The worry about wood fires

Do wood fires have a place these days in an efficient home?

2020-07-03

By Richard Keech

Introduction

In discussion about efficiently heating our homes, the question often comes up about whether burning wood is appropriate for heating. My view is that wood burning is highly problematic because of:

  • Air quality problems,
  • Broader environmental impact,
  • Low heating efficiency,
  • Induced draughts.

Air quality

Burning wood has an enormous impact on health because of it pollutes the air both inside and outside our homes. Off the back of a summer which saw unprecedented and even debilitating air pollution from bush fires, it’s all the more problematic to be faced with wood smoke in our neighbourhoods again in winter. It only takes a very small proportion of households burning wood to impact the whole neighbourhood.

There’s been a lot written about wood fires and air quality. Here’s a smattering:

Resources. A good resource to learn more about the impact of wood smoke is the web site of Doctors and Scientists Against Wood Smoke Pollution. A good Australian resource of information is the Australian Air Quality Group here and here.

Doctors and Scientists Against Wood Smoke Pollution logo

Broader environmental impact

Carbon emissions

One line of argument goes along the lines “burning wood is part of the active carbon cycle, i.e. burned wood is liberating CO2 that was recently in the atmosphere, so no harm done.”

The problem with this line of argument is that it fails to acknowledge that, at least where old-growth forest is concerned, for each 1kg of wood recovered, there is considerable additional loss of sequestered carbon trapped in the soil and other parts of the forest.

Habitat and biodiversity impact

Some may say that picking fallen wood is utilising an otherwise wasted resource. However this assumes that the fallen wood serves no useful purpose, when in fact it has considerable value in supporting local biodiversity, for example as habitat for birds and insects.

Efficiency

Direct: heating from wood

Burning wood as a source of heat is problematic because it is very inefficient at turning the embodied chemical energy into useful heating. Clearly there’s a range of efficiencies from:

  • (least-efficient) open fires;
  • slow-combustion fires; and
  • pellet heaters.

However, in each case, most of the energy within the wood fuel ends up going up the chimney.

Indirect: induced draught

As discussed here, a very common downside from using wood-fired heating is that, in the process of burning wood, we draw cold air into our homes from outside. So in our mental reckoning of the warming benefit, we should balance that ambience of the glowing fire in front of you, with the cold draught around your ankles that’s a consequence of the fire. Remember, most fires are draught amplifiers.

External air source. The only way of having an internal wood fire not induce draughts is by using one that has an external air source. This is never possible with an open fire. However some brands of slow-combustion wood burner have the provision for an external air source. This means that it’s possible have a dedicated pipe from the firebox of the wood burner to outside. So the firebox doesn’t need to draw air directly from the room in which it operates.

Year-round problems. Most wood fires don’t have an external air source. In such places there needs to be a vent allowing air to be drawn from outside. Without such vents the risk to occupants would be very high, and the fire would not draw. The trouble with these vents is that they’re usually not controlled vents, i.e. they’re open 24×7. So a side effect is allowing draughts all year round, whether or not the fire is burning.

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