Go ventless! you will keep all the heat in your house rather than losing it up the chimney. We had ventless gas logs installed last year. Our fireplace was empty, but we could see where the old gas line was, and the very small flue was just open. I bought a metal cap – made for a wood-burning stove – and capped the flue. the ventless log installation was fairly easy, but took a long time – permits to pull to install a new gas line. It was probably about $1500 to $2000. We run the fire all the time, it heats our whole house and has lowered our gas bill. Ventless logs have a built-in CO2 sensor, but this is unlikely to be a problem anyway if you have the right size log set and the BTUs are below a certain number (I think 40,000).