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Green
Fireplaces
Efficiency
Full
OMNI Report (PDF)
Explanation: The graph shows EPA certified
stoves to have an average efficiency of about 63%
- the default number. Masonry fireplaces tested
between 20% and 50% efficient whether they were
tested open or with closed glass doors. The one
"zero-clearance" metal fireplace tested lowest at
7% efficient - again with or without doors.
Room for Improvement: The scatter might
indicate that there is a lot of room for improvement
in fireplace efficiency. Fireplaces have not been
tested for efficiency very often. I think clean-burning
fireplaces probably will be about 63% efficiency
- just as clean-burning stoves are.
Methodology: While the study compares the
efficiencies of various fireplaces and stoves, it
only tells half the story - maybe not even half.
The method used to test for efficiency is called
the "flue gas loss method". The energy lost up the
chimney, which can easily be determined by the volume
and temperature of the flue gasses, is deducted
from the total energy known to be in the fuel burned.
All the rest of the energy is assumed to be useful.
Heat Transmission: That doesn't address
the vast differences in the way the heat is transmitted
- or even if it's transmitted. How much heat is
wasted into the brickwork if the chimney is outside
the house? How much heat is wasted when cold air
is sucked down the chimney to cool a light weight
air cooled metal chimney? In what situations would
the heat be delivered more effectively radiantly
or by convection? How does the air lost up the chimney
relate to the ventilation system?
The amount of heat transferred by infra-red
radiation versus heat transferred by convection
and/or conduction is small. The actual amount
of matter heated by infra-red (ie, what you feel
on your skin) is very small but because it gets
delivered directly to your skin it is many times
more effective. It gets heat exactly where it
is needed and most easily absorbed and sensed
by humans. - Tiegs
Summary: Fireplaces - even cheap poorly
designed fireplaces - are not negatively efficient,
as some would contend. In fact there is reason to
believe fireplaces can be as efficient as other
wood-burning appliances and can meet level-playing-field
efficiency standards.
Rumford
Fireplace Efficiency
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Jim Buckley,
Buckley
Rumford Company
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