Using permaculture ethics & design principles to transform an old energy guzzling bungalow into a showcase of sustainable design. It's about energy cycling, building community, self-reliance,creatively using & reusing materials... all without spending heaps of money.

Monday, August 8, 2011

Making the most of firewood

Principle 3: Obtain a yield
Principle 4: Apply self regulation and accept feedback
Gourmet combustion stove in full swing, cooking on stove top, drying clothes, boosting hot water, heating oven and warming our house
We obtain multiple benefits from using our combustion stove, because it is designed as a stove / oven / heater and hot water booster, which helps to maximise our yields. We also get the added benefit of a glass door, which creates a fantastic ambiance and allows us to more easily monitor the fire to ensure that it burns hot, key to a longevity and low emissions. One of the inefficiencies with this style of cooker is that we use more fuel than we would for a specific task focused model, because we are performing many functions from the one task. In our case, the advantages multiple functions outweigh the disadvantages.
Some of the other functions that we use our stove for is to dry clothes, dry citrus peel (in oven overnight) for use as firelighters, warm our sourdough bread before baking, keep a large pot of water on the boil for cooking or topping up the bath.
Firewood: sustainable and appropriate energy source
This is an extract from the article of the same name from the eBook David Holmgren: Collected Writings & Presentations 1978 to 2006 available as a free download (article 41 written in 2005).
 ...wood heating has lower greenhouse gas emissions than any other fuel heating. Even poorly managed woodlands supplying wood heaters (60% efficient) up to 400 kms distant, have a net greenhouse gas production of one third that of natural gas and one tenth that of electricity. The figures for sustainably managed regrowth native forest are one third that of woodland systems while new plantations managed for timber and firewood have no net carbon dioxide emissions and actually take 0.17kg of CO2 out of the atmosphere for every kWhr of heat produced.
Heating only requires low quality energies such as passive solar gain or firewood. If we use high quality energies such as electricity for space or water heating then this is wasteful (and therefore environmentally damaging in some way) whether that electricity is from coal or renewable sources. We should reserve electricity for lighting, communications and electric motors.
In a more enlightened sustainable and low energy future, the very real problems of localised air pollution from wood burning in cities needs to be addressed by better training of firewood users, better designed wood burning stoves and most importantly, use of cleaner burning charcoal produced in sustainably managed forests using modern wood gasifer technology which recovers the waste heat.
Our fuel supply
Most of our wood was sourced from the large red gum that was on site, but we have also been using off-cuts from the building of the new house, scrap timber from the deconstruction of the old bungalow and prunings from other trees on site. We also use scrap paper / card and oven dried citrus peel (high oil content) to start the fire. The wood stove provides us with the service of being able to use this renewable natural resource that would otherwise need to be disposed of (or composted).
I expect that this intial supply will keep us going for a number of years, but in the longer term we need access to a good wood source, this means either buying it in (very expensive) or cutting it up ourselves from appropriate locations. If my only option was to buy firewood from a expensive or unsustainable source I would think twice about using a wood stove as my primary heating and hot water back-up system.
Living in the country and having established good contacts throughout the local community makes cutting wood ourselves a good option. We'll source it either from friends or with permission from the appropriate authority from public land. I've learnt how to use and maintaining a chainsaw by working with friends who know what they are doing, gaining enough confidence to operate the machine on my own. Since moving here I've acquired a 30 year old chainsaw and purchased the accessories and safety gear to be able to use it as safely as it can be used.

Storage location
Originally I stacked our firewood in the backyard, against a south facing wall, thinking that this would be the best spot. This was based on the belief that the space wouldn't receive any sun, so it wasn't good for growing, was fairly close to the house and would have good access to the fireplace.
After our first year I realised that this was not a good spot at all. In order to collect wood in winter I needed to get out into the elements, putting on boots and sometimes braving the rain. The other issue with this location was that I would need to transport any new wood that I collected all the way around to the backyard to stack it.
I have since begun stacking wood at the front of the property, where there is better access and it's close to the covered entryway where a smaller stack of wood is stored on a stand near the front door. Using the stand I can measure how much wood we use over the cold season, so I can estimate how much wood I need for the future. I measured the stand space in cubic metres and mark how many times I fill it over the season. By locating our wood stack in a good spot we've saved ourselves a lot of time and effort, a good example of design principle 4.

