Depends on what kind you are talking about and how it is installed. We have a gas log installed in a fireplace originally designed as a wood burning fireplace. Even with the flue closed, its super inefficient because (surprise) the hot air goes straight up mostly. Its a nice luxury, makes the room cozy and warm, and when our power goes out its a good emergency room heating option but it does NOT save us any money. In fact, before we had kids we would accidentally get drunk and go to sleep with it on, and then get $200 gas bills.