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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
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