We would like to convert the coal fireplaces (2) in our old Victorian to natural gas. The fireplaces don't have dampers so I'm sure that must be added. What else do I have to know?
The optimal natural gas system is the type called "direct vent." This is an insert that fits into your firebox and contains 2 vents that go up the chimney--one to exhaust the fumes and one to bring in fresh air that feeds the combustion. The system is sealed, which means that it does not communicate with the ambient air inside your house.
These are the most energy efficient and safest gas fireplace systems on the market, and they can put out quite a lot of really serious heat. It's best to get a remote thermostat that typically comes as an option at extra cost. Otherwise you'll be busy turning the thing off and then on again periodically.
These units are costly, usually $1500 or more and take the better part of a day to install. And you really need to vet the installer to make sure he knows how to do it right.
The less expensive gas system would be a vented one that makes a nice flame but sends most of the heat up the chimney. These do communicate with the air in the indoors of you house, but not too badly because they are vented.
The thing you should stay away from is any system that is described as unvented. These consume the oxygen in your room and dump exhaust fumes into the room, and are therefore a health and safety hazard. Buidling codes often prohibit installations of this kind especially in sleeping rooms because some people have been killed in their sleep with these systems on. Besides if you have any kind of breathing problems, even a minor one, it will make it worse.
If you can afford it and you want realy serious heat, not something that just looks pretty, go DIRECT VENT. There are many on the market. Shop around on the web and then in a local merchant-installer.
Good luck with you search and decision.