Chopping, stacking and seasoning
Wood needs to be cut into pieces that can easily fit into our stove. Big rounds may need to dry for quite a while before you can chop them up. I like to leave pieces as big as possible as they will take longer to burn as they have less surface area, but you need smaller sizes too to get the fire up to temperature. I like to use smaller diameter branches for this purpose. I've found that its easy to cut branches to size using loppers while wood is green or with a drop saw when its dry, or if it's too thick for loppers.
It's important to stack the wood so that air can circulate and carry away moisture as it evaporates through both ends of the piece. I make sure that wood stacks lean slightly back to the wall and that the pieces fit tightly with each other. By stacking the wood well I can be sure that it wont easily fall over and can be covered to help keep it dry.
Freshly cut wood has up to 80% moisture content, it must be seasoned (dried) to 20-25% before it should be used. Leaving wood in the rain will cause the wood to reabsorb moisture, reversing the drying process, so it need to be covered for at least 6-9 months to dry sufficiently. Leaving big chunks will take much longer to dry, 2-3 years is not a bad guide, if wood sizzles when it burns then it's not dry enough.
Red Gum and building offcuts stacked and later covered with a tarpaulin to dry
Cutting up and stacking dried red gum branches for firewood using a drop saw
Small branches are cut to length when pruned and stacked for use as kindling. Smaller pieces are chopped up finely and left on site for mulch.
Oven dried citrus skins makes great fire starters because of the high oil content

Efficient burning
Most of the remaining moisture in wood is the resins. When the wood heats up in the firebox the resins emit combustible gases, when these ignite they can account for as much as half the heated output of the fire. If the wood is not properly seasoned then steam is emitted when the wood heats up, preventing the gases from igniting. These unignited gases build up on the glass door, if you have one, and inside the chimney as creosote.
Creosote is highly combustible and condenses in liquid form as the wood exhaust cools up the chimney, solidifying as it dries. As it builds up it increases the risk of a chimney fire, which can burn for an extended period and reach temperatures that can destroy your chimney. Creosote is also caustic and will reduce the life of your chimney.
By seasoning firewood and giving it enough oxygen to properly combust you will reduce creosote formation by burning the gases and sending more heat up the chimney, thereby reducing flue gas cooling.

Method
Learning how to use a wood stove takes time, and each stove has its own intricacies. Practice makes perfect, but these tips that I've picked up may help.
  •  If the firebox is filled with ash it may be a good idea to remove some. With our stove we leave a 25mm bed of ash to prevent the oven below getting too hot, but too much ash prevents the oven getting hot enough. With kitchen stoves it may be best to remove all ash, but the wood heaters it's sometime best to leave a nice bed. I like to leave any coals in the firebox to give them a second chance to burn.
  • Start with well dried citrus peel or eucalyptus leaves as a fire starter, lightly scrunched paper with small dried twigs and sticks stacked around the paper like a tepee. Open up the flue damper and air controls before lighting the paper, and leave the door open until the fire takes. Then close the door, leaving all vents fully open.
  • The fire should get roaring pretty quickly. Once it begins dying down a bit you can flatten out the remaining material and add some larger sticks on the small bed of coals, criss-crossing them to ensure that there is plenty of air around them. Leave all the vents fully open still.
  • Let the fire roar some more. Once it begins to die down again, after 5-10 minutes, add some small logs onto the bed of red hot coals. You can then close the flue damper, but leave the air vents fully open until the logs are well alight.
  • Once there are plenty of hot coals you can add big logs and control the heat of the fire using the air vent. With our combustion stove it's important to open the flue damper before opening the door to add more wood, otherwise smoke will flood into the house setting off the smoke alarm.
  • Burning the fire hot initially helps clean the chimney and glass door and gives you a good coal bed that allows large pieces of wood to burn. Once the wood box is hot it's much easier to burn the combustible gases and make the most of burning wood. You can see this happen with a glass fronted firebox, as flame dances all around the space - which is beautiful to watch. There should be little to no smoke.
  • If the fire dies down and smokes a lot then you will need to stoke it up and add some smaller branches to get it fired up again. Smoldering wood creates pollution and doesn't have the heat to fully combust resins, reducing efficiency. 
  • I tend not to completely close the air vents (we have two) overnight unless there are just hot coals left in the firebox. The thermal mass of the stove keeps the chill off the air overnight after the fire has gone out.
Some other information in this post was sourced from the fantastic article: Wood Burning Basics by Tom Oyen

1 comment:

HAZEL said...

When we moved in here, we replaced the wood burning heater. I have been kicking myself that I didn't replace it with a cooking stove! I love yours...it looks so compact and practical. Oh, well, maybe next time.

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