Wood fires: The
cold evenings at the beginning of October caused us to have a fire in our wood
burning stove in the lounge for several evenings running. Somewhat belatedly, I had a look up the
chimney whilst relaying to fire ready to relight it later that day. It really ought to be swept, as it has not
been swept before. After a phone call,
it was arranged for a chimney sweep to clean the chimney in 5 weeks’ time. I should have organised this last summer!
Open fires
are very inefficient, as most of the heat from the fire, and in the room, is
drawn up the chimney and lost. Typical fuel
efficiency for an open fire is around 10% of useful heat, compared to 80-90%
for an efficient enclosed stove. Fires need
an air supply, and it is more effective/efficient to supply the fire with air
ducted from outside, instead of drawing air from the room. Air drawn from the room by the fire has to be
replaced from air drawn into it as cold draughts.
The
effectiveness of a wood fire is dependent on the type of wood, and how dry the
wood is. Freshly felled ash contains
about 35% moisture, whereas poplar wood can contain up to 60%. Trying to burn damp timber produces steam,
and less heat as a result. Damp wood can
also cause more smoke, and tar deposits in the chimney.
Timber should
be stored in a ventilated area and under cover for at least 6 months to allow
the timber to dry before being burnt.
Some of the
best timbers for burning are ash, beech, hornbeam, hawthorn, apple and
cherry. Oak and elm are dense timbers,
but can be slow to burn. These two
timbers are better burned as a mixture with other timbers. Spruce and horse chestnut woods tend to spit,
so should not be used in open fires for this reason. Softwoods such as pine and spruce are easy to
light, and burn quickly, so are best for starting a fire.
A rough
guide to the size of a stove is that a 5kW stove should be able to heat a room
2.4m high by 25 square metres floor area, but the insulation of walls, ceiling,
and windows, plus draught proofing, and air supply, all have an influence on
this estimate.
Information
about wood burning stoves is available from:
www.energysavingtrust.org.uk/generating-energy/choosing-a-renewable-technology/wood-fuelled-heating.
