The Weekly Review

If it's firepower you want ...

JOHN FOXX / STOCKBYTE / THINKSTOCK

Using sustainably sourced wood for heating represents a cheap, low-carbon, renewable energy source. The scent in the air while walking around Camberwell on a winter’s evening shows that many wood fires are burning.

Open fireplaces are commonly used in Melbourne’s older suburbs. Unfortunately, they are inefficient: they’re effective at taking air from a room and putting it up a chimney but not much good at warming.

As a rule of thumb, an open fireplace has the lowest efficiency rate among wood heaters in converting the energy in the wood to useful heat. The negative efficiency is in situations where the fire is taking warmed air from a room and sending it up the chimney faster than the fire is warming the room. And open fireplaces can be even worse with respect to making a house cold and uncomfortable if the chimney is left open when the fire is not in use.
A better solution is a well-designed firebox. These enclosed systems have several features that significantly improve their efficiency. They can be free-standing in a room or be placed in an existing chimney as an insert.

Efficiencies of these systems range from 40 per cent to 70?per cent. This means that, compared with an open fireplace, you could potentially heat a much larger area with the same amount of wood, or use much less wood.

The Masport we have is rated at about 10 kiloWatt thermal (kWt) in general operation. It could potentially heat about 20 squares in the old money. Expect to pay about $2000-$3000 for a well-built firebox. We burn about four to five cubic metres of wood each year. As we get our wood from arborist waste we don’t pay for our fuel, but if we bought it, it would cost us about $600 a year.

A newer technology is the pellet stove. These stoves burn small wood pellets that are about the size of chicken feed. The units generally come with hoppers that contain the pellets. The stove automatically feeds the pellets to the combustion chamber as required. So no more trips outside in the rain to fetch firewood.

A good-quality pellet stove is well engineered and very efficient. The general figure quoted for fuel use is about one tonne of pellets a year for a 10kWt stove. The small amount of fuel used and efficient combustion means that the stoves produce smaller quantities of ash. A tonne of pellets will cost about $600. Pellet stoves are more expensive than fireboxes, costing about $4000-$5000.

In Austria and many other European countries it’s common to have what are called district heating systems burning wood to heat water that is reticulated around a grid of pipes connecting several buildings. These burners are very efficient (about 85 per cent). These would be a useful solution in apartments, shopping strips and commercial buildings in Melbourne, but for general residential use in, say, Camberwell the relatively low concentration of buildings makes these systems less relevant.