I don't know if I'd call them efficient, unless you have an enclosed shell with circulation around the firebox. That would be the only way you'd really get any use out of one, but that ruins the effect of having a fireplace.
Well, I got a 35 year old fireplace, sucks the heat right out, inserts are much better. I havent kept up but know some of them suck air from outside instead of inside, and have heat exchangers.
The kind that bring in air from outside are really more of a open-face woodstove. I guess if you want to call that a fireplace...
The one in my folk's home, built in 77, is very pretty - it was a locally cut sandstone face that was supposed to be the model for the builder (that went out of business shortly after) but it's totally useless because it does the same thing - sucks the heat right out of the house and up the chimney. We used a Wonder Coal stove vented into the chimney for many years before replacing it with a whole-house model and vent pipe/registers.
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