For the past 20 years, I've been heating two floors (2,800 sq ft) of my house with this. It has dual external air feeds. It has a blower that pulls air from the room, heats it and blows it back out. I run several ceiling fans and the blower on the furnace to distribute the warm air. It provides all the heat I need, down to around 10*. Below 10*, I supplement with a propane furnace.
Putting it in was the best decision I made, when building the house.
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My last two houses had standard masonry fireplaces.
The were both ineffective as a heat source.