I'm not sure how much a blower fan can increase the efficiency of a natural gas fireplace, but it seems like it would help extract at least some energy that would otherwise go out the exhaust.
My house has 2 identical 12,600-18,000 BTU gas fireplaces. One has a blower and the other does not, though there is electrical already installed.
The squirrel cage blower consumes 56 watts and is moderately noisy to run. I think these are typically rated to 130 CFM. My high school electronics teacher said squirrel cage blowers aren't very efficient.
As a proof of concept, I rounded up all the spare PC cooling fans I could find and set them into the intake of the fireplace. The 4 fans (3x 160mm, 1x 120mm) consumed 6 watts, was whisper quiet, and seemed to move about half the air.
I could purchase a purpose built squirrel cage blower with rheostat and temperature switch for $60, or I could purchase 6x 120mm fans for around the same price. I'm thinking if I mounted 6x 120mm fans, I could get equivalent air movement for 18 watts.
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