We’ve taken down the window screens and fired up our two woodstoves now that more seasonable weather has settled in. We burn about six cords of wood each winter on our farm, a fact that I have always included on the “green” side of our environmental ledger. By burning wood, we’re not burning oil, and by not burning oil, we’re not contributing to climate change.
So I’ve been surprised in recent years to have numerous friends and relations question the environmental benefits of the twin iron stoves that heat our house. Some have even suggested I tear out the woodstoves, replacing them with a “clean” oil or gas furnace.
The knock on burning wood in woodstoves is that it’s a dirty affair, producing more smoky particulate gunk than either oil or gas. The evidence for this is readily apparent: we clean our woodstove chimneys twice a year and our oil-furnace chimney about once every five years, and only then perfunctorily (it never appears to need it). The pro-gas folks are self-evidently correct.
Or are they?
The wood-versus-gas/oil question reveals a hidden truth of environmental problems: we are much better at noticing messes that are right under our noses than those that are hidden from sight. Or, in this case, messes that are created in our name somewhere far away.
The fossil fuel that burns so cleanly in the home furnace has traveled thousands of miles to get here and has made its way through pipelines, refineries, possibly ocean-going tankers, storage tanks, delivery trucks, and drilled wells, each stage of which has been far from clean or emissions-free. Our six cords of wood, meanwhile, have all come from within sight of the house, requiring lots of sweat and calories to wrestle into the shed but less than five gallons total in fossil fuel, to power the chainsaw and run the truck.
In this larger context, which is the cleaner energy for us: the one whose mess we can see or the one that’s hidden thousands of miles away?
The wood-versus-gas question brings up the point that what’s environmentally beneficial for one person in one place may not be true for another person in another place. If every residence in Boston, for example, were converted from oil to wood, air quality in the city would plummet, respiratory illness would increase, and untold fossil fuel would be spent transporting the wood from where it grows – here in northern New England – down to the city to be burned. Nothing green about that.
Wood burning makes sense here in the rural corners of New Hampshire and Vermont because wood is growing near at hand and because our occasional woodstoves are not cheek by jowl from those of our neighbors. Particulate pollution is a local, not a regional, problem. Particulates are relatively heavy and tend to fall to the ground close to where they were emitted; they don’t join the regional weather flow and end up causing acid rain or other problems downwind.
Which isn’t to say that all woodsmoke is created equal. To gain the full environmental benefit of burning wood – the avoidance of greenhouse gas emissions along with minimized particulates – wood should be burned as hot as possible. There are three ways to accomplish this in a woodstove.
First, make sure your firewood is as dry as you can get it. Wood that is saturated with water requires energy to evaporate that water – energy that goes up the chimney as water vapor. Green wood also leads to smoldering fires, which pollute more and heat less.
Second, run your woodstove with as much air as you can (without heating the stove into the danger zone). A constant air supply feeds a hot, clean fire with reduced emissions. If your stove is prone to over-firing, use less wood per stoke. Two or three sticks at full flame will yield equivalent heat to a half-dozen sticks with the air choked down, but with far fewer emissions.
Finally, if the prospect of frequently re-lighting your stove every time the two sticks burn out doesn’t appeal to you, operate your stove with a good inch of ash in the bottom at all times. The ash insulates the coals as they burn down and keeps them ready for the next stoking. This is a much cleaner approach than trying to preserve the coals by closing the damper for the all-night smolder, which is when woodstoves are at their most polluting.
Woodstove efficiency ratings are nearly as good as those of fossil-fuel furnaces when the stoves are burning hot and clean. But these efficiency ratings only tell part of the story: the part that’s right under our noses. They don’t include the benefits that come from using – and, in many cases, improving – a local resource that’s not creating distant messes, out of sight and out of mind.