Ventless fireplaces utilize catalytic converters to oxidize fuel, producing heat without the need for a chimney or flue. They use propane, natural gas, or ethanol as fuel and burn it at high temperatures within the converter, converting it into water vapor, carbon dioxide, and heat. The heat is then released into the room through a vent near the top of the fireplace, while the water vapor and carbon dioxide are expelled through a small exhaust pipe.