Gas Logs - Vented
The two basic types of log you can buy for your gas fireplace are vented and vent free. The vented logs require some means of venting the fumes into the outside air. Some varieties will use outdoor air for their burn as well, thus needing an incoming as well as an outgoing vent. Vent free logs use indoor air for their burn and control the amount of fumes so that its safe to vent back into the home.
The vent free option may sound attractive, but these logs are illegal in some states. And even where they are legal, they can cause condensation in the home which leads to mold and mildew. A vented log may be more difficult to install up front, but in the long run it will probably give you more peace of mind.
Some vented gas logs will give you a realistic yellow flame, but you pay a price for that. The yellow flame is not as efficient and produces soot. A blue flame burns hotter and cleaner at the expense of a bit of realism.
If you're installing the gas log in a traditional masonry fireplace, that will take care of your venting needs right there. You can get a vented log that burns either propane or natural gas, install it in the fireplace, and use it with the damper open. Just remember that the chimney needs to be in good working condition. If the chimney is damaged, dirty, or doesn't produce a draft, a vented gas log is not a solution to these problems. In any event, you should always have your chimney thoroughly cleaned before installing a gas log. While that's being done, the chimney sweep can alert you to other problems or potential problems with the system.
If you install the gas log in the fireplace, building codes may require that the damper be permanently blocked open. It's just too easy to forget to open the damper when "building a fire" involves flicking a switch, sometimes by remote control. This may be a good idea in any event. You'll need a tight seal on the glass doors to reduce the heat loss, though.
Many times, of course, the reason you want a gas log is because you don't have a traditional chimney. In that case, there are a couple of options available-top venting, and direct venting.
Top venting is the best option if it's possible. This involves installing a special pipe that vents the appliance from the top. In effect, you're installing a small but efficient chimney. Any store that sells gas logs should also sell top venting pipes and can recommend someone who knows how to install the pipe.
Where top venting isn't possible, direct venting can be. Direct venting vents the fireplace directly to the outside via a pipe that runs out the back or sometimes from the top and then out the back wall. This is the option mentioned above that actually requires two pipes (though usually one is placed inside the other. One pipe draws air from the outside to fuel the fire. The other pipe vents gases and fumes.