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 scatter might indicate that there is a lot of room for improvement in fireplace efficiency. I think clean-burning fireplaces probably will be about 63% efficiency just as clean-burning stoves are. The one "zero-clearance" metal fireplace tested lowest at 7% efficient - again with or without doors. The Rumford (I think it was "C") did not stand out becasue, by the NSC rules the fuel load was based on hearth area, and was about half what it should have been. See Rumford Fueling Protocol.
While this table compares the efficiencies of various fireplaces and stoves, it only tells half the story - maybe not even half. The efficiency is a gift, really. It's 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 in the fuel burned. All the rest of the energy is assumed to be useful.
That doesn't address the vast differences in the way the energy is transmitted - or even if it's transmitted. Is a fireplace with an outside chimney as efficient as one with an inside chimney? Does a stove heat as efficiently outdoors? To be rediculous, how efficient is that 93% efficient gas furnace if it were located outside with ten feet of uninsulated ductwork?
My point isn't to be rediculous but radiant heat is delivered very efficiently, as Paul Tiegs notes, so in some situations it costs less to heat radiantly than it does to heat the air with a more "efficient" appliance.
This fact became apparent to me when, in the test lab, our Rumford fireplace was at least as efficient by the flue gas loss method when it was tested with the doors closed and obviously was not heating as effectively. See the picture of one of our 1996 closed door tests. Had the door been open with that fire the cat would have been singed and the beer boiled. That's what led me to invesitigate the nature of radiant heat. Now we know that glass doors block over 90% of the radiant heat from a fireplace - yet the fireplace is just as "efficient"?
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