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!