We will have two wood heaters in the house – 1 large fan forced, 1 small convective. The large one in the living room will see a lot of use, the other much less.
We don’t want to collect firewood from our bush – fallen branches and dead trees are part of the ecosystem, with many creatures and other vegetation depending on it. But we also don’t want to buy it in – expensive and not necessarily sustainably sourced.
Since we have a couple of small clearings, we decided to plant some dedicated firewood trees that we could harvest. But what sort and how many?
To be self sufficient in wood as fast as possible, we need trees that grow quickly. Red Gum is a really popular wood, but it’s slow growing and not suited to our area. We do have peppermints and messmate on the block, but they only grow well in certain locations – they don’t like wet feet. Most of the seedlings we’ve planted in the damaged area of the block have not survived.
Blue gums are local to the region, although we don’t have any, grow really quickly, generate good heat and from experience take longer to burn than messmate. They also coppice well – we can cut them down and they will regrow. So blue gums it is.
Method 1 – Anecdotal volume, based on past usage
For the year we lived in an old drafty house, with large single glazed windows, in an enormous living room, we used around 8 – 10 m3 wood, much of it messmate. We hope that in the new 7.5 star house we won’t use anywhere near as much. Based on living in a 6 star house in the same area, we use the heating probably half as often, frequently at low settings. So we shouldn’t have to light the fire as often or keep it roaring as we did in the old house. Our house is also designed to minimise heating requirements with the black roof, waffle pod slab etc since we’re not worried about it heating up in summer, as we’ll have lots of power then for the air conditioning. Our new wood heaters will also be much more efficient at distributing the heat into the house, rather than sending it up the chimney. We’re hoping that this will drop the use to around 4m3 of good quality wood.
Method 2 – more scientific calculated volume based on Star Rating
Our star rating says we will need 18,000 MJ per year, to heat the entire house. Assuming well air dried wood has a calorific value of around 13 MJ/kg, and that our heater is 65% efficient, we would need 2.2 m3 of blue gum.
Given that we won’t perfectly control the temperature and are likely to heat it more than the heat rating suggests, even though we won’t heat the entire house, the 4m3 looks like a pretty good target.
So how many blue gums to plant to get 4m3/year?
If our trees grow well (yet to be proven), we hope they may have a 20 cm diameter (at 1.4m above ground) after 10 years and be over 10 m tall – based on what I can find on forestry sites. Just assuming the trunk is a cylinder (although it tapers, it should be 12 -13m tall, and there are branches….) then that would give us around 0.3m3 wood/tree. Hopefully a lot more!
But our soil is pretty poor, and the seedlings will be competing with bracken and other plants) so we don’t want to bank on more. We just may not get the growth.
So in theory we’d need 13 trees for one year’s supply. Over our 10 year plant to harvest time, we would need to plant 130 blue gums. By the time we’re through all the trees, the first ones should have coppiced and grown back. We should be sustainable.
Last year we planted around 60 blue gums, enough for almost 5 years wood supply, although we likely won’t want to leave some of them for 15 years – they might be too big to handle.
It seems like a lot of big trees to feed one small fire, but if we have too much, there are plenty of people who would like the excess. We will supplement the plantation with branches growing over the tracks or trees growing in the defendable space – we have to keep the entrance track clear of overhang, and the tree canopies in the defendable space separated by 5m, with no branches within 2 metres of the ground. If this extra wood is still not enough, we’ll have to plant more than the target 130.
The wood will need to be seasoned (dried) before we burn it – ideally stored and left for two years for the moisture to evaporate. Burning green wood is hard, and a lot of the heat goes into evaporating the moisture in the wood, so it is doesn’t provide anywhere near as much useable heat.
So, in theory, 12 years after planting (10 years after we move into the house), we should be wood self-sufficient. A long term project, but one that’s worth it.
We’ve got about 12m3 drying at the moment from the defendable space and road clearing & trimming. Not sure what we’ll do for wood before our first harvest….