the amout of wood I need to shift on a daily basis.

There is a lot of folklore around firewood. Not without reason either as in the days before central heating (whats that?), double glazing (must do something about that soon) and cavity wall insulation, having poor firewood could mean freezing to death or even burning your house down rather than sitting next to the fire in a cosy warm room.

The good news is that all wood burns. Even better, all wood delivers about the same heat energy per Kg. It’s just that some woods are less dense so  a 1 tonne pile of  birch will take up twice the volume of 1 tonne of oak, for example. If  you notice that you burn your way through more birch, willow or softwoods than oak then that’s the simple explanation, you need to burn roughly twice as many logs. Most other woods are somewhere in between in terms of density. Since most firewood is delivered in volume – it’s something to be aware of.

Of course it’s not quite that simple as any wood you care to burn will also contain moisture. Even the floors and doors in the house will contain some moisture which is dependent upon the atmospheric moisture content – the wood will adjust which is why things tend to shift through the year from summer to winter as it swells and contracts. When it’s cut wood has a moisture content somewhere between 30 and 60%. Some woods are wetter than others depending upon the season of cutting and the type of tree. Ash has a relatively low moisture content which combined with it’s medium density and a relative lack of tars probably accounts for it’s reputation as the best firewood of all.

Trying to burn water is going to reduce the efficiency of the combustion as heat is diverted to turning water to steam and the wood resists burning when its too wet so you have to increase the amount of air flowing into the fire and up the chimney which only further reduces heat flowing to where you want it. Steam in the flue combines with the unburnt wood products in the partially combusted smoke and can condense as creosote, with the danger of blocking the flue and causing a chimney fire.

Burning the driest wood you can get makes your fire most efficient, reduces the amount of wood you need to cut (or buy in) and store, saves you time and money and releases less emissions into the atmosphere. Most of all on a really cold night you’ll be cosier and warmer.

There are more ways of cutting, splitting, stacking, seasoning and storing firewood than I could possibly cover in a short post that is already getting too long. But if you have the inclination you can find an old post of mine looking at firewood stacks in Southern Germany here – Firewood stacks of Mahringen pt II.

If you don’t happen to have a spare barn to act as a logstore then it’s no problem to stack the wood outside. The key to drying wood outside is to make the most of the sunshine (as I was reminded recently it dries wood for free and with no carbon emissions) and allow the air to circulate through the stack. I have pallets on the top of this stack to provide an air gap between the wood and the ground sheet which is protecting it from the rain, as well as a raft of old posts on the bottom to improve the drainage.

When you come to burn it, if your firewood is not as dry as you’d like then you can do some simple things to help. Make sure you bring firewood into the house before you want to burn it – preferably stack it by the fire for a day or two if you can do so safely. The smaller you chop the wood the quicker it will dry before you burn it – and the more efficiently it will burn once it’s on the fire.

You’ll find that the lower density woods are easier to dry out in a hurry so if you do have to burn wetter wood than you’d like you’ll find chopping up Ash, Birch, Willow etc into small sticks and drying on top of the stove before burning is a better bet than trying to burn green Oak.

To keep a fire working at optimum efficiency you should be continually adding small pieces of firewood to it. But generally that’s a bit like hard work and we all prefer to put a big log on now and then through the evening. If your fire is just for effect then that’s fine but if you are relying on it to heat the room, house or your water then big logs are bad, and big wet logs are the worst.

 

The combustion is most efficient when just the right amount of air is mixed with the fuel to burn up all of the fuel with the minimum amount of heat escaping through the flue with the water and carbon dioxide. An open fire is normally quite inefficient compared with a stove as it normall draws too much air into the fire and much of the heat is lost into the chimney and more warm air is drawn out of the room. But not all stoves are very efficient and some open fires can be almost as efficient as a stove.

In our living room we have a ‘Jetmaster’ open fire and even though I’ve been using it for 18years I’ve only recently learned that a part of the reason for it’s metal fire box construction is that it has a convector heater built in as room air is drawn under the firebox and up through a void at the back, coming back out through radiating fins at the top of the firebox and into the room as hot air! This, together with the variable damper, can raise the overall efficiency of the fire to 50% – which is exceptionally good for an open fireplace, though nowhere near the 72% of our woodburning stove.

I trust you are all enjoying the warmth of your labours in front of your fires right now!

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Of fires and firewood

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